Evolution of the Hydrobiological Communities of a Coastal Lake in the Novaya Zemlya Archipelago (Southern Island, Arctic Russia) in Relation to Climate Change Following the End of the Little Ice Age

Abstract

There are very few data linking recent climatic changes to changes in biological communities in the Russian Arctic, and no palaeoecological data are available from the Novaya Zemlya archipelago (NZ). We studied chironomid, cladoceran, and diatom communities from a 165-year-old sediment core from a lake on Southern Island, NZ. Sixteen diatom and four cladoceran species new to NZ were found in the lake. Significant changes occurred in biological communities; species turnover was highest for diatoms (2.533 SD), followed by chironomids (1.781 SD) and cladocerans (0.614 SD). Biological communities showed a correlation with meteorologically recorded climate parameters. For chironomids, the strongest relationships were found for T_June, T_July, and T_ann. Both planktonic proxies, diatoms, and cladocerans showed a relationship with summer and annual air temperature and precipitation. The largest shifts in communities can be linked to recent climatic events, including the onset of steady warming following the variable conditions at the end of the LIA (ca. 1905), the cooling associated with the highest precipitation on record between 1950 and 1970, and, probably, the anthropogenic influence specific to Novaya Zemlya at this time. The new data provide a valuable basis for future ecological studies in one of the least explored and remote Arctic regions.

1. Introduction

The terrestrial and freshwater ecosystems of the Arctic are often considered to be species-poor and fragile [1,2]. Modern climate change has caused significant shifts in terrestrial, freshwater, and coastal ecosystems. In the Arctic, the situation is approaching irreversibility due to hydrological changes caused by glacier retreat or permafrost thaw [1,2]. Any study of high-latitude islands is challenging due to the remoteness of these sites. In addition to transport inaccessibility, special permits are often required to visit most Arctic islands [3]. Much about their fauna is therefore still poorly understood, including how climatic processes influence their development in isolated archipelagos. These questions can best be answered by a combined study of species biodiversity, biogeography, and ecology in relation to available data on climate variability at these sites using palaeoecological methods [4]. Freshwater invertebrates and phytoplankton are important contributors to ecosystem functioning, which includes detritus decomposition, self-purification, animal–microbe interactions, and energy transfer to consumers at higher trophic levels [5,6]. They are also used as indicator groups in monitoring the environmental status of water bodies and maintaining the health of aquatic ecosystems [7].

The Novaya Zemlya (NZ) archipelago is located in the Arctic Ocean in the extreme northeast of Europe, and it consists of two main islands and numerous smaller islands.

The first data on the freshwater invertebrate fauna and algae of NZ were collected in the second half of the 19th century by A. E. Nordenskiöld during the Swedish expedition of 1875 [8,9] and later by the Danish expedition of 1887 [10]. Further studies were carried out in the first half of the 20th century by the Norwegian expedition to the NZ in 1921 [11,12,13,14,15], etc. In the 1920s, the Soviet Union organized two expeditions to the archipelago: the Floating Marine Scientific Institute Expedition [16] and the expedition of the Institute for Northern Studies [17].

The study of the chironomid fauna of the NZ was started by A.E. Holmgren [18], continued by J.J. Kieffer [19,20], and later followed by Russian specialists [21,22,23,24,25]. At present, 68 species and larval forms of chironomids have been recorded in the NZ.

Since 1954, NZ has been designated as a nuclear test site, and its territory has been assigned a regime of strictly limited access [26,27]. Consequently, scientific visits have not been permitted. Following the cessation of nuclear testing in 1990, research in NZ was resumed. Despite the fact that the majority of the archipelago within the boundaries of the Central Test Site of the Ministry of Defence remains inaccessible, a national park, Russian Arctic, was established in the northernmost part of Severny Island in 2009 [28], and the southernmost part of Southern Island can be accessed with a special permit. Consequently, there is a paucity of data regarding the biodiversity of the archipelago. In order to address this knowledge gap, a short sediment core was retrieved from a small coastal lake in the south of Southern Island in summer 2015.

The objective of the present study was to investigate the taxonomic composition of the hydrobiological communities (chironomids, cladocerans, and diatoms) inhabiting the lake on the Southern Island of NZ using fossil material preserved in the sediment core. Furthermore, the study sought to assess the influence of sub-recent environmental conditions on the succession of the biota of this lake.

2. Materials and Methods

2.1. Regional Settings

The NZ archipelago is located in the Arctic Ocean between the Barents and Kara seas (73.9525 N; 56.34861 E; Figure 1). It consists of two major islands, the Northern and the Southern Islands, and numerous small inshore islands.

The landscapes in NZ are characterized by a prevalence of stony plains, barren rubble areas, and polygonal permafrost soils, supporting sparse vegetation. In the alluvial plains of the Southern Island, which are composed of moraine deposits eroded by the sea, swamps with a continuous moss–grass cover and patchy tundra can be found. Approximately 25% of the archipelago is covered by glaciers, the majority of which are currently located on the Northern Island [29].

The lakes of the NZ archipelago are distinguished by their varied origins and chemical compositions. The freshwater lakes are predominantly situated on the Southern Island. Relict or thermokarst lakes are primarily located in the plains. Lakes situated along the coastline are frequently the remains of former lagoons, whilst those found in mountainous regions are typically of glacial origin [30].

The climate of NZ is maritime, with cool summers that are frequently accompanied by fog and light rain. The winter seasons are marked by relatively mild frosts and slightly higher precipitation levels compared to the more continental regions of the Arctic. The most characteristic feature of the climate in NZ is frequent strong local winds, the so-called Novaya Zemlya bora, which create extremely harsh weather. Gusts of bora can reach 50–60 m/s [30,31].

The Southern Island is located in the Arctic climate zone. Summers are short, cool, and very humid; winters are cold and long [30,32]. The average air temperature in recent decades has been −5.4 °C. The coldest months are January and February, with an average temperature of −14 °C, and the warmest month is July, with an average temperature of 7 °C. The annual rainfall is 295 mm (http://www.pogodaiklimat.ru/history/20744.htm; accessed on 25 March 2025). The duration of the ice-free period on the Southern Island of the NZ archipelago is 2.5–3 months, from the end of June to the end of September [33]. Air temperatures over 0 °C occur from June to August (http://www.pogodaiklimat.ru/history/20744.htm; accessed on 25 March 2025).

The mean monthly temperature and precipitation data used in this study were obtained from a meteorological station, Malye Karmakuly, located on the Southern Island (72.3742 N, 52.7158 E, 18 m a.s.l.; http://www.pogodaiklimat.ru/history/20744.htm; accessed on 25 March 2025). This is the first Arctic weather station in Russia and the second in the world where regular meteorological observations are carried out [34]. The monthly temperature record is partially available since 1876. Since 1897, monthly mean data have been recorded regularly, except for 1901, 1911, and 1918–1920, where the data are available but not complete. The precipitation record is available since 1921 (Figure 2). A second meteorological station is located in closer proximity to the sampling site (20,946; 70°26′ N, 59°5′ E). However, it is situated on a different island, and the T record commenced in 1914, while the precipitation record only began in 1959 (http://www.pogodaiklimat.ru/; accessed on 25 March 2025).

The research comprised an analysis of the bottom sediments from a lake without an official name situated on the coast of Cape Sakhanin, NZ, Southern Island (70.5511 N, 55.1915 E). The surface area of the lake is 0.31 km². A narrow, flat ridge, with a height of 1–1.5 m, separates it from the sea. A minor watercourse flows into the lake to the north of the sampling site (Figure 1).

2.2. Sampling, Lithology, Geochemistry, and Age Model

A short sediment core (12.5 cm) was collected from 1 m depth using a soil tube [35] during the summer expedition in 2015. The sampling site was selected based on visual inspection of the lake’s sediments. The bay where sampling took place was the only place where the sediments contained a silty component. The rest of the accessible parts of the lake had gravelly sediments where biological remains are poorly preserved [36]. Unfortunately, no data on lake morphometry are available, as these investigations could not be carried out due to time constraints in the expedition schedule.

The lithological description of the core was based on visual observation of the sediments. The core was cut in the field at 0.5 cm intervals for further laboratory analysis.

Selected sediment intervals were analyzed for 210 Pb at the Geochronology Laboratory of St. Petersburg State University. The age–depth model for the core was based on the results of the 210 Pb analysis and produced with the Bacon 2.2 package [37] with R version 4.4.2 software [38].

