Distinguishing nutrient-depleting effects of scallop farming from natural variabilities in an offshore sea ranch

Abstract

While bivalve farming is well recognized in terms of modifying the biogeochemical cycle through filter-feeding and biodeposition, its impacts on water column nutrient concentrations in various ecosystems may vary from depletion to addition. Hypothesizing that the variability arises from overlap of farming-associated impacts and pelagic processes, nutrient concentrations and phytoplankton biomass were investigated monthly in an offshore sea ranch, to detect the real impacts of bottom-seeding scallop farming. Nutrient concentrations in water column average increased from July to December following successive surface freshening and disappearance of thermal stratification. Abrupt decrease was observed in March, along with increase in chlorophyll a concentration. The silicate concentration decreased below 2 μM in all depth layers in the mariculture area in March and April, whereas silicate limitation in open waters was commonly observed in April. Compared to open waters, inorganic nitrogen and phosphate concentrations were significantly lower in the mariculture area on an annual cycle, whereas silicate concentration was lower in spring and higher in summer. Our results indicate that scallop farming in frequently refreshed waters can act as nutrient sink through top-down effects. Without exogenous supply, it can also introduce the limitation of nutrient (silicate) with low regeneration rate and stimulate shift in the phytoplankton community structure.