Whether sediments act as sinks or sources of nutrients depends partly on the oxygen conditions at the seafloor. Laboratory experiments on coastal Gulf of Finland (Baltic Sea) sediment tested the sensitivity of denitrification to a 2 wk anoxia exposure and subsequent reoxidation of the bottom waters. At the same time we followed the rapidly (1 d) and more slowly (9 d) emerging changes in different forms of sediment P after oxic conditions were restored. The total denitrification rate (Dtot) did not change during anoxic incubation, but shifted from coupled nitrification-denitrification (Dn) towards water column nitrate dependence (Dw). As the Dn rate did not decrease at the same rate as the Dw rate increased, the overall effect of 2 wk exposure to anoxia was an increase in Dtot rate. Nitrification was enhanced in the manipulated sediment compared to natural conditions, despite anoxia. Anoxia quickly caused a release of dissolved P from its 2 most labile forms. The effect was readily reversible, but in nature the replenishment of oxygen stores is usually linked to an intense mixing of the water column, and it is possible that part of the P released during anoxia reaches the productive layer. In our experiments, anoxia affected P cycling more than N cycling.
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