Anthropogenic impacts on nutrient variability in the lower Yellow River.

Excessive nutrient discharges and changes in nutrient ratios caused by global change and anthropogenic activities have been reported in global rivers; however, the actual alterations occurring in the Yellow River environment is too fast to catch up with. From 2001 to 2018, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) concentrations showed decreasing trends in the lower Yellow River throughout the study period. Dissolved organic phosphorus (DOP) concentrations increased since 2009, reaching up to 95% of the total dissolved phosphorus. Annual minimum dissolved organic nitrogen concentrations increased with time. We observed extremely low nutrient concentration events since 2014 in response to the retention effect of large reservoirs; this significantly reduced the downstream water discharge and sediment load and increased phytoplankton uptake. To further analyze the variability of nutrient fluxes, we quantified the fluxes to the Yellow River from natural (runoff, precipitation deposition, and sediment load from the Loess Plateau), anthropogenic (recharged water, fertilizer application, and vegetation coverage), social and industrial (population urbanization, GDP, and sewage effluents) sources. The highest contributions of total nutrient fluxes emptied into the Yellow River was fertilizer losing (44-48%) for DIN, sewage effluents (85-88%) for DIP, and runoff (35-65%) for DSi, respectively. Strictly controlling the amount of fertilizer and improving the application methods, improving sewage treatment technology, and vigorously promoting "green travel" might reduce nutrients emptied into the Yellow River based on the main sources of nutrients. Our study may help policy makers formulate strategies and it is possible to own a better water quality in the Yellow River.

[Influence of Ruditapes philippinarum aquaculture on benthic fluxes of biogenic elements in Jiaozhou Bay].

Nutrient excretion rates and the impact of Ruditapes philippinarum on benthic nutrient fluxes were measured by incubation experiments, and these results were used to assess the effect of R. philippinarum aquaculture on nutrient recycling in Jiaozhou bay. Our research indicates that the bioturbation of R. philippinarum would modify biogeochemical progresses in sediment, and restrain nutrients excreted by R. philippinarum releasing to overlying water. Only 37% DIN and 34% PO4(3-) -P excreted by R. philippinarum were released to seawater across the sediment-water interface due to bioturbation of R. philippinarum. According to the reduction of nutrient exchange across the sediment-water interface by bioturbation, only 19% (N) and 17% (P) of biogenic elements fed by R. philippinarum can be recycled and returned to seawater, while the others would be removed from seawater in Jiaozhou Bay. The ratios of Si: N and Si: P of nutrients released from sediment to seawater with bioturbation of R. philippinarum were about 5:18 and 5:1, which were far less than Redfield ratios. Therefore, aquaculture of R. philippinarum should promote BSi biodeposition to sediment and reduce the nutrient ratios of Si: N and Si: P in seawater of Jiaozhou bay.