Assessment Of The Influence Of Large-Scale Aquaculture On Benthic Environment In Sanggou Bay

As an important part of marine ecosystem,benthic environment is “source” and “sink” of nutrient elements in the water,and is of great importance to the whole water environment health.Long time and high-intensive marine aquaculture creates organic pollution,which exert an effect on the marine ecosystem.With a weak circulating and exchange power,organic accumulation in the benthic environment has a long-term effect.Sanggou Bay has an aquaculture history of several decades and large-scale aquaculture area.It is the seaweed culture area、bivalve-seaweed integrated area、and bivalve culture area from the bay mouth to the bay,and there is a small-scale net cage culture area in the Southwest corner of the bay.Therefore,to ensure the healthy of the whole marine ecosystem,and continuous and efficient aquaculture activity,it’s necessary to survey and assess the benthic environmental quality.Two surveys about benthic environment in September 2016 and July 2017 in raft bivalve-seaweed culture area,and four surveys in September and November 2016、April and July 2017 in fish cage culture area in Sanggou Bay were carried out.By combining field observation and assessment using MOM-B system 、 macrobenthic community healthy assessment(macrobenthic community diversity index、macrobenthic Abundance and Biomass Curves、marine biotic index and multivariate-AMBI index)and sediment nutritional status assessment,benthic environmental healthy condition in Sanggou Bay aquaculture area was assessed.The results are as follows:(1)Raft bivalve-seaweed culture area in Sanggou Bay: Biological parameters of MOM-B system assesses the sediment acceptable status;Chemical parameters of MOM-B system assesses the sediment quality one grade;Sensory parameters of MOM-B system assesses the sediment quality two grade;After synthesizing chemical and sensory parameters,the MOM-B system assesses the sediment quality in bivalve-seaweed culture area in Sanggou Bay very good(one grade).Macrobenthic diversity index assesses the sediment clean or mild contamination;Abundance and Biomass Curves decides the macrofaunas suffered moderate disturbance;Biotic index shows the macrofaunas suffered mild but tending to be moderate disturbance;And the sediment environment quality was good.After synthesizing several macrobenthic assessment index,the results shows the macrofaunas in the raft bivalve-seaweed culture area in Sanggou Bay suffers mild disturbance,and the sediment quality is good(two grade).Organic pollution index reveals the sediment was mild but tending to be middle polluted;Nitrogen pollution index reveals the same results to organic pollution index;Phosphorus pollution index reveals the sediment was mild polluted;Nitrogen and phosphorus integrated pollution index reveals the sediment was mild polluted.Sediment nutrient parameters shows the sediment in the raft bivalve-seaweed culture area in Sanggou Bay is mild(two grade)but tending to be middle(three grade)polluted,and phosphorus contamination is relatively light(two grade).(2)Fish cage culture area in Sanggou Bay: Biological parameters of MOM-B system assesses the sediment acceptable status;Chemical parameters of MOM-B system assesses the sediment quality one grade in both culture period and non-culture period,but the score in culture period is lower than non-culture period;Sensory parameters of MOM-B system assesses the sediment quality two grade;After synthesizing chemical and sensory parameters,the MOM-B system assesses the sediment quality in fish cage culture area in culture period in Sanggou Bay better(two grade),and in non-culture period very good(one grade).Macrobenthic diversity index assesses the sediment mild or middle contamination;Abundance and Biomass Curves decides the macrofaunas didn’t suffer any disturbance;Biotic index shows the macrofaunas suffered mild disturbance;And the sediment environment quality is good.After synthesizing several macrobenthic assessment index,the results shows the macrofaunas in the fish cage culture area in Sanggou Bay didn’t suffer disturbance or suffered mild disturbance,and the sediment quality is good(two grade).Organic pollution index reveals the sediment was mild but tending to be middle polluted;Nitrogen pollution index reveals the sediment was middle or heavy polluted;Phosphorus pollution index reveals the sediment was mild but tending to be middle polluted;Nitrogen and phosphorus integrated pollution index reveals the sediment was mild or middle level polluted.Sediment nutrient parameters shows the sediment in the fish cage culture area in Sanggou Bay was mild(two grade)or middle(three grade)polluted,and nitrogen contamination was relatively heavy(four grade).(3)Analyzing sediment environment in bivalve-seaweed culture area and fish cage area in Sanggou Bay synthetically,the results are as follows: If analyzing from physical parameters(MOM-B system assessment)、chemical parameters(sediment nutrition index assessment),every index in bivalve-seaweed area is better than in fish cage area,apart from organic matter.If analyzing from biological parameters,macrobenthic community diversity index is higher in bivalve-seaweed area than in fish cage area.But,macrobenthic community composition、Abundance and Biomass Curves、marine biotic index all has a better assessment result in fish cage area than in bivalve-seaweed area.And multivariate-marine biotic index which combining diversity index has an accordant result in different culture areas.Whatever bivalve-seaweed area or fish cage area,the sediment quality in Sanggou Bay is good,which indicates that the influence of large-scale aquaculture on benthic environment is relatively mild.(4)Assessment of benthic environment quality must consider physical、chemical and biological parameters,and the weighting coefficient of biological parameters should be higher than others.If nutrient content of environment has negative impacts on macrobenthic community health,the concentration reaches the level of pollution.Whereas,if the nutrient content plays a positive role to the macrobenthic community,the concentration does not reach the pollution level.In consideration of the limitations in popularizing biological assessment method,it is suggest that using biological parameter assessment under the condition of a bad physical or chemical evaluation result.