Nitrate and Phosphate Toxicity on Post Larvae of the Giant Fresh Water Prawn (Macrobrachium rosenbergii) Individual and Synergistic Effects

Abstract

The indiscriminate use of fertilizers has resulted in nutrient enrichment giving rise toeutrophic conditions in many freshwater bodies in Sri Lanka. Few studies have investigatedeffects of field levels of nitrate and phosphate on commercial freshwater species. The presentstudy therefore attempted to assess the individual and synergistic effects of nitrate andphosphate on post larvae of the Giant fresh Water Prawn Macrobrachium rosenbergii, whichare released in large quantities into reservoir systems of the dry zone of Sri Lanka.

The post larvae were collected from a culture facility in Pambala, Chilaw, and wereacclimatized. After the measurement of the initial body lengths the larvae were introducedinto 500 ml glass beakers (n=10 per beaker) containing five field based concentrations ofeither nitrate (0.01 – 100 mg l-1) or phosphate (0.001 – 10 mg l-1). Controls and treatmentswere maintained in triplicate. Synergistic effects were also tested for 0.1 mg l-1 of phosphatewith three concentrations of nitrate (0.1, 1, 10 mg l-1). Survival, growth and the feedingbehaviour of the post larvae were monitored as end points of the empirical acute toxicity (96hr) trials. Results revealed that both nitrate and phosphate enhances mortality levels in prawnlarvae when acting alone at concentrations of 0.01 mg l-1and above for nitrate and 0.001mgl-1and above for phosphate. For instance, mortality in controls was 13.3 %, and for the twonutrients at the lowest test levels i.e. 0.01 mg l-1 of nitrate and 0.001 mg l-1 of phosphate, were23% and 20%, respectively. The synergistic effects with both nitrate and phosphate werelower than the additive effects of each, at all tested levels, but were greater than the recordedindividual effects. The synergistic mortality with 10 mg l-1 of nitrate and 0.1 mg l-1 ofphosphate was greater than mortality induced by each nutrient acting alone. In contrast tomortality, effects on growth were not apparent with either nitrate or phosphate acting alonenor synergistically, while the phosphate alone and both nutrients acting synergistically,appeared to cause reduced feeding. These findings highlight the need for investigating effectsof nutrient enrichment, and specifically their synergistic effects, on commercially exploitedspecies in order to predict levels of sustainability of the inland fishery resources of Sri Lanka.