Benthic foraminifera such as Trochammina hadai and Saccammina sp. occur along with biological aggregates(marine snow)that suspended in the brackish waters of Lake Nakaumi. These species are common in organic rich sediments at greater depths in this lake. The biological aggregates were mainly composed of a dinoflagellate Prolocentrum minimum, which formed a red tide in the spring of 2003. The presence of these foraminiferal species strongly suggests that strong wind stress caused turbulence that disturbed bottom sediments leading to dispersion at middle water depths. In addition to the observations based on this foraminiferal occurrence, the mixing of deeper and shallower water at the halocline is a significant factor leading to outbreaks of red tide.