Impacts Of Scallop Farming On Nutrient Limitation In The Zhangzi Island Area

As mariculture expands offshore in response to the increasing demand for seafood,a new set of ecological concerns arises.While bivalve farming is well recognized modifying biogeochemical cycle in water column through filter-feeding and biodeposition,its impacts on nutrient concentrations in various ecosystems may vary from depletion to addition.Located in northern Yellow Sea,waters around the Zhangzi Island?50 km offshore?is a typical offshore shellfish farming area in China,where bottom-seeding aquaculture of Japanese scallops Patinopecten yessoensis has been performed since 1998.As natural variability in this area has been well documented,it was selected as an ideal place to investigate the ecological consequences of shellfish farming.Annual variations of nutrients,Chlorophyll-a?Chl-a?and size-fractionated Chl-a concentrations were investigated from July 2009 to June 2010,and compared between mariculture area and open waters,in order to distinguish the effects of scallop farming from influences of natural variabilities.Furthermore,in order to figure out the causes of silicate limitation in spring,the temporal and spatial distribution of nutrients,biogenic silica,Chl-a concentrations as well as physical conditions were investigated in this area from March to May in 2014.We analyzed the occurrence frequency of nutrients limitation and the way in which it regulates the phytoplankton community structure.Additionally,a new method of Danish Unisense microelectrode was applied in our study to determine the physicochemical properties of sediments in the Laizhou Bay,the Muping area,the Jiaozhou Bay and the Zhangzi Island area,so as to explore the correlation between biodeposition and shellfish farming in water columns.According to the annual survey in the Zhangzi Island area,strong vertical stratification was observed from June to October and disappeared in winter with vertical homogeneity.Significant decrease of salinity was observed in the upper layers from June to August.Nutrient concentrations in monthly average showed similar trends in mariculture area and open waters,increasing continuously from July to December.This can be attributed to the coefficient supplement by freshwater discharge of the Yalu River and the collapse of the YSCWM.From February to March,nutrient concentrations decreased dramatically and net consumption occurred overwhelmingly in spring.Correspondingly,increase in Chl-a concentration was recorded in March.Silicate concentration lower than the minimum threshold for phytoplankton growth occurred in March in all stations in mariculture area,while in open waters silicate limitation was recorded firstly in April in upper layers.Dissolved inorganic nitrogen?DIN?and phosphate concentrations were significantly lower in mariculture area compared to those in open waters all through the year?P<0.01?.Silicate concentration,however,was higher in mariculture area in summertime?July to September?and lower during November-June?P<0.05?.According to our results,shellfish farming can work as nutrient sink through top-down control on nutrient concentration and structure.Nutrients removal was extremely significant in spring when exogenous supplement is scarce,leading to silicate limitation and shift in size-fractionized phytoplankton community structure.Along with silicate limitation,dominance shift from diatoms to dinoflagellates was recorded in phytoplankton community.The monthly averaged DIN concentration was comparatively high,with the maximum of 5.74?M in March and slight change during the sampling period.Phosphate and silicate deficiencies were recorded in spring in all layers?except the bottom layer in May for phosphate?.Silicate limitation presented at up to 71.2%of stations,with the average concentration as low as 1.7?M in March.Meanwhile,phosphate concentration decreased from 0.12 to 0.05?M in the upper layers from March to May.Stoichiometric ratios and absolute concentrations indicate that 77.3-90%of stations showed phosphate limitation in April and May.Accordingly,phytoplankton abundance decreased sharply in spring,from 7.16×10⁴cells L^-1 in March to 1.70×10⁴ in May.The dominant species in phytoplankton community changed from diatoms to dinoflagellates.On species level,both increased dominance of dinoflagellate and shift in dominant diatom species were observed.The dominant diatom species changed from Paralia sulcate in March to Coscinodiscus granii in May.It suggested that diatom/dinoflagellate shift in dominance was triggered by silicate limitation and further promoted by phosphate deficiency.The dominance shift was proposed to be determined by bottom-up control of nutrient concentrations rather than selective feeding of scallop?top-down?.Specifically,the silicate limitation in spring was attributed to net consumption of photosynthesis and defficiency in local re-mineralization.During continuous observation in spring,silicate concentration in water column was significantly higher in nighttime than daytime,whereas that of DIN and phosphate showed no diel difference.Comparing to dissolved silicate,biogenic silica concentration was lowest in April?one month later?,and bottom enrichment was not evident.The sediment type in the Zhangzi Island area is silty sand,and the organic matter content was slightly higher than that in the Laizhou Bay,where the sediment type is a mix of silty sand and sandy silt.Compared with the Jiaozhou Bay,another bottom seeding shellfish farming area,the probability of hypoxic conditions is lower in the Zhangzi Island area,but that of nutrient limitation is higher.It is thus concluded that,even in the same shellfish farming area,the observed ecological consequences may vary with seasons.In summer and autumn,the farmed shellfish population helped in removal of extra DIN and phosphate,without any significant negative impacts on nutrient structure and phytoplankton community,whereas in spring it resulted in silicate limitation and in turn diatom/dinoflagellate shifted.On future perspective,presence of silicate limitation can be used as an index of over cultivation in shellfish farming for both seafood production and eutrophication mitigation.On the other hand,the“bottle-neck”effect of nutrient limitation on food availability in spring suggests that carrying capacity might be originally overestimated,when calculated from averaged annual primary production.