Ecological niches of bloom-forming cyanobacteria in brackish Baltic Sea coastal waters

Cyanobacterial blooms occur regularly during summer and fall in various freshwater and brackish environments worldwide causing harm to aquatic flora, fauna, and water quality in natural ecosystems as well as in aquaculture. They also interfere with industrial fisheries, tourism, and human health. Meanwhile, up to date limited knowledge exists about these detrimental phenomena in brackish waters, and patterns of their expansion across the freshwater-to-marine continuum remain unclear. In this study, we analyzed long-term (1972-2016) dynamics of cyanobacterial harmful algal blooms (cyanoHABs) in the brackish southern Baltic Sea coastal waters, refined the cyanoHAB criteria, and for the first time measured ecological niches of the dominant bloom-forming cyanobacteria. The ecological niches of selected cyanoHAB species were determined considering a set of abiotic parameters as niche dimensions: water temperature, salinity, and concentrations of basic nutrients (PO43-, SiO44-, dissolved inorganic nitrogen - DIN, total phosphorus - TP, total nitrogen - TN, and TN/TP ratio), with special emphasis on salinity. We hypothesized that in highly variable brackish environments, the cyanobacterial species with broader ecological niches occur and bloom more often compared to those with narrower niches. For testing this hypothesis, we evaluated the effects of ecological niche width on the occurrence of the dominant cyanoHAB species and frequency of their blooms. The results revealed a pronounced statistically significant (p < 0.05) positive correlation between salinity-niche width and frequency of cyanoHABs. Moreover, the occurrence of cyanoHAB species with wider DIN and PO43- concentration-niches was substantially higher than that of the species with narrower nutrient-concentration niches. The effect of temperature-niche width on the occurrence of cyanoHAB species, on the contrary, was negligible and statistically insignificant. The latter result concurs with the fact that most of these cyanobacteria are highly thermophilic: on average, they bloom at temperatures above 21 degrees C, and their realized temperature niches in the southern Baltic coastal waters are relatively narrow.