Identification of nitrate sources in Taihu Lake and its major inflow rivers in China, using δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) values

In the present study, δ(15)N and δ(18)O-NO(3)(-) values, as well as concentrations of some major ion tracers were determined in seasonal water samples from Taihu Lake and major watersheds to investigate the temporal and spatial variations of nitrate sources and assess the underlying nitrogen (N) biogeochemistry process. The results lead to the conclusion that the nitrate concentrations in Taihu Lake are lower in summer than that in winter due to the dilution effect of wet deposition. In winter, sewage and manure were the primary nitrate sources in major inflow rivers and North Taihu Lake (NTL), while nitrate sources in East Taihu Lake (ETL) probably derived from soil organic N. In summer, atmospheric deposition and sewage/manure inputs appear to play an important role in controlling the distribution of nitrates in the whole lake. The δ(18)O-NO(3)(-) values suggest that the nitrate produced from microbial nitrification is another major nitrate source during both winter and summer months. The variations in isotopic values in nitrate suggest denitrification enriched the heavier isotopes of nitrate in NTL in winter and in ETL in summer.     

The distribution,density, and biomass of the zebra mussel (Dreissena polymorpha) on natural substrates in Lake Winnipeg 2017–2019

The distribution, density, biomass and size-structure of the zebra mussel (Dreissena polymorpha) population in Lake Winnipeg were examined between 2017 and 2019. Zebra mussels have colonized most of the available hard substrate in the south basin and Narrows region, but colonization of the north basin remains low at present, even on suitable substrate. Numerical densities and shell free biomass peaked at 5530 ± 953 m^?2 and 64.7 ± 57.9 g shell free dry mass m^?2 respectively. The distribution appeared to be strongly limited by substrate type and availability, with further limitations on the distribution imposed by physical disturbance in shallow waters and unsuitable substrate in deeper areas of the lake. Zebra mussels <1 year old dominated the populations, and individuals >18 mm were exceedingly rare. Poor recruitment was observed at sites along the eastern side of the south basin compared to elsewhere in the lake. The proximate causes of these differences in colonization success and recruitment are not clear, but may be in part due to heterogeneous patterns of key physico-chemical environmental conditions such as calcium concentrations required for successful development of juvenile mussels and colder water temperatures in the north basin. This study provides a baseline of information on which to track further expansion of zebra mussels in Lake Winnipeg and assist efforts to develop an understanding of how zebra mussels may affect the ecology of Lake Winnipeg.