Recurrent red-tides in the Southampton Water estuary caused by the phototrophic ciliate Mesodinium rubrum

In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliate Mesodinium rubrum (=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophyll a (chl a) of over 100 mu g/l, cell numbers of M. rubrum of over 1 x 10 super(3)/ml, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH super(+) sub(4) from the water column, whereas some NO super(-) sub(3) persisted, presumably maintained by freshwater input. NO super(-) sub(3) and NH super(+) sub(4) gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass of M. rubrum appeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a/m super(2) within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order ( approximately 15 mu mol photons/m super(2)/s) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20-30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed.