REFINING NITROGEN AND PHOSPHORUS FERTILIZATION STRATEGIES FOR CONTROLLING THE TOXIGENIC ALGA PRYMNESIUM PARVUM

Previous studies have shown that three times weekly applications of phosphorus (30 mu g P/l) and nitrogen (300 mu g N/l) were effective at reducing the density and toxicity of the alga Prymnesium parvum in limnocorrals simulating a 40-day moronid (e.g., striped bass, Morone saxatilis, and palmetto bass, M. saxatilis xMorone chrysops) fingerling culture period. However, this fertilization regime produced high pH and unionized ammonia-N concentrations that are detrimental to the survival of moronid fry and fingerlings. In two follow-up experiments we changed the source of N from ammonia to nitrate, reduced fertilization rates, and examined the effect of N-only or P-only fertilization. In the first experiment P fertilization rates were reduced by one-half to 15 mu g P/l and NO3-N was substituted for NH3-N at the previously used rate of 300 mu g N/l. In the second experiment, N fertilization rates were reduced to 150 mu g N/l and the frequency of fertilization was determined by pH and P. parvum responses. Nitrate appeared to be as effective as ammonia as a source of N and when used in combination with P reduced P. parvum cell density and ichthyotoxicity. However, reduced N and P application rates and lower pond water temperatures during the study appeared to have decreased the speed at which fertilization produced these effects. While lower fertilization rates reduced algal productivity, high pH remained a concern for fish culture although pH was reduced to levels that might be acceptable with careful management of fish culture activities. Neither N-only nor P-only fertilization had a measurable effect on algal productivity or eliminated P. parvum and its toxicity. Furthermore, P-only fertilization may have increased P. parvum density and toxicity. For controlling P. parvum density and ichthyotoxicity we recommend a fertilization rate of 212 mu g NO3-N/l plus 30 mu g PO4-P/l applied three times weekly for aquaculture ponds where high pH is not a concern. Where high pH is a concern we recommend a fertilization rate of 117 mu g NO3-N/l plus 16 mu g PO4-P/l applied three times weekly with careful attention to afternoon pond pH.