The content of total carbon (TC) and total nitrogen (TN) was measured at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (Potsdam, Germany), using a Vario EL III CNS and a Vario MAX C analyzer (Elementar, Langenselbold, Germany). The accuracy of the device was 0.05% for C and 0.1% for N. The data were used to calculate the C/N ratio using a factor of 1.167, which is the ratio of the atomic weights of nitrogen (14.007 atomic mass units (amu)) and carbon (2.001 amu) to obtain the atomic ratio of C/N according to [39]. The type and amount of sedimentary organic matter can be used to reflect past fluctuations in lake productivity and terrestrial inputs associated with climate-induced environmental changes [40]. The C/N ratio of land-derived organic matter is typically greater than 14–20, whereas that of phytoplankton and aquatic macrophytes is typically between 4 and 10 [39].

2.3. Chironomids

The treatment of sediment samples for chironomid analysis followed standard techniques described in [36]. The total number of chironomid head capsules was counted and expressed as 100%. Chironomids were identified at the highest possible taxonomic resolution, with reference to [36,41]. The full list of chironomid taxa and their probable modern analogues are presented in

Table A1. List of chironomid taxa found in the sediments of the investigated lake on the Southern Island of the Novaya Zemlya archipelago. Species that can be possible analogs in the modern fauna around the investigated lake. Biotopes where the possible modern analogues species have been found: Lo lotic, Le lentic, ST semi-terrestrial. Each cross (+) represents one sample where the taxon was found.

Taxa [36]	Possible Analogue Species in the Modern Fauna	Biotope	Chironomid Zone (Sediment Depth, cm; Age, CE)
			I (12.5–9.5; 1850–1880)	II (9.5–7.5; 1880–1906)	III (7.5–4.5; 1906–1955)	IV (4.5–2.5; 1955–1975)	V (2.5–0; 1975–2015)
Chaetocladius type B	Chaetocladius s. str. binotatus	Lo	+++	+		+
Chaetocladius piger-type	Chaetocladius s. str. glacialis	Loc					+
Chironomus anthracinus-type	Chironomus s. str. albimaculatus	Le			++		++
Chironomus plumosus-type	Chironomus s. str. sp.	Le					+
Chironomini larvula	-			+
Cladotanytarsus mancus-type	Cladotanytarsus sp.	Lo/Le	+
Corynocera oliveri-type	Corynocera sp.	Le					+++
Corynoneura arctica-type	Corynoneura arctica	Lo/Le					++
Cricotopus bicinctus-type	Cricotopus gr. bicinctus	Lo			+
Cricotopus cylindraceus-type	Cricotopus gr. cylindraceus	Lo/Le	+		+
Cricotopus intersectus-type	Cricotopus (Isocladius) intersectus	Le				+
Cricotopus laricomalis-type	Cricotopus (Isocladius) laricomalis	Le	+
Eukiefferiella devonica-type	Tvetenia duodenaria	Lo	++	+	+	++
Eukiefferiella fittkaui-type	Tvetenia discoloripes	Lo	+
Eukiefferiella claripennis-type	Tokunagaia rectangularis	Lo/Le	+	++		+	++++
Hydrobaenus johannseni-type	Hydrobaenus gr. pilipes	Le					+
Hydrobaenus lugubris-type	Hydrobaenus gr. lapponicus	Le		+
Limnophyes–Paralimnophyes	Limnophyes sp.	ST	+++++	++++	++	+	++
Metriocnemus eurynotus-type	Metriocnemus sp.	ST	+++++	+++++	++++	++	++
Oliveridia	Oliveridia tricornis	Lo/Le		+
Orthocladius oliveri-type	Orthocladius s. str. sp.	Lo/Le	+
Orthocladius trigonolabris-type	Orthocladius s. str. sp.	Lo/Le			+
Orthocladius type I	Orthocladius s. str. sp.	Lo/Le		+
Orthocladius type S	Orthocladius (Eudactylocladius) sp.	ST/Lo/Le		+			++
Paraphaenocladius	Paraphaenocladius sp.	ST/Lo		+
Parakiefferiella triquetra-type	Parakiefferiella sp.	Lo/Le					+
Paratanytarsus austriacus-type	Paratanytarsus austriacus	Lo/Le	+++	++++	++++	++	++++++
Paratanytarsus	Paratanytarsus kaszabi	Lo/Le			+
Psectrocladius flavus-type	Psectrocladius (Allopsectrocladius) sp.	Le			+
Psectrocladius sordidellus-type	Psectrocladius s. str. sp.	Le					+
Pseudosmittia	Allocladius nanseni	ST/Le	+++	+++++	++++	++++	+++++
Sergentia coracina-type	Sergentia coracina	Lo/Le					+
Smittia–Parasmittia	Smittia sp.	ST					++
Stempellinella–Zavrelia	Chironomus s. str. sp.	Le			+
Symposiocladius	Orthocladius (Symposiocladius) sp.	Lo/Le					+
Stictochironomus	Stictochironomus sp.	Lo/Le		+
Tanytarsus lugens-type	Tanytarsus sp.	Lo/Le					+
Tanytarsus no spur	Tanytarsus gracilentus	Lo/Le					+
Trissocladius	Hydrobaenus gr. lapponicus	Le					+
Zalutschia type A	Zalutschia sp.	Le					+
Chironomini undiff.	-			++
Orthocladiinae undiff.	-			+++	++	++	++

and

Table A2. Species that have been found in the modern fauna around the lake investigated.

	Species	Present in the Core
1	Allocladius nanseni (Kieffer, 1926)	+
2	Chaetocladius (Chaetocladius) binotatus (Lundström, 1915)	+
3	Chaetocladius (Chaetocladius) + (Lundström, 1915)	+
4	Chironomus s. str. albimaculatus Shobanov, Wülker et Kiknadze, 2002	+
5	Corynoneura arctica Kieffer, 1923	+
6	Diamesa arctica (Boheman, 1865)	no
7	Hydrobaenus lapponicus (Brundin, 1956)	+
8	Limnophyes brachytomus (Kieffer, 1922)	as Limnophyes
9	Limnophyes minimus (Meigen, 1818)	as Limnophyes
10	Limnophyes pumilio (Holmgren, 1869)	as Limnophyes
11	Metriocnemus (Metriocnemus) brusti Saether, 1989	as Metriocnemus
12	Metriocnemus (Metriocnemus) sternerectus Makarchenko et Makarchenko, 2013	as Metriocnemus
13	Orthocladius (Eudactylocladius) olivaceus (Kieffer, 1911)	no
14	Orthocladius (Eudactylocladius) subletteorum Cranston, 1999	no
15	Orthocladius (Orthocladius) decoratus (Holmgren, 1869)	no
16	Orthocladius (Orthocladius) glabripennis (Goetghebuer, 1921)	no
17	Orthocladius (Orthocladius) hazenensis Soponis, 1977	no
18	Orthocladius (Orthocladius) oblidens (Walker, 1856)	no
19	Paraphaenocladius impensus (Walker, 1856)	as Paraphaenocladius
20	Paraphaenocladius pseudirritus Strenzke, 1950	as Paraphaenocladius
21	Paratanytarsus kaszabi Reiss, 1971	+
22	Pseudokiefferiella parva (Edwards, 1932)	no
23	Tanytarsus gracilentus (Holmgren, 1883)	+
24	Tokunagaia rectangularis (Goetghebuer, 1940)	+
25	Tvetenia duodenaria Kieffer, 1922	+

. Information on the ecology of chironomid taxa was derived from [36,42,43,44,45,46,47], with additional sources referenced in the text.

2.4. Diatoms

Diatom slides were prepared according to standard methods using the water bath technique [48]. Diatom slides were mounted with Naphrax and identified and counted under an Axioplan Zeiss light microscope (Oberkochen, Germany) equipped with an oil immersion objective, with 300 to 500 valves per sample. The identification of diatoms was conducted at the highest possible taxonomic level, primarily in accordance with [49,50,51], according to the modern taxonomy from the AlgaeBase database [52], and according to the classification of diatoms used in Russia [53] with the latest revisions [54,55]. The full names of diatom species are presented in

Table A3. List of diatom species found in the sediments of the lake investigated on the Southern Island of the Novaya Zemlya archipelago. *—species found in Novaya Zemlya for the first time.

