Can microbenthic photosynthesis influence below-halocline oxygen conditions in the Kattegat?

Abstract

Abstract Gross benthic oxygen uptake at 20 m depth in the Laholm Bay, SE Kattegat (measured in the laboratory on darkened cores) varied between 3.5 and 6.5 ml O2 · m−2 · h−1 for the period May through September. The integrated oxygen uptake for this period was calculated to 17.6 l O2 · m−2 or 8.0 g C · m−2 if RQ = 0.85. At 14–16 and 17–18 m depth a microbenthic primary production of 15 and 3.5 g C · m−2 · yr−1, respectively, has been measured with the 14C-technique. This corresponds to 28 and 6.5 l O2 · m−2 if PQ = 1.0. At 20 m production was 1 g C · m−2 · yr−1 (1.9 l O2). Microbenthic primary production is concentrated to the period May to September. Laboratory experiments show that a light intensity of approximately 10 µE · m−2 · s−1 (16L/8D) is sufficient to make the benthic community autotrophic (P/R> 1) when the heterotrophic activity corresponds to 10 ml O2 · m−2· h−1, i.e. above the maximal benthic respiration found during summer at 20 m depth in the Laholm Bay. Light intensity in summer often exceeds 10 µE · m−2 · s−1 at 15 m depth. We conclude that for coastal areas in SE Kattegat oxygen in below-halocline waters (>14 m depth) may be significantly influenced by photosynthesis of microbenthic algae. Eutrophication may initially improve benthic oxygen conditions through increased microbenthic photosynthesis. However, with more advanced stages of eutrophication plankton blooms are likely to cause a deterioration of the benthic light climate, diminishing benthic photosynthesis.