Nutrient Fluxes, Porewater Profiles and Denitrification in Sediment Influenced by Algal Sedimentation and Bioturbation by Monoporeia affinis

The effects of addition of algae (340 mg C m-2) and of the amphipod Monoporeia affinis (1500 ind m-2) on denitrification; porewater nutrient profiles; nutrient fluxes between sediment and water; and bacterial production were studied. A laboratory experiment was conducted using cores filled with sediment from the Gulf of Finland (Baltic Sea). Denitrification (isotope pairing method) was dramatically reduced in the cores enriched with algae (from 200-300 to 10-20 mu mol N m-2 day-1). However, the addition of M. affinis together with algae negated this effect. Monoporeia affinis alone increased the coupled nitrification-denitrification as compared to the control cores. Bioturbation by the amphipods oxidised the sediment surface layers and consequently, PO43-concentration in the porewater decreased. In the cores enriched with algae, the loss of N from the sediment porewater was dominated by the efflux of NH4+which formed 90% of the loss. In the M. affinis enriched cores, the share of the NH4+efflux was 60% of which the excretion by M. affinis was 5-10%. The effects of M. affinis were found to be caused mainly by the improved O2conditions in the sediment.