Significant impacts of artificial regulation on nutrient concentrations and transport in Huanghe River

Human perturbations such as dam regulation have led to significant changes in nutrient flux,structure,and spatiotemporal distribution through rivers and have greatly influenced coastal and estuarine ecological environments.Based on the data from 2001-2019,the impacts of the water-sediment regulation scheme(WSRS)in the Huanghe(Yellow)River on nutrient concentrations,forms,fluxes,and potential ecological effects on the Huanghe River estuary and Bohai Sea were analyzed.Nutrient concentrations and forms were significantly influenced by the different regulatory modes and were associated with hydrological and biogeochemical processes.The concentrations and forms of nutrients were strongly influenced by the riparian floodplains and scouring process of the downstream riverbed at the water draining stage,while they were influenced mainly by the mixing process in the Xiaolangdi Reservoir at the sediment desilting stage.Compared with those in non-WSRS years,the seasonal distributions of water discharge and nutrient fluxes in WSRS years significantly changed,with peaks occurring at least one month earlier than those in non-WSRS years.Nitrate(NO_(3)-N),phosphate(PO_4-P),and silicate(SiO_(3)-Si)fluxes during the WSRS,which were strongly controlled by water discharge at Lijin(the nearest hydrological station to the river mouth),accounted for more than 20%of the annual fluxes;these fluxes were more than 5-9 times greater than those during the same period in non-WSRS years.WSRS exacerbated nutrient imbalances and increased Pyrrophyta/Bacillariophyta ratios by 231%in the Huanghe River estuary,resulting in chlorophyll-a peaks in the Bohai Sea shifting from spring to autumn in the 20~(th)century to spring-summer in the 21^(st)century.

Composition and Distributions of Nitrogen and Phosphorus and Assessment of Eutrophication Status in the Maowei Sea

Maowei Sea(MWS) is the largest oyster maricuture bay in Southwest China. Surface water from 12 sampling sites were collected during the dry season to study the composition and distribution of different forms of nitrogen(N) and phosphorus(P) beginning from the inner bay to the bay mouth in the MWS. An improved multi-parameter eutrophication index was developed and applied for the evaluation of the water quality in the MWS. Dissolved inorganic nitrogen(DIN), dissolved organic nitrogen(DON), and particulate nitrogen(PN) averagely accounted for 11.28%, 65.32%, and 23.39% of total N(TN) pool, dissolved inorganic phosphorus(DIP), dissolved organic phosphorus(DOP), and particulate phosphorus(PP) averagely accounted for 54.58%, 30.31%, and 15.12% of total P(TP) pool, respectively. DON and DIP were respectively the dominant forms of N and P in surface water of the MWS, during the study period. Concentration trends of DIN, DIP, DOP, PN, dissolved silicate(DSi), total organic carbon(TOC), chemical oxygen demand(COD) decreased from the inner bay to the bay mouth, suggesting that the MWS may be largely influenced by land-based inputs. Based on nutrient contents and stoichiometry, it likely that phytoplankton growth in the MWS is strongly limited by DIP during the dry season. Results from the improved eutrophication index revealed that the water quality in the MWS is in a state of moderate to poor eutrophication(average EI = 0.953). The eutrophication state decreased from the inner bay to the bay mouth. Nitrate(NO3-N), DON, DIP, DOP, and DSi showed significant positive correlations with eutrophication index(r2 = 0.52 – 0.78, P < 0.05), implying that these nutrients are among the significant factors, which could be used in evaluating the eutrophication status of the MWS.