Response of Tetraselmis suecica to nutrient and grazer manipulation

Three methods of algal quantification (direct cell counts, chlorophyll a ex traction, in vivo fluorescence) were used to evaluate the response of the u nicellular green flagellate Tetraselmis suecica to nutrients and grazers. N utrient enrichment enhanced total cell counts, chlorophyll a concentration and in vivo and DCMU-fluorescence. Photosynthetic efficiency was reduced in the complete F2 medium as indicated by the high level of in vivo fluoresce nce, whereas photosynthetic efficiency was increased by the introduction of mussels to the F2 medium. The addition of mussels significantly increased the proportion of non-motile cells, but did not reduce the total cell count . The effect of mussel grazing on algae could be underestimated if only tot al cells were counted or only the chlorophyll a concentration was measured. The results indicate that these three methods measure different properties of an algal culture and are complementary to each other in assessing the q uality and quantity of an algal population. Direct algal counting offers a reliable numerical assessment for cell population abundance. Chlorophyll a concentration was closely correlated to the total cell count. In the presen ce of mussels, in vivo fluorescence did not correlate with either algal cel l counts or chlorophyll a concentration, indicating that the measurement of in vivo fluorescence may be misleading for estimating algal abundance unde r different culture conditions.