Nitrogen Biogeochemistry of Aquaculture Pond Sediments.

Abstract

Aquaculture pond water quality is dominated by planktonic processes, although the high surface-to-volume ratio suggests that sediment has the potential to effect important changes in water quality. The objectives of this research were to evaluate annual variation of sediment nitrogen biogeochemistry and develop a model of annual variation in ammonia concentration in a commercial channel catfish pond. In addition, the effects of physical and chemical sediment management techniques on water quality in aquatic mesocosms were evaluated. Sediment nitrogen was dominated by organic (92-96%) and adsorbed ammonium (2-4%) fractions. Porewater ammonium comprised the smallest yet most dynamic component of sediment N, with maximum concentrations measured during summer. Porewater ammonium concentrations were 4-20x higher than in the water column. Sediments were sinks for oxygen and oxidized nitrogen and sources of carbon dioxide and ammonium. Molecular diffusion accounted for only 1.4-14.9% of ammonium flux suggesting that mineralization of organic matter at the sediment surface was rapid. Denitrification rates were limited by low ambient oxidized nitrogen concentrations, although denitrification potential was substantial. A simulation model accurately described annual variation of catfish pond ammonium concentration. Ammonium production was partitioned between fish excretion (67-75%) and sediment diffusion (25-33%). Phytoplankton ammonium uptake exceeded nitrification during the growing season (April-October); nitrification was a more important removal mechanism than phytoplankton uptake during the winter (November-March). Model output was most sensitive to changes in the partition of nitrogenous excretion, average feeding rate, and the phytoplankton specific uptake rate. Sediment disturbance in organically-enriched aquatic mesocosms improved water quality for fish production. Dissolved oxygen concentration was higher and ammonia concentration was lower in mesocosms stocked with fish. Whole-tank respiration was inversely related to sediment disturbance. Nitrification associated with suspended particles was inversely related to C:N ratio of organic inputs. Nitrate enrichment of aquatic mesocosms did not improve water quality for fish production. Elevated concentrations of ammonia and nitrite were associated with high dissimilatory nitrate reduction rates. Substantial quantities of organic matter mineralization mediated by nitrate were inferred from alkalinity accumulation. Dissolved oxygen concentrations were increased and soluble phosphorus concentrations were decreased only slightly as a function of nitrate enrichment.