Biogenic Silica Record In The Yangtze Estuarine Sediments: A Response To The Recent Human Activity And Eco-modification In Catchment

In recent decades, material inputs of the Yangtze River Basin have changed dramatically. Dam construction reduces sediment load and the DSi concentration of the estuary. Human activities have contributed to the increase of nutrient N and P. Changes of the material input can inevitably affect the sea surface productivity and the ecological environment of the Yangtze Estuary. In this paper, biogenic silica is used to reveal sea surface productivity changes affected by human activities in the Yangtze estuary.In this paper, two vibrocores (Cx21 and Cx38) were obtained from the muddy sediments off the Yangtze estuary to examine changes in sediment inputs from the river basin. Core sediment samples were analysized for 210Pb and 137Cs, grain size, biogenic silica (BSi), organic carbon (TOC) and nitrogen (TN) content. Chronological sediment patterns of about one-hundred years of the two cores were achieved on the basis of sedimentation rates derived from 210Pb and 137Cs test and associated sediment lithology. Four sediment stages of the past century can be identified as shown below (bottom upward):Stageâ… can correspond to the core depth 100-70cm of Cx21 and 200-125cm of Cx38. This sediment stage is characterized by the high content of sand and the general lows of BSi, TOC and TN. Sedimentation rates suggest a time period of late 19th century to 1950s. The average BSi contents of the two vibrocores are 0.76% and 0.64%, the average TOC contents are 0.49% and 0.50%, the average TN contents are 0.063% and 0.066%; BSi fluctuates synchronously with TOC and TN.Stageâ…¡can correspond to the core depth 70-55cm of Cx21 and 125-95cm of Cx38. This sediment stage is denoted by the markedly decrease of the sand content, and the general highs of BSi, TOC and TN. Sedimentation rates of vibrocores suggest the time period of 1950s-1960s. The average BSi contents are 0.87% and 0.82%, the average TOC contents are 0.56% and 0.54%, the average TN contents are 0.074% and 0.069%; In Cx21, BSi fluctuates synchronously with TOC and TN, but in Cx38, they begin to fluctuate unsynchronously.Stageâ…¢can correspond to the core depth 55-25cm of Cx21 and 95-55cm of Cx38. This sediment stage occurs the high content of sand and the general lows of BSi, TOC and TN. Sedimentation rates of the two vibrocores infer to the time period of 1960s-1980s. The average BSi contents are 0.80% and 0.65%, the average TOC contents are 0.51% and 0.50%, the average TN contents are 0.065% and 0.066%; In Cx21, BSi fluctuates still synchronously with TOC and TN, but in Cx38, they fluctuate unsynchronously.Stage IV can be linked to the core depth 25-0cm of Cx21 and 55-0cm of Cx38. This sediment stage corresponds to 1980s-2005, in which sand content has significantly reduced, but, BSi increased in the late 1980s-1990s and lowed after the mid-1990s. In Cx21, TOC and TN still fluctuate synchronously with BSi. But the average TOC and TN contents of the vibrocore Cx38 reached the highest in the sediment profile. The average BSi contents of the two vibrocores are 0.82% and 0.71%, the average TOC contents are 0.60% and 0.56%, the average contents of TN are 0.068% and 0.073%.Four phases of BSi, TOC and TN in the Yangtze River basin over the past century may reflect the natural-human activities processes, especially the human activities of watershed: DSi declined to the sea after the constructions of reservoir, and the nutrients N and P increased to the estuary with the industrial and agricultural development recent years.From the early 20th century to 1950s, human activities made less impact on sediment. load and nutrient inputs, and diatoms were main planktonic algae in the Yangtze estuary. In the two vibrocores, BSi were consistent with organic matter. Otherwise, lower SST (the sea surface temperature) associated with lower nutrients, may result in lower content of BSi and organic matter.After 1950s, with the growth of population and production, human activities of watershed began to increase, and the obvious changes of estuarine environment began from the late 1970s, when the DIN, DIP began to increase. Especially after 1985, with the completion of Gezhouba reservoir, DSi decreased because of drastically reduction of sediment load into the sea. On the contrary, with the strengthening of industrial and agricultural activities, the DIN, DIP increased dramatically. In this study, BSi presents high content at late 1980s then significantly reduces, while TOC and TN continue to increase, reflecting that with the increase of nutrients during the first period, the large population of diatoms present, but with the consumption of DSi, diatoms are silicon-limited. Too much nitrogen and phosphorus lead to toxic algae such as Heterocapsa circularisquama.This phenomenon appears in Cx38 which is closer with the Yangtze estuary. In Cx21, which is farther away from the Yangtze estuary, BSi presents a peak at the late 80s, and then decreases. TOC and TN fluctuate synchronously with BSi, reflecting that toxic algal bloom have not been serious at the location area. Vibrocore Cx38 is in the hypoxic zone with larger water depth and finer grain size, while Cx21 is not in the hypoxic zone with shallower water depth and coarser sediments, which may be the main reason of the difference.