Organic and inorganic carbon levels in recycled seawater during the culture of tropical prawns Penaeus sp.

Suspended particles and dissolved organic carbon in laboratory recirculation systems were broadly categorised by size filtration and ultraviolet absorption. Their concentrations in recycled seawater increased during the culture of tropical prawns with increase in feeding rate and also with a ‘load’ factor calculated to take account of the volume of water used to culture a known biomass of prawns. In systems operated without periodic water renewal the proportion of organic matter in filterable material and the integrated ultraviolet absorbance (Σ A 250–330 nm) of filtrate increased with time. The increasing absorbance observed, particularly in the higher wavelengths (Σ A 290–330 nm), indicated the accumulation of metabolically derived organic material that was resistant to microbial degradation. Mean numbers of bacteria varied immensely from 7 to 25 000 × 10³ ml^?1 but were lower in foamed than unfoamed systems.The relation between the concentration of dissolved inorganic carbon (a measure of carbonate alkalinity) and pH in the systems varied according to the type of media in the biological filter and the method used to control pH. Results closest to natural seawater were obtained in systems containing filters with limestone media periodically dosed with sodium hydroxide solution. Higher levels of inorganic carbon for a given pH above pH 7·9 were, however, noted when sodium hydroxide was used with plastic media filters, a feature beneficial to heavily loaded systems.