Fragilaria longwania sp. nov. (Bacillariophyceae), a New Araphid Diatom from Sihailongwan Maar Lake, Northeastern China

Abstract

A new species, Fragilaria longwania sp. nov. from Sihailongwan Maar Lake in northeastern China is described on the basis of observations made using light microscopy (LM) and scanning electron microscopy (SEM) that reveal the distinctive features that separate this new taxon from the other species currently placed in the genus Fragilaria. Fragilaria longwania sp. nov. has relatively short valve length, high stria density, spindle-shaped valves with a swollen, and sometimes deformed central area and capitate valve apices, short spines and weakly developed apical pore fields. This combination of character was found to be unique when the new species was compared with morphologically similar taxa. This species was found in various samples collected from Sihailongwan Maar Lake between 2003 and 2017. The analysis of the diatom assemblages collected in the sediment trap samples allowed an assessment of the cell size variation, seasonal occurrences and ecological requirements of this species. Fragilaria longwania appears to be a planktonic diatom most often associated with the spring season.

1. Introduction

In the past few years, considerable progress has been made on the revision of the genus Fragilaria Lyngbye (1819: 185) [1], with the reanalysis of historic samples and type materials. These analyses have provided a better understanding for some of the most reported species belonging to Fragilaria, as well as the description of numerous new species (e.g., [2,3,4,5]).

By contrast, few new species have been described from material collected in China. In their recent catalog on freshwater diatom families and genera of China, Wang et al. [6] listed 41 species and 61 varieties for the genus Fragilaria. According to the list compiled by Kociolek et al. [7], only one new species of Fragilaria has been described from China between 2000 and 2019 (i.e., Fragilaria crenophila var. sinensis Rioual [8]). Since the publication of this list, Fragilaria huebeneri A. Schwarz, KJ Krahn et CE Wetzel [9] and Fragilaria sphaerophorum Luo et Wang [10] were described from material collected on the Tibetan Plateau, from Lake Namco and the Yarlung Zangbo River, respectively. When compared with the large number of new species belonging to Ulnaria and Hannaea, two genera closely related to Fragilaria, that were recently described from China (e.g., [11,12,13]), it is very likely that the diversity of the genus Fragilaria in China has been understudied.

As part of an ongoing research project on the modern and down-core diatom assemblages of Sihailongwan Maar Lake (Northeast China), a species of Fragilaria that was found mostly in sediment trap samples could not be identified using the currently available literature. On the basis of observations made using light microscopy (LM) and scanning electron microscopy (SEM), and after comparisons with known species of the genus, this Fragilaria is described as new-to-science.

2. Materials and Methods

2.1. Study Site

The study site, Sihailongwan Maar Lake, is located in the Long Gang volcanic field Jilin Province, in Northeast China (42°17′ N, 126°36′ E, 790 m a.s.l., Figure 1). This lake is a typical maar lake, i.e., a volcanic lake formed by a phreatomagmatic explosion. Its morphology is therefore shaped like a bowl, with steep rocky slopes and a flat bottom, a relatively small surface area (0.5 km²) but a deep water depth (maximum 50 m and relative depth of 6.3%). Detailed information on the geology, recent climate and vegetation of the region around the lake, as well as on the lake limnology, can be found in previous studies on this lake [14]. Lake Sihailongwan is a dimictic lake with two periods of circulation in the water column, in spring and in autumn. The winter ice-cover melts around mid-April, and the spring overturn, induced by wind mixing, is short-lived as the temperature of the surface waters rises quickly. Thermal stratification develops by the end of April, and the surface water reaches its maximum temperature of 25 °C in August. Afterwards, a cooling period starts, and the thermocline deepens. The autumn overturn period occurs in mid-November before the return of the ice-cover [15].

As shown by the analyses on short cores and trap material, the sediment sequence of this lake is composed of seasonal laminations, or varves, which correspond to the alternation of allogenic atmospheric dust inputs that correspond to the winter/spring season, and authigenic material, mainly composed of algal remains including diatoms, deposited during the spring, summer and autumn seasons [15,16]. The high quality of its sediment sequence, which stores a record of environmental and human impacts at a continental and global scale over the past millennia [14], made this site particularly suitable for palaeolimnological investigations. Considering these characteristics, Lake Sihailongwan was selected as a candidate site to become a Global Boundary Stratotype Section and Point (GSSP) for the demarcation of the Anthropocene [17].