	Class Centrophyceae
	Family Aulacoseiraceae
1	Aulacoseira ambigua (Grun.) Simons.
2	Aulacoseira crenulata (Ehrb.) Simons. *
3	Aulacoseira islandica (O. Müll.) Simons. *
	Family Stephanodiscaceae
4	Lindavia bodanica (Eulenstein ex Grunow) T.Nakov, Guillory, Julius, Theriot & Alverson *
5	Cyclotella iris Brun et Hérib. *
6	Cyclotella krammeri Håkans. *
7	Pantocsekiella ocellata (Pantocsek) K.T.Kiss & Ács. *
8	Pantocsekiella rossii (Håkansson) K.T.Kiss & E.Ács. *
9	Pliocaenicus costatus (Log., Lupik. Et Churs.) Flower, Ozornina et Kuzmina
10	Stephanodiscus alpinus Grun. *
11	Stephanodiscus astraea (Ehrb.) Grun. *
	Class Pennatophyceae
	Family Achnanthidiaceae
12	Planothidium delicatulum (Kützing) Round & Bukhtiyarova
13	Planothidium lanceolatum (Brébisson ex Kützing) Lange-Bertalot
14	Achnanthidium minutissimum (Kützing) Czarnecki
	Family Bacillariaceae
15	Denticula kuetzingii Grun.
16	Hantzschia amphioxys (Ehrb.) Grun. in Cl. et Grun.
	Family Cavinulaceae
17	Cavinula scutelloides (W.Smith ex W.Gregory) Lange-Bertalot
	Family Catenulaceae
18	Amphora libyca Ehrb. *
19	Amphora pediculus (Kütz.) Grun. ex A. Schmidt *
	Family Cocconeidaceae
20	Cocconeis lineata Ehrenberg
	Family Cymbellaceae
21	Cymbella aspera (Ehrb.) Perag. in Pelletan
22	Cymbella cistula (Ehrb.) Kirchn.
23	Cymbopleura inaequalis (Ehrenberg) Krammer *
24	Placoneis elginensis (W.Gregory) E.J.Cox
25	Cymbella sp.
	Family Diploneidaceae
26	Diploneis elliptica (Kütz.) Cl. *
27	Diploneis interrupta (Kütz.) Cl.
	Family Encyonemataceae
28	Encyonema mesianum (Cholnoky) D.G.Mann *
29	Encyonema silesiacum (Bleisch) D.G.Mann
	Family Eunotiaceae
30	Eunotia circumborealis Lange-Bert. et Nörp.
31	Eunotia praerupta Ehrb.
32	Eunotia subarcuatoides Alles, Norpel et Lange-Bert.
	Family Fragilariaceae
33	Fragilaria capucina Desm.
	Family Gomphonemataceae
34	Gomphonema acuminatum Ehrb.
35	Gomphonella olivacea (Hornemann) Rabenhorst
	Family Naviculaceae
36	Caloneis silicula (Ehrb.) Cl.
37	Gyrosigma attenuatum (Kütz.) Rabenh.
38	Navicula digitoradiata (Greg.) Ralfs in Pritchard
39	Pinnunavis elegans (W.Smith) Okuno
40	Navicula vulpina Kütz.
41	Navicula sp.
	Family Neidiaceae
42	Neidium bisulcatum (Lagerst.) Cleve
43	Neidium productum (W. Sm.) Cl. *
	Family Pinnulariaceae
44	Pinnularia lata (Bréb.) W. Sm.
45	Pinnularia lundii Hust.
46	Pinnularia microstauron (Ehrb.) Cl.
47	Pinnularia viridis (Nitzsch) Ehrb.
	Family Rhopalodiaceae
48	Epithemia argus (Ehrb.) Kütz.
49	Epithemia argus var. alpestris (W. Sm.) Grun.
	Family Sellaphoraceae
50	Sellaphora bacillum (Ehrenberg) D.G.Mann
51	Sellaphora pupula (Kützing) Mereschkovsky
52	Fallacia pygmaea (Kützing) Stickle & D.G.Mann
	Family Stauroneidaceae
53	Stauroneis anceps Ehrb.
54	Stauroneis phoenicenteron (Nitzsch) Ehrb.
	Family Staurosiraceae
55	Pseudostaurosira borealis (Foged) M.L.García, E.Morales, Ector & Maidana
56	Staurosirella leptostauron (Ehrenberg) D.M.Williams & Round
57	Staurosirella pinnata (Ehrenberg) D.M.Williams & Round
58	Staurosira construens Ehrenberg
	Family Tabellariaceae
59	Diatoma vulgaris Bory *
60	Tabellaria fenestrata (Lyngb.) Kütz. *
	Family Ulnariaceae
61	Hannaea arcus (Ehrenberg) R.M.Patrick
62	Ulnaria ulna (Nitzsch) Compère

. For easier reading, the authorships of species’ descriptions are omitted from the text’s body. The total number of valves was taken as 100%. We defined taxa with abundances ≥ 10% and ≥5% as dominant and subdominant, respectively [56,57]. The biogeographical and ecological characteristics of the taxa were described following [58,59,60], with additional sources referenced in the text.

2.5. Cladocera

The thermochemical treatment of sediment samples for cladoceran analysis followed standard methods [61,62]. Subsamples of bottom sediments were dissolved in 10% KOH solution. The solution was heated at 75 °C for 30 min, and the resulting suspension was filtered through a 50 µm mesh sieve. Subfossil Cladocera remains were stained with safranin and fixed in 96% ethanol. Due to the low concentration of Cladocera remains in the bottom sediments, all available samples were fully analyzed. Cladocera remains (head shields, carapaces, postabdomens, postabdominal claws, ephippia, etc.) were identified and counted to the highest taxonomic resolution possible. The most common remains of each species were used to reconstruct the number of individuals [63]. An adequate minimum number of cladoceran remains for statistical analysis is 200–300 remains [61,62,63], but, for low abundance lakes (<20 taxa), counts of around 50 individuals are likely to be sufficient to record the presence of most taxa occurring at >1% abundance in the observed assemblages [64]. Cladoceran remains were identified using specialized keys of modern and subfossil cladocerans [63,65,66,67,68,69], as well as publications on individual taxa [70,71,72]. The full names of Cladocera species are presented in

Table A4. List of cladoceran taxa found in the sediments of the investigated lake on the Southern Island of the Novaya Zemlya archipelago. Range types: ARC (P)—Subarctic and Arctic of Palearctica, C—cosmopolite or widespread unrevised species, HOL—Holarctic, PAL—Palearctic, EWS—Europe and West Siberia, *—species noted for the first time.

	PAL	[33]	[8]	[129]	[9]	Present Study
Subclass Cladocera
Order Anomopoda
Family Bosminidae
Bosmina (Eubosmina) cf. longispina (O.F. Müller, 1785)	EWS	−	−	−	−	+
Bosmina obtusirostris G.O. Sars, 1862 (synonym of Bosmina (Eubosmina) cf. longispina)	EWS	+	+	−	−	−
Bosmina sp.	-	−	−	−	−	+
Family Eurycercidae
Eurycercus (Teretifrons) glacialis (O.F. Müller, 1776)	ARC (P)	+	+	+	+	+
Family Chydoridae
Alona guttata Sars, 1862	C	+	+	+	−	−
* Alona werestschagini (Sinev, 1999)	ARC (P)	−	−	−	−	+
* Alonella excisa (Fischer, 1854)	C	−	−	−	−	+
* Biapertura affinis (Leydig, 1860)	PAL	−	−	−	−	+
Camptocercus fennicus Stenroos, 1898	ES	+	−	−	−	−
Chydorus cf. sphaericus (O.F. Müller, 1785)	C	+	+	+	+	+
Coronatella rectangula (Sars, 1862)	C	+	+	+	−	+
* Pleuroxus uncinatus (Baird 1850)	PAL	−	−	−	−	+
Tretocephala ambigua (Lilljeborg, 1901)	ES	−	+	−	−	−
Family Macrothricidae
Macrothrix hirsuticornis Norman & Brady, 1867	HOL	+	+	+	−	−
Family Daphnidae
Daphnia longiremis G. O. Sars, 1862	HOL	+	+	−	−	−
Daphnia longispina gr. O.F. Müller, 1776	PAL	−	−	−	−	+
D. (D.) middendorffiana Fischer, 1851	ARC (P)	+	+	+	−	+
D. (D) pulex Leydig, 1860	C	+	+	+	−	−
D. (D.) pulex gr.	C	−	−	−	−	+
D. (D.) pulex tenebrosa (synonym of D. (D.) middendorffiana)	ARC (P)	+	−	−	−	−
Daphnia sp.		−		−	−	+
		11	10	7	2	13

. For easier reading, authorship and the year of description are omitted from the text’s body. Information on the ecology of the geographical distribution of Cladocera taxa was taken from [66,69,73].

2.6. Numerical Analysis

The meteorological data and the results of the chironomid, diatom, and cladoceran analyses are presented in graphs made with C2 ver. 1.7.7 [74]. The zoning of the stratigraphy was conducted through the implementation of cluster analysis (constrained single linkage method with chord distances measured) utilizing PAST software version 2.07 [75] and supported by principal component analysis (PCA) [76], which was used to assess overall changes in species composition across the sediment core based on square root transformed data.

