A comparison of cyanobacterial bloom dynamics in freshwater, estuarine and marine environments

Abstract

Cyanobacterial blooms are diagnostic of accelerating eutrophication in freshwater (FW) and estuarine (E) ecosystems. In full salinity marine (M) ecosystems blooms are more closely controlled by physical than trophic conditions. FW and E blooms are initiated and exacerbated by (1) excessive nutrient (N and P) loading. (2) high surface water temperatures (> 20°C). (3) persistent water column stratification. (4) long water residence time. and (5) organic matter enrichment. Physical–chemical forcing features can act synergistically to control bloom potential. Factors additionally favouring cyanobacterial blooms include (1) low dissolved inorganic carbon concentrations (especially in FW), (2) low total and dissolved N: P ratios. (3) close metabolic coupling between sediments and water column, and (4) high irradiance, long daylength. FW and E blooms are closely controlled by hydrodynamic and salinity alterations. The ability of nuisance taxa (nuisance characteristics include hypoxia and anoxia, toxicity, odour and taste problems and food web alterations) to dominate these waters can be traced to exploitation of physical-chemical ‘extremes’ in nutrient loading, stratification and irradiance. In subtropical and tropical oligotrophic, N-deficient oceanic environments, blooms are dominated by N2-fixing genera (Trichodesmium, Rhizosolenia–Richelia) that often accumulate as buoyant surface-dwelling ‘mats’ under low wind stress (i.e. low turbulence and non-mixed) conditions. Nuisance conditions are not encountered in the open ocean, largely because nutrient limitation (Fe, P) and frequent episodes of wind mixing preclude massive accumulations of biomass over time scales exceeding a few days. In all environments, the ability to rapidly (within minutes to hours) migrate to high irradiance near-surface waters by gas vacuolation can confer an advantage over less-motile, non-cyanobacterial taxa. Bloom genera exhibit adaptive biochemical, physical and biotic mechanisms, which promote dominance under anthropogenically-stressed conditions. These include the following: N2 fixation, cellular nutrient (P, N) storage, metal (Fe, Cu) sequestration by siderophore chelators, mucilage and sheath production to counter desiccation, buoyancy regulation, photoprotection by carotenoid accessory pigments, and consortial associations with other microorganisms and higher plants. Management strategies aimed at arresting problematic blooms should consider and manipulate the linkages and synergistic interplay of physical, chemical and biotic variables favouring nuisance genera.