Microbial and phytoplankton processes, and their effect on water quality were examined over a 3-week period in five high-intensity (120 animals m-2) shrimp (Litopenaeus vannamei) ponds of varying crop ages at Belize Aquaculture Ltd., (BAL) in Central America. These ponds were characterized by zero water exchange throughout the crop, plastic lining and high aeration rates. Nitrogen (N) and organic carbon (C) inputs, in the form of fishmeal-based feed, grain-based feed and molasses, resulted in high concentrations of dissolved organic and inorganic N (2.29-5.56 and 0.17-10.66 mg l-1, respectively) and dissolved organic C (14.20-48.10 mg l-1). Phosphate levels were also high, ranging from 0.07 to 1.17 mg l-1. The high nutrient concentrations promoted the growth of bacteria, phytoplankton (mostly autotrophic flagellates) and protozoa. Up to 40% of the bacteria were associated with flocculated matter. However, bacterial numbers and oxygen (O2) consumption in the water column did not appear to increase with crop age. This may be due to a reduction in the C/N ratio below the optimum for bacterial growth. Up to 22% of the O2 consumption was due to nitrification and there was some indication of lowering of total ammoniacal N (TAN) concentrations and an increase in nitrite and nitrate levels in older crops. Both phytoplankton and bacteria were responsible for high rates of ammonium uptake. In ponds with high nitrate concentrations, nitrate uptake rates were also high. Phytoplankton productivity remained high irrespective of crop age and ponds fluctuated between net O2 production (autotrophy) and net O2 consumption (heterotrophy) irrespective of crop age. This reflected the highly dynamic nature of the bacterial and phytoplankton populations with frequent blooms and crashes of individual phytoplankton species. The high mixing rates resulted in phytoplankton and other detritus remaining suspended in the water column. However, a small area of sludge (<2% of pond area) did accumulate containing a high N and C content, and high pore water TAN. This study showed that despite what is generally considered as poor water quality in the ponds, i.e. high nutrient concentrations, high and unstable phytoplankton numbers, and high bacterial numbers, shrimp production was high relative to conventional ponds. There appeared to be scope for increasing bacterial production in these systems by increasing the C/N ratio, and hence C availability for bacterial growth. However, it remains to be established which microbial processes are likely to be promoted, and if the benefits of this outweigh the costs.