Phytoplankton Bloom Mechanism in an Area Affected by Eutrophication: Tokyo Bay in Spring 1999

Abstract

Although extensive research on annual cycles of phytoplankton communities in the open sea has been conducted, there have been less continuous measurements on short term variations in semi-enclosed bays. To estimate the conditions necessary for red tide occurrences in an area affected by eutrophication, we carried out continuous field measurements in the inner part of Tokyo Bay at three stations where red tides have often been observed in Spring. The blooms of phytoplankton occur under high solar radiation conditions. Mixed layer thickness and the vertical distribution of PAR are also significant in accounting for the levels of phytoplankton blooms. Under optimum conditions of mixed-layer thickness and the euphotic zone, phytoplankton increased rapidly even under average solar radiation. At this time, north-wind induced outflow and vertical mixing result in diluting phytoplankton and terminating blooms. These bloom conditions will not continue due to self shading of phytoplankton, even if there isn't a strong wind. Therefore, these physical conditions are significant in controlling the levels of blooms in an area affected by eutrophication. Following the phytoplankton blooms, dissolved oxygen and phosphate concentrations show greater temporal variability through decomposition processes of the phytoplankton.