An extensive survey was carried out in the Westerschelde estuary in 1989 to estimate phytoplankton primary productivity. High nutrient concentrations year round and very high turbidity resulted in light-limited growth of phytoplankton. This was reflected in a conservative behaviour of silicate and phosphate. The same was true for total inorganic nitrogen. However, in the inner part of the estuary nitrification took place. The calculated annual primary production was 200-300 gCÂ·m-2 at the range 29-20 salinity, 100-150 gCÂ·m-2 at 18-10 salinity and increased sharply to 900 gCÂ·m-2 at 2.5 salinity due to freshwater populations. In the inner part of the estuary the vertical mixing zone was frequently larger than 5 times the euphotic zone. The high rates of primary production we measured contrast with generally expected values. This corroborates Grobbelaar's hypothesis that the critical depth can be seriously underestimated. Phytoplankton copes with the turbid conditions by adjusting light-utilization efficiencies.