Annual dynamics of pelagic primary production and respiration in a shallow coastal basin

Abstract

The Wadden Sea (North Sea, Europe) is a shallow coastal sea with high benthic and pelagic primary production rates. To date, no studies have been carried out in the Wadden Sea that were specifically designed to study the relation between pelagic respiration and production by comparable methods. Because previous studies have suggested that the import of primary-produced pelagic organic matter is important for benthic Wadden Sea carbon budgets, we hypothesised that on an annual average the northern Wadden Sea water column is autotrophic. To test this hypothesis, we studied annual dynamics of primary production and respiration at a pelagic station in a shallow tidal basin (List Tidal Basin, northern Wadden Sea). Since water depth strongly influences production estimates, we calculated primary production rates per unit area in two ways: on the basis of the mean water depth (2.7 m) and on the basis of 1 m depth intervals and their respective spatial extent in the List Tidal Basin. The latter more precise estimate yielded an annual primary production of 146 g C m − 2 y − 1 . Estimates based on the mean water depth resulted in a ∼40% higher annual rate of 204 g C m − 2 y − 1 . The total annual pelagic respiration was 50 g C m − 2 y − 1 . The P/R ratio varied between seasons: from February to October the water column was autotrophic, with the highest P/R ratio of ∼4–5 during the diatom spring bloom in April/May. In autumn and winter the water column was heterotrophic. On an annual average, the water column of the List Tidal Basin was autotrophic (P/R ∼3). We suggest that a large fraction of the pelagic produced organic matter was respired locally in the sediment.