Evaluation of Phytoplankton Growth in Atsumi Bay as an Effect of Nutrient Input during Rainfall

Abstract

Eutrophication is a global environmental issue and has been studied for more than 40 years in Atsumi Bay, Aichi Prefecture, Japan. The distribution of nutrients in watersheds and estuaries is strongly influenced by meteorological factors, especially rainfall. Our observations in summer 2010 found that high river discharges during rainfall altered nutrient concentrations throughout the bay, and furthermore resulted in a red tide due to nutrient uptake by phytoplankton. In order to study nutrient dynamics and phytoplankton growth in estuaries, with a focus on the effects of river inputs during rainfall, we developed a depth-averaged two-dimensional ecological model. The model was conceived with the ability to simulate nutrient dynamics, in the form of dissolved nutrient concentrations, and phytoplankton growth throughout an estuary. The model provided reasonable results and agreed well with observed data. Our evaluation showed that the increase in availability of dissolved nutrients and favorable irradiance post-rainfall stimulated production of phytoplankton in the bay, compared to pre-rainfall.