The decay of Zostera marina, representative for beach wrack, at the micro-tidal southwestern Baltic Sea coast on the island of Poel in Germany

Beach wrack is defined as any material washed ashore by wind and wave actions plays a vital part in coastal ecosystems: it fosters dune formation and prevents beach erosion at land, provides food and shelter to beach fauna, both in and off the water. Nevertheless, it is often seen as nuisance and therefore removed at recreational beaches. So far, no complex data has been retrieved regarding the decay of beach wrack when left at the beach and in the shallow water.
To gain insight into the decomposition processes of beach wrack local seagrass of the species Zostera marina was chosen as prime example species and vital part of beach wrack at the southwestern German Baltic Sea coast at the island of Poel. In different seasonal experiments, seagrass was filled into white and black litterbags. In another experiment, seagrass in white litterbags was left in the shallow water until total disintegration of biomass. Sampling was conducted in regular intervals. The seasonal experiments were moved from water to land and land to water every week to mimic the movement between these interfaces in tidal seas for a period of six weeks. Constant exposure to the shallow water lasted for between 140 and 210 days. With every sampling, the biofilm was scratched off the decaying leaves of seagrass. The development of its microbial community as well as biomass loss were investigated. C/N-ratio was measured with the respective changes over time. The influence of abiotic parameters like light, salinity and temperature for the microbial community during the decay were then evaluated. 
In respect to decomposition processes at micro-tidal coasts, these studies provide an important insight into seagrass decay, at land and in the shallow water. Beach management processes at recreational beaches, and the possibilities of nutrient back-flow, need to be implemented into nature-compatible approaches. A corresponding system based on this research results can be promoted for leading to a balanced coexistence between man and nature.