2.2. Hydrochemistry of Lake Sihailongwan

Between August 2020 and November 2014, chemical variables such as pH, alkalinity (Gran titration), electrical conductivity and the concentrations for the main cations, anions and nutrients were measured on surface water samples collected in the middle of the lake at the same time as sediment trap samples were retrieved. On a few occasions, the depth of the photic zone was assessed using a Secchi disk. Details on the water chemistry methods that were used to measure these variables are provided in Rioual et al. [18], and a summary of the results is given in

Table 1. Summary of the chemical and limnological measurements made on surface water samples collected from Lake Sihailongwan between August 2010 and November 2014.

Variable	Unit	nb	Min.	Max.	Median	St. Dev.
pH		99	5.53	7.21	6.55	0.24
Elec. Conductivity	µS/L	99	51	105	62	7
Alkalinity	µeq/L	99	338	660	478	52
Tot. Phosphorus	µg/L	99	5.0	88.6	14.9	14.4
Tot. Nitrogen	µg/L	97	7	2443	360	294
Diss. Organic Carbon	mg/L	92	0.2	10.0	1.7	1.2
Cl	mg/L	78	0.6	3.8	1.2	0.6
NO3	mg/L	73	0	2.4	0.6	0.5
SO4	mg/L	78	3.4	7.8	5.7	1.0
Ca	mg/L	78	2.7	28.8	5.3	5.3
K	mg/L	78	0.6	3.3	1.3	0.6
Mg	mg/L	78	1.2	3.6	2.3	0.6
Na	mg/L	78	1.2	4.4	2.2	0.7
Si	mg/L	99	0.1	0.8	0.4	0.2
Secchi Disk depth	m	22	2.4	7.6	3.8	1.3

.

2.3. Diatom Sampling and Sample Preparation

Material for diatom analysis was obtained from sediment traps deployed at the center of Lake Sihailongwan between May 2003 and July 2017. The mooring was designed so that the traps collected all material, organic and inorganic, that settles into the lake water column, but that the content of the trap would not be affected by possible resuspension from the bottom sediment. To this effect, the mooring system includes an anchor and a rope that is stretched by a surface buoy. The traps were disposed on the rope several meters above the anchor that lies on the surface sediment [15] (Figure S1A). The design of the traps themselves follows the general recommendations of Håkanson et al. [19], i.e., simple cylinders, with a high aspect ratio and a diameter larger than 4 cm. The initial design used between May 2003 and June 2009 consisted of a set of three non-transparent PVC tubes, 14.5 cm in inner diameter and 78 cm high, closed at their bottom and fixed to the rope by crossbar and wires (Figure S1(B1)). Then, from June 2009, a much lighter design that could be operated by a single person was used and consisted of four PVC rubes of 4.6 cm inner diameter, 50 cm high, closed at their bottom with tight-fitting collecting jars (Figure S1(B2)). These differences in design did not affect the amount of collected material as measured by dry weight on a unit area basis. The same observation was reported by Kirchner [20]. Samples were collected monthly between 2002 and 2005 and bimonthly from 2006 until the end of the study in 2017. In total, 187 sediment trap samples were collected and analyzed for this study.

In addition to the trap samples, three periphyton samples were also analyzed, scrapped from the surface of the buoy that maintains the trap mooring in the water column; two surface sediment samples (the top ~1 cm, representing recent epipelic assemblage as well as several years of sedimentation) that were collected from the middle of the lake (~50 m water depth) on the 21 October 2005 and 14 August 2007; and a series of benthic samples (epilithon, epiphyton, epipelon and epixylon) collected in the littoral zone of the lake at different times during this study. Details of these samples are given in Table 2. The purpose of analyzing these additional samples was to determine the preferred habitat of the new species.

Diatom samples were prepared according to the standard procedure described in Battarbee et al. [21] using hydrogen peroxide. In the trap samples, diatom concentration was estimated using the microsphere method [22] and expressed as number of valves per gram of dry matter (valves gdm⁻¹). Diatom fluxes were then calculated by dividing the diatom concentration by the number of days of the trap period and by the surface area of the trap. Diatom fluxes are then expressed in number of valves per day and per centimeter square (valves day⁻¹ cm⁻²).

