From Red Tides to Healthy Ecosystems by NutrientManagement in Ise Bay and Mikawa Bay

Ise Bay and Mikawa Bay, situated near major urban and industrial areas, have been increasingly affected by water pollution due to human activities. Over time, eutrophication--primarily driven by excess phosphorus (P) and nitrogen (N)--has led to frequent occurrences of red tides and blue tides, significantly impacting coastal fisheries. To mitigate environmental pollution, efforts have focused on purifying the rivers and brackish lakes that flow into these bays. However, as pollution loads have decreased, these bays have transitioned from a eutrophic to an oligotrophic state, leading to a decline in the populations of benthic organisms and their higher consumers. In this study, we examine the relationship between phytoplankton growth--indicated by chlorophyll a (Chl-a)--and nutrient concentrations, particularly P and N. Through this analysis, we determine the optimal levels of P and N necessary to support healthy phytoplankton growth while preventing harmful algal blooms. Based on our findings, we propose a balanced approach to enhancing the nutritional conditions of enclosed coastal waters without triggering red or blue tides.