# Zooplankton Community Responses to Oxygen Stress
## 2 Study Area and Methods
The metalimnion temperature was \(14.2\pm 3.4\)\({}^{\circ}\)C. The hypolimnion temperature was \(7.7\pm 2.0\) m, and there were significant differences between the harmonic and dystrophic lakes (One-way ANOVA: F = 25.1; \(p<0.0001\)). The hypolimnion temperature in the dystrophic lakes was \(6.6\pm 1.5\)\({}^{\circ}\)C, compared to \(8.0\pm 1.9\)\({}^{\circ}\)C in the harmonic lakes. Additionally, deep harmonic lakes (\(>\)30 m) were characterized by lower temperatures in the hypolimnion (One-way ANOVA: F = 16.4; \(p=0.0001\)), which was \(7.3\pm 1.5\)\({}^{\circ}\)C, while in shallower harmonic lakes it was \(8.9\pm 2.2\)\({}^{\circ}\)C.  
Each lake was sampled during the peak of summer stratification (end of July and beginning of August) in 2018 and 2019. The sampling stations in each lake were located close to the deepest point. The field measurements included Secchi disc visibility (SDV), dissolved oxygen, pH, and electrical conductivity using an HQ40D Multi Meter (Hach-Lange GmbH, Germany). The photosynthetic active radiation (PAR) was measured using a LI-193 SA Spectral Quantum sensor (LI-COR Biosciences, Lincoln, NE, USA). The temperature and chlorophyll-\(a\) concentrations were recorded by continuous measurements using a submersible spectrofluorometer FluoroProbe (bbe-Moldaenke, Germany). This probe also determines four phytoplankton classes: green algae (Chlorophyta and Euglenophyta), cyanobacteria (phycocyanin- rich cyanobacteria), diatoms (Heterokontophyta, Hartopophyta, and Dinophyta), and cryptophytes (Cryptophyta and the phycoverthrin-rich cyanobacteria) [26; 39]. Field measurements of the temperature, oxygen, and chlorophyll-\(a\) were made at 1 m intervals from the surface to 30 m, while PAR measurements were made at 1 m intervals from the surface to 10 m. In situ  
Figure 2: Oxygen profiles in harmonic lakes (**a**) and dystrophic lakes (**b**). Different types of lines represent different groups of lakes according to oxygen deficits. In harmonic lakes (**a**) solid lines represent lakes without anoxia (G1), dash-dot lines for lakes with anoxia in hypolimnion (G2); dashed line for lakes with anoxia from metalimnion (G3); dotted lines for lakes with anoxia from epilimnion (G4). In dystrophic lakes (**b**) solid lines for lakes with anoxia from 3–5 m (G5); dotted lines for lakes with anoxic conditions from 1–2 m (G6); Detailed list of lakes in Table 1.  
measurements of chlorophyll-_a_ and PAR were used to calculate the gross primary production using the DCMU (3-(3,_A_-dichlorophenil)-1,1-dimethylurea) method described below. The zooplankton samples and water for phosphorus and carbon analysis were collected using a 5-L Limnos sampler from the different layers (epilimnion, metalimnion, and hypolimnion) of each lake. For zooplankton samples, ten liters of water were taken from each of the three layers and individually filtered through a 50-\(\upmu\)m plankton net and fixed with 4% formaldehyde. The analyses of total phosphorus (TP) were conducted in the laboratory according to the conventional photocolorimetric method [40]. The concentrations of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were analyzed via high-temperature catalytic combustion using a TOC-L Series (Shimadzu, Japan). Rotifers and crustaceans were identified to species and all individuals in the samples were enumerated. Ten length measurements were also made for each species and used to estimate the wet weight of crustaceans by applying the equation from [PERSON] and [PERSON] [41]. The biomass of rotifers was established following the work of [PERSON] [42]. Cladocera were further divided into large (_Daphnia_ sp. and _Diaphanosoma brachyum_) and small-bodied species (_Ceriodaphnia_ sp., _Bosmina_ sp., _Chydorus sphaericus_, etc. ), as body size is an important attribute of their biology and ecology [43], and they respond differently to predation pressure and algal resources [44; 45].