12. Long-Term Investigations in Brackish Ecosystems

Variability and complexity in brackish systems require long-term measurements in order to define base conditions, from which deviations can be ascertained. Long-term observations in three systems, lagoons, the Baltic Sea, and the Chesapeake Bay, are examined to identify system changes, unlikely detectable with sampling in single years or in temporally and spatially heterogeneous sampling. One basic condition in brackish systems is the gradient in salinity, which may be large-scale and rather stable stretching over the entire sea (marine gradient), or meso-scale and highly variable such as those in river plumes (estuarine gradient), and upwelling cells (upwelling gradient). For the first two gradients, and in some cases the third, distinct boundaries separate stenohaline taxa from more eurytopic taxa resulting in spatially explicit distributions of plankton, nutrients, and food web characteristics. The natural variability has to be ascertained through repeated long-term sampling in order to fix a baseline for shifts and trends in the ecosystem. A general trend during the last decades is cultural eutrophication, leading to increased phytoplankton biomass and sedimentation, and hypoxia in bottom water. In some areas, eutrophication was repressed in the 1990s, e.g., stabilization of chlorophyll concentrations in the Baltic Proper, recovery of macrophytes in the Darss-Zingst Bodden Chain (DZBC). In the coming years, the effects of declining nutrient loads are expected to cause a return to mesotrophic conditions in the DZBC, resulting in a return of nutrient limitation. Further monitoring will be performed to follow this unique event. It is therefore imperative that the community support long-term observations in these complex systems particularly as increasing human populations exacerbate impacts of global climate change that slowly warms waters, changes intensities and frequencies of meteorological events and responsive hydrologies, and shifts biogeographic ranges of many cosmopolitan taxa.