Assessing relationship of degradation of coastal zones and phytoplankton species structure of Lake Uvildy and Lake Turgoyak (South Ural, Russia)

The current paper aims at analyzing the species structure of the phytoplankton community of two reservoirs on the territory of South Ural. The given water bodies are of different trophicity, namely, oligotrophic Lake Turgoyak and oligomesotrophic Lake Uvildy. Both lakes, being natural monuments, are subject to intensive recreational use. A large flow of tourists is a serious problem for the ecological state of the water bodies coastal areas. Also, tourist activities can affect the state of aquatic biocenoses. In this work, the degree of recreational degression of the selected study sites was assessed. The analysis was carried out according to the state of landscapes, vegetation and trampledness of the surveyed area. Based upon the research the current paper reveals spatial patterns in the distribution of recreational load on the coastal geosystems and the level of anthropogenic transformation of the territory. Mainly due to the high density of recreational centres and camping areas, the coastal area is characterized by a high degree of regression. Some signs allow forecasting changes in the species composition of phytoplankton communities and the trophic status of the lakes depending on the degradation of the coastal zones. Diatoms (Bacillariophyta) have been dominant in plankton phytocoenosis of Turgoyak and Uvildy. At the same time, in the oligomesotrophic lake (Uvildy), the number of diatom species is significantly reduced in areas with a higher degree of regression. Thus, diatoms are sensitive indicators of anthropogenic impact on aquatic ecosystems. Green and blue-green algae biodiversity has grown in the water body. However, no relationship was found between their species diversity and the level regression. But the following tendency is observed for the oligotrophic Lake Turgoyak. The species diversity of green algae grows with an increase in the degression degree. To forecast the further state of the studied water bodies, the revealed patterns in the change of the phytoplankton species structure depending on the degradation level of the coastal territories can be used as eutrophication signs.

Unusual massive phytoplankton bloom in the oligotrophic Lake Tanganyika

Background and aims – Massive algae growth resulting in a phytoplankton bloom is a very rare event in the meromictic and oligotrophic Lake Tanganyika. Such a bloom was observed in the north of the lake in September 2018. Phytoplankton species composition during this bloom is compared to a documented bloom in 1955, and to the composition in September 2011–2013. Meteorological observations suggest hydrodynamics could explain the occurrence of the 2018 bloom.Material and methods – Phytoplankton net samples were taken in the pelagic and littoral zone near Uvira during five consecutive days of the bloom in 2018. For the period 2011–2013, quantitative phytoplankton samples were obtained during a weekly sampling at the same sites. Samples were analysed with an inverted microscope and relative abundances of the algal species were compared. Key results – Dolichospermum flosaquae (Cyanobacteria) initially dominated the bloom followed by high relative abundance of Limnococcus limneticus (Cyanobacteria) on the third sampling day in September 2018. In the pelagic zone an increase of Nitzschia asterionelloides (Bacillariophyta), and Dictyosphaerium and Lobocystis (Chlorophyta) was observed while in the littoral zone increasing abundances of dinophytes were noted. Dolichospermum flosaquae was also responsible for the bloom reported in 1955, but was only sporadically observed in the 2011–2013 samples. Although Limnococcus limneticus was present in 2011–2013, it never reached relative abundances as high as during the 2018 bloom. Meteorological data indicate that 2018 experienced different conditions compared to previous years: strong south-east winds from May to September with a more eastern direction of the wind, and a well-marked drop in atmospheric pressure between August and September.Conclusion – After a very windy season, the combination of strong hydrodynamics, calmer lake conditions, and high solar radiation and air temperature in September 2018 was favourable for a massive Cyanobacteria bloom in the north of Lake Tanganyika.

