Nutrient Burial And Environmental Changes In The Yangtze Delta And The Yellow River Delta In Response To Recent River Basin Human Activities

Deltas are important interfaces between terrestrial and oceanic ecosystem,highly sensitive and vulnerable to global climate change and human activities.Over the past decades,human activities,coupled with the global climate change,have had profound impacts on ecology and environment of river and deltaic systems.Intensive human activities have substantially increased the discharge of agricultural,domestic,and industrial sewage into the aquatic environment,further the input of inorganic N?DIN?and P?DIP?,coupled with the change of riverine nutrient cycles which is caused by hydrological engineering,thus resulting in the eutrophication,the change of aquatic nutrient construction and the adjustment of community structure in the estuary.In addition,the development of hydrological engineering in the river basin result in the trapped of sediment,contributing to the adjustment of sedimentary environment in the estuary area,thereby changing the ecology and environment.Therefore,how the environment and ecology in the estuary have responded to human activities in the Yangtze River Basin,the alteration of nutrient burial fluxes and the degree to which this would affect nutrient budgets require further evaluation.Two sediment cores in the Yangtze Estuary and Yellow River Estuary?the Bohai?were selected respectively as the research objects,then further analyzed for sediment dating,grain size composition and sediment biogenic component parameters,in order to:?1?reconstruct high resolution sediment records of the sedimentary environment,aquatic nutrient level,primary productivity and so on,and elucidate the environmental changes.?2?explore the response of the environmental changes to human activities in the river basin.?3?evaluate the alteration in nutrient burial fluxes and its potential impacts on nutrient cycling.The main conclusions are as follows:?1?Nutrient changes in the Yangtze Delta front sediment profile were coincident with increased human activities in the watershed during previous decades.This suggested that the delta shifted from a natural climate change prevailing system towards an anthropogenic-dominated one.Increases in OP,BSi,TN,and TOC began in the1950s,along with?¹³C depletion towards a marine organic origin,and reduction in?¹⁵N.This indicated that strengthened anthropogenic activities including chemical fertilizer application and population growth increased nutrients fluxes to the river mouth and stimulated primary productivity over that of natural weathering dominated fluctuations.The depletion of TOC,TN,and BSi since the 1980s,together with the decline in sedimentation rate and coarsening of sediment in the profile,was largely consistent with increasing hydrological engineering practices in the river basin.Sediment retention caused by the operation of reservoirs and dams reduced the sediment discharge of the Yangtze River.This resulted in sediment coarsening and reduction in sedimentation rate,and riverine TOC and TN influxes in the Yangtze River Estuary.The variations in the BSi/TOC molar ratio were most likely attributed to the change in primary productivity toward non-diatom species.The trap of Si in the reservoirs substantially decreased the riverine discharge downstream to the river mouth,and consequently modified primary productivity in the delta.?2?The differences in the grain size parameters of the sediment core B66 in the Yellow River Estuary?the Bohai?between the period before and after the 1850s,might be effective response to the environment changes caused by the diversion of the Yellow River in 1855.The relatively stable state and low fluctuation of biogenic components in the sediment core B66 before 1950s suggested a minor influence of human activities and climatic-or natural process-dominated condition.Increases in TOC,TN,BSi,TP and IP since the 1950s suggest increased nutrient deposition in the delta,corresponding to the increase of the growth of primary productivity stimulated by eutrophication affected by human activities in the river basin.At the same time,the continued decrease in the BSi/TOC molar ratio in the sediment core B66 suggested a reduction in diatom productivity result from the change of aquatic nutrient construction.The greater fluctuation in the biogenic components since the mid-1970s respond to the frequent diversions and special events of the Yellow River.Since 1990s,the Yellow River Cut-off,in the near term,led to the decrease of the primary productivity in the Yellow River Estuary and adjacent water.The sediment dilution effect,caused by project of water-sediment regulation in the Yellow River in the early 21st century,might be the reason of the decrease in biogenic component at the top of the sediment core B66.?3?It is estimated that the change of biogenic components burial fluxes of the sediment core A5-4 in the Yangtze Estuary was mainly constrained by sedimentation rate,while the change of biogenic components burial fluxes of the sediment core B66in the Yellow River Estuary?the Bohai?were mainly constrained by nutrient influxes and primary productivity.Recent dramatic decline in nutrients and C burial in the delta area suggested low efficiency for nutrient removal in this area.Consequently,more nutrients have been further transported to the inner shelf and open waters instead of being buried in the delta sediment,might aggravating the eutrophication and increasing the environmental pressure in the estuary and adjacent waters.