Changes over flood season in turbidity maximum zone in a mountainous macrotidal estuary from 1986 to 2020

The construction of channel regulation projects,reservoirs,and other human activities have led to significant changes in channel geometry and hydrodynamic conditions in mountainous macrotidal estuaries.However,their impact on the long-term evolution of the turbidity maximum zone(TMZ)in these estuaries is still unclear.Therefore,the Minjiang Estuary(ME)was selected as the study area and using the Gabor filter and surface suspended sediment concentration(SSSC)data retrieved from GF PMS/WFV and Landsat-TM/ETM+/OLI images in the flood season from 1986 to 2020,the flow direction of Chuanshi Waterway,the spatiotemporal evolution characteristics of TMZ in the ME,and the influence of human activities on these were analyzed.The results indicate that during flood tides in the past 35 years,the TMZ was mainly distributed in sections from the Changmen to the Chuanshi and Meihua waterways.The construction of the Shuikou Reservoir caused the SSSC to decrease by 65 mg/L at the Chuanshi Tidal Gauge Station in the ME.The TMZ in the ME waterway channel notably migrated toward the sea due to the waterway regulation project,with the landward and seaward boundaries moving by 2.5 km and 3 km seaward,respectively.The main distribution area moved from Jinpaimen to the section from Chuanshi Waterway to the mouth of the ME.These variation characteristics were basically consistent with the annual average TMZ in the flood season.Through the interactions between nature and human interventions,the flow regime of the ME tended to converge in the flood season.Therefore,human activities have significantly impacted the long-term evolution of the TMZ in the ME.

Suspended sediment load in the turbidity maximum zone at the Yangtze River Estuary： The trends and causes

Based on the analysis of suspended sediment elements at estuaries, influence of human activities and estuarine regulation projects on the turbidity maximum zone was studied according to the measurement data between 1959 and 2011. It was found that human activi- ties had little effect on the seaward water while the sharp decrease of sediment volume and concentration in runoff led to the sharp decrease of turbidity maximum zone in the estuary. The concentration at outside sea and Hangzhou Bay did not change, and that along the Subei coast also decreased a little, which had no influence on the turbidity maximum zone. Com- pared with the concentration between 1959 and 1999, the peak of concentration moved up- stream in the estuary, and the concentration in 2000-2009 decreased by about 24.73% with a narrower variation range along the river to the sea. The suspended sediment concentration in North Passage was low in upstream and downstream because of the decrease of seaward sediment and coarsening of bed material, while it was relatively high in the middle due to the influence of sediment cross the north jetty.

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

Nutrients,dissolved and particulate organic carbon and plankton（bacterio-,phyto-and zoo-）were compared in the turbidity maximum zone（TMZ） and adjacent areas（non-TMZ） in the Pearl River estuary.Our results showed that high levels of suspended substances had marked effect on dynamics of nutrients and plankton in the TMZ.Based on the cluster analysis of total suspended solids（TSS） concentrations,all stations were divided into two groups,TMZ with average TSS of 171 mg/L and non-TMZ of 45 mg/L.Suspended substances adsorbed PO^（3-）_4 and dissolved organic carbon,resulting in higher particulate phosphorus and organic carbon（POC）and lower PO^（3-）_4 and DOC in the TMZ,compared to the non-TMZ.However,suspended substances had limited effect on nitrogenous nutrients.Phytoplankton growth was light-limited due to high concentrations of suspended substances in the TMZ and a peak of phytoplankton abundance appeared in the non-TMZ.In contrast,the highest bacterial abundance occurred in the TMZ,which was likely partly responsible for low DOC levels.Two peaks of zooplankton abundance observed in the TMZ and non-TMZ in the Pearl River estuary were primarily supported by bacteria and phytoplankton,respectively.Our finding implied that high levels of suspended solids in the TMZ affect the trophic balance.

Heavy metals in suspended matters during a tidal cycle in the turbidity maximum around the Changjiang(Yangtze) Estuary

In order to discuss the content distributions and fluxes of heavy metals in suspended matters during a tidal cycle in the turbidity maximum around the Changjiang （Yangtze） Estuary, the contents of heavy metals （Zn, Pb, Cd, Co and Ni） have been analyzed. During a tidal cycle, the average contents of heavy metals are in the order of Zn〉Ni〉Pb〉Co〉〉Cd. The average contents in ebb tide are generally higher than that in flood tide. However, at the inshore Sta. 11, influenced by the contamination from the nearby waste treatment plant, the average contents of Zn and Ni in flood tide are higher than those in ebb fide and at the offshore Sta. 10, the content of Cd in flood tide higher than that in ebb tide due to marine-derived materials. The five heavy metals, mainly terrigenous, are transported towards east-northeast, and settle down with suspended matters in the area between Sta. 11 and Sta. 10. Influenced by marine-derived materials, the flux value of Cd does not alter significantly with obviously changing in flux direction towards northwest. The source of heavy metals, the salinity of water and the concentration of suspended matters are the main factors controlling the content distributions of heavy metals during a tidal cycle. There is a positive correlation between the contents of heavy metals （Zn, Pb, Co and Ni） and the salinity of water, while the opposite correlation between the contents and the concentrations of suspended matters. Because of marine-derived materials, the content of Cd is not correlated with the concentration of suspended matters and the salinity of water.

The important role of Turbidity Maximum Zone in sedimentary dynamic of estuarine mangrove swamp

Sedimentation is a key process affecting wetland sustainability and carbon burial flux.In context of sea level rise,climate change and human activities,further understanding about the sedimentary dynamic in wetland is critical in predicting the landscape evolution or the change in carbon burial flux.In this study,based on the field hydrological observation in a mangrove system in the Nanliu River estuary,we found the net flux of suspended sediment to mangrove is 39−72 kg/m in tidal cycles with Turbidity Maximum Zone(TMZ)forming in surface layer and only is 9−18 kg/m in tidal cycles without TMZ.The higher net flux of suspended sediment to mangrove in tidal cycles with TMZ forming in surface layer is attributed to high SSC in rising tide and intense flocculation in mangrove.The significant discrepancy in sedimentation rate in the mangrove patches also can be explained by the probability of TMZ forming in the surface layer of estuary.In future,rapid sea level rising may lead to the change of TMZ pattern in estuary,which will result in non-negligible variation in sedimentation rate in wetlands.According to the present data of sedimentation rate in wetlands,the fragility of wetlands in river estuary may be miscalculated.