Marine eutrophication induced oxygen deficiency: Effects on soft bottom Fauna, Western Sweden

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Abstract
Marine eutrophication is studied in a multidisciplinary project in the Kattegat, western Sweden. Input of nutrients has increased several fold during this century; nitrogen ≈ 6 times, phosphrorus ≈ 10 times. This has caused elevated concentrations of nutrients in the water and increased primary production. Some adverse effects at the bottom have been noted annually from about August through October in the 1980's, e.g. fish catches have dropped and mortalities have been reported for fish and benthic animals including Nephrops noruegicus. In this paper we evaluate the effects on benthic macrofauna which are suggested to be caused mainly by oxygen deficiency occurring especially around the depth ofa strong halocline (≈ 15m), but also along a bottom transect down to 57 m. At some sites the benthic communities were impoverished, and at others only some species seemed to be especially sensitive, e.g. the bivalve Abra alba and the brittle star Amphiura filijormis. In shallow < 10 m) exp.sed waters an annually occurring mortality of molluscs was observed over large areas from 1980 onwards. Affected species were mainly the bivalves Cardium edule and Mya armaria. The causes behind this mortality may also be oxygen deficiency occurring during calm weather periods.