Phytoplankton responses to meteorological and hydrological forcing at decadal to seasonal time scales

One of the challenges for predicting global change effects on aquatic ecosystems is the vague understanding of the mechanisms of multiple controlling factors affecting phytoplankton dynamics at different time scales. Here we distinguish between hydrometeorological forcing of phytoplankton dynamics at time scales from days to decades based on a 54-year monthly phytoplankton time series from a large shallow Lake Võrtsjärv (58°16′N, 26°02′E) in Estonia, combined with daily data on forcing factors—thermal-, wind-, light- and water-level regimes. By using variance partitioning with linear mixed effect modelling (LME), we found a continuum from the large dominant K-selected filamentous cyanobacteria with strongest decadal scale variation (8–30%) to r-selected phytoflagellates with large stochastic variability (80–96%). External forcing revealed strong seasonal variation (up to 80%), while specifically water level and wind speed had a robust decadal variation (8% and 20%, respectively). The effect of external variables was proportionally manifested in the time scales of phytoplankton variation. Temperature, with a clear seasonal variation, had no impact on the dominant cold tolerant filamentous cyanobacteria in Lake Võrtsjärv. We found the LME as a reliable method for resolving the temporal cross-scale problem. It yielded quantitative results that matched our intuitive understanding of the dynamics of different variables.

Bioaccumulation of Metals in Eels Taken From Lakes Köyceğiz (Turkey) and Võrtsjärv (Estonia) and Health Risk Assessments

Monitoring heavy metal contaminants in fish is important for the assessment of environmental quality as well as food safety. In this study, European eel samples were collected from Lake Köyceğiz and Lake Võrtsjärv in 2017 and 2018. The concentrations of Mn, Cd, Zn, Pb, and Cu metals were measured by using GF-AAS in four selected tissues of eel, including liver, gill, skin, and muscle in both lakes. The pollution indexes (Pi, MPI) values were calculated for both lakes and the health risk for consumers was assessed for both adults and children in Turkey and Estonia. The estimated weekly intake (EWI), hazard index (HI), and lifetime cancer risk values ​​(CRs) for the metals were calculated for both lakes. According to the results of this study, a significant difference was determined the between the metal concentrations (especially Cu, Cd and Pb) in the tissues of the eel samples taken from the two lakes. These results show that besides the pollution levels in the aquatic environment, physiological needs, and metabolic activities in different habitats have a significant effect on metal accumulation in eels. In addition, HI was found to be < 1 for both adult and child consumers in both lakes, which indicates that consumers would not experience non-carcinogenic health effects. However, the values of CR for Pb and Cd were found negligible in Lake Köyceğiz, while the CR value for Pb was found to be very close to the danger limits in Lake Võrtsjärv.

Comparison of Lake Optical Water Types Derived from Sentinel-2 and Sentinel-3

Inland waters play a critical role in our drinking water supply. Additionally, they areimportant providers of food and recreation possibilities. Inland waters are known to be opticallycomplex and more diverse than marine or ocean waters. The optical properties of natural waters areinfluenced by three different and independent sources: phytoplankton, suspended matter, andcolored dissolved organic matter. Thus, the remote sensing of these waters is more challenging.Different types of waters need different approaches to obtain correct water quality products;therefore, the first step in remote sensing of lakes should be the classification of the water types. Theclassification of optical water types (OWTs) is based on the differences in the reflectance spectra ofthe lake water. This classification groups lake and coastal waters into five optical classes: Clear,Moderate, Turbid, Very Turbid, and Brown. We studied the OWTs in three different Latvian lakes:Burtnieks, Lubans, and Razna, and in a large Estonian lake, Lake Võrtsjärv. The primary goal of thisstudy was a comparison of two different Copernicus optical instrument data for opticalclassification in lakes: Ocean and Land Color Instrument (OLCI) on Sentinel-3 and MultispectralInstrument (MSI) on Sentinel-2. We found that both satellite OWT classifications in lakes werecomparable (R2 = 0.74). We were also able to study the spatial and temporal changes in the OWTs ofthe study lakes during 2017. The comparison between two satellites was carried out to understandif the classification of the OWTs with both satellites is compatible. Our results could give us not onlya better overview of the changes in the lake water by studying the temporal and spatial variabilityof the OWTs, but also possibly better retrieval of Level 2 satellite products when using OWT guidedapproach.

The structure of psammic rotifer communities in two boreal lakes with different trophic conditions: Lake Võrtsjärv and Lake Saadjärv (Estonia)

Psammon communities are still poorly studied worldwide. The aim of the present study was to establish the structure of psammic rotifer communities including their diversity and quantitative parameters. A total of 41 rotifer taxa were found in hydro-, hygro-, and euarenal zones of eutrophic Lake Võrtsjärv and mesotrophic Lake Saadjärv during the study carried out in Estonia in 2008, including 11 rotifer species new to Estonia. In L. Võrtsjärv, the predominant rotifer taxa were Bdelloidea and Lecane psammophila. L. psammophila dominated in July and August and bdelloids prevailed in the rest of the year. In L. Saadjärv, the most abundant taxa were Lepadella ovalis, Keratella hiemalis, and Cephalodella megalocephala. The psammic rotifer community of L. Saadjärv proved to be more diverse than that of L. Võrtsjärv. Microphagous rotifers were dominant in L. Võrtsjärv. Raptorial feeders dominated in L. Saadjärv in summer and autumn and were more abundant at sites with coarser sand and low plant density.

Food spectrum of the omnivorous rotifer Asplanchna priodonta in two large northeastern European lakes of different trophy

In this paper we examine the stomach contents of the omnivorous rotifer, Asplanchna priodonta, to evaluate possible trophic interactions between this rotifer and its potential prey in two large northeastern European lakes: moderately eutrophic Lake Peipsi and strongly eutrophic Lake Võrtsjärv. Our results show that the A. priodonta diet consisted of Bacillariophyta, Chlorophyta, Cyanobacteria, and Dinoflagellata. Ciliata were detected on rare occasion. Bacillariophyta were the most frequent food items in stomachs. There were no between-lake differences in the consumption of Chlorophyta and Cyanobacteria, whereas Bacillariophyta were consumed more in Lake Peipsi. However, neither Dinoflagellata nor Ciliata were found in stomach samples in Lake Võrtsjärv. We conclude that A. priodonta is an opportunistic feeder that is capable of influencing the phytoplankton community structure in large lakes.