Phytoplankton primary production and nutrients in the Oosterschelde (The Netherlands) during the pre-barrier period 1980–1984

Phytoplankton primary production, nutrient concentrations and turbidity were monitored at three stations in the Oosterschelde during 1980–1984 as part of an ecosystem study. From comparisons of dissolved nutrient ratios with the nutrient requirements of phytoplankton, and of ambient nutrient concentrations with half-saturation constants for nutrient uptake by natural phytoplankton populations it was concluded that silicate was a limiting nutrient for diatoms after the spring bloom until the end of the summer. Dissolved inorganic nitrogen and phosphate were not considered to be limiting to phytoplankton growth. In general, the phytoplankton growing season started during the first fortnight of April and ended at the end of September. Column production in the whole Oosterschelde varied between 201 and 540 g C m−2 yr−1 and was, on average, 25% higher in the western part than in the eastern part. ‘Basin’ production in the Oosterschelde varied between 120 and 466 g C m−2 yr−1 and was, on average, 55% higher in the western part than in the eastern part; this difference could be explained by differences in the ratio of euphotic depth to mean depth of the compartments. Estimated carbon-specific growth rates in the eastern part varied between < 0.1 and 3 d−1 and in the western part between < 0.1 and 1 d−1. This difference could be explained by the great differences in depth of the compartments. Carbon-specific growth rates are discussed in relation to phytoplankton loss rates. It is suggested that in the eastern part sedimentation must be an important sink for phytoplankton.