THE SEASONAL ABUNDANCE OF THE PHOTOTROPHIC CILIATE MESODINIUM-RUBRUM IN SOUTHAMPTON WATER, ENGLAND

The abundance of the marine phototrophic planktonic ciliate Mesodinium rubrum was monitored throughout an annual cycle at two stations in the Southampton Water estuary. Seasonal changes in the concentration of nitrate, ammonia and phosphate were monitored both at the inner estuary station (NW Netley) and outer estuary station (Calshot). Nutrient levels in the winter were similar at both stations, and were diminished during sequential diatom blooms dominated initially by Skeletonema costatum and then by Rhizosolenia delicatula. Nitrate was reduced to a seasonal minimum in the outer estuary following these spring diatom blooms, but in the inner estuary was sustained >500 mug l-1 until the onset of the M.rubrum bloom. During the development of a visible red tide of M.rubrum in June/July at NW Netley, nutrient concentrations were considerably reduced. Cell numbers of M.rubrum varied between 2 and 3 cells ml-1 during winter to >400 cells ml-1 during the bloom at NW Netley, whereas at Calshot cell abundance did not increase above 25 cells ml-1 at any time of the year. At NW Netley, dense accumulations of the ciliate occurred over restricted depth intervals during the bloom and possible factors influencing the observed vertical distribution of cells are considered.