Pelagic processes in extensive shrimp ponds of the Mekong delta, Vietnam

Abstract

Rates of pelagic primary production, respiration, and bacterioplankton dynamics were measured in relation to water quality parameters in three extensive shrimp ponds in the Mekong delta, Vietnam. There were few consistent differences in pelagic characteristics among different locations within these ponds, among the three ponds, or between the ponds and adjacent river water. Rates of primary production ( 14 C uptake) ranged from <1 to 94 mg C m −3 d −1. Rates of gross primary production (light–dark bottle technique) ranged from −184 to 2697 mg C m −3 d −1. Rates of pelagic respiration ranged from 60 to 3783 mg C m −3 d −1. Primary production rates measured by oxygen flux were greater than those measured via 14 C uptake, with P/ R ratios varying widely (−0.45 to 3.4), but the mean P/ R at each site was <1 with a grand mean among ponds of 0.7. Bacterioplankton numbers (mean range: 0.2 to 27.2×10 8 cells ml −1) and productivity (range: 2.5 to 297.3 mg C m −3 d −1) did not vary consistently among ponds with season. Mean bacterioplankton growth rates were fastest ( μ=0.29 d −1) in the pond with highest shrimp production and slowest in the poorest yielding pond ( μ=0.08 d −1). The primary production and respiration rates, and bacterioplankton dynamics, indicate that these pond waters are net heterotrophic. Plankton measurements rarely correlated with changes in physicochemistry (pH, dissolved O 2, salinity, temperature) or nutrient concentrations as within-site and temporal variability were large for most parameters. The diurnal cycles of physicochemical characteristics were similar to those measured in other unfertilized, low-alkalinity ponds, but these cycles were dampened by intense rainfall. Our data indicate that low net primary production, high rates of respiration, moderate rates of bacterioplankton production, high suspended solid and nutrient concentrations, low and very variable pH and dissolved O 2 concentrations, and variations in salinity due to intense rainfall episodes limit shrimp production in these extensive ponds.