Phytoplankton primary production was studied in the turbid estuary of the river Schelde (The Netherlands). Measured rates of gross primary production were comparable to other estuaries. Respiration rates were calculated as being a fixed percentage of the maximum rate of photosynthesis (P-max(B)). We calculated that net primary production would be possible only when the rate of respiration was less than 1.5% of P-max(B). This respiration rate seemed too low to be realistic, but was necessary to explain the observed growth of phytoplankton. Near the maximum turbidity zone, no net annual primary production could be calculated. Maximum rates of photosynthesis were similar to those reported in the literature. Phytoplankton had relatively high rates of photosynthetic affinity [alpha(B), 0.032 to 0.043 mg C mg(-1) chl h(-1)