Narrow Results

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.

The temporal and spatial variations of a hypoxic water mass in Tokyo Bay were investigated through intensive field surveys coupled with numerical simulations in summer 2007. The measurement results showed that the hypoxic water mass developed in the bottom layer from the head to the bay mouth in the first half of August. This water mass rose to the middle layer in the last half of August and had disappeared by the beginning of September. The analysis of observed seawater characteristic variations (temperature and salinity) and numerical simulation results suggested that these variations of the hypoxic water mass were regulated by a combination of the oceanic water variations at the mouth of Tokyo Bay and the wind-driven current.