Effect of Dissolved Organic Nitrogen in a Reservoir of Reclaimed Land on Phytoplankton Growth in Isahaya Bay

Abstract

Isahaya Bay has suffered from severe red tides, and previous reports revealed that red tides have continued to exist under dissolved-inorganic-nitrogen (DIN) -depleted conditions in the bay. One possible explanation for this phenomenon is that the influx of dissolved organic nitrogen (DON) from a reservoir on reclaimed land facilitates the persistence of red tides in the bay. In this study, DON enrichment bioassays were conducted to test whether the growth of phytoplankton in Isahaya Bay can be stimulated through the addition of DON isolated from the reservoir. Microsized phytoplankton (> 10 μm) growth rates in both the presence and absence of antibiotics, a bacterial inhibitor, were stimulated by DON addition. The results suggest that microsized phytoplankton in Isahaya Bay can be stimulated by DON isolated from the reservoir directly and indirectly. Thus, the influx of DON from the reservoir might be an important process supporting the existence of red tides under DIN-depleted conditions.