Arenicola marina (Polychaeta) and organic matter mineralisation in sandy marine sediments: In situ and microcosm comparison

Effects of the polychaete Arenicola marina on sediment porewater profiles and organic matter mineralisation were studied both in situ and in manipulated laboratory microcosms. Comparison of the results from the two methods showed differences in the shape and magnitude of porewater profiles and solute fluxes. Porewater profiles revealed a significant effect of irrigation on solute transport. The effect was most evident in the homogenised laboratory microcosms where a significant accumulation of porewater solutes with depth was observed in the absence of A. marina irrigation. In contrast, the limited reactivity of old organic matter stored deep in anoxic in situ sediment resulted in low accumulation of solutes at depth, whereas a rich benthic microalgal community present at the sediment surface contributed to high reaction rates and nearsurface solute peaks. Reaction rates were more homogenous with depth and similar or even higher in A. marina sediment than the control despite a lower organic content in the former. Continuous reworking by A. marina results in the rapid burial of the small, but reactive and nitrogen-rich, organic pool derived from the benthic microalgae at the sediment surface. Addition of seagrass detritus to the A. marina sediment resulted in a further increase of reaction rates compared to the unamended A. marina sediment, while reworking and irrigation activities maintained the organic pool and porewater profiles as low as in the unamended treatment. Benthic respiration in laboratory microcosms was stimulated almost two-fold in the presence of A. marina. No such stimulation was apparent in situ. Here the stimulated capacity for carbon mineralisation was evident instead as an almost 50% lower sediment organic content in the presence of A. marina. Despite the strengths and weaknesses of each method, the results are valuable when combined as they clearly demonstrate the effect of irrigation on solute transport and the potential of A. marina to redistribute and facilitate the removal particulate organic matter via microbial degradation.