Detrended Canonical Correspondence Analysis (DCCA), the direct form of DCA with changes in species assemblages constrained to sediment age as the only environmental variable, was used to develop quantitative estimates of compositional turnover as beta-diversity (BD), scaled in standard deviation (SD) units for each taxonomic group. This technique has previously been used in paleolimnological studies in Arctic regions to quantitatively assess the response of biological indicators to environmental stressors, including climate warming and permafrost thaw [77,78,79]. Species diversity was estimated using Hill’s N2 index, which is commonly used as a measure of ‘effective’ diversity [80].

To assess the rate of change of mean monthly temperatures and annual precipitation, and to find possible statistically significant relationships between species composition (chironomids, diatoms, and cladocerans) and climatic variables, we performed ordinary least square regression using PAST [75,81]. This method has previously been used in our palaeoecological studies in the Bol’shezemelskaya Tundra region, just 400 km south of NZ [4,82]. Using the same method allows for better comparison of the results. Due to the relatively short time span, we assumed a linear response of species composition along the main underlying gradient. The species composition data were summarized as PCA axes before being used as response variables. All ordinations were carried out using CANOCO 4.0 for Windows [83].

3. Results

3.1. Climate in Novaya Zemlya Since the Beginning of Observation

The instrumental record of the Malye Karmakuly meteorological station shows that the mean annual and mean monthly temperatures of all months have increased over the observation period. A particularly strong increase occurred after 1985–1990 (Figure 2). Monthly mean temperature changes were highest in June and September (

Table 1. Results of ordinary least squares regression analysis (p ≤ 0.01) illustrating the change in mean monthly temperatures, based on meteorological data (1876–2015). The slope of the regression line represents the rate of change [81]; T °C is the difference between the mean monthly temperature at the start of the observations and the modern data preceding the sampling campaign (2005–2015).

	Months
	I	II	III	IV	V	VI	VII	VIII	IX	X	XI	XII	Tann
slope	0.17	0.14	0.14	0.22	0.23	0.37	0.26	0.15	0.40	0.22	0.27	0.30	0.33
T, °C	1.8	1.5	1.5	2.5	2.6	4.3	2.9	1.6	4.5	2.4	3.0	3.4	3.5

).

Meteorological observations show that between 1920 and 1940, the annual precipitation rate (Pann) is the lowest on record. Precipitation remains low, especially during the winter months, and starts to increase after 1940. The strongest increase is observed for September and October (Figure 2). Precipitation shows a positive trend until ~1950 (min = 152; max = 428; median = 220; skew= 1.4), the wettest period is recorded between 1950 and 1975 (min = 292; max = 586; median = 426; skew = 0.05), with a decreasing trend thereafter (min = 200; max = 505; median = 321; skew= 0.4) (Figure 2), and the highest precipitation rate is recorded in 1968 (568 mm year⁻¹).

3.2. Age Model

Data on the 210 Pb content of the core are presented in

Table 2. AM 210 Pb concentrations and calculated age of the sediments from the studied lake (NZ, Southern Island).

Depth, cm	210 Pb, Bq kg−1	Pb Ages, Years
0–0.5	54.05 ± 2.01	3.4 ± 0.6
0.5–1	50.17 ± 1.87	10.1 ± 1.8
1–1.5	50.17 ± 2.2	16.9 ± 3.1
1.5–2	33.23 ± 3.4	23.6 ± 4.3
3–3.5	36.35 ± 1.39	43.9 ± 6.7
4–4.5	29.22 ± 1.29	57.4 ± 9.2
8.5–9	27.3 ± 1.76	118.2 ± 20.2
9–9.5	25.67 ± 1.81	125.0 ± 21.4
9.5–10	22.89 ± 1.6	131.8 ± 23.9

. Concentrations of 210 Pb in the core decreased gradually with depth, as determined by the half-life of 210 Pb (T1/2 = 22.2 years). The decrease in activity down the column is generally consistent. The similarity of values obtained for layers 0.5–1 and 1–1.5 cm is most likely due to local mixing. The lower value for the 1.5–2 cm layer compared to the layer below it may be due to the coarse granulometric composition of the sediments, which results in diffusion of the isotope being determined from this layer [84].

In addition, when the confidence intervals are considered, the overall pattern remains unchanged. Using models of constant initial concentration [84] and radioactive decay, it was calculated that the bottom sediment layers between the 0–0.5 cm and 4–4.5 cm horizons accumulated over 53.8 ± 9.47 years. According to the adopted model, the sedimentation rate remained constant throughout this period, enabling an estimate of 0.74 ± 0.13 mm/year to be made (Figure 3). All dates in the manuscript are given in years of the Common Era (CE).

3.3. Lithology and Sediment Organic Chemistry

The lithological composition of the core is presented in Figure 4. The lower 3.5 cm (1850–1897) consists of loamy sand. Between 9 and 7.5 cm, sediments consist of sand with a touch of clay. The transition to the upper section is smooth. Between 7.5 and 4 cm, sediments are composed of sand mixed with coarse clastic material and shells; the number of shells and coarse material increases towards the upper part of the section. The uppermost 4 cm of the core comprises coarse sand with an admixture of coarse clastic material, a small amount of organic matter, and macro residues; the lower boundary is clear.

The carbon content varies slightly from 7.3 to 9.4 wt%, increasing to a maximum of 3 cm in 1971. N varies between 0.2 and 0.4 wt%. It increases from the bottom of the core up to 8 cm (1904) and then decreases. The C/N atomic ratio shows high peaks at 10 cm (1876) and 3 cm (1971); it then decreases at 2 cm (1985) before increasing again at the sediment surface. High C/N ratios indicate an increase in terrestrial influence [39].

3.4. Chironomid Stratigraphy

We found 42 chironomid taxa in the examined core, most of which could be attributed to the modern species. The complete list of chironomid taxa found in the core and the list of chironomid species found during the field expedition in NZ in 2015 are presented in the Appendix A (Table A1 and Table A2). The chironomid paleo record was divided into five chironomid assemblage zones (CH I-V) (Figure 5).

CH I (1850–1885). This zone is dominated by semi-terrestrial taxa that can also be attributed to lotic environments: Limnophyes—Paralimnophyes and Metriocnemus eurynotus-type and the semi-terrestrial taxon Pseudosmittia. The lotic taxon Chaetocladius type B has the highest core abundance (up to 15.8%). Hill’s N2 diversity reaches 5.8, which is the highest in the core.

CH II (1885–1905). This short period is characterized by a strong increase in the abundance of the Paratanytarsus austriacus-type and a decrease in the lotic and semi-terrestrial fauna. The diversity of chironomid assemblages decreases towards the top of the zone, accompanied by an increase in the dominance of the P. austriacus-type and a decrease in the concentration of chironomid capsules in the sediments.

CH III (1905–1955). During this period, the concentration of head capsules (HC) in the sediments increases, while the diversity decreases. The semi-terrestrial Pseudosmittia dominates. The Paratanytarsus austriacus-type is less abundant but still constitutes 10 to 33% of the communities. Lotic taxa, including semi-terrestrial lotic taxa, with the exception of the Metriocnemus eurynotus-type, appear at low abundances or disappear.

CH IV (1955–1975). This period is characterized by the lowest concentration of chironomid HC. Only six chironomid taxa were found here, including the unidentifiable remains of the family Orthocladiinae. Pseudosmittia and Paratanytarsus austriacus-type dominate.

CH V (1975–2015). The diversity of the chironomid communities is low at the beginning of the zone but gradually increases, reaching a high value around 2005. The Pseudosmittia and Paratanytarsus austriacus-type dominate until ca. 1995. After 1995, a more diverse chironomid fauna develops in the lake. Several lotic taxa appear or reappear; Eukiefferiella claripennis-type reaches its highest abundance in the core, and Trissocladius and Symposiocladius appear. After 1995, semi-terrestrial taxa decline sharply, although they are still present in the communities. A new semi-terrestrial lotic taxon, Chaetocladius piger-type, emerges. A more diverse fauna evolves: Tanytarsus lugens-type, Orthocladius type S, Parakiefferiella triquetra-type, Corynoneura arctica-type, Corynocera oliveri-type, and Chironomus anthracinus-type (Figure 5). These taxa are mainly found in the littoral zones of the lakes, with the exception of T. lugens-type and C. anthracinus-type, which can be found in both littoral and profundal zones of the lakes.

