Occurrence and effects of a spring oxygen minimum layer in a stratified coastal water

Abstract

The combined effect of pelagic respiration and stratification resulted in the development of an oxygen minimum layer in early April 1988 in the Kattegat, Denmark. Strong wind mlxing in midMarch created a cold and homogeneous surface layer above a saltier and warmer bottom layer This was followed by a large outflow of low salinity Baltic Proper water, which covered the higher salinity Kattegat water and created a surface layer trapping the cold former surface water a s an intermediate layer for several weeks. Wind forcing was insufficient to mix the intermediate layer with the water column above. In this cold layer, 1 or 2 subsurface chlorophyll a maxima were observed, and the composition of the plankton community changed during the spring period towards heterotrophy, culminating with a maximum of 2200 ind. I ' of rotifers in the pycnocline a t the end of April. In the bottom part of the cold layer a distinct layer exhibiting an oxygen minlmuln developed. In early April the layer was observed 10 km off the coastline and covered approximately 500 km2 The upper 10 m of the water column was at the same time supersaturated, with a maximum of 152% 0, (554 pM 02), presumably due to spring bloom photosynthesis. The oxygen saturation In the bottom water was 70% 02, whereas an oxygen minimum was recorded below the chlorophyll a peak between the surface and bottom layers. Here, the oxygen saturation was 32% O2 (120 pM 02) with a plankton respiration of 14 pM O? dl , yielding an oxygen turnover of 8 d. Where the pycnocline intersected the bottom, maxima of phytoplankton biomass (33 pg chlorophyll a I ' ) , plankton respiration rates (16 pM 0, d l ) and bacterial net production rates (3.6 FM C d' ) occurred. Including the sediment oxygen uptake, an oxygen turnover of approximately 3 d was estimated in this zone, and only 2 "h of full oxygen saturation (6 FM 0, concentration) was found. Microzooplankton and flagellates accounted for approximately '/3 of the total oxygen consumption. The vertical position of the pycnocline changed during April and May, which resulted in fish mortality when fish trapped in the bottom nets were exposed to the oxygen minimum layer. At the end of May, the oxygen minimum layer was no longer distinguishable from the Kattegat bottom water.