Climate Regime Shift and Phytoplankton Phenology in a Macrotidal Estuary: Long-Term Surveys in Gyeonggi Bay, Korea

Phytoplankton are finely tuned to the seasonality of their environment, and shifts in the timing of phytoplankton phenology provide some of the most compelling evidence that species and ecosystems are being influenced by global climate change. Evaluation of a 50-year dataset of climatic parameters, a 12-year dataset of nutrients, and a 15-year dataset of phytoplankton biomass and composition in Gyeonggi Bay of the Yellow Sea revealed that the climate has shifted from a cold to a warm phase in the last few decades and that recent warm climatic and eutrophication trends are affecting phytoplankton biomass, phenology, and structure. In Gyeonggi Bay, climatic and ecological regime shifts were detected during the 1990s and 2000s, respectively. The asymmetric relationship between climate and ecological regime shift probably depends on macrotidal system configurations that are more resistant to environmental perturbation. The spring diatom blooms observed in the 1990s have moved forward to winter blooms in the 2000s because early winter warming has been induced by higher light and precipitation, which has removed prior light limitation and control of diatom blooms. Summer blooms are triggered by enhanced nutrients, which leads to frequent and recurring dominance of dinoflagellates and diatoms, supporting the hypothesis that summer phenology might be brought about by local processes such as eutrophication, as well as by climate change. Overall, differences in phenological trends can be brought about by differences in the underlying drivers of seasonality. Based on the results of this study, perspectives are drawn regarding the utility of phenology as an organizing principle for analysis of pelagic ecosystems.