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Scientists continue to document many effects of climate change on freshwaters, estuaries, and marine environments, and those effects are predicted to be exacerbated in the future. These effects, along with nutrient pollution, might cause algal blooms to become more severe and to occur more often in more waterbodies. Blooms with the potential to harm human health, pets, livestock, or aquatic ecosystems are referred to as harmful algal blooms, or HABs, and they can also have wide ranging economic impacts.
In freshwaters, cyanobacteria (microscopic photosynthetic bacteria previously known as blue-green algae due to their color) are the most common HAB producers. Some cyanobacterial HABs, or cyanoHABs, produce toxins that cause illness in humans and other animals.
HAB forming cyanobacteria thrive in warm, slow-moving water, and typically occur when water temperatures are warmer. As a result, increases in water temperature with climate change are expected to increase the magnitude and duration of cyanoHABs. Warmer water temperatures favor cyanoHABs in several ways, including:
In some regions, climate change is predicted to reduce freshwater runoff due to drought and increased evaporation. These effects will combine to increase salinity in inland waters. This can impact irrigation, harm crops, contaminate drinking water, and allow the invasion of salt-tolerant and marine algae to inland lakes. In the southwestern and south-central United States, more salt-tolerant algal HABs known as “golden algae” have regularly recurred, expanded into lagoons, and killed fish in freshwater lakes since 2000.
In other regions, increased runoff from more intense rainfalls may increase freshwater flows to coastal areas. This freshwater sits on top of saltwater because it is lighter. This can prevent mixing of oxygen rich surface water with deeper layers where the oxygen has been removed by the decomposition of HABs after they die and sink to the bottom layer. These low oxygen (or hypoxic) zones harm or kill animals that require oxygen to live, including many commercially important species.
Algae need carbon dioxide to survive. Higher levels of carbon dioxide in the air and water can lead to rapid growth of algae, especially cyanoHABs that can float to the surface of the water and use the increased carbon dioxide. Increased levels of carbon dioxide also increase the acidity of the water, which affects competition among algal species and impacts the organisms that graze on algae. These effects can combine to increase the competitive advantage of HAB species.
Climate change is affecting rainfall patterns, increasing both rainfall intensity and the duration of drought. Increased rainfall causes higher nutrient runoff from land into waterbodies fueling HABs like those observed in Lake Erie in 2011 and 2015. If followed by extended drought, waterbodies can retain those nutrients for longer, favoring HAB species that compete better under higher nutrient conditions.
Moreover, increases in extreme rainfall and subsequent increases in freshwater flows may flush large loads of nutrients, freshwater HABs and their associated toxins into estuaries and marine areas. As a result, estuarine and marine waters may be at greater risk of developing HABs or seeing an exacerbation of existing HABs.
Scientists predict that sea level could rise one meter by the year 2100. This would increase the area of shallow, stable coastal waters that provide more favorable conditions for HABs.
Coastal upwelling occurs when wind pushes warm, surface water offshore and deep, nutrient rich waters rise to replace it. Climate change is expected to alter the timing and intensity of coastal upwelling. Along the west coast of the United States, excess nutrients delivered by upwelling might lead to more algal blooms, including HABs, especially when combined with increased runoff of nutrient pollution from the land.