Abstract
The vertical distribution and photosynthetic activity of the fast swimming phototrophic ciliate Mesodinium rubrum were studied periodically between 1983 and 1988 in a 20 m deep stagnant brackish inlet, Inre Verkviken on Åland, southwestern Finland. Density maxima of M. rubrum and associated peaks in chlorophyll a were observed both near the surface and at the interface of the anoxic zone at 12-14 m. Viable M. rubrum cells were also observed in samples from anoxic layers rich in phototrophic bacteria. Sampling at close intervals showed indications of vertical migration at least on sunny days in October 1983. Primary productivity measurements with incubations at the surface indicated that both the surface and the depth populations of M. rubrum were photosynthetically active. In September 1984 a surface maximum and an associated primary productivity peak were recorded. Depth maxima of M. rubrum were also detected in June 1986 and in September 1988. M. rubrum can migrate vertically over tens of metres per day and exploit layers low in dissolved oxygen but rich in nutrients. It is suggested that M. rubrum may form a significant mechanism for transport of nutrients from deep layers into the euphotic zone when it occurs in high concentrations in stratified waters.