| Sansha Bay is characterised by the highest production of large yellow croaker Pseudosciaena crocea in China,accounting for about 70%of its total national yield in2020.In Sansha Bay,P.crocea is mostly cultured by cage farming,which produced aquaculture-derived organic matter（AOM）in the farming process.The AOM is mainly consisted of waste feed（WF）and fecal matter（FM）.It is of great theoretical and practical significance to study the sedimentation characteristics and benthic ecological effects of AOM in Sansha Bay.There were several studies about the sedimentation characteristics and benthic ecological effects of AOM in Sansha Bay which focused on the sedimentation rate and the impacts of macrobenthos community structure of AOM.In this paper,the proportion of AOM in sediments and the impacts of AOM sedimentation on the trophic characteristics of dominant macrobenthos in Sansha Bay were investigated.The main research methods and conclusion are as follows:1.The proportion of AOM in sediments and its relationship with environmental factorsThe sampling stations were set up along the dominant current direction in the near-bottom layer of a typical experimental cage area in July 2020 and September 2021 which consisted of six stations（0 m,10 m,50 m,100 m,250 m,500 m away from the experimental cage area,respectively）.Sediments and its potential source samples（WF,FM,seaweed（SW）,and estuarine particulate organic matter（e POM））were collected.The stable carbon and nitrogen isotope ratios(δ13C andδ15N),total organic carbon（TOC）,total nitrogen（TN）and carbon-to-nitrogen（C/N）ratio of these samples were analysed.The proportion of AOM in sediments（the sum of the proportion of WF and FM）was quantified by the Bayesian model.The content of aquaculture-derived organic carbon（AOC）and aquaculture-derived nitrogen（AN）caused by cage farming was calculated.The typical physicochemical characteristic factors of sediment（physical characteristic factor:water content;Chemical characteristic factor:sulfide content）were analysed,and regression analysis was used to analyse the influence of AOM sedimentation on these physicochemical factors.The δ13C,δ15N,TOC,TN and C/N ratio in sediments ranged from–23.60‰to–21.95‰,4.09‰to 6.73‰,7.27 to 14.96 mg g–1,0.80 to 1.78 mg g–1 and 7.56 to 13.52,respectively.However,none of the five sediment parameters showed significant temporal-spatial differences in our study area（Scheirer-Ray-Hare test,P>0.05）.The Bayesian Markov chain Monte Carlo analysis showed that the mean proportion of WF,FM,SW and e POM to SOM were 13.1±6.3%,5.5±3.6%,8.8±1.4%and 72.6±5.0%in two years,respectively.The mean proportion of AOM was 18.6±4.8%in two years.The proportion of AOM showed no significant temporal-spatial discrepancy in two years（Scheirer-Ray-Hare test,P>0.05）.The results of the temporal-spatial differences of AOC and AN contents supported this conclusion,which the calculation of these two parameters based on the proportion of AOM.In sediments,AN and AOC contents are positively correlated with acid volatile sulfide（AVS）concentration and water content.The AVS concentration which reflected benthic environment pollution conditions showed a significant decrease within 50 m from the cage area in fall 2021.2.Impacts of AOM on the trophic and ecological characteristics of dominant macrobenthosThe sampling stations were set up along the dominant current direction in the near-bottom layer of different hydrodynamic characteristics of three experimental cage areas in September 2021,which consisted of five stations（0 m,400 m,800 m,1500 m,2300m away from the experimental cage area,respectively）.Dominant macrobenthos（polychaetes Onuphis eremiata,Sternaspis scutata,Ophelina acuminata,Lumbrineris sp.and Heteromastus filiformis）and their potential food sources were collected.The potential food sources of macrobenthos in Sansha Bay mainly included sedimentary organic matter（SOM）,POM,SW,zooplankton,benthic microalgae（BMA）,WF,FM.Theδ13C andδ15N of these samples were analysed.The trophic level（TL）of dominant macrobenthos and the contribution of food sources to dominant macrobenthos were analysed by the formula of TL and the Bayesian model.The δ13C andδ15N of different potential food sources ranged from–25.51‰to–12.71‰and 3.30‰to 12.91‰,respectively.Theδ13C andδ15N of different potential food sources showed significant differences,respectively（Kruskal-Wallis test,P<0.001）.Theδ13C andδ15N of SOM showed significant differences among the three transects （Kruskal-Wallis test,P<0.05）.Theδ15N of POM showed significant difference among the three transects（Kruskal-Wallis test,P<0.05）.Theδ15N of BMA showed significant difference between the two transects（Wilcoxon test,P<0.05）.The δ13C andδ15N of dominant macrobenthos ranged from–21.82‰to–16.87‰and 7.05‰to 12.98‰,respectively.Theδ15N of five dominant macrobenthos showed significant difference（Kruskal-Wallis test,P<0.001）.Theδ13C andδ15N of O.eremiata showed significant differences among the three transects,respectively（Kruskal-Wallis test,P<0.001）.Omnivorous Lumbrineris sp.and O.eremiata occupied the highest TL（2.79 and2.78,respectively）.The TL of deposit-feeding H.filiformis,O.acuminata and S.scutata were 2.62,2.41 and 1.81,respectively.Based on SIMMR results,AOM was an important food source for dominant macrobenthos,contributing 68.4±17.2%,61.7±22.0%,54.9±13.4%,36.4±8.3%and18.3±13.2%to O.eremita,O.acuminata,Lumbrineris sp.,H.filiformis and S.scutata,respectively.Benthic microalgae and particulate organic matter contributed to the diets of dominant macrobenthos as well.3.ConclusionOur study results showed that the proportion of AOM from the cage farming in sediments was low in Sansha Bay,and the spatial distribution of the proportion of AOM in sediments was relatively homogeneous.The deposition of AOM increases the concentration of acid volatile sulfide in sediments,and increases the water content in sediments.The AOM in sediments had served as an important food source for macrobenthos in Sansha Bay.The sedimentation of AOM had an obvious environmental and ecological effects in Sansha Bay. |
