Zooplankton Standing Stock Biomass and Population Density: Data from Long-Term Studies Covering Changes in Trophy and Climate Impacts in a Deep Subalpine Lake (Lake Maggiore, Italy)

Abstract

Lake Maggiore is a deep subalpine lake that has been well studied since the last century thanks to a monitoring program funded by the International Commission for the Protection of Italian–Swiss Waters. The monitoring program comprises both abiotic and biotic parameters, including zooplankton pelagic organisms. In this study, we present a dataset of 15,563 records of population densities and standing stock biomass for zooplankton pelagic taxa recorded over 43 years (1981–2023). The long-term dataset is valuable for tracing changes in trophic conditions experienced by the lake during the last century (eutrophication and its reversal) and the impact of global warming. Zooplankton samples (Crustacea and Rotifera Monogononta) were collected within 0–50 m depth by vertical hauls with an 80 µm light plankton sampler. The sampling frequency was monthly, with the exception of the 2009–2012 period, which employed seasonal frequency. The estimation of zooplankton taxon abundance and of its standing stock biomass is crucial in order to quantify the flux of matter, energy, and pollutants up to the upper trophic levels of the food web. The dataset provided is also suitable for food web analysis because the zooplankton taxa have been classified according to their ecological roles (microphagous organisms; primary and secondary consumers).

Dataset: https://doi.org/10.5281/zenodo.15082827

Dataset License: CC-BY 4.0

1. Summary

Assessing population density and standing stock biomass (SSB) of aquatic organisms is of basic importance for measuring productivity and quantifying the flux of matter, energy, and persistent pollutants through food webs [1,2,3]. Along with basic environmental variables, such as water temperature and nutrient concentrations, data on SSB provide a basis to assess the availability of matter and energy in time. Zooplankton is a key component of aquatic food webs because it is the main link between phytoplankton, the primary step in organic matter production, and fish as secondary consumers. Comprising both primary and secondary consumers, zooplankton SSB incorporates two levels of organic matter, primary (herbivorous species and filter feeders) and secondary (predatory taxa, consuming zooplankton primary consumers) consumers, which both represent a source of food for zooplanktivorous fish. The distinction between these two zooplankton components and the availability of SSB are not common in ecological datasets, mainly because their estimation is time-consuming and involves microscopic analysis with distinctions between taxa and between developmental stages, requiring zooplankton experts with taxonomic skills. In fact, not only are primary and secondary consumers taxon-specific, but in the case of copepods, their food requirements change with developmental stages, with cyclopoids’ nauplii being primary consumers, while adult stages are mainly secondary consumers.

In the present study, we provide monthly data for zooplankton SSB, collected over 43 years (from 1981 to 2023) in the framework of the long-term CIPAIS (International Commission for the Protection of Italian–Swiss Waters) project. The project is aimed at identifying changes in the zooplankton community of the deep, subalpine Lake Maggiore during its eutrophication reversal and under the recent impact of climate change. These data have been analyzed and discussed in several scientific papers, and they represent a unique dataset, providing information on the different components of the zooplankton community with the calculation of monthly SSB [4,5]. The data are also highly valuable for the monitoring methodology utilized for zooplankton in the field, with sample collection through the water column that is not restricted only to superficial waters. This allows us to take into account the vertical distribution of plankton communities, which, in deep lakes such as Lake Maggiore, comprises the euphotic layer for phytoplankton and extends up to the upper part of the hypolimnion for zooplankton.

Study Site

Lake Maggiore is the second deepest (Depth_max = 370 m) and largest (area = 212 km²; volume = 37.5 km³) subalpine lake located in the northwestern part of Italy (Figure 1). The lake is shared with Switzerland.

The lake rarely undergoes complete vertical mixing and is classified as holo-oligomictic [6,7,8], with a mean theoretical water renewal time of more than 4 years [7]. The Lake Maggiore watershed is quite large (6599 km²) and mostly mountainous. The lake has more than 33 tributaries and one outlet (Ticino River). Originally oligotrophic, the lake became eutrophic during the second half of the 20th century [9]. The concern arising from the eutrophic state of the lake resulted in a program to monitor the water quality of the lake and its drainage basin and tributaries, funded by the International Commission for the Protection of the Italian–Swiss Waters. The recovery of the lake has been achieved thanks to the construction of sewage treatment plants and a ban on phosphorus from detergents, which led to a decrease in phosphorus load and concentrations in the water column [10]. Over the last twenty years, the impact of global warming has become evident, leading to a sharp increase in water temperature and changes in the lake’s thermal stratification and hydrodynamics.

2. Data Description

The dataset provides population density data (ind m⁻³) and SSB (mg m⁻³) data for Lake Maggiore zooplankton pelagic taxa: Daphnia, Eubosmina, Diaphanosoma, Bythotrephes, and Leptodora among cladocerans, cyclopoids, and diaptomids, with a distinction in their developmental stages (nauplii and copepodites) among copepods and rotifers. The data also comprise the ecological roles of the organisms by distinguishing microphagous organisms (copepods nauplii and rotifers, i.e., organisms that mostly compete for food items no larger than 15–20 µm [11,12]), primary (herbivores, i.e., Daphnia, Eubosmina, Diaphanosoma, and copepod diaptomids), and secondary consumers (predators, i.e., Bythotrephes, Leptodora, and copepod cyclopoids) [13,14,15,16,17,18].

The dataset consists of a total of 15,563 records, with 7791 population density estimates and 7772 SSB estimates.

3. Methods

Zooplankton samples were collected monthly at the lake’s deepest station (“Ghiffa”; 45°58′30″ N; 8°39′09″ E; Figure 1) representative of the pelagic zone. From 1981 to 2016, we sampled zooplankton with an 80 μm light Clarke-Bumpus plankton sampler, towed from a 50 to 0 m depth. After 2016, we collected the samples with a plankton net (80 µm light nylon nets) equipped with a flowmeter [4,18]. Both plankton samplers allowed for the filtration of at least 1000 L of lake water.

The nylon nets used enabled us to collect both small (basically, Monogononta rotifers, except for the smallest species, the immature stages of copepods, and small Bosminidae) and large zooplankton organisms [4,19]. The samples were fixed in pure (96% volume) ethanol to allow for further genetic analyses. Sampling was performed monthly from 1981 to 2008 and from 2013 to 2023, while during the 2009–2012 period, the sampling frequency was seasonal. Individual body length measurements for each sample (at least 25 individuals each) were used to apply length/weight regression equations [20,21] to each taxon. The population density of each taxon was thereafter used to obtain taxon-specific biomass (dry weight, mg m⁻³). The total standing stock biomass was obtained by summing up all taxon-specific biomasses of each sampling date.