Mechanistic drivers of cyanobacterial dominance and eukaryotic algal decline in estuarine ecosystems: integrating nutrient stoichiometry and thermal stratification

Cyanobacteria in estuarine ecosystems outcompete eukaryotic algae through the synergistic interaction of nutrient flexibility, buoyancy control, and thermal stratification. Although these changes in the ratio of nitrogen to phosphorus (N: P) are usually detrimental to eukaryotic taxa, specific cyanobacteria either fix atmospheric nitrogen (N2) or can store phosphorus, and are able to grow during nutrient-limited phases. This review identifies critical low N: P thresholds and temperature ranges that trigger a shift from diatom-rich to cyanobacteria-dominated states. Rising global temperatures and stratification of water columns aid the utilization of gas vesicles and help cyanobacteria to reach the surface, where they absorb sunlight and effectively suppress competing organisms. Their high tolerance to warmer water and formation of toxins protect them against grazing pressures that normally regulate the algal populations. The resulting feedback loop involves thick cyanobacterial mats which increase surface temperatures and cause hypoxia through decomposition, which in turn leads to loss of biodiversity and collapse of existing algal-based food web. These findings provide a mechanistic predictive framework for estuarine management, emphasizing that climate-induced warming necessitates more stringent, dual-nutrient (N and P) reduction targets to prevent the onset of toxic monocultures.