SEASONAL AND INTERANNUAL DYNAMICS OF CHLOROPHYLL IN PLANKTON OF THE RYBINSK RESERVOIR (2015–2019)

New data on chlorophyll content at standard stations in the Rybinsk reservoir, that continue series of long-term observations, were obtained in May–October 2015–2019. A fluorescent method is used in this study which makes it possible to determine the total amount of chlorophyll (ΣChl) by its content in cyanoprokaryotes, di-atoms, and green algae (ChlCyan, ChlBac, ChlChl, respectively). These taxa are the main contributors to ƩChl which show interannual and seasonal variation. In different years, the average content of ChlBac makes 2.4–6.6 μg/L or 24–50% of the ΣChl pool, ChlCyan – 3.1–10.9 μg/L (50–70%), ChlChl – 0.3–1.2 μg/L (3–12%). The distribution of ChlBac in the water column is generally uniform while the amount of ChlCyan decreases with depth. With the average ΣChl content for the growing season 14.8±1.0, 13.0±1.3, 7.8±0.6, 13.6±1.6, and 11.9±1.0 μg/L in 2015–2019, the trophic status of the reservoir varied from moderately eutrophic to mesotrophic.

Seasonal and interannual dynamics of soil microbial biomass and available nitrogen in an alpine meadow in the eastern part of Qinghai–Tibet Plateau, China

Soil microbial activity varies seasonally in frozen alpine soils during cold seasons and plays a crucial role in available N pool accumulation in soil. The intra- and interannual patterns of microbial and nutrient dynamics reflect the influences of changing weather factors, and thus provide important insights into the biogeochemical cycles and ecological functions of ecosystems. We documented the seasonal and interannual dynamics of soil microbial and available N in an alpine meadow in the eastern part of Qinghai–Tibet Plateau, China, between April 2011 and October 2013. Soil was collected in the middle of each month and analyzed for water content, microbial biomass C (MBC) and N (MBN), dissolved organic C and N, and inorganic N. Soil microbial community composition was measured by the dilution-plate method. Fungi and actinomycetes dominated the microbial community during the nongrowing seasons, and the proportion of bacteria increased considerably during the early growing seasons. Trends of consistently increasing MBC and available N pools were observed during the nongrowing seasons. MBC sharply declined during soil thaw and was accompanied by a peak in available N pool. Induced by changes in soil temperatures, significant shifts in the structures and functions of microbial communities were observed during the winter–spring transition and largely contributed to microbial reduction. The divergent seasonal dynamics of different N forms showed a complementary nutrient supply pattern during the growing season. Similarities between the interannual dynamics of microbial biomass and available N pools were observed, and soil temperature and water conditions were the primary environmental factors driving interannual fluctuations. Owing to the changes in climate, seasonal soil microbial activities and nutrient supply patterns are expected to change further, and these changes may have crucial implications for the productivity and biodiversity of alpine ecosystems.

Trophodynamics of nekton communities in the epipelagic layer of the North-West Pacific in summer and winter seasons

Trophic links between nekton and plankton are analyzed using the data collected in trawl surveys conducted by Pacific Fish. Res. Center (TINRO) in the Pacific waters at Kuril Islands and East Kamchatka in June-July of 2004-2016 and in the central and western parts of the Subarctic Front zone and adjacent subarctic waters in February-April of 2009-2011. Spatial, seasonal and interannual dynamics of nekton communities are detected, as well as changes in their composition and abundance. The most significant changes were caused by mass migrations of subtropical species, as in summer of 2014-2016. In these cases, new components were involved in the food web and energy flows between the low and upper trophic levels were changed, in the first place concerning to copepods, then to euphausiids, amphipods, and small-sized nekton prey. There were no such significant interannual changes in the energy flows in winter. The estimates of daily and seasonal energy flows between low and upper trophic levels, as well as the consumption rate of forage resources indicate no excess of allowable grazing pressure upon forage resources in the upper pelagic layer of the surveyed areas.

Seasonal and interannual dynamics of soil microbial biomass and available nitrogen in an alpine meadow in the eastern part of Qinghai-Tibet Plateau, China

Soil microbial activity occurs seasonally in frozen alpine soils during cold seasons and plays a crucial role in available N pool accumulation in soil. The intra- and interannual patterns of microbial and nutrient dynamics reflect the influences of changing weather factors, and thus provide important insights into the biogeochemical cycles and ecological functions of ecosystems. We documented seasonal and interannual dynamics of soil microbial and available N in an alpine meadow in the eastern part of Qinghai-Tibet Plateau, China between April 2011 and October 2013. Soil samples were collected in the middle of each month and were analyzed for water content, microbial biomass C (MBC) and N, dissolved organic C and N, and inorganic N; soil microbial community compositions were measured by the dilution-plate method. Fungi and actinomycetes dominated the microbial community during the non-growing seasons, and the number of bacteria increased considerably during the early growing seasons. Consistently increasing trends of MBC and available N pools were observed during the non-growing seasons. MBC sharply declined during soil thaw and was accompanied by a peak of available N pool. Induced by soil temperatures, significant shifts in the structure and functions of microbial communities were found during the winter-spring transition and largely contributed to microbial reduction. Divergent seasonal dynamics of different N forms showed a complementary nutrient supply pattern during the growing season. Similar interannual dynamics were observed between microbial biomass and available N pools, and soil temperature and water condition were the primary environmental factors driving these year-to-year fluctuations. Under the background of changing climate, the seasonal soil microbial activity and nutrient supply patterns will be further changed, having important implications to the productivity and biodiversity of alpine ecosystems.