Combined nitrogen and phosphorus fertilization for controlling the toxigenic alga Prymnesium parvum

The toxigenic alga Prymnesium parvum has caused significant fish kills in Texas reservoirs and fish hatchery ponds since 2001. Copper sulfate and ammonium sulfate can control P. parvum. but they provide short-term improvement and have undesirable side effects. Copper sulfate also kills desirable algae and invertebrates, and ammonium sulfate can be harmful to fish. Because dominance of the phytoplankton community and toxin production by P. parvum appear to be nutrient related, we evaluated two nitrogen (N) and phosphorus (P) fertilization regimes for their efficacy in controlling P. parvum populations and toxicity. The experiment included two N:P fertilization treatments (300:30 or 300:60 mu g/L three times weekly) and an unfertilized control group conducted in limnocorrals suspended in a fish hatchery pond. Ammonium chloride (NH4Cl) and phosphoric acid (H3PO4) were used as the sources of N and P, respectively. In the unfertilized control, P. parvum cells and toxicity persisted throughout the 40-d experiment. In both fertilization treatments, P. parvum cell densities and toxicity declined below detectable levels within 2 weeks and remained undetectable for the duration of the study. The fertilization regimes promoted pH and un-ionized ammonia N levels that may be detrimental to the survival of crustacean zooplankton and sensitive fish species, such as fry and fingerlings of temperate basses Morone spp. However, this approach may be suitable for production of less-sensitive species, such as advanced fingerlings of channel catfish Ictalurus punctatus. The next steps in refining this strategy are reducing the fertilization rates while still achieving P. parvum control, investigating alternative sources of N fertilizer to eliminate the potential confounding effect of un-ionized ammonia, and integrating pH reduction measures for sensitive fish species.