Phytoplankton community structure, biomass, and off-flavor: Pond sizerelationships in Louisiana catfish ponds

摘要

Many aquaculture studies are conducted in relatively small research ponds and the results are then extrapolated to larger commercial ponds. Implicit in this research is the assumption that there is no relationship between pond size and phytoplankton composition. Study objectives were to assess phytoplankton composition and biomass by several methods in 17 channel catfish Ictalurus punctatus ponds at the Aquaculture Research Station, Louisiana Agricultural Experiment Station in Baton Rouge, Louisiana, USA. Pond size ranged from 0.04-0.60 ha. Sampling occurred weekly from 10 September - 1 October 1997. Water temperatures coincided with a transition from summer to fall-winter conditions. Biomass was assessed by cell counts and quantification of photopigments. Concentrations of dissolved off-flavor compounds (2-methylisoborneol and geosmin) were assessed by gas chromatography/mass spectroscopy of water column samples. Cell count data showed differences in dominant species, biovolume, and diagnostic pigment signatures among ponds. The smaller ponds had more diverse phytoplankton composition compared to the larger ponds, whereas chlorophyll levels were nearly an order of magnitude lower in the smaller ponds. Ultraplanktonic (2-20 #mu#m) unicellular cyanobacteria dominated the numerical counts on most sampling dates; however, biovolume transformations of cell count data reduced the dominance of this component relative to cryptophytes, diatoms, and filamentous cyanobacteria. Pigment and microscopic analyses were well correlated. Unialgal isolates of dominant taxa from these samples indicated the presence of at least five genera of off-flavor producers in these ponds; these taxa included Anabaena, Aphanizomenon, Pseudanabaena, as well as two species of Oscillatoria. Care in extrapolating results from smaller-sized research ponds to larger commercial ponds is warranted, as is the potential for taxa other than Oscillatoria and Anabaena in forming off-flavor compounds.