Effect of salinity on growth, cell size and silicification of an euryhaline freshwater diatom: Cyclotella meneghiniana Kütz

1 - The rivers transport plankton seawards which finally enters estuaries and often experiences lethal osmotic stress along an increasing salinity gradient. The tolerance of river phytoplankton to salinity increase can modify the nutrient cycles in estuaries. In this experimental study based on two culture experiments, the effect of salinity on growth, cell size and silicification of the euryhaline freshwater diatom Cyclotella meneghiniana was studied. 2 - Diatoms were first grown in batch cultures at 8 different salinities ranging from 0 to 33. The maximal growth rate (0.92 d-1 ±0.04) occured at salinity 18 and decreased continuously from this optimum toward higher or lower salinities reaching 0.44 (±0.06) and 0.64 d-1 (±0.13) at salinities 0 and 33 respectively. 3 - In a second experiment two continuous cultures of the diatom were studied at salinities of 1 and 30. At equilibrium, the difference in the amount of Si per cell between the two cultures was due to a decreased cell height at salinity 30 and did not reflect any change in cell silicification on a surface basis. 4 - This work reveals the importance of accounting for the effect of salinity on freshwater euryhaline diatoms in the study of the Si cycle in the river-sea transition zone.