Impact of finfish aquaculture on biogeochemical processes in coastal ecosystems and elemental sulfur as a relevant proxy for assessing farming condition.

We conducted experiments to investigate the effects of finfish aquaculture and to propose appropriate proxies for assessing their environmental impact. Due to enhanced fish feed input, sulfate reduction (SR) and the resulting metabolic products (H2S, NH4+, PO43-) were significantly greater at the farm than at the control site. Benthic release of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from farm sediment accounted for 52-837% and 926-1048%, respectively, of the potential DIN and DIP demand for phytoplankton production. The results suggest that excess organic loading in fish farms induces deleterious eutrophication and algal blooms in coastal ecosystems via benthic-pelagic coupling. Direct SR measurement provided the most useful information of all the parameters on organic contamination in fish farms. However, given its abundance, relatively lower chemical reactivity and relative ease of analysis, elemental sulfur was regarded as the most appropriate proxy for assessing the environmental impacts of finfish aquaculture.

Impacts of typhoon-induced heavy rainfalls and resultant freshwater runoff on the partitioning of organic carbon oxidation and nutrient dynamics in the intertidal sediments of the Han River estuary, Yellow Sea.

Occurrence of typhoons accompanied by heavy precipitation has increased for the past 40 years in northeast Asia. To elucidate the impact of three consecutive typhoon-induced heavy rainfall events and resultant freshwater runoff on the partitioning of organic carbon (Corg) oxidation and nutrient dynamics, we investigated the geochemical constituents, the rate of anaerobic Corg oxidation, sulfate reduction (SR), iron reduction (FeR) and P speciation in the intertidal mud flat of the Han River estuary, Yellow Sea. Corg oxidation by SR and FeR and their metabolic products (∑CO2, NH4+, H2S, Fe2+) decreased significantly (P < 0.05) during and immediately after the heavy rainfall. Additional mesocosm experiments demonstrated that potential N2 production rates increased up to 2.4 times with increased nitrate concentrations during freshwater runoff. The results suggest that denitrification becomes a significant Corg oxidation pathway substituting for SR during high-nitrate freshwater runoff, which may remove substantial portion of the N introduced into the estuary. P speciation analysis further revealed that the concentrations of iron bound P decreased by 2.2 fold during the heavy rainfall compared to that measured before the rainfall. The results suggest that an excess supply of riverine Si keeps P from binding to Fe, thereby stimulating P release. Taking projections of enhanced rainfall events in the future into account, our results suggest that the intensified storm events and resultant riverine runoff induces a shift of Corg oxidation pathways in the sediments, which ultimately alters C-N-P-S-Fe dynamics and may deepen N-limiting conditions in coastal ecosystems of the Yellow Sea.