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Abstract
Eutrophication causes seasonally anoxic bottom waters in coastal environments, but
we lack information on effects of onset of anoxia and subsequent reoxygenation on
benthic fluxes of redox-sensitive minerals and associated organic carbon (OC). As
the first study, we determined the effect of inducing anoxia and subsequently restoring
oxic conditions in mesocosms with surface sediment and water from a coastal environment.
These concentration changes were compared with those in an oxygenated control. We
determined water column concentrations of dissolved organic carbon (DOC), particulate
organic carbon (POC), iron, manganese, and phosphate. Benthic fluxes of DOC, POC,
and iron increased at the onset of anoxia in oxygen-depleted treatments. DOC and iron
concentrations increased concomitantly towards maxima, which may have indicated reductive
dissolution of FeOOH and release of associated OC. The subsequent concomitant concentration
decreases may have been the result of coprecipitation of OC with iron-containing minerals.
In contrast, the phosphate-concentration increase occurred several days after the
onset of anoxia and the manganese concentration was not affected by the onset of anoxia.
Restoring oxic conditions resulted in a decrease in DOC, POC, and phosphate concentrations,
which may indicate coprecipitation of OC with phosphate-containing minerals. The high
DOC fluxes at the onset of anoxia indicate that redox oscillations may be important
in OC degradation. Further, our results indicate a close coupling between OC cycling
and dissolution/precipitation of iron-containing minerals in intermittently anoxic
sediments.