High ammonium production from sediments in hypereutrophic shrimp ponds

Abstract

Intensive shrimp ponds are hypereutrophic ecosystems with high nutrient loading rates. The contribution of sediment nutrient fluxes to nutrient cycling in shrimp ponds was studied. Benthic processes were quantified using benthic chambers (clear and dark) on the sediment of intensive shrimp ponds at 3 farms. The ponds contained 2 distinct sediment zones: an outer zone of sand and rubble swept clean by the action of paddlewheels and other aerators (65 to 85% of the pond area), and an inner zone where settled particulate matter formed a sludge pile. The high nutrient loads, in the form of formulated feed, coupled with high sediment deposition resulted in an anoxic sediment in the pond center with a low respiratory quotient (oxygen:carbon dioxide ratio). The carbon dioxide effluxes were 2- to 4-fold higher than oxygen consumption. The highest nutrient flux rates from the sediment were those of ammonium (NH4+), particularly in the inner sludge zone (up to 182 mmol m-2 d-1). Inferred denitrification efficiency was low (<2%), particularly in the inner sludge zone. Denitrification rates, as measured by the acetylene block method, confirmed this. Denitrification may be limited by nitrate (NO3-) availability. It is unclear why NO3- concentrations were low, but nitrifiers either may have been inhibited by pond conditions or were unable to become established due to the routine water discharges to maintain adequate water quality. Fluxes of urea and PO43- were also low. This study shows that low rates of nitrogen loss from denitrification, relative to the high nitrogen inputs, results in high NH4+ concentrations in the water column. The discharges of pond water mean that sediment processes in shrimp ponds ultimately affect the nitrogen loads discharged into the aquatic environment, exacerbating the potential for eutrophication caused by shrimp farming activities.