Slides for light microscopy were prepared by mounting the cleaned suspension in Naphrax^® (Brunel Microscope Ltd., Chippenham, UK). LM observations were made at ×1000 magnification using a Leica DM 2500 equipped with immersion objectives (N Plan, 1.25 numerical aperture) and differential interference contrast (DIC). Photomicrographs were captured using a Leica DFC450 camera (Leica Microsystems CMS GmbH, Wetzlar, Germany). For scanning electron microscope analyses, aliquots of cleaned suspensions were filtered and rinsed through a 3 µm Isopore™ polycarbonate membrane (Merck, Darmstadt, Germany). The membrane filters were then mounted on aluminum stubs using double-coated carbon conductive adhesive tape and coated with gold using a Quorum Q150R S Plus sputter coater (Quorum Technologies, Laughton, UK). To illustrate the morphological variability of the species, we provide images captured from various samples collected at different locations in the lake (e.g., sediment trap, surface sediment from the deepest point, epipelon from the littoral zone of the lake) and in different years and seasons. The type of sample and the time of collection are given in the captions of the images. Through the years, different scanning electron microscopes were used. Most of the images reproduced in this paper were taken with a Phenom XL desktop SEM (Nanoscience Instruments, Phoenix, AZ, USA) at IGG-CAS (Beijing, China) operated at 10 kV.

3. Results

3.1. New Species Description

Division: Bacillariophyta

Class: Fragilariophyceae—F.E. Round 1990 [23].

Subclass: Fragilariophycidae—Round and Crawford

Order: Fragilariales—Silva, 1962 sensu emend [24].

Family: Fragilariaceae—Greville 1833 [25].

Genus: Fragilaria Lyngbye 1819 [1].

Fragilaria longwania Rioual sp. nov.

Description: LM (Figure 2): Frustules in girdle view rectangular, solitary or, occasionally, two cells connected to each other (Figure 2V). Valves spindle-shaped to elongated, narrowly lanceolate with clearly convex, never-parallel margins, tapering from a broadened central area towards protracted, distinctly capitate apices, often deflected. Teratological forms with irregularly undulating or bent valve outlines often observed (Figure 2B,T). Valve dimensions: length 15–44 µm (n = 1078, LM and SEM), width 2.0–3.5 µm (from SEM images only, n = 36). Sternum distinct but narrow, gradually widening towards the central area. Central area swollen, forming small, irregularly shaped hyaline zone, occasionally absent. Striae alternating, parallel throughout entire valve, 21–25 in 10 µm. Areolae sometimes discernible in LM under differential interference contrast.

SEM (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8): Series of marginal spines, generally emerging from vimines, but sometimes from virgae, occurring more or less evenly from apex to apex (Figure 3C, Figure 5C, Figure 6C and Figure 7B). Spines acute, shaped like shark tooth (Figure 3B,C and Figure 8A). Up to three tiny spines present at apices (Figure 4B, Figure 5B and Figure 6B). Valve face flat with weakly raised virgae (Figure 3A, Figure 5C and Figure 7C). Central area with ghost striae (Figure 3D, Figure 5C, Figure 6C and Figure 7B). Striae uniseriate, composed of several (up to six) rounded areolae (Figure 5C, Figure 6C and Figure 7B), becoming transapically elongated on the mantle (Figure 8A,B), 62–74 in 10 µm (n = 35). Virgae much wider than the striae. Apical pore field of the ocellulimbus type, not well developed, composed of only one or two rows of small rounded poroids (Figure 3A,E, Figure 4B,C, Figure 5B,D and Figure 8B,C). One rimoportula is present per valve, positioned at a distal end (Figure 3A, Figure 4A, Figure 5A and Figure 7A). Externally, rimoportula opening small, rounded but not rimmed, generally located at the center of the last stria (Figure 3B, Figure 4B, Figure 5B, Figure 6B and Figure 7D). Internal rimoportula opening more or less elongated, oblique (Figure 4C). Girdle composed of only two or three open bands, ligulate, tapering and opening at the valve pole (Figure 8A–C). Each band with one row of poroids (Figure 4C and Figure 8A–C).

Holotype: 20 April 2005, collected by P. Rioual, slide number IGGDC-DB-SIHA-0504 (herb. IGGDC) partly shown here in Figure 2, holotype specimen: illustrated in Figure 2Z, located using England Finder J27/1.

