Impacts of shrimp farm effluent on water quality, benthic metabolism and N-dynamics in a mangrove forest (New Caledonia)

Water quality parameters, sediment oxygen demand (SOD), dissolved organic and inorganic nutrient fluxes, and N-cycle processes (nitrification; denitrification; dissimilatory nitrate reduction to ammonium (DNRA)) were determined in a New Caledonian mangrove receiving shrimp farm effluent and a natural mangrove. Effluent was enriched in nutrients and organic matter, and significantly stimulated SOD and nutrient regeneration rates in the receiving sediments. All N-cycling processes were stimulated between ∼2 and 12-fold in the sediments receiving effluents compared to the natural mangrove. However, due to the preferential enhancement of DNRA compared to denitrification, there was no significant increase in net nitrogen elimination compared to the significant increase in sediment nutrient regeneration rates. These results indicate that the mangroves are only a partial filter for the shrimp farm effluent, as confirmed by the elevated nutrient concentrations measured in an external, marine creek of the effluent receiving mangrove.