Short Communication: Assessing phytoplankton species structure in trophically different water bodies of South Ural, Russia

Kostryukova A, Mashkova I, Belov S, Shchelkanova E, Trofimenko V. 2021. Short Communication: Assessing phytoplankton species structure in trophically different water bodies of South Ural, Russia. Biodiversitas 22: 3530-3538. The study aims to analyze the species structure of the phytoplankton communities of four water bodies in South Ural (Lakes-Turgoyak, Uvildy, Ilmenskoe and Shershnevskoe reservoir). These water bodies are characterized by different trophic states and levels of anthropogenic impact. Lake Turgoyak is oligotrophic; Lake Uvildy is oligomesotrophic. Both water bodies are protected areas and natural monuments. But tourism and recreation are not prohibited on their territories. The mesoeutrophic Lake Ilmenskoe is partially located within the Ilmen State Reserve, and it experiences less anthropogenic impact. The eutrophic Shershnevskoe reservoir is located within the boundaries of the city of Chelyabinsk. It is used as a source of drinking water. Cyanobacteria was the dominant division in the eutrophic Shershnevskoe reservoir. Bacillariophyta dominated in the plankton communities in oligotrophic Lake Turgoyak and oligomesotrophic Lake Uvildy. Green and blue-green algae biodiversity increased in the oligomesotrophic Lake Uvildy. For the mesoeutrophic Lake Ilmenskoe, diatom algae are dominant, but a higher proportion of Chlorophyta (especially Cyanobacteria) was revealed. Species similarity among phytoplankton communities in the given lakes was studied through cluster analysis based on the Sorensen-Czekanowski coefficient. The higher level of similarity between plankton species compositions could be explained by their locality rather than their trophic status.

Trophic status and local conditions affect microbial potential for denitrification versus internal nitrogen cycling in lake sediments

The nitrogen (N) cycle is of global importance as N is an essential element and a limiting nutrient in terrestrial and aquatic ecosystems. Excessive anthropogenic N fertilizer usage threatens sensitive downstream aquatic ecosystems. Although freshwater lake sediments remove N through various microbial transformation processes, few studies have investigated the microbial communities involved. In an integrated biogeochemical and microbiological study on a eutrophic and oligotrophic lake, we estimated N removal rates in the sediments from porewater concentration gradients. Simultaneously, the abundance of different microbial N transformation genes was investigated using metagenomics on a seasonal and spatial scale. We observed that contrasting nutrient concentrations in the sediments were reflected in distinct microbial community compositions and significant differences in the abundance of various N transformation genes. Within each lake, we observed a more pronounced spatial than seasonal variability. The eutrophic Lake Baldegg showed a higher denitrification potential with higher nosZ gene (N2O reductase) abundance and higher nirS:nirK (nitrite reductase) ratio, indicating a greater capacity for complete denitrification. Correspondingly, this lake had a higher N removal efficiency. The oligotrophic Lake Sarnen, in contrast, had a higher potential for DNRA and nitrification, and specifically a high abundance of Nitrospirae, including some capable of comammox. In general, the oligotrophic lake ecosystems had a higher microbial diversity, thus acting as an important habitat for oligotrophic microbes. Our results demonstrate that knowledge of the genomic N transformation potential is important for interpreting N process rates and understanding the limitations of the N cycle response to environmental drivers.

Nitrogen removal processes in lakes of different trophic states from on-site measurements and historic data

Freshwater lakes are essential hotspots for the removal of excessive anthropogenic nitrogen (N) loads transported from the land to coastal oceans. The biogeochemical processes responsible for N removal, the corresponding transformation rates and overall removal efficiencies differ between lakes, however, it is unclear what the main controlling factors are. Here, we investigated the factors that moderate the rates of N removal under contrasting trophic states in two lakes located in central Switzerland. In the eutrophic Lake Baldegg and the oligotrophic Lake Sarnen, we specifically examined seasonal sediment porewater chemistry, organic matter sedimentation rates, as well as 33-year of historic water column data. We find that the eutrophic Lake Baldegg, which contributed to the removal of 20 ± 6.6 gN m−2 year−1, effectively removed two-thirds of the total areal N load. In stark contrast, the more oligotrophic Lake Sarnen contributed to 3.2 ± 4.2 gN m−2 year−1, and had removed only one-third of the areal N load. The historic dataset of the eutrophic lake revealed a close linkage between annual loads of dissolved N (DN) and removal rates (NRR = 0.63 × DN load) and a significant correlation of the concentration of bottom water nitrate and removal rates. We further show that the seasonal increase in N removal rates of the eutrophic lake correlated significantly with seasonal oxygen fluxes measured across the water–sediment interface (R2 = 0.75). We suggest that increasing oxygen enhances sediment mineralization and stimulates nitrification, indirectly enhancing denitrification activity.