3.5. Diatom Stratigraphy

In the investigated core, 62 diatom taxa from 22 families and 39 genera were identified (Table A3). A total of 11 species were identified within the class Centrophyceae, with the remaining species belonging to 20 families of the class Pennatophyceae. The Stephanodiscaceae family was the most species-rich family in the class Centrophyceae, with a total of eight species documented. Within the Pennatophyceae, the most diverse families were Naviculaceae (six species), Cymbellaceae (five species), Pinnulariaceae, and Staurosiraceae (four species each). In comparison with the paucity of data concerning diatoms in NZ [85], our study has identified 17 species for which no prior records existed on NZ (Table A3).

The diatom stratigraphy was divided into five diatom zones (DI–DV) (Figure 6).

D I (1850–1860). Only two species were recorded in this zone. In the lower part of the zone, valves of the cosmopolitan, benthic, mesohalobic, pH-indifferent species Diploneis interrupta were recorded. In the upper part of the zone, valves of the planktonic freshwater species preferring oligotrophic alkaline waters, Pliocaenicus costatus, were noted.

D II (1860–1905). Twenty-nine diatom species were identified within the zone. The dominant species is the cosmopolitan planktonic-benthic Pantocsekiella ocellata. Small quantities of the alpine zone freshwater inhabitant Stephanodiscus alpinus are also present. The boreal species Aulacoseira islandica, Cyclotella iris, and Staurosirella leptostauron were only recorded in very low abundances. In the middle of the zone, at around 1885, the highest proportion of Diploneis interrupta valves was recorded.

D III (1905–1950). Within this zone, 27 species of diatoms were identified. The cosmopolitan Pantocsekiella ocellata, which was dominant in the previous zone, is replaced by Pantocsekiella rossii. The cold-stenotherm acidophilic species Eunotia praerupta replaces the pH-indifferent species Aulacoseira islandica. Valves of the arcto-alpine species Hannaea arcus appeared. In the lower part of the zone (around 1910), Cymbella aspera has the highest abundance. After about 1924, Epithemia argus var. alpestris, Staurosirella pinnata, and Pinnularia lundii dominate. In the upper part of D III (ca. 1944), all of these species decrease, and Staurosira construens dominates.

D IV (1950–1970). Only nine diatom species were recorded in this zone. The concentration of diatoms in the sediments is very low. The mesohalobic species Diploneis interrupta dominates the assemblages. The thermophilic species Planothidium lanceolatum, the benthic Pinnularia lundii, and the alkaliphilic Cymbella aspera occur in remarkable abundances (10–20%). Other species occur at very low abundances.

D V (1970–2015). Within this zone, 34 species of diatoms were recorded. At the beginning of the zone, Pinnularia lundii and Staurosira construens dominate until about 1980. Aulacoseira ambigua has the highest abundance in the core. Since 1980, the valves of the planktonic species Pantocsekiella rossii have strongly increased. Boreal Cyclotella iris and mesohalobic Navicula digitoradiata and Fallacia pygmaea appear. In relation to the pH of the environment, valves of the alkalibiont Stephanodiscus astraea dominate. In relation to the geographical distribution, valves of Holarctic species dominate, with Pantocsekiella rossii and Stephanodiscus astraea.

3.6. Cladocera

We examined 228 remains of 114 individuals of Cladocera. A total of 13 taxa of the order Anomopoda of the Cladocera were identified in the studied lake of the NZ. The list of the taxa with the main distribution types is presented in Table A4. In addition to microcrustacean species, three species of Rotifera (Keratella quadrata, Kellicotia longispina, Keratella cochlearis), remains of Copepoda, and numerous spicules of freshwater sponges were found in the uppermost layers of the core (0.5–4.0 cm) (Figure 7).

There were no Cladocera remains in the lower layers of the lake sediments between 10.5 and 12.5 cm (1850–1870). We identified three zones (Figure 7) within the cladoceran biostratigraphy (CLI–CLIII).

CL I (1870–1905). The diversity and abundance of cladocerans in the zone are low, and only three taxa were recorded. The concentration of cladoceran remains did not exceed 1.2 ind./g (mean 0.5 ind./g). The zone is characterized by the dominance of the littoral taxon Chydorus cf. sphaericus (33–100%). A littoral taxon often associated with submerged vegetation, Eurycercus glacialis, and the planktonic Daphnia sp. were recorded.

In general, the zone reflects extremely unfavorable conditions for the development of cladoceran assemblages. The low total abundance of cladocerans during this period indicates an oligotrophic environment with low nutrient levels.

CL II (1905–1951). Six cladoceran taxa were recorded within this zone. Towards the top of the zone, both taxonomic richness (N0 = 1–5, mean = 2.3) and the concentration of cladoceran remains increased significantly, reaching 10.6 ind./g (mean = 3.9).

The littoral taxon Chydorus cf. sphaericus is consistently present and dominant, accounting for 50–100% of the community. The second most significant taxon in the community is the planktonic species Daphnia sp., which increases in abundance in the middle of the zone. The eurythermic littoral species Coronatella rectagula is only present at a depth of 5.5 cm (1937). In the upper part of the zone, the planktonic taxa Daphnia middendoffiana, Bosmina longispina, and Bosmina sp. have been observed. There is a marked increase in the diversity of planktonic taxa within this zone, particularly in the upper section. The conditions in this zone are more favorable for the development of cladoceran organisms than those of the previous one.

No cladoceran remains were found in layers 4.0–5.0 cm (1951–1964).

CL III (1964–2015). Ten species were identified within this zone (median of 3.8 species per sample). The highest core alpha diversity and relatively high concentrations of cladoceran remains were observed in this zone, reaching their peak around 2005 (12.6 ind./g).

The dominant chydorid Chydorus cf. sphaericus decreases sharply to 14% in the lower part of the zone and then increases towards the top of the zone. The glacial relict Eurycercus glacialis reappears after being absent in the previous zone. Only in this zone, at a depth of 2.5 cm (1978), were the littoral species Alona werestschagini and Pleuroxus uncinatus recorded for the first time in the core and in general for the NZ archipelago. In the upper part of the zone (1.5–0.5 cm, 1991–2005), the acidophilic or acid-tolerant littoral taxa Alonella excise and Biapertura affinis were also recorded for the first time for NZ.

Although the ratio of littoral to planktonic taxa varies considerably, there is a general trend towards an increase in littoral taxa. The increase in biodiversity and Cladocera concentration during this period reflects more suitable conditions for this group of invertebrates, including an increase in ambient temperature and nutrients.

3.7. Changes in Biological Communities

The diversity of chironomid communities (N2) shows relatively high values at the beginning of the record (1850–1860), with a median of 4.5–5.9 (Figure 8). It then decreases to around 2.5 before increasing again to 5.1 in 1883. Following this, the N2 value of the chironomid communities remained low, with a median of 1.8 ± 0.8, increasing to 3.9 between 1944 and 1950. In the upper part of the record, between 1995 and 2005, N2 increases to 5.9 before decreasing to 2.2 by 2015. There is an overall downward linear trend in chironomid N2 diversity (Figure 8).

The diversity of diatom communities remains low (median 1.5 ± 0.6) until 1883, when it rises to 6.5 before decreasing again (median 1.4 ± 0.6). The next increase in diatom diversity occurred between 1924 and 1937 (median 4.7 ± 0.4). Diatom N2 decreased to 1 between 1958 and 1964, increasing to a median of 4.2 ± 1.0 in the upper part of the core.

The N2 diversity of cladoceran increases from the lowest values in the lower part of the core to three in 1883. It then decreases, varying between 0 and 2.6 (median 1.0 ± 0.9) until 1978, when it rises to a maximum of 5.2. In the upper part of the core, the median cladoceran N2 value is 2.5 ± 1.4. In the uppermost sediment layer, N2 decreases to one.

The overall changes in species composition during the ca. 165 years of sedimentation were the highest for diatoms (2.533 SD). The species turnover for chironomids was 1.781 SD, and for cladoceran assemblages it was the lowest (0.614 SD).

Regression analysis showed a relationship between variations in chironomid communities and mean monthly air temperatures from the instrumental record. The relationship was not strong, but it was significant (p ≤ 0.01) (

Table 3. Results of regression analysis (p ≤ 0.01). R²: coefficient of determination; T_June: mean June air temperature; T_July: mean July air temperature; T_August: mean August air temperature; T_September: mean September air temperature; T_ann: mean annual air temperature; P_ann: mean annual precipitation; ns: not significant.

