Intensive penaeid prawn ponds are characterised by high stocking densities of animals whose nutritional requirements are met by the addition of formulated feed. Penaeid prawns have a nutritional requirement for high protein levels, and hence formulated feeds contain significant amounts (5 to 9%) of nitrogen (N). However, much of the N is not retained by the prawns but enters the pond system stimulating the growth of the natural biota and elevating nutrient levels in the water column and sediment. While it is well established that the efficiency of feed conversion to prawn biomass is low, prior to this study little was known of the transformation and ultimate fate of dietary N in intensive prawn ponds.
In this study, a conceptual model of N cycling in intensive prawn ponds late in the growth season, when the prawn biomass and feed inputs were highest, was developed and tested to determine the dominant pathways of N transformation. The experimental work focussed on: quantifying the forms of dissolved N (DN) originating from feed; determining the role of the microbial (bacteria, phytoplankton, heterotrophic flagellates) community in processing DN and particulate N (PN) in the water column and sediment; and the ultimate pathways of N removal from the system.
Penaeus monodon is the dominant species cultured in Australian and southeast Asian intensive aquaculture systems. The prawn pond systems in this study had high concentrations of ammonium (NH4+ and dissolved organic N (DON), and a high biomass of both heterotrophic and autotrophic nanoplankton (< 10 µm), and bacteria. By feeding prawns 15N-enriched formulated feeds, it was established that only 20 to 30% of the dietary N consumed by prawns was retained by the prawns after 2 weeks. While most of the nutritional requirements of the prawns were met by the formulated feed, the natural biota also contributed. This was demonstrated by rapid 15N-enrichment of prawns following the addition of 15N-NH4+ to pond water. However, prawns relied less on the natural biota as they grew.
The N not retained by the prawns entered the water column and sediment of ponds. Soluble products from prawn gill excretion, as well as faecal excretion and uneaten feed in the form of NH4+ and urea. Significant amounts of complex DON compounds were also produced from prawn feeding and metabolic processes. The bacterial community appeared to play a minor role in assimilating DON and it accumulated in the water column.
Particulate waste products from feeding and other detritus, as well as inorganic particles scoured from the pond edges, were concentrated on the sediment in the pond centre via the action of aerators. This accumulation of sludge was highly anoxic and much of the PN was converted to NH4+ which rapidly diffused across the sediment-water interface. A significant amount of the NH4+ regenerated in the pond system was from the sediment, particularly the inner sludge zone, with prawn gill excretion and microbial regeneration processes in the water column contributing to a lesser extent. Phytoplankton uptake of NH4+, principally by the nanoplankton, was rapid. The phytoplankton therefore, played the dominant role in preventing the accumulation of NH4+. There appeared to be little conversion of NH4+ to NO3- via the nitrification pathway.
Gaseous loss of N (denitrification) from the prawn pond system was minimal relative to dietary N inputs. Inferred denitrification efficiency, based on benthic chamber experiments, was low. Denitrification appeared to be limited by NO3- availability since the NO3" concentrations in the water and sediment were relatively low.
It is common practice at prawn farms to routinely discharge water from ponds when the water quality declines. Since little of N was retained in the prawns and sediment, and the denitrification efficiency was low, this study suggests that most of the dietary N must ultimately be discharged from ponds.
This study identified that dietary N was cycled in prawn ponds through two major DN compounds: and DON. In the case of NH4+, cycling through the microbial community was rapid with high assimilation rates of NH4+, principally by nanoplankton uptake, and high regeneration rates, principally by the microbial community in the sediment. In contrast, most DON was produced directly from the feeding process and slowly assimilated by the microbial community. However, urea (a simple form of DON) was the exception to this; it was rapidly assimilated by the nanoplankton community.
These findings demonstrate that there is considerable scope to reduce waste N discharges by: enhancing pond processes which reduce dissolved N concentrations, e.g. promoting higher rates of nitrification and denitrification; increasing the incorporation of DON into microbial protein; and minimising NH4+ regeneration in the sediment. In addition, this study has highlighted that there is scope to reduce dissolved N concentrations by lowering the protein content of formulated feeds, and improving feed stability to minimise N leaching. This study has provided a scientific basis for testing strategies to reduce dietary N waste with the ultimate aim of improving prawn farming profitability and reducing the environmental effects of the discharge of pond waters into adjacent waterways.