Response of Gonyaulax tamarensis to the presence of a pycnocline in an artificial water column

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Abstract
In nature, large concentrations of the toxic bloom-forming dinoflagellate, Gonyaulax tamarensis, are frequently observed in the vicinity of the pycnocline. In the absence of a pycnocline the organism is usually recorded near the surface, where light levels are more advantageous for photosynthesis. In this paper we examine the swimming behaviour of G.tamarensis when exposed to varying degrees of stratification and investigate whether the maintenance of a subsurface (pycnocline) population is the result of retention of the algae by a physical barrier or active accumulation of the organisms at a density interface. The study indicates that G.tamarensis cells presented with a halocline of ΔS<˜6–7‰ (occurring over a few centimeters) cross this salinity barrier and accumulate at the highest available photon flux density (˜100 μmol m−2 s−1). Cells exposed to a gradient of ΔS>˜7‰remain at the halocline (pfd=˜40 μmol m−2 s−1). However, when light above the pycnocline is attenuated by the addition of food colour to the medium, the cells cross a halocline of ΔS=10‰ and accumulate at the highest available photon flux density. In the absence of added nutrients (inorganic N and P) the organism fails to exhibit a phototactic response. Thus, the presence of a strong halocline does not represent an inpenetrable physical barrier for G.tamarensis and the development of pycnocline populations of this organism is a function of density, light and nutrient climate.