Algal Blooms in Freshwater Systems: Causes, Impacts, and Cyanobacteria Focus

The term algae encompasses a diverse group of organisms unified by the presence of the photosynthetic pigment chlorophyll-a. They are further classified into distinct taxonomic groups based on cell wall composition and photosynthetic storage compounds. This broad category includes both eukaryotic organisms, which possess membrane-bound organelles like a nucleus and chloroplasts, and prokaryotic organisms, which lack such structures. Cyanobacteria, often called blue-green algae, are prokaryotic bacteria that perform photosynthesis using chlorophyll-a, similar to eukaryotic algae and higher plants. As primary producers, they are foundational to aquatic food webs, utilizing inorganic nutrients, carbon dioxide, and sunlight for growth.

Algae inhabit all aquatic environments, from freshwater to marine systems, and exist either as planktonic organisms floating freely in the water column or as periphytic organisms attached to substrates like rocks or plants. When algal growth surpasses its consumption by grazers or other loss factors, excessive accumulations can form. These proliferations are termed blooms for planktonic varieties and benthic or floating mats for periphytic types. Only a select number of algal and cyanobacterial species are responsible for these events, with most blooms occurring as free-floating masses in freshwater, estuarine, and marine systems.

A subset of these events is classified as harmful algal blooms (HABs), with those involving cyanobacteria specifically termed cyanoHABs. Other common names include red tides and brown tides. These blooms can significantly impair water quality by releasing compounds that cause unpleasant tastes and odors and by depressing dissolved oxygen concentrations. Most critically, some blooms produce potent toxins that pose serious risks to fish, wildlife, and human health, leading to widespread ecological and public health concerns.

A frequent and damaging consequence of algal blooms is hypoxia, or the severe depletion of oxygen in the water, which can cause mass mortality of fish and invertebrates. While photosynthetic organisms within the bloom produce oxygen during daylight hours, they continuously consume it through respiration. During the night, when photosynthesis ceases, oxygen depletion accelerates and can become severe if the bloom's biomass is high enough to affect the entire water column. Furthermore, nonphotosynthetic bacteria associated with the bloom actively contribute to this oxygen consumption, particularly as the bloom declines, "crashes," or settles out of the sunlit photic zone.

To provide a focused examination, this article concentrates on algal blooms within freshwater systems, with specific emphasis on cyanobacteria. Understanding these events requires knowledge of the organisms involved and their seasonal dynamics. In many temperate freshwater bodies, a natural seasonal succession dictates which algal groups dominate. During colder months, lakes and reservoirs are typically dominated by diatoms and chrysophytes (Figure 1), which can form nuisance blooms that cause taste, odor, and filter-clogging issues, though they are generally non-toxic.

Figure 1. Two common bloom-forming organisms during winter in freshwater systems. (a) The filamentous diatom, Melosira varians; (b) the colonial chrysophyte, Dinobryon divergens. Source: Photo credit: Barry H. Rosen

As seasons change, the winter flora transitions to motile green algae, cryptophytes, and dinoflagellates in early spring. This is often followed by a summertime assemblage of green algae, which subsequently gives way to cyanobacteria. Cyanobacteria generally exhibit growth maxima at warmer temperatures, which contributes to their late-summer dominance. This shift in organism dominance is driven by a complex interplay of factors including temperature, nutrient dynamics, zooplankton grazing pressure, day length, and water column stability. Although these seasonal patterns are common, blooms of certain species, such as some from the genus Planktothrix, can sometimes proliferate under the ice during atypical seasons.

Emerging research indicates that secondary metabolites play a crucial role in determining the intricate intra- and interspecies interactions within assemblages of eukaryotic algae and cyanobacteria. These chemical interactions can influence competition and succession within the phytoplankton community. Much work remains to fully elucidate these complicated relationships among organisms in aquatic environments. For cyanobacteria specifically, population dynamics are not only about which genera or species dominate; a shift in the strains within a single species can also occur, where one strain is a toxin producer and another is not, directly determining the presence or absence of toxins in the water.