Importance of benthic macrofauna and coastal biotopes for ecosystem functioning – Oxygen and nutrient fluxes in the coastal zone

The coastal zone, often a mosaic of biotopes, plays a number of important roles that ensure the proper functioning of the entire marine ecosystem. In this transition zone between the land and the open sea, the processes of nutrient transformation and retention take place. The objective of the present study was to determine the effect of benthic fauna and environmental parameters on solute fluxes at the water-sediment interface. Furthermore, we have made an attempt to investigate the role of various biotopes in the coastal zone, including Zostera marina and Mytilus trossulus beds, and to determine whether they are a sink or source of oxygen and nutrients. For this purpose, bare soft-bottom biotopes and biotopes with engineering species, Z. marina and M. trossulus were examined. The study was carried out in autumn, at six sites with water depths ranging from 2 to 23 m, located in Puck Bay (southern Baltic Sea). Oxygen and nutrient fluxes across the sediment-water interface were used as a measurement of ecosystem functions, resulting from numerous processes that take place in the benthic environment and combine biotic and abiotic pathways. We observed a high variability of fluxes in the coastal biotopes. Soft sediments inhabited by Z. marina and fauna were generally a source of nutrients. Soft sediments inhabited by microphytobenthos and fauna were also a source but to a lesser extent. The results of our study have demonstrated a significant effect of epifauna inhabiting the surface of sediment or its topmost layer on solute fluxes and thus on the ecosystem functioning. This applies in particular to macrofaunal communities dominated by M. trossulus, which had a large impact on ammonia and phosphate effluxes. The largest efflux of silicate was observed at the deepest site with a high organic matter content as well as in Z. marina beds.