From Red Tides to Healthy Ecosystems by Nutrient Management in Enclosed Sea Areas

The enclosed sea areas of Ise Bay and Mikawa Bay have long been impacted by anthropogenic water pollution, with eutrophication driven by excess phosphorus (P) and nitrogen (N) leading to frequent red and blue tides and severe damage to coastal fisheries. In response, efforts to purify inflowing rivers and brackish lakes have successfully reduced pollution loads, shifting these bays from eutrophic to oligotrophic conditions. However, this transition has resulted in declines in benthic organisms and their higher-level consumers, raising concerns about ecosystem health. Water quality data reveal a shift from a phosphorus-limited (eutrophic) state to a nitrogen-limited (oligotrophic) state, offering critical insights into how changes in nutrient-limiting factors affect ecosystem structure. This study examines the relationship between phytoplankton growth measured by chlorophyll-a (Chl-a) and nutrient concentrations, identifying optimal ranges of P and N that support healthy phytoplankton productivity while minimizing the risk of harmful algal blooms. Response surface analysis determined suitable concentrations to be 0.026–0.046 mg/L for P and 0.95–1.08 mg/L for N. To maintain ecological balance and prevent red and blue tides, we propose managing nutrient levels within these ranges, offering a scientifically grounded strategy for restoring and sustaining the health of enclosed sea areas.