Paratype: BM 101 673 in the Natural History Museum, London, UK. (Note: this is the same slide as for Staurosira longwanensis that was described in Rioual et al. [26].

Registration: http://phycobank.org/105857 (accessed on 6 August 2025).

Type locality: China, Lake Sihailongwan, Jilin Province (E: 126°35′51″–126°36′24″, N: 42°17′0″–42°17′24″; Elevation: 797 m above sea level)

Etymology: For naming this species we followed Dubois [27], who recommends keeping species names as simple and short as possible in order to be euphonious in all languages and easily memorable. The epithet of the new species refers to “Long wan”, the Chinese name for the maar lakes of the Long Gang volcanic field in Jilin Province and which means “Dragon bowl” in English.

Distribution: So far only observed in the type locality.

3.2. Analysis of the Trap Samples

In Lake Sihailongwan, during the 15-year period of investigation, two centric diatoms, Discostella stelligeroides (Hustedt) Houk and Klee and Lindavia balatonis (Pantocsek) Nakov, Guillory, Julius, Theriot and Alverson, largely dominated the sediment trap assemblages. Fragilaria longwania was present in 139 of the 187 trap samples analyzed, with a maximum abundance of 19.8% recorded at the beginning of the study in June 2003 and a mean abundance of 2.1%. Over the study period, a general decline in flux and relative abundances is observed (Figure 9A,B). On the other hand, during the same period, the mean valve length follows a positive trend (Figure 9C).

The seasonal distribution of F. longwania was investigated by plotting its flux and relative abundances against Julian day on a year-to-year basis (Figure 10). The highest flux value was observed during spring 2007 (127 millions valve/day/cm²). The highest fluxes of F. longwania occurred during the spring period (from April to mid-June) in 2005, 2008, 2010, 2011, 2103 and 2014, so in 7 of the 15-year-long study. In the region of Lake Sihailongwan, the spring period is characterized by low temperature, low precipitation, high wind speed and long daily sunshine duration (due to low cloud cover during this season). In some years, however, the highest flux values were observed during summer (in 2003, 2004, 2009, 2012) or during autumn (in 2006, 2008, 2015, 2016). This suggests that F. longwania does not have a very strong seasonal preference and/or that meteorological conditions are not the only factors driving the population dynamic of this species.

3.3. Analysis of Additional Samples

D. stelligeroides and L. balatonis, the two centric diatoms that dominate the assemblages collected in the sediment trap samples, are also present and often dominant in all the other types of samples collected in the lake. These include periphyton samples collected from the mooring buoy and benthic samples from the littoral zone that were collected from various substrata, including from surface sediment (epipelon), stone (epilithon), aquatic plant (epiphyton) and wood from dead trees that have fallen into the lake (epixylon) (Table 2).

For F. longwania, the highest percentages were found in sediment trap samples (up to 19.8%). Unsurprisingly, considering its abundance in trap samples, F. longwania is also among the most abundant species in the surface sediment samples collected from the middle of the lake, reaching 1.8% in October 2005 and 1.2% in August 2007 (Table 2). It is also found in the periphyton samples collected from the mooring buoy, although its abundance in this type of sample is also declining and therefore follows the same trends observed in the trap samples. F. longwania can be completely absent from the benthic samples collected from the littoral zone and, when present, this species never reaches 3% of the assemblages.

Table 2. Summary results of the diatom analysis of the samples collected for this study.

