Continuous cultures of mixed phytoplankton populations were subjected to gradual salinity changes. The phytoplankton was exposed to defined regimes of high, low or fluctuating salinity, in artificial brackish media. In several experiments ammonium was the limiting nutrient. A rapid selection process was observed in natural phytoplankton assemblages. A gradual freshening caused the dominance of Chaetoceros mülleri at low salinity (S=5). Skeletonema costatum became dominant at higher, constant or fluctuating salinities, accompanied by Ditylum brightwellii in low cell numbers. Ammonium limitation was not achieved in this experiment. Competition for ammonium was studied in a second experiment, using an inoculum of two species. At S=18 D. brightwellii became the dominant species in this competition. A minor shift towards S=15 reversed the affinities for ammonium, and S. costatum won the competition. At S=8 S. costatum had the highest affinity for ammonium after a period of osmotic adjustment. Ammonium became limiting when salinities arrived at constant meso-or polyhaline levels. Both species were able to grow in fluctuating osmotic environments (S=5 to 19). The growth of D. brightwellii decreased below S=8 and after repeated variations of the salinity. These salinity fluctuations suppressed growth and ammonium uptake of both species, thus preventing ammonium limitation. These competition experiments indicate that unstable salinity may stimulate the mass development of S. costatum in brackish lakes.
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