Distributions And Ecological Significance Of Biological Elements In Core Sediments From Aquaculture Areas Of Sanggou Bay

Vertical distrabutions of biological elements (N,P,TOC,BSi)in three core sediments from aquaculture areas of Sanggou Bay were analysed,and the ecological significance were shown. The pollution from the organic matter and its sources were discussed. The contents of biogenic silica (BSi) were used to rebuild the productivity. The potential eutrophication, the self-pollution from organic matters and the transitions of the phytoplankton which was the dominant food of bivalves were discussed, and they were reflected by the long-term changes of the biological elements. The scientific basis and the guide were provided.The conclustions are as follows:(1) In the core sediments from aquaculture areas of Sanggou Bay, the mean content of total nitrogen (TN) and total phosphorus (TP) in station outside the bay was higher than that of stations in the bay. The fluctuations of TN's contents in station were small. The content of TP increased at first and then decreased with depth. In the depth with shell deposits of stations inside the bay, the contents of TN and TP fluctuated greatly, and the upper contens were higher than the background contens.The contents of TOC in core sediments of upper layer were higher than that of lower layer. The contents of TOC in station outside the bay showed great flutuations in upper layer, and the peaks appeared at the depth of 20cm to 30cm. The contents in stations inside the bay fluctuated greatly at layers with shell deposits, and they were higher than the background ones, the upper ones were also higher than the background ones.The contents of BSi in core sediments of upper layer were higher than that of lower layer. The aquaculture restrained the amount of phytoplankton.The contents of BSi in station outside the bay increased first and then decreased,and increased again with depth, and two peaks was shown at the depth of 100cm and 220cm. The contents at shell deposits zones in sediments of station outside the bay fluctuated greatly and were low, which means that bivalves restained the amout of phytoplankton.(2) The exchangeable nitrogen accounted for 27.55%,8.72%and 7.38% of TN separately,which indicates that the contents of potential bioavaliable nitrogen were low. In core sediments from aquaculture areas of station outside the bay, the contents of IEF-N,CF-N and IMOF-N decreased with depth , but the contents of CF-N increased first and then decreased with depth. The contents of IEF-N,CF-N and OSF-N decreased with depth in core sediments of station at the north of the bay,but that of IMOF-N increased with depth. The contents of IEF-N,CF-N and IMOF-N increased with depth in station at the south of the bay,but that of OSF-N decreased. All forms of nitrogen fluctuated greatly in layers with shell deposits. The contents of potentially bioavaliable phosphorus accounted for 60.07%,61.16% and 71.00% of TP separately. It means that most of the phosphorus may be actived on the water-sediment interface and provide the nutritions the algal need.In station outside the bay, the content of Ex-P,Au-P and Or-P increased first and then decreased with depth, but the depth they changed differed. The content of Fe-P decreased with depth, but that of De-P increased. In station at the south of the bay, the contents of all forms of phosphorus decreased with depth, but the depth where they changed differed.The contents of all forms of phosphorus also decreased with depth, and the depth they changed differed. The contents of all forms of phosphorus kept steady in the lower layers, and they are lower than that in the upper layer. The contents of all forms of phosphorus fluctuated greatly in the layers with shell deposits.The contents of Fe-P in core sediments from aquaculture areas of Sanggou Bay accounted for 1% or less than 1% of TP, which indicates that the pollution of the environment were in normal range. The organic phosphorus was the main form of phosphorus in core sediments, and it accounted for more than 50% of TP. The contents of Or-P in surface sediments were higher than that of other areas, and the aquaculture araised the contents of Or-P.The correlations between nitrogen (phosphorus) forms and BSi showed that none of the forms of nitrogen has close relation with productivity. But Ex-P has close relationship with productivity.(3) The mean contents of TOC in core sediments were lower than 2%, which means that the qualities of the sediments were good.The mean single standard indexes of TN in sediments of station outside the bay and at the north were lower than 1, and didn't fluctuate greatly with depth. The indexs of TN were above 1 between 0～23cm, which exceeded the standard index, but they were still below the high risks level. The indexes below 23cm were all below 1.The sigle standard indexes of TP in sediments of the station outside the bay were between 0.9 and 1.2, and the indexes in middle layer were higher than that of upper layer and lower layer. But the result indicated that the contents of TP were below the risky content. The mean indexes of station inside the bay were below 1, and the indexes in upper layer were higher than that in lower layer. We got the conclusion that aquaculture in this area raised the concent of the nutrious, but didn't result in self-pollution.The TOC/TN ratios reached 35.39 at 3cm, and others was 10, which show organic matter is the mixture of aquatic and terrestrial sources. The ratios were steady with depth except the shell deposits layer in station outside the bay. The organic matter were aquatic sources in the sediments above the shell deposits layer in station S2 and S3, and were terrestrial sources under the layer with shell deposits.(4) Deposit velocityes of the three stations were 0.374cm/a, 0.357cm/a and 0.699cm/a. When 1960s to 1980s, the culture of kelp diminished the amount of phytoplankton and diatom. When 1908s to 1990s, the culture of bivalves increased the biogenic deposits, and the increase of nutrious promoted the amount of phytoplankton and diatom. But when the density of the bivalve's aquaculture was higher than the bay's carrying capacity, the feeding of the bivalves diminished the amount of phytoplankton and diatom. After 1990s, the performance of the mode of polyculture led to the increase of the amount of phytoplankton and diatom.