Date	Sample	Dominant Species	% Fragilaria longwania
From 19 May 2003 to 15 July 2017	Sediment trap samples (center of the lake, −45 m water depth), (n =187)	Discostella stelligeroides (43.3, 0.4–92.0%), Lindavia balatonis (26.2, 1.7–76.3%) 1	2.1 (0–19.8%) 1
26 April 2005	Epilithon (small stones, near shore, <0.5 m depth)	Lindavia balatonis (19.3%), Achnanthidium sieminskae (16.0%), Encyonopsis perborealis (7.6%), Discostella stelligeroides (5.9%), Encyonema silesiacum (5.0%)	Not observed
21 October 2005	Surface-sediment (−50 m water depth, lake center)	Discostella stelligeroides (57.2%), Lindavia balatonis (17.6%), Pseudostaurosira parasitoides (3.2%), Nanofrustulum trainorii (2.2%), Fragilaria pectinalis (1.9%)	1.8%
18 November 2006	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (50.6%), Brachysira neoexilis (15.2%), Encyonopsis subminuta (7.2%), Staurosirella ovata (5.9%), Discostella stelligeroides (4.6%)	3.0%
19 November 2006	Epilithon (small stones, near shore, <0.5 m depth)	Nitzschia cf sociabilis (12.6%), Achnanthidium sieminskae (9.6%), Lindavia balatonis (9.1%), Nitzschia archibaldii (7.8%), Tabellaria hercynica (7.4%)	2.6%
19 November 2006	Epiphyton (−10 m water depth, filamentous green)	Discostella stelligeroides (18.7%), Lindavia balatonis (9.6%), Pseudostaurosira pseudoconstruens (8.7%), Staurosirella ovata (6.5%), Sellaphora chistiakovae (3.0%)	1.3%
19 November 2006	Epipelon (−17 m water depth)	Discostella stelligeroides (32.0%), Staurosirella ovata (15.4%), Lindavia balatonis (10.6%), Pseudostaurosira parasitoides (5.9%), Staurosira longwanensis (5.9%)	1.2%
14 August 2007	Surface-sediment (−51 m water depth, lake center)	Discostella stelligeroides (80.4%), Lindavia balatonis (6.3%), Fragilaria longwania (1.2%)	1.2%
22 April 2008	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (38.5%), Discostella stelligeroides (16.3%) Brachysira neoexilis (14.4%), Encyonopsis perborealis (4.4%), Lindavia balatonis (3.0%)	0.4%
22 April 2008	Epilithon (small stones, near shore, <0.5 m depth)	Encyonopsis perborealis (23.6%), Encyonema silesiacum (14.6%), Discostella stelligeroides (12.2%), Lindavia balatonis (10.6%), Achnanthidium sieminskae (8.9)	Present (<0.1%)
3 August 2016	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (27.5%), Encyonopsis perborealis (19.0%), Lindavia balatonis (14.2%), Eucocconeis laevis (13.3%), Rossithidium pusillum (7.3%)	Not observed
3 August 2016	Epilithon (small stones, near shore, <0.5 m depth)	Encyonopsis perborealis (29.3%), Achnanthidium sieminskae (9.30%), Lindavia balatonis (8.7%), Achnanthidium sp cf duriense (8.0%), Nitzschia lacuum (6.0%)	1.3%
3 August 2016	Epixylon (dead tree, near shore, <0.5 m depth)	Encyonopsis perborealis (32.8%), Achnanthidium sieminskae (18.0%), Lindavia balatonis (14.8%), Nitzschia gessneri (6.9%), Brachysira neoexilis (4.8%)	Not observed

¹ average and range values from 187 sediment trap samples.

Table 2. Summary results of the diatom analysis of the samples collected for this study.

