Plankton community responses in earthen channel catfish nursery ponds under various fertilization regimes

Abstract

We evaluated fertilizer effects on phytoplankton and zooplankton concentrations in channel catfish Ictalurus punctatus nursery ponds. In 2001, three ponds were not fertilized and three ponds were fertilized with inorganic (8.4 kg/ha N, 2.0 kg/ha P followed by twice a week applications at half the initial rate) and organic fertilizer (224 kg/ha cottonseed meal followed by once a week applications of 28 kg/ha) for 4 weeks. Total P and soluble reactive P (SRP) were significantly higher in fertilized ponds ( P<0.05), however no differences in algal populations resulted. Of the zooplankton, only copepod nauplii were higher in fertilized ponds. In 2002, 26 ponds were used. Sixteen ponds were newly constructed; 10 ponds were old (>10 years). Within each age group, treatments of no fertilizer (controls), organic fertilizer only, inorganic fertilizer only, or both organic and inorganic fertilizers were used for 3 weeks. In 2002, the same P but higher N rates were used (20.2 kg/ha N followed by twice a week at half rate). Several differences occurred among treatments ( P<0.05). Both total P and total N were significantly higher in ponds fertilized with inorganic fertilizer and both inorganic and organic fertilizers than in control ponds and organically fertilized ponds. Also, total P, total N, SRP, and nitrite were all significantly higher in old ponds than in new ponds. All phytopigment concentrations were also higher in old ponds than in new ponds. Chlorophyll a (total algal biomass), lutein (green algal biomass), and fucoxanthin (diatom biomass) were significantly higher in ponds fertilized with inorganic fertilizer or both inorganic and organic fertilizers than in control or organically fertilized ponds. Old ponds contained significantly more rotifers, copepod adults, and ostracods than new ponds. Rotifer and ostracod concentrations were not different among fertilizer treatments. Copepods adults and nauplii were significantly increased in ponds fertilized with both organic and inorganic fertilizers than in organically fertilized ponds. Cladocerans were significantly higher in inorganically fertilized ponds than in all others. Results indicated little benefit of organic fertilizer addition at the rates used. Also, ponds appear to be more N limited than P limited. Old ponds are more fertile than new ponds. Applying only inorganic fertilizer at an initial rate of ∼20 kg/ha N and 2 kg/ha P, followed by subsequent applications of half the initial rate for 3–4 weeks increased zooplankton concentrations desirable for fry with larger mouth gapes.