Blooms of Mesodinium rubrum in southampton water - Do they shape mesozooplankton distribution?

Abstract

Blooms of the red-tide ciliate Mesodinium rubrum occur every year in Southampton Water from late May to August, peaking in abundance in July. During blooms, M. rubrum concentrates in subsurface layers during the day and and also undergoes diurnal vertical migration; this results in supersaturation in the near-surface water and significant deoxygenation at lower depths. During the period of M.rubrum bloom, the mesozooplankton community of Southampton Water is typically dominated by nauplius larvae and calanoid copepods. The true nature of M. rubrum's impact on mesozooplankton and fish communities is equivocal, with contrasting reports of either bloom- associated avoidance or mortality by zooplankton or fish, or both, or alternatively grazing by micro- and mesoplankton on M. rubrum. A temporal sequence of zooplankton hauls conducted between mid-May and early August indicated that vertical profiles of mesozooplankton composition and abundance typically exhibited lower near-surface densities, with numbers increasing significantly below 3 m. During blooms, this pattern occurred both inside and outside patches, and therefore the reduced near-surface zooplankton numbers are considered to be caused primarily by turbulent mixing rather than by the associated oxygen supersaturation induced by M. rubrum. Equally, no clear pattern of mesozooplankton density differences was observed at lower depths, associated with the low concentrations of oxygen caused by M. rubrum respiration. The results of the sampling programme undertaken show that M. rubrum does not significantly exclude zooplankton from the vicinity of patches, nor is there any associated substantial increase in zooplankton numbers. It is concluded that M. rubrum blooms have no immediate significant impact on the broad structure of the mesozooplankton community within Southampton Water.