Beach-cast algae communities as a proxy for evaluating coastal water eutrophication

Coastal ecosystems, located at the intersection of land and sea, are subject to multiple human-induced stressors that affect both terrestrial and marine environments. Among the 12 core priorities of global policies and biosphere monitoring efforts, eutrophication indicators play a key role. However, developing and implementing these indicators at high resolution and large spatial scale poses significant logistical, technological, and economic challenges. Satellite-based chlorophyll-a measurements provide insights into the immediate effects of coastal eutrophication, but assessing its impacts on benthic habitats remains a significant hurdle. An alternative lies in examining beach-stranded materials, particularly macrophyte wracks, which may serve as proxies for shifts in the diversity and composition of coastal benthic habitats. This study explored whether macroalgae communities in beach wracks reflect the condition of marine habitats affected by coastal eutrophication. Using empirical data (for 84 taxa on 212 sites), two community-level indicators were developed: Community Turbidity Tolerance (CTTI) and Community Thallus Length, derived from algae found in beach wracks. Both indicators were shown to correlate with coastal chlorophyll-a concentrations, a widely used proxy for eutrophication. Both metrics also aligned with trends in the Water Strategy Framework Directive’s Good Ecological Status (GES) indicator, even though the CTTI only had a significant relationship with extreme GES levels. The macrophyte wracks derived indicators of coastal eutrophication we developed here further promise increased sampling resolution, cost-efficiency, and logistical simplicity. Moreover, beach wrack monitoring offers strong potential for inclusion in citizen science programs, further enhancing its scalability and impact.