Ammonium accelerates the growth rate of Skeletonema spp. in the phytoplankton assemblage in a heavily eutrophic embayment, Dokai Bay, Japan

High ammonium loading in Dokai Bay produces large diatom blooms and therefore laboratory experiments was conducted to determine if ammonium stimulated the occurrence of the observed massive phytoplankton blooms. The presence of ammonium as a nitrogen source significantly increased the growth rate of Skeletonema spp. compared to nitrate. Growth rate (μ _chl) on ammonium was significantly higher (~13-15%) than on nitrate. However, the effect of ammonium on growth rate acceleration was species specific, because the effect was not observed when the field assemblage was a mixture of Skeletonema spp. and other diatoms. In addition, the magnitude of the growth acceleration effect varied, depending on the irradiance level. The largest significant increase in growth rate on ammonium compared to nitrate occurred when irradiance was at irradiance around 200 μ mol photons m^-2 s^-1. Our results suggested that the growth of Skeletonema spp. which was dominant species in Dokai Bay was accelerated by ammonium and particularly under an irradiance which occurs in the mixed surface layer of this bay in summer. This bay may act like a selective growth incubator for certain diatoms such as Skeletonema that are subsequently exported to nearby coastal waters