	Span	R2
		Chironomids	Diatoms	Cladocera
TJune	0.5	0.37	0.26	0.14
	0.3	0.36	0.26	ns
	0.2	0.32	ns	ns
TJuly	0.5	0.32	0.40	ns
	0.3	0.33	0.26	ns
	0.2	0.28	ns	ns
TAugust	0.5	ns	0.42	ns
	0.3	ns	0.47	ns
	0.2	ns	0.45	ns
TSeptember	0.5	ns	0.26	ns
	0.3	ns	0.28	ns
	0.2	ns	0.31	ns
Tann	0.5	0.27	0.21	0.20
	0.3	ns	0.26	0.22
	0.2	ns	0.27	0.21
Pann	0.5	ns	0.27	0.25
	0.3	ns	ns	0.24
	0.2	ns	ns	ns

). The strongest relationships were found for T_June, T_July, and T_ann. Diatoms showed a significant relationship (p ≤ 0.01) with all mean monthly summer and annual temperatures. The strongest correlation was found with T_June and T_July, and weaker but significant relationships were found with T_August, T_September, T_ann, and P_ann. Cladocera had a weak but significant relationship (p ≤ 0.01) with T_ann and P_ann (Table 3).

4. Discussion

4.1. Climate and Its Relationship to Biological Indicators

This study of a short sediment core from Noname Lake, located on the Southern Island of the NZ archipelago, was carried out using a complex of palaeobiological methods, including chironomid, diatom, and cladoceran analyses, supplemented by information on the organic matter content of the sediments. The investigated sediment core encompassed a time span from the mid-nineteenth century to the present. This time interval is recognized as a period of substantial ecological transformation from the end of the Little Ice Age (LIA) to modern climate warming [86]. As there were no other paleolimnological studies carried out in the region, we compared our results with the study by Solovieva et al. [82] and Nazarova et al. [4] carried out on the lakes in the Bol’shezemelskaya Tundra, a continental Arctic area situated about 400 km to the south of NZ.

Our analysis of the instrumental meteorological record indicated that the climate of the South Island of NZ had undergone several major changes since the end of the LIA. Temperatures were fluctuating at the end of the 19th century, when temperature records began to be kept. No clear trend could be discerned. However, it is known that air temperatures began to rise around this time in many regions of Europe [87] and the European part of the Russian Arctic [4,82]. Prior to the onset of the evident warming trend as well in NZ, temperatures here exhibited greater variability.

A rising trend in the T record was observed after 1900–1905, particularly in the summer months, including September, when temperatures were still above 0 °C and the lakes remained ice-free. From 1905 to 1950, there was a moderately positive trend in temperature, accompanied by an increase in precipitation. The period between approximately 1950 and 1970 was characterized by a decline in temperatures, with precipitation levels reaching unprecedented heights and exhibiting significant variability. A comparable interval characterized by cool and wet conditions was documented in the Bol’shezemelskaya Tundra [4]. After 1970, a moderate upward trend in air temperatures began. Precipitation continued to decrease. After 1990–1995, the climate became much warmer (T_ann increased from −5 in 1995 to −1.7 °C in 2015, span 0.3) and dryer (P_ann decreased from 383 mm in 1995 to 291 mm in 2015; span 0.3).

There is a similarity in climate development in NZ and the Bol’shezemelskaya Tundra [82], where there has been a general increase in T_July since the mid-19th century, with a particularly marked increase after 1980–1985. However, it seems that in NZ the most pronounced warming started some years later.

Significant changes have occurred within the biological communities. Our study supports the influence of the climate on these communities, as evidenced by correlations between climate parameters and taxonomic shifts in chironomid, diatom, and cladoceran data.

For chironomids, the strongest relationships were found for T_June, T_July, and T_ann. Diatoms showed a relationship with all summer mean monthly and annual air temperatures. Cladocera had a weak but significant relationship with T_ann. The correlation of the development of all components of the studied hydrobiological communities, especially chironomids, with summer temperatures is in agreement with the findings of many studies, e.g., refs. [88,89,90,91,92,93,94], including studies of the closest to NZ regions in northern European Russia [4,46,47].

Temperature is known to be important as well for the development of cladoceran assemblages in many Arctic regions [95,96,97]. A significant correlation was observed between T_July and the distribution of subfossil cladocerans in lakes in northwestern Yakutia in eastern Siberia [98,99,100].

Air temperature is the main driver of the temporal variability of ice cover [101]. The freeze-up period of lakes can be significantly affected by changes in air temperature during winter and summer [102]. The duration of ice cover affects the amount of available light, the depth to which water mixes, and the amount of nutrients that enter the system from the catchment area [103,104]. All of these factors influence the abundance and dynamics of hydrobiological communities, establishing a link between variations in annual temperature and the development of chironomid, cladoceran, and diatom assemblages.

No relationship was found between variations in the taxonomic composition of chironomid communities and precipitation in the NZ and Bol’shezemelskaya tundra. In contrast, both planktonic proxies, diatoms and cladocerans, exhibited a correlation with summer and annual air temperatures, as well as with precipitation, in both regions [4]. Precipitation increases catchment runoff, providing lakes with additional allochthonous mineral and organic matter. Increased atmospheric precipitation at the beginning of the growing season may result in changes in littoral zooplankton that are similar to those typically associated with increased organic matter and nutrients [105]. In tundra regions, precipitation-induced surface runoff from the catchment may be facilitated by thawing permafrost [106]. Heavy rainfall can lead to extreme inflows of water into lakes, which can result in the washing away of flora and fauna, particularly in shallow biotopes [107,108]. Heavy rainfall is responsible for around 65% of floods [109]. In northern latitudes, snow-covered areas are vulnerable to floods caused by snowmelt, which are sometimes exacerbated by rainfall [109,110].

4.2. Chironomids

The chironomid communities of the investigated lake have undergone substantial changes over the last 165 years, as reflected by the high BD (1.781 SD). These taxonomic changes in chironomid communities are greater than those observed in our previous studies of high-latitude regions in the European part of the Russian Arctic and elsewhere. The chironomid communities of two glacial lakes in the Bol’shezemelskaya Tundra [77,82] showed smaller species turnover from 1850 to 2000: 1.08 and 1.15 SD. In the Lake Bolshoy Kharbey, the largest lake in the Bol’shezemelskaya Tundra, it was 1.331 SD for the last 180 years [4]. A higher total species turnover (BD = 1.47 SD) was known for Col Pond on Ellesmere Island in the High Canadian Arctic [77].

Three major groups of chironomid taxa have been found in the lake. The most abundant were semi-terrestrial taxa, a group of taxa frequently found in lotic environments of different kinds, and a group of the taxa with different ecology, which was mainly represented by taxa characteristic of cold oligotrophic lakes at high latitudes or altitudes.

A semi-terrestrial taxon, Limnophies-Paralimnophies, was especially abundant in the lower part of the core (1850–1885). This taxon is usually associated with very shallow water in lake littorals and streams, and it can be a useful indicator of lake level fluctuations [111]. Some species of this taxon are terrestrial or semi-terrestrial [112]. The dominance of this taxon, together with the significant representation of several lotic taxa in the lower part of the core, suggest significant and unstable surface runoff under the variable climatic conditions at the end of the LIA.

The most frequently found taxon in the lake was Pseudosmittia. It was present in all horizons investigated but had the highest abundances from 1905 to 1990. This taxon is relatively common in lake sediments, and many species are terrestrial, semi-terrestrial, or can occur in littoral and splash zones of the lakes. The larvae can live submerged for a long time. After inundation, they seem to remain on the bottom and do not swim [43]. Some species live in marches, inundated floodplains, or rain puddles and can emerge from water [43,113]. Most probably, Pseudosmittia inhabited the shore zone of the NZ lake or had been brought to the lake with an inflowing river.

The Metriocnemus eurynotus-type was also common (18 out of 24 samples) in the lake sediments. This genus occurs among plants [114] in the littoral of temperate lakes but also in streams or fast-flowing springs [43]. Some species are semi-terrestrial, living in wet mosses and other hygropetric habitats [112,115]. This taxon was present in the lake at notable abundances at the end of the 19th century and between 1920 and 1960 and can probably be attributed to variable lake levels, together with Limnophies-Paralimnophies.

Most taxa in the lotic group (genera Chaetocladius and Eukiefferiella, Stempellinella–Zavrelia) are characteristic of dynamic environments, such as small, fast-flowing streams and brooks. Symposiocladius is known as a wood miner in running water [112], and Trissocladius species are mainly found in slow flowing streams or even in temporary puddles [43]. These taxa were mainly found in the lower part of the core and indicated dynamic environments.