Date	Sample	Dominant Species	% Fragilaria longwania
From 19 May 2003 to 15 July 2017	Sediment trap samples (center of the lake, −45 m water depth), (n =187)	Discostella stelligeroides (43.3, 0.4–92.0%), Lindavia balatonis (26.2, 1.7–76.3%) 1	2.1 (0–19.8%) 1
26 April 2005	Epilithon (small stones, near shore, <0.5 m depth)	Lindavia balatonis (19.3%), Achnanthidium sieminskae (16.0%), Encyonopsis perborealis (7.6%), Discostella stelligeroides (5.9%), Encyonema silesiacum (5.0%)	Not observed
21 October 2005	Surface-sediment (−50 m water depth, lake center)	Discostella stelligeroides (57.2%), Lindavia balatonis (17.6%), Pseudostaurosira parasitoides (3.2%), Nanofrustulum trainorii (2.2%), Fragilaria pectinalis (1.9%)	1.8%
18 November 2006	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (50.6%), Brachysira neoexilis (15.2%), Encyonopsis subminuta (7.2%), Staurosirella ovata (5.9%), Discostella stelligeroides (4.6%)	3.0%
19 November 2006	Epilithon (small stones, near shore, <0.5 m depth)	Nitzschia cf sociabilis (12.6%), Achnanthidium sieminskae (9.6%), Lindavia balatonis (9.1%), Nitzschia archibaldii (7.8%), Tabellaria hercynica (7.4%)	2.6%
19 November 2006	Epiphyton (−10 m water depth, filamentous green)	Discostella stelligeroides (18.7%), Lindavia balatonis (9.6%), Pseudostaurosira pseudoconstruens (8.7%), Staurosirella ovata (6.5%), Sellaphora chistiakovae (3.0%)	1.3%
19 November 2006	Epipelon (−17 m water depth)	Discostella stelligeroides (32.0%), Staurosirella ovata (15.4%), Lindavia balatonis (10.6%), Pseudostaurosira parasitoides (5.9%), Staurosira longwanensis (5.9%)	1.2%
14 August 2007	Surface-sediment (−51 m water depth, lake center)	Discostella stelligeroides (80.4%), Lindavia balatonis (6.3%), Fragilaria longwania (1.2%)	1.2%
22 April 2008	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (38.5%), Discostella stelligeroides (16.3%) Brachysira neoexilis (14.4%), Encyonopsis perborealis (4.4%), Lindavia balatonis (3.0%)	0.4%
22 April 2008	Epilithon (small stones, near shore, <0.5 m depth)	Encyonopsis perborealis (23.6%), Encyonema silesiacum (14.6%), Discostella stelligeroides (12.2%), Lindavia balatonis (10.6%), Achnanthidium sieminskae (8.9)	Present (<0.1%)
3 August 2016	Periphyton on floating buoy (lake center)	Achnanthidium sieminskae (27.5%), Encyonopsis perborealis (19.0%), Lindavia balatonis (14.2%), Eucocconeis laevis (13.3%), Rossithidium pusillum (7.3%)	Not observed
3 August 2016	Epilithon (small stones, near shore, <0.5 m depth)	Encyonopsis perborealis (29.3%), Achnanthidium sieminskae (9.30%), Lindavia balatonis (8.7%), Achnanthidium sp cf duriense (8.0%), Nitzschia lacuum (6.0%)	1.3%
3 August 2016	Epixylon (dead tree, near shore, <0.5 m depth)	Encyonopsis perborealis (32.8%), Achnanthidium sieminskae (18.0%), Lindavia balatonis (14.8%), Nitzschia gessneri (6.9%), Brachysira neoexilis (4.8%)	Not observed

¹ average and range values from 187 sediment trap samples.

4. Discussion

4.1. Generic Placement of the New Species

As discussed by Williams [28], the genus Fragilaria is currently aphyletic, i.e., without identified synapormophy, and so far, recent attempts to separate new genera from Fragilaria have not been entirely convincing [8,29,30]. Nevertheless, F. longwania shares many characters with what Williams [28] refers to as ‘classical’ species of Fragilaria, including the presence of one rimoportula on one apex, apical pore fields of the ocellulimbus type, uniseriate striae and open girdle bands. F. longwania is therefore best placed in that genus. Among the groups of Fragilaria species that Williams [28] recognized according to the occurrence and placement of their marginal spines, F. longwania is closest to the group 3, i.e., ‘classical’ species with spines that emerge from the vimines, even if some spines appear to emerge from virgae. In that group, Fragilaria tenera var. nanana and Fragilaria billingsi are included, two of the species that closely resemble F. longwania (see below).

4.2. Comparison with Other Species of the Genus Fragilaria

The new species described here is comparable to several other species of Fragilaria (

Table 3. Selected morphological characteristics of Fragilaria longwania sp. nov. and of other species of Fragilaria sharing a broadly similar outline.

