EFFECTS OF DESTRATIFICATION ON AUTOTROPHIC AND HETEROTROPHIC MICROPLANKTON PRODUCTIVITY IN EUTROPHIC AQUACULTURE PONDS

The effect of vertical mixing on microbial production rates in the water column of freshwater prawn (Macrobrachium rosenbergii) aquaculture ponds in Hawaii [USA] was examined using data collected over a period of 17 months. Ponds were judged as being stratified or well mixed based on the temperature difference between the top and bottom (depth 1 m). During stratified and well-mixed conditions this difference averaged 1.9 .+-. 0.8.degree. C and 0.1 .+-. 0.2.degree. C, respectively. The seston of stratified ponds was found to be characterized by a significantly higher carbon/nitrogen (C/N) ratio (8.2 .+-. 1.2 versus 4.0 .+-. 0.6 by weight) and a significantly lower carbon/chlorophyll a (C/chl a) ratio (27 .+-. 4 versus 80 .+-. 55 by weight) than the seston of well-mixed ponds. However, there was no significant difference between stratified and well-mixed ponds in terms of mean chl a concentration, or surface primary production rates and productivity indices. Vertical profiles of microbial DNA and RNA synthesis over a 40-h period during typical stratified and well-mixed conditions revealed a pronounced vertical gradient in the lower 0.5 m of the water column during stratified conditions, but only a 20-30% difference between stratified and well-mixed conditions in water-column-integrated microbial DNA and RNA synthesis rates, and no significant difference in microbial growth rates as inferred from DNA/RNA synthesis rate ratios. Stratification events in these ponds are evidently of such short duration that microbial production and growth rates are rarely limited by the availability of organic and inorganic substrates.