The Paratanytarsus austriacus-type was most abundant between 1890 and 1900, at the beginning of post-LIA climatic amelioration, and after 1975. This taxon is often found in cold oligotrophic lakes at high latitudes or altitudes. Paratanytarsus taxa are often associated with macrophytes [36,116]. However, according to [117,118], Paratanytarsus larvae can be found in a wide variety of habitats, including marshes, ponds, lakes, springs, streams, rivers, and even drinking water systems. According to [119], this species is an eurythermal, obligate inhabitant of shallow waters. It has been found in Lapland [120] and in several places in Germany in spring-fed habitats among epiphytic algae on stones in strong or weak currents [121]. Klink [118] found Paratanytarsus austriacus in cold streams and small ponds fed by groundwater. It was often collected from solid substrates above stagnant water [118]. In the Northern Russian datasets, this taxon was mainly present in the northernmost shallow permafrost lakes, such as, for example, those on the Novosibirsk Islands (75°16′00″ N, 145°15′00″ E). T optima in the North Russian dataset is 10.1 °C, and in the Far Eastern part of the dataset, which includes colder environments, it is as low as 3.3 °C [44,45].

The abundances of Paratanytarsus austriacus-type seemed to occur in antiphase with the representation of semi-terrestrial fauna, at least with Pseudosmittia and Limnophies-Paralimnophies. A strong increase in the abundance of the Paratanytarsus austriacus-type and a decrease in the lotic and semi-terrestrial fauna between 1890 and 1900 and after 1975 may indicate a higher lake level. Between about 1905 and 1955, the semi-terrestrial/lentic fauna dominated, and it could be assumed that the lake level was lower with less surface discharge. After 1945, however, the abundance of the lotic/semi-terrestrial Metriocnemus eurynotus-type and the Paratanytarsus austriacus-type increased, reflecting more dynamic conditions with increasing discharge. The reappearance of lotic taxa, the dominance of the Paratanytarsus austriacus-type, and the appearance of taxa characteristic of deeper environments may indicate an increase in lake depth after 1975.

4.3. Diatoms

The highest rate of taxonomic change was observed in diatom communities (2.533 SD) and, similarly to chironomids, it was higher in NZ than in the three previously studied lakes in the Bol’shezemelskaya Tundra: Lake Harbey (1.701 SD), Lake Mitrofanovskoe (1.23 SD), and Lake Vanuk-ty (1.49 SD) [4,77,82].

In the lowermost part of the core (1850–1860), a very low concentration of diatoms may reflect unfavorable conditions for the development of diatom flora. The presence of valves of the mesohalobic, cosmopolitan, benthic, pH-indifferent species Diploneis interrupta in the lower sediment horizon (ca. 1850) may indicate the possible influence of seawater. Pliocaenicus costatus, which is present only in this part of the core, is a planktonic freshwater species that prefers oligotrophic alkaline waters and is known only from mountainous deep-water reservoirs in northern regions of the Holarctic [122].

After 1860, diatom diversity increased, and the number of species rose sharply. There was clear dominance of the cosmopolitan planktonic-benthic Pantocsekiella ocellata. The decreasing abundance of the alpine freshwater species Stephanodiscus alpinus, which is indifferent to salinity and pH, the low abundance of boreal species (the planktonic Aulacoseira islandica, Cyclotella iris and the benthic Staurosirella leptostauron), as well as the increase in the mesohaline species Diploneis interrupta could reflect an increase in the water level in the lake and an influx of biogenic material from the catchment area, possibly due to a slight temperature increase.

From 1905 to 1950, cosmopolitan alkaliphilic planktonic-benthic species, indifferent to salinity and adapted to nutrient-poor environments, still dominated. Abundant around 1905–1910, Cymbella aspera is widespread in alkaline waters with low nutrients and low to moderate electrical conductivity but rarely found in large numbers. It is often found in lakes, ponds, bogs, springs, and streams [123]. Rising after 1924, Epithemia argus, like the other Epithemia species, may contain nitrogen-fixing endosymbiotic cyanobacteria that allow the cells to become abundant in areas of low nitrogen concentration [123]. This species tolerates highly alkaline, nutrient-poor conditions. The lentic Pantocsekiella ocellata disappeared, and the abundance of current-indifferent species with the dominant Staurosira construens and the subdominant Tabellaria fenestrata increased. Around 1944, the number of valves of the halophobic species Eunotia praerupta and Tabellaria fenestrata increased.

Another taxonomic shift could be seen between 1950 and 1970. Here, the diversity and concentration of diatoms in the sediments were very low. Almost all previously abundant diatom species disappeared from the assemblages, with only the mesohalobic species Diploneis interrupta thriving. There could be several reasons for such a sharp decline in diatom assemblages, one of which could be the influence of seawater entering the lake during strong storms [26,30].

After 1970, the diatom flora became more diverse and abundant. Valves of planktonic species dominated. Valves of cold-water species were absent during this period. Valves of stagnant water species, indifferent to salinity, predominated, but the proportion of valves of mesohalobic species increased with the appearance of new species (Navicula digitoradiata, Fallacia pygmaea). Aulacoseira ambigua, which was abundant only after 1980, was reported as a freshwater species [124,125] widespread in the plankton of oligotrophic [126], mesotrophic [127], and eutrophic waters [125]. This taxon prefers large and deep waters [127,128] but has also been reported from rivers [126]. Changes in diatom assemblages indicated that the lake ecosystem became more stable, which was supported by the development of more diverse diatom fauna. The presence of species characteristic of planktonic and lotic environments suggested an increase in water depth.

4.4. Cladocera

Cladoceran communities showed the lowest species turnover (0.614 SD), which was also lower than in Lake B. Kharbey (0.966 SD), Bol’shezemelskaya Tundra [4,77,82], the only lake in the European part of the Russian Arctic where palaeoecological studies of sub-recent changes in cladoceran communities have been conducted so far. The low compositional turnover of the cladoceran communities was a consequence of the generally low diversity and abundance of cladoceran remains in the NZ lake studied. Cladoceran remains were present at very low abundances, with concentrations ranging from 0 to 12.8 ind./g. Between 12.0 and 10.5 cm (1850–1870) and at 5.0–4.0 cm (1951–1964), no cladoceran remains were found in lake sediments. The lowest amount of cladoceran remains was found between 12 and 8 cm (1850–1905).

Cladoceran species’ richness in the studied core was also low, with 2.04 taxa per sample on average (ranging from 0 to 8). The most abundant taxa in the investigated core were the cladocerans Chydorus cf. sphaericus, Daphnia sp., and Bosmina cf. longispina. Only Chydorus cf. sphaericus was observed in more than half of the samples of the investigated core (17 samples, 71% of the samples), with Daphnia sp. in 10 samples (42%) and Bosmina cf. longispina in 5 samples (21%). More than half (62%) of the total list of taxa was rare, occurring in only one or two layers of the core. The species richness of Cladocera was lowest in the lower part of the core, gradually increasing towards the upper part.

Four species of the family Chydoridae were recorded for the first time for the NZ archipelago (Biapertura affinis, Alona werestschagini, Alonella excisa, Pleuroxus uncinatus).

Although littoral taxa dominated the record, planktonic taxa, such as Bosmina and Daphnia, were also present in significant quantities, accounting for 40% of the total abundance, and they were relatively diverse. Six of the cladoceran taxa belonged to the planktonic group.

The NZ archipelago had very poor fauna, consisting mainly of eurybiotic taxa that were highly adapted to inhabiting a wide variety of permanent and ephemeral biotopes in all zones of the world or cold-water relict taxa with disjunct ranges. Some of these taxa had only been found in the Subarctic or Arctic regions of the Palearctic or in high mountain regions (Eurycercus (Teretifrons) glacialis, Daphnia middendorffiana, Alona werestschagini). Taxa with wide cosmopolitan distributions had lower representation.

Chydorus cf. sphaericus prevailed in the cladoceran community (mean abundance 55.3%). It was the most abundant taxon in our studies. It was also the most abundant taxon in other studies of Arctic areas [4,99,100,129,130], where climatic conditions became unacceptable for other members of the cladoceran group. This taxon is characterized as a highly adaptive, pioneering species complex with a wide habitat range. It is widely distributed in Europe and reported to live in all types of water. It lives among vegetation near the bottom (especially during the day) and in the pelagic zone of oligotrophic to eutrophic waters. It is saltwater tolerant up to a concentration of >10 [73,99,100].

According to the literature, only two taxa of cladocerans have been recorded in the northern part of NZ: Chydorus sphaericus and Macrothrix hirsuticornis [129]. The number of species increased significantly in the south of the archipelago. According to the previous study by N.V. Vekhoff, 11 valid species of Cladocera were found [8,33,129] in the fauna of the islands of the NZ archipelago. Two species that he found, D. (D.) pulex tenebrosa and Bosmina obtusirostris, are now not valid and combined with other taxa from his list.

