Anthropogenically mediated salinization is altering the relative abundances and absolute concentrations of elements in freshwater systems. Salinization can degrade freshwater systems by altering the structure and function of phytoplankton communities thereby altering the flow of basal food resources through aquatic food webs. However, physiological mechanisms driving taxon-specific responses remain poorly connected to ecosystem processes, limiting our ability to predict community responses to salinization.To bridge this gap, we conducted mesocosm experiments comparing growth rates, sodium homeostasis, and cellular C:N:P ratio plasticity for two cosmopolitan phytoplankton genera, Dolichospermum (prokaryotic, cyanobacteria) and Scenedesmus (eukaryotic, green algae) across NaCl gradients. We hypothesized that (H1) low level NaCl additions would stimulate algal growth, with further additions would depress growth, (H2) both algal taxa would exhibit non-homeostatic responses to NaCl, and (H3) growth declines from NaCl would cause algal C:N:P to deviate from values observed in optimal conditions. (H1) Contrary to our hypotheses, growth was not subsidized by NaCl addition and only decreased phytoplankton growth. Reduced growth rates appeared to be due to stoichiometric trade-offs between growth and homeostatic regulation. (H2) Scenedesmus showed stronger homeostatic regulation of their cellular Na content across NaCl gradients than Dolichospermum, which coincided with greater reductions in Scenedesmus growth rate and (H3) increased variability in C:N:P stoichiometric ratios. Conversely, (H2) nonhomeostatic Na regulation allowed Dolichospermum to sustain higher growth rates, which appeared to (H3) constrain variation in their stoichiometric C:N:P ratios along with their stronger physiological regulation of intracellular P storage molecule production. Differences we observed in phytoplankton growth rate align with stoichiometric theory and support field observations that indicate shifts from green algae to cyanobacteria under freshwater salinization but suggest this occurs below current threshold limits. While our findings highlight the need to consider ion effects in freshwater management, the development of mechanistically informed policies should consider the systems-level interactions among organisms and their environment of all elements