The nutrient characteristics of intensive shrimp farm effluent
High concentrations of nutrients and solids, particularly during shrimp harvest, are released to receiving environments. The reductions of pond water depths that are followed by shrimp collections using cast nets are common harvest strategies applied in Indonesia. However, these strategies elevate concentrations of nutrients and solids in effluent and eventually may affect the surrounding waters. This study was aimed to analyze nutrient characteristics of intensive shrimp farm effluent during shrimp harvest. Three harvested ponds stocked with 113 and 120 shrimp m-2 were sampled in the wet and dry seasons, respectively in a shrimp farm of Lampung Province, Indonesia. Samples of effluent were collected from sluice gate during shrimp harvest when the depths of pond waters were 0.12, 0.9, 0.6, 0.3, 0.2 and 0.1 m, respectively. The harvest strategies adopted in this studied farm triggered sediment erosion indicated by significantly high concentrations of solids represented by TOC, TSS and chlorophyll a in effluent during the reduction of pond water depth. In contrast, concentrations of dissolved nutrients showed different responses where PO4 that were not affected by harvest strategies indicated the ability of pond bottoms in retaining phosphate during sediment erosions. TAN and NO3-N were only in the wet season whereas NO2-N was in the dry season. Concentrations NO2-N and NO3-N in effluent were significantly higher in the wet season; in contrast, concentrations of TAN, PO4 and TOC were higher in the dry season, whereas there were no significant differences in concentrations of TSS between the wet and dry seasons. The N:P ratios of effluent discharged during harvest in the wet and dry seasons which were 95:1 and 130:1, respectively, indicated that the phosphorus was limiting nutrient for phytoplankton growth. Concentrations of TAN and TSS that exceeded concentration thresholds for shrimp pond effluent required an improvement in harvest strategies and effluent treatments before the effluent was released to the receiving environments.