Our study has added four new species to the list of Cladocera taxa inhabiting the NZ archipelago: Alona werestschagini, Biapertura affinis, Alonella excisa, and Pleuroxus uncinatus. Taking into account our new findings, the cladoceran fauna of the NZ comprises 15 valid species, which is 27% more than the previously known species list [8,9,33,129].

Alona werestschagini currently has a narrow, disjunct distribution, occurring in high-latitude lakes in northern Norway, Finland, Iceland, and the Kola Peninsula, as well as in high-altitude lakes in the Pamir, Mongolia, the Tien Shan, and the Altai. It is absent from temperate lowlands [69,131]. Such a patchy distribution in cold climates suggests that this species is a postglacial relict adapted to cold climates [67]. A. werestschagini seems to be a good indicator of oligotrophic conditions and low temperatures in paleolimnological studies [132].

Biapertura affinis is a euryhaline and common species widely distributed in cold regions [4,73,99,100]. It is reported from lakes, ponds, reservoirs, and river floodplains in lowlands. The species is benthic, living in littorals near the bottom and among vegetation. It has a wide conductivity range (54–2050 μS cm⁻¹) and tolerates saline water up to 5.25 ‰ [66,73]. The species has wide pH tolerance (3.6–8.2) but is most abundant at pH < 5. Paleolimnological studies showed that B. affinis was an indicator of cold conditions and classified as a ‘subarctic’ species due to its relatively high frequency in the Arctic [99,133].

Alonella excisa is a common eurybiontic species widely distributed across Europe. It has been found in littoral areas among vegetation, such as Chara and Sphagnum, as well as near the bottom of lakes, ponds, lowland rivers, and bogs. It can also be found in small water bodies in mountainous regions at altitudes of up to 3000 m. The species prefers slightly eutrophic waters [73,99,100] and is acidophilic, recorded in waters with pH as low as 3.8 [66,69].

Pleuroxus uncinatus is widely distributed across northern Eurasia. It is found in open, sandy lake littorals and ponds, as well as moorland pools and rivers, and it is saltwater tolerant [66,69,73].

4.5. Diversity

There was no clear common trend in the development of the diversities of the investigated components of the hydrobiological communities. The linear trends in the diversity of the planktonic proxy (diatoms and cladocerans) were positive, and there was a decreasing linear trend in the N2 diversity of chironomids. However, the difference could be explained by the asymmetry in the evolution of the planktonic and benthic proxies at the beginning of the record. Chironomid diversity was high between 1850 and 1863. At the same time and until 1876, diatom abundance and diversity were low, and cladocerans were absent.

Following the different beginnings of the diversity record, several phases were documented during which the diversity of all three proxies increased or decreased in a similar manner. Between 1890 and 1917, the N2 diversity of all proxies was low, with the lowest values found around 1905. This date is a border between stratigraphic zones, and there is a main shift in PCA 1 of all proxies studied: chironomid, diatom, and Cladocera. Prior to 1905, climatic conditions were unstable, and it was possible that the lake level was also unstable, with the lake being filled with inflowing stream water. Evidence of this could be seen in the low abundance or complete absence of planktonic proxies in the lower part of the core, alongside a predominance of lotic semi-terrestrial chironomid fauna.

According to the meteorological record, the period between 1890 and 1917 corresponds to the beginning of steady climate warming after preceding changeable climatic conditions. The C/N ratio was lowest in 1905, indicating a decrease in allochthonous influence, most likely related to low surface runoff and the establishment of oligotrophic conditions in the lake. This was supported by the dominance of semi-terrestrial chironomids, the absence of lotic chironomid taxa, the prevalence of littoral cladoceran species (Chydorus cf. sphaericus), and the increase of the oligotrophic diatom species Cymbella aspera after 1905.

A further period of declining diversity was documented between 1950 and 1970. A distinct lithological sediment boundary has been dated to 1958. The C/N ratio increased during this period, indicating an increase in allochthonous/terrestrial influence [39]. According to meteorological data, the period between 1950 and 1975 was characterized by climatic cooling and the highest precipitation rate in the available record. Additionally, the years 1956 and 1957 experienced the highest rainfall in June at 64 and 59 mm, respectively. In comparison, the June precipitation rate for the previous 15 years (1941–1955) was 22 ± 10 mm, and for the following 14 years (1958–1972) it was 23 ± 12 mm. Given that June is the month when lakes and other bodies of water in NZ open up from the ice, such a sudden increase in precipitation could be one of the reasons for stronger currents and increased water inflow, causing changes in sedimentation processes.

Therefore, the decline in diversity and abundance of hydrobiological communities during the two-decade period may be attributed to sporadic intense to catastrophic surface runoff events. These runoff events may be caused by both climatic conditions and specific anthropogenic influences associated with nuclear tests, which will not be further discussed here [26].

The diversity of all proxies recovered after 1970 and exhibited positive trends for chironomids and diatoms, while N2 diversity for cladocerans demonstrated a slightly lower trend. Nevertheless, the total diversity, abundance, and alpha diversity of cladocerans were highest in the latter part of the lake history studied. Several studies in the European part of the Russian Arctic have observed positive trends in species diversity and richness for hydrobiological proxies [4,133,134].

5. Conclusions

Our investigation is the first ever multi-proxy study of sub-recent (~165 years) variations in the hydrobiological communities of a lake located on the Southern Island of the NZ archipelago and the first attempt to trace the influence of recent climatic conditions on the hydrobiological communities of this remote and difficult-to-access region of the Russian Arctic.

The chironomid fauna included 42 taxa, most of which could be attributed to modern species. There were three groups of chironomid taxa in the lake: semi-terrestrial taxa, which were the most abundant group, dominated by Limnophies-Paralimnophies and Pseudosmittia; a group of taxa frequently found in lotic environments; and a group of taxa with different ecology, mainly represented by taxa characteristic of cold oligotrophic lakes at high latitudes or altitudes, dominated by the Paratanytarsus austriacus-type.

Our study added 16 new species to the list of freshwater diatom species known for NZ. The diatom assemblages are dominated by cosmopolitan, benthic, alkaliphilic species that are indifferent to salinity, current, and temperature. The succession of diatom complexes shows a decrease in the proportion of cold-stenothermic species that are characteristic of the northern regions of the Holarctic (e.g., Pliocaenicus costatus, Aulacoseira islandica), an increase in the proportion of temperature-indifferent diatom species or species preferring warm conditions, and an increase in the proportion of species reflecting a rise in water level and lake productivity, such as Pantocsekiella rossii, Navicula digitoradiata, Fallacia pygmaea, Stephanodiscus astraea, and Planothidium lanceolatum.

Four species of cladocerans new to the NZ archipelago were found in the lake studied: Alona werestschagini, Biapertura affinis, Alonella excise, and Pleuroxus uncinatus. The cladoceran fauna was mainly represented by eurybiotic taxa with wide cosmopolitan, Palearctic, or Holarctic distributions, though some taxa were found only in the Subarctic and Arctic regions of the Palearctic or in high mountain regions. Ecologically, most cladocerans are highly adapted to inhabiting a wide variety of permanent and ephemeral habitats in all natural areas. Others are rare, cold-water relict taxa with disjunct ranges.

Chironomids and diatoms exhibited higher levels of species turnover in the period under investigation when compared to the lakes situated in more southern Russian Arctic regions. The overall species turnover was highest for diatoms (2.533 SD), while for chironomids the species turnover was 1.781 SD and for cladoceran assemblages it was the lowest (0.614 SD). The influence of the climate on lake fauna and flora was evident in our study through significant correlations between climate parameters and taxonomic variations in hydrobiological community components. The strongest relationships for chironomids were found for T_June, T_July, and T_ann. Both planktonic proxies, diatoms and cladocerans, showed relationships with not only summer and annual air temperature but also with annual precipitation.

Taxonomic richness and diversity (expressed as Hill’s N2) were generally low, especially for Cladocera. Variations in the diversity of all three proxies during the sub-recent period could be related to the climatic parameters and associated hydrological regime of the lake studied. Strong shifts in taxonomic composition and declines in diversity of chironomids, diatoms, and cladocerans occurred around 1905, when the region had just begun to experience a steady warming trend following a period of unstable climatic conditions at the end of the LIA in the late 19th century. A strong decline in diversity between 1950 and 1970 could be related to slight cooling of the climate combined with the highest precipitation rate on record and, probably, the anthropogenic influence specific to NZ at this time. Species richness and biodiversity were rising in the most recent period for all proxies.

The newly obtained data can form the basis of future investigations.