Phytoplankton growth stimulation and nutrient puls by natural rainfall during spring and summer in Sagami Bay, Japan

Abstract

To assess the consequences of high-nutrient freshwater input on the phytoplankton community structure during the rainy season, the phytoplankton levels were monitored daily together with environmental factor variables from 12 April to 23 July 2003 in Sagami Bay, Japan. The low salinity conditions lasting 2-3 days after heavy rainfalls resulted in a significant loading of nutrients, such as dissolved inorganic nitrogen (DIN), into the coastal area. Also, a few days after the decrease in salinity, increases in Chlorophyll-a (Chl-a) concentration were frequently observed. Based on the high values of total Chl-a concentration, the time was divided into three periods, which were from 1 to 11 May (Period A), from 26 May to 9 June (Period B) and from 30 June to 22 July (Period C). The phytoplankton assemblages during Period A were dominated by two dinoflagellates, Ceratium furca and Ceratium fusus. Prior to the blooming of these species, the heterotrophic dinoflagellate Noctiluca scintillans was dominant. During Period B, the phytoplankton communities were dominated primarily by the diatoms Rhizosolenia delicatula, Hemiaulus sinensis and Navicula spp. Finally, Cerataulina dentate, Rhizosolenia spp., Lauderia borealis and Neodelphineis pelagica were the dominant species during Period C. The increase in their phytoplankton abundances had consumed and exhausted the available nitrogen (N) and phosphorous (P), as the two main nutrient levels measured during almost all of the high Chl-a periods were low enough to indicate N and P limitation. In addition, this may have also been the cause of a shift in the dominant organisms from high diatoms to pico- and nano-plankton in the low Chl-a environment. In particular, silicate (Si) was not a major limiting factor for phytoplankton production, since the plots of the Si: DIN and Si: P ratios clearly demonstrated that there were not any potential stoichiometric limitations created by Si, and almost all silicate concentrations were > 2.0