Seasonal variation in net benthic C‐mineralization in a shallow estuary

Seasonal changes in net benthic C‐mineralization were measured as sediment O2 uptake, as well as ΣCO2 flux from the sediment to the water column. Oxygen uptake varied between 26 and 181 mmol m−2 d−1 through the year, and ΣCO2 efflux varied between 17 and 132 mmol m−2 d−1. Both parameters were related to temperature, and the Q10 for O2 uptake and ΣCO2 efflux were 2.5 and 2.0, respectively. The ΣCO2/O2 ratio varied between 0.8 and 1.0 during most of the year. The ΣCO2/O2 ratio was, however, less than 0.8 in late summer and autumn, which might be due to reoxidation of accumulated reduced sulfur compounds and carbonate precipitation. Calcium carbonate deposition by molluscs seemed quantitatively important for the ΣCO2 efflux in September 1990 and August 1991. The ΣCO2/O2 ratio reached a maximum of 1.3 in mid‐April during decomposition of benthic and sedimented microalgae. On an annual basis, all the oxidants which were reduced during anaerobic mineralization (e.g., Mn2+, Fe2+, HS−), apparently were reoxidized by O2, as illustrated by an annual ΣCO2/O2 ratio of 0.8. Benthic macrofauna stimulated C‐mineralization, indicated by a positive relationship between O2 uptake and ΣCO2 efflux rates, respectively, and macrofauna biomass. Sediment chlorophyll‐a+ pheopigment content and C/N ratio served as good indicators both of algal sedimentation from the water column and of the presence of benthic microalgae. The sediment parameters, including the organic carbon and nitrogen content, C/N ratio, and chlorophyll‐a+ pheopigment content, clearly reflected the seasonal variation in sediment mineralization rates. There was a temporal phase displacement between input of organic matter and benthic C‐mineralization, which resulted in a temporary build up of organic matter during periods with high primary production and/or low C‐mineralization (spring, late autumn, winter). The accumulated organic matter was mineralized during periods when the actual supply of organic matter was insufficient (summer, early autumn).