Nutrients and Saltwater Exchange as Drivers of Environmental Change in a Danish Brackish Coastal Lake over the Past 100 Years

Many northwest European lake systems are suffering from the effects of eutrophication due to continued loading and/or poor, ineffective management strategies. Coastal brackish lakes particularly difficult manage complex nitrogen, phosphorus, and salinity dynamics that may exert varying influence on biological communities, but long-term data how these important often biodiverse respond change rare. In this study, palaeolimnological (including sedimentary parameters, diatoms, plant macrofossils) environmental monitoring (for last ~40 years) have been used assess over 100 years in Kilen, a Jutland, Denmark. Kilen has regularly monitored for (since 1972), TP (from 1975), TN 1976), since 1989 (phytoplankton, zooplankton, macrophytes), which allows robust comparison contemporary paleolimnological at high temporal resolution. The indicate nutrient rich years, with peaking mid-1980s late 1990s. Reduced concentrations occurred 1990s, though is not reflected sediment core diatom assemblage, highlighting caution must be taken when using quantitative transfer functions paleolimnology. Lake recovery 20 driven by reduction catchment shows improvements water clarity and, recently, macrophyte cover. after 2004 also changed composition dominant community within lake. low N:P ratio indicates summer, predominately N-limited, likely explaining why previous management, mainly focusing measures, had modest effect quality Despite slight recovery, still nutrient-rich, future system continue reduce loads both ensure sustained recovery. This study provides an exceptional opportunity validate record demonstrates power combined approach understanding key aquatic ecosystems.