AbstractIn the past decade, sinking-out of phytoplankton in the late phases of bloom periods has come to be regarded as an important source of energy-input to the bottom in coastal regions. Estimates of rate of phytoplankton sinking-out were made from studies of the temporal and vertical variations in pelagic chlorophyll-a and combined inorganic nitrogen in the northern Øresund (Denmark) during 1976–1978. It is concluded that, in this almost permanently stratified water body, sinking-out of phytoplankton to the bottom only took place to a significant degree during the late phase of the spring bloom following a nearly complete depletion of combined nitrogen. This organic input seemed nearly sufficient to cover the annual requirements of the benthos, other sources being at most supplementary. It is hypothesized, that these findings could also apply to at least the deep easternmost part of the Kattegat and that entrainment of deeper water by the surface current could be an important mechanism for reintroducing sinking algal cells to the euphotic zone in the near-permanently stratified inner Danish waters. It appears that, due to the hydrographic conditions, the bottom of the Øresund serves as a trap for phytoplankton from the neighbouring Kattegat. This could explain the comparatively large biomass of benthic invertebrates in the northern Øresund.