Algae

Algae encompass a diverse group of plants. The majority are microscopic (microalgae), but some larger forms (macroalgae) resemble higher plants in size. The distinction between algae and higher plants relates to a lesser degree of cell differentiation in the former. Algae include both eukaryotic (membrane-bounded internal cell structures) and primitive prokaryotic forms. For the purposes of this text, algae in five divisions are recognized: Chlorophyta, Chromophyta (seven classes), Rhodophyta, Cyanophyta, and Charophyta. Some groups contain exclusive or varied proportions of photosynthetic pigments, which impart a range of colours to particular algae in nature, through green, red, yellow and brown. The world of algae contains a vast range of cell shapes and sizes, from non-motile and motile single cells to complex three-dimensional, multicell forms. These latter colonial forms may be visible to the naked eye. Colonial forms have a greater degree of complexity in shape, ranging from branched filaments to clumps of undifferentiated cells. Some may be characterized by a fixed number of cells, while others comprise variable mass and shape, with breakaway cells forming new colonies. Interestingly, some colonial forms may lose their tendency for cellular aggregation when maintained in laboratory cultures. This speaks to the role of species interaction, competition and predation, etc., in the ecology and physiology of algae in nature. The same is true of pigment composition, which may shift in response to light regimes, temperatures, and available nutrients. The nutritional status and form dimensions of algae may influence their quality as a food source to grazers.

Biologists further differentiate algae on the basis of where they are most commonly found. The major split is into planktonic (or floating/drifting) types, and benthic (bottom dwelling) types. Benthic algae can only grow as deep as light penetration permits, whereas viable planktonic forms can occur throughout the water column, well below the boundary of critical light availability, due to mixing of the water by various turbulent forces. The species composition and abundance of algae is very cyclical in nature. Typically, one sees marked seasonal oscillation in abundance and composition of algae. A series of complex interactions sets the stage for the ecology of algae, including among others: water temperature; availability of nutrients such as carbon, phosphorus, nitrogen, silica, iron, and vitamins; salinity light; water depth and mixing regime; and Zooplankton grazing. Base conditions vary seasonally, and with the type of rocks, soils, vegetation and drainages found across the landscape.

The full range of inland water types can be found in Saskatchewan. A transition of conditions extends from northern cold-water systems in the boreal Shield, to cool and warm-water systems in the southern Prairies. This cline of inland water types means that species occurrence and abundance of algae is highly varied. In cases of high nutrient availability, algae may reach nuisance densities, which may interfere with desired uses such as drinking water sources, fisheries, and recreation. Many “problem” species do well in nutrient-rich warm waters, especially cyanophytes (blue-green algae, also known as cyanobacteria). Some of these species can supplement their nitrogen requirements by fixing atmospheric nitrogen - a considerable advantage. They can also regulate their buoyancy, thereby optimizing light exposure. Bloom-forming cyanophytes may produce a range of toxic compounds acting on the liver, kidneys, and nervous system of higher animals. High levels of algae may lead to wide diurnal swings in dissolved oxygen concentrations, as well as oxygen depletion during bloom die-offs.

Ken Scott

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Further Reading

Wehr, J.D. and R. Sheath (eds.). 2003. Freshwater Algae of North America. San Diego: Academic Press; Wetzel, R.W. 2001. Limnology. San Diego: Academic Press.