Taxon	Length (µm)	Width (µm)	Striae in 10 µm	Valve Outline	Central Area	Pore Fields	Distribution	Reference
F.longwania	15–44	2.0–3.5	21–25	spindle shaped	clearly swollen	small, only one or two rows of small poroids	Lake Sihailongwan, NE China	This study
F.campyla	35–45	2.5–3.0	19–20	linear to linear-lanceolate	not swollen	well-developed, several rows of small poroids	Europe	[3]
F. parva	20–40	3.0–3.5	19–21	linear to linear-lanceolate	swollen	well-developed, four rows of large poroids	Europe	[3]
F. metcalfeana	60–120	2.5–3.5	ca. 18	linear but strongly elongated	distinctly inflated	large, 2–3 rows of large poroids	Fossil, Neogene, Dubravica, Croatia	[3]
F. pseudofamiliaris	30–50	2.0–3.0	18–19	elongated, narrowly lanceolate	not swollen	well-developed, at least four rows of small poroids	Artesian well, Dresden, Germany	[3]
F. scotica	65	4	15	linear-lanceolate	clearly inflated	not observed	Loch Leven, Scotland	[3]
F. nanoides	40–90	1.8–2.4	22.5–24	linear-lanceolate	weakly inflated	not observed	Lake Julma Ölkky, Finland	[26]
F.billingsii	54–76	2.0–2.5	17–20	narrow lanceolate	bilaterally gibbous	well-developed	Reservoir, Sao Paulo State, Brazil	[27]
F.huebeneri	99–121	2.0–4.0	17–21	spindle to needle-shaped, narrowly lanceolate	indistinct	well-developed, 3–5 rows of poroids	Lake NamCo, Tibet, China	[8]
F.stoermeriana	60–110	1.5–3.0	20–24	long, lanceolate	distinctly swollen	small ocellulimbi	Lake Superior, Ontario, Canada	[28]
F. tenera var. nanana	29–85	2.0–2.3	18.5–20	linear to linear-lanceolate	barely inflated	well-developed, three rows of poroids	cosmopolitan	[29]

), especially to species of the Fragilaria rumpens complex, which have been recently reviewed by Van de Vijver et al. [3] who analyzed the type of material of numerous species described in the 19th century.

One of the most similar species is Fragilaria campyla (Hilse) Van de Vijver, Kusber et DM Williams, which largely overlaps with F. longwania in terms of valve length (35–45 µm vs. 15–44 µm) and width (2.5–3.0 µm vs. 2.0–3.5 µm). F. campyla, however, lacks a swollen area and its valves are linear to linear-lanceolate, not spindle-shaped to elongated and narrowly lanceolate. In F. campyla, the ocellulimbi are also larger than in F. longwania [3].

Fragilaria parva (Grunow), Tuji et DM Williams, is another species of the F. rumpens complex with similar ranges of valve dimensions compared to F. longwania. In addition, F. parva also possesses a swollen area and shark-tooth-shaped spines. However, F. parva differs by having linear to linear-lanceolate valve outline, a large hyaline rectangular zone in the central area and well-developed ocellulimbi [3].

Fragilaria metcalfeana Van de Vijver, DM Williams et Kusber, is a fossil species originally described from Neogene material collected in Dubravica (Croatia). This taxon also has a distinctly inflated central area, but its valves are much longer (60–120 µm) and characterized by a well-delimited hyaline central area and large ocellulimbi at the apices [3].

Fragilaria pseudofamiliaris Van de Vijver, TM Schuster, Kusber et DM Williams, has similar valve dimensions (length 30–50 µm and width 2.0–3.0 µm) and protracted subcapitate to capitate apices. However, this species differs from F. longwania by having elongated, narrowly lanceolate valves, with broad linking spines that help this species in producing long ribbon-like colonies that we never observed in F. longwania. In addition, this species has a lower stria density (18–19 striae in 10 microns) and well-developed ocellulimbi composed of at least four rows of small poroids [3].

Last among the species of the Fragilaria rumpens complex that could be usefully compared to F. longwania is Fragilaria scotica (Grunow) Van de Vijver, CE Wetzel et Ector. This species also possesses a clearly inflated central area. Its outline is, however, not spindle-shaped, but linear-lanceolate with gradually narrowing margins and protracted, rostrate apices. In addition, F. scotica has much lower stria density than F. longwania, with 15 instead of 21–25 striae in 10 microns [3].

Outside the F. rumpens complex, several more species can be compared to F. longwania. Fragilaria nanoides Lange-Bertalot, which was described from Julma Ölkky, an oligo-dystrophic lake from Finland, has a very similar silhouette, and overlapping ranges of stria density (22.5–23 vs. 21–25 striae in 10 µm). However, F. nanoides has longer (40–90 µm) and narrower valves (1.8–2.4 µm) [31].

Fragilaria billingsi Wengrat, C.E.Wetzel et E.Morales, that was described from a reservoir in southern Brazil, is another species with an inflated central area, but its valves are longer (54–76 µm), have a lower stria density (17–20 striae in 10 microns), clearly raised costae and well-developed ocellulimbi [32].

