Net population growth of some dinoflagellates is inhibited by fluid shear at shear stresses comparable with those generated during oceanic turbulence. Decreased net growth may occur through lowered cell division, increased mortality, or both. The dominant mechanism under various flow conditions was determined for the red-tide dinoflagellate Lingulodinium polyedrum (Stein) Dodge. Cell division and mortality were determined by direct observation of isolated cells in 0.5-mL cultures that were shaken to generate unquantified fluid shear. Larger volume cultures were exposed to quantified laminar shear in Couette-flow chambers (0.004-0.019 N.m(-2) shear stress) and to unquantified flow in shaken flasks. In these larger cultures, cell division frequency was calculated from flow cytometric measurements of DNA.cell(-1). The mechanism by which shear inhibits net growth of L. polyedrum depends on shear stress level and growth conditions. Observations on the isolated cells showed that shaking inhibited growth by lowering cell division without increased mortality. Similar results were found for early exponential-phase cultures exposed to the lowest experimental shear stress in Couette-flow chambers. However, mortality occurred when a late exponential-phase culture was exposed to the same low shear stress and was inferred to occur in cultures exposed to higher shear stresses. Elevated mortality in those treatments was confirmed using behavioral, morphological, and physiological assays. The results predict that cell division in L. polyedrum populations will be inhibited by levels of oceanic turbulence common for near-surface waters. Shear-induced mortality is not expected unless shear-stress levels are unusually high or when cellular condition resembles late exponential/stationary phase cultures.
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