Drifts and Blooms

Large accumulations of cyanobacteria and algae can result from either drifts or blooms.


During storms and windy periods, cyanobacteria and macroalgae may be ripped from the rocks, seagrasses and other structures on which they grow. These unattached thalli drift in the sea until they are stranded in shallow water and on the shore. Algal drifts are typically comprised of 10 to 50 species of green, red and brown seaweeds of different growth forms including large frondose or leathery species. In contrast, algal blooms are dominated by one or two filamentous or sheet-like species.


High levels of the nutrients, nitrogen and phosphorus, may trigger algal blooms. Some blooms are natural events such as phytoplankton blooms, fuelled by the episodic upwelling of nutrient-rich water from the ocean floor into the well lit surface waters. However, other blooms, which have increased in intensity and frequency over the last five decades are attributed to human-related activities. Altered land use practices in catchments, development in the coastal zone, groundwater seepage and sewer and stormwater discharges have increased nutrient loading into estuaries and bays, causing excessive algal growth.

All blooms cause environmental degradation by depleting dissolved oxygen to levels too low for the survival of fish and aquatic invertebrates, disrupting marine food webs, altering marine community structure and decreasing biodiversity with some organisms unable to tolerate bloom conditions. Many slow growing algal species (eg Sargassum species) are lost from habitats experiencing high nutrient loading and blooms. Some bloom-forming species are toxic to humans or other animals, causing skin conditions, asthma-like symptoms, diarrhoea, and in extreme cases, death.

Relatively few cyanobacterial and algal species form blooms. Most bloom-forming species are characterised by high nutrient uptake rates and high growth rates due to their filamentous and sheet-life growth forms. These species are tolerant of varying and often extreme environments associated with the bloom, including dramatic fluctuations in nutrient levels, pH, and turbidity, low dissolved oxygen levels from the degradation of organic matter and high levels of hydrogen sulphide, ammonia and other toxic metabolites. Being an ancient life form on Earth, cyanobacteria evolved in extreme and fluctuating environments, and now often become the unchallenged, dominant organisms in polluted environments.