Fragilaria huebeneri A. Schwarz, KJ Krahn et CE Wetzel, which was described from NamCo, a large subsaline lake in Tibet, has spindle-shaped valves that are often curved, like what can be observed in F. longwania. However, in F. huebeneri the central area is less inflated, the valves are more narrowly tapering towards subcapitate apices. In addition, F. huebeneri has much longer valves (99–121 µm), possesses raised costae and spatula-shaped spines that allow this species to form ribbon-like colonies. The ocellulimbi are also well-developed with 3–5 rows of poroids [9].

Fragilaria stoermeriana Alexson, Reavie et Van de Vijver, described from plankton samples collected in the eastern basin of Lake Superior (Ontario, Canada), also shares some characteristics with F. longwania such as a distinctly swollen central area, protracted capitate ends, weakly raised virgae, acute spines irregularly scattered along the entire margin of the valve and small ocellulimbi at the apices. The two species also have similar stria density (20–24 vs. 21–24 striae in 10 microns). However, F. stoermeriana is much longer (60–110 µm) and can form stellate colonies [33].

Finally, the apparently cosmopolitan Fragilaria tenera var. nanana (Lange-Bertalot) Lange-Bertalot et S.Ulrich partly overlaps with F. longwania in terms of valve dimensions as length and width of its valves range between 29 and 85 µm and 2.0–2.3 µm, respectively. However, the stria density of the variety nanana is lower (18.5–20 instead of 21–24 in 10 microns), as reported by Lange-Bertalot and Ulrich [34] and the outline of the valves, even for the short ones, is not spindle-shaped but lanceolate, with a barely inflated central area.

4.3. Ecology, Seasonality, Cell-Size and Distribution

From the results of our water chemistry analyses (Table 1), Lake Sihailongwan has circumneutral, weakly alkaline water. Considering the median values for total phosphorus concentration and Secchi disk depth of ~15 µg/L and 3.8 m, respectively, the trophic status of Lake Sihailongwan can be considered mesotrophic. The highest abundances of this species were observed in trap samples. It is also found in the periphyton attached to the mooring buoy and in the subfossil assemblages found in the surface sediment collected at the bottom of the water column. Together, these data on the occurrences and abundances in samples collected from the middle of the lake and the fact that it was never found in large abundance in the benthic samples collected from the littoral zone suggest that this species is part of the diatom plankton.

From the series of trap samples, F. longwania appears to be associated with the spring season (and in some years, to the autumn). In the region of Lake Sihailongwan meteorological conditions during spring are characterized by low temperatures, high wind speed and sunny conditions (Figure 10). During the 15-year-long period we investigated for this study, valve length of F. longwania tended to increase with decrease in valve abundance (Figure 9). A similar inverse relationship between cell size and density was observed for another species of Fragilaria by Soumya et al. [35] in their investigation of a natural planktonic population from Lake Biwa (Japan). According to these authors, this relationship indicates occurrence of cell division. If this is true for F. longwania in Lake Sihailongwan, it would suggest that conditions at the beginning of the study period (from 2003 to 2013) were more favorable for the growth of the population by vegetative cell division than at the end of the study period (2014–2017), when less numerous and longer cells were found, suggesting lower rates of cell division. Temperature and nutrients are thought to be key factors that affect such changes in cell size and density, but our water chemistry data only cover a short part of the study period, and a more detailed investigation would be required to unveil such relationships.

F. longwania is the second diatom species described from Lake Sihailongwan after Staurosira longwanensis Rioual, Morales and Ector [26]. So far, F. longwania has not been observed in other lakes from NE China or anywhere else but, considering the relatively understudied diversity of the genus Fragilaria in China, it is probably premature to consider it as a species endemic to Lake Sihailongwan.

Finally, preliminary diatom analyses of the long sedimentary sequence of Lake Sihailongwan [14] indicate that Fragilaria longwanensis has been present in the diatom flora of this lake for at least the past 20,000 years. Its population, however, appears to be declining, as suggested by the trends we observed (Figure 9 and Figure 10) in the trap samples we collected over a 15-year-long period.

In conclusion, this study allowed the description of a species of Fragilaria that is new to science, and thus expanded the known diversity of this genus in China.