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Impacts of Algal Blooms in Freshwater Ecosystems

Algal blooms can be detrimental to aquatic life

Algal blooms frequently occur in coastal ecosystems, and in slow flowing rivers and lakes. The impact on these environments can be quite dramatic, and is often associated with high fish mortality and loss of other aquatic life. But what are the causes and consequences of algal blooms?

Environmental Conditions that Stimulate Algal Blooms

Algal blooms develop during favorable conditions that allow algae cells to rapidly multiply due to an increase in the availability of nutrients (nitrates and phosphates) to fuel their growth. This usually occurs in summer when there is increased light intensity – another factor necessary for their development. When there is a combination of intense light availability and abundant nutrients in freshwater systems, the algae cells take advantage of the situation and rapidly multiply to form a thick luminous green carpet that rapidly engulfs the surface of the waterbody.

Nutrients are essential for both plant and animal growth. Herbivores get their nutrients from plants, and carnivores get their nutrients from herbivores. But as primary producers, plants and algae get their nutrients from the environment in the form of nitrates, phosphates and minerals. By transforming these nutrients into carbon by harnessing energy from the sun in the process of photosynthesis, plants are able to sustain other life on earth. Nutrients occur naturally in the environment as a result of decomposition and other processes, but are  also added to the environment by man made activities. These include runoff of fertilizer and animal waste products from agricultural practices, from sewage runoff and discharge from wastewater treatment plants. When excessive nutrients flow into a waterway, the algae cells quickly multiply to form dense blooms.

 

Freshwater algal bloom

An Ecological Imbalance Can Result in Algal Blooms

However, while excess nutrients are the primary cause of algal blooms, overload of nutrients flowing into the system from human sources is not the only reason for this. It has recently been discovered that ecological imbalances can also cause algal blooms.  Recent studies have revealed that there is a link between calcium levels in aquatic systems and the frequency and intensity of algal blooms. Daphnia, a common aquatic invertebrate that is a primary grazer in aquatic ecosystems, requires calcium to reproduce. When calcium is not available Daphnia populations decline, and are replaced by another grazer Bosmina, that consumes algae at a much slower rate than Daphnia. As the key grazer is replaced by a less ferocious grazer that is not able to control the algae populations as effectively, the algae quickly flourish to form extensive blooms. This can have massive implications for the whole trophic food chain in lake ecosystems.

Similarly, the recent massive algal bloom experienced in Lake Erie has been partly blamed on trophic imbalances that have risen as a result of introduced invader zebra- and quagga mussels. These mussels feed on phytoplankton but seem to prefer other algae over Microcystis, a toxic blue-green algae that is commonly found in lakes. As the mussels consume the algae other than Microcystis, the Microcystis cells are allowed to outcompete their competition and multiply to form dense algal mats. In addition, the mussels add nutrients to the system through their waste products, which further enhances the growth of Microcystis.

Microcystis and other blue-green algae very often contain taste-and-odor compounds and toxins that can effect water quality; can be lethal to fish, wildlife, and domestic animals; and can pose a health risk to humans if exposed to the toxins through recreational activities or from consuming contaminated fish or shellfish. Blooms of toxic algal species are commonly referred to as harmful algal blooms (HABs). Depending on the type of toxin present in the algae, these toxins can either attack the nervous system or the liver of animals and humans that are exposed to them. Symptoms of exposure through  contract and inhalation include breathing difficulties; eye, nose and throat irritations, skin rashes and other allergic reactions. If toxins are ingested it can result in toxicity of the kidneys, liver and nervous system. Symptoms of exposure through ingestion include diarrhea, vomiting, abdominal cramps, excessive salivation, tingling fingers, sensations of feeling numb or dizzy, breathing difficulties, and can be fatal. Toxic algal blooms can kill fish, shellfish and aquatic invertebrates, and as these toxins are stored in the tissue of organisms, they can also be fatal to animals higher up the food chain that consume contaminated prey.

However, even non-toxic algae can be fatal to aquatic wildlife. The algal cells rapidly grow and reproduce while nutrients and light are abundant, but once the nutrients in the surface water are depleted they have to move deeper to find nutrients to sustain them. Eventually there is insufficient nutrients in the surface layers, and as light cannot penetrate the deeper water layers, there is insufficient light in the deeper, nutrient rich layers. When this point is reached the algae cells die off and sink to the bottom of the lake where they decompose. Oxygen is stripped from the water during the decomposition process, resulting in water that is low in oxygen, which can result in mass mortality of fish and other aquatic organisms that require oxygen to survive.

Will Algal Blooms Occur More Frequently in the Future?

Algal blooms are likely to occur more frequently in the future as a result of climate change, more intensive agricultural activities, ecological imbalances, or as a result of invader species changing ecosystem dynamics. We therefore need to take care to protect our family and pets from exposure to harmful algal blooms and contaminated food in order to prevent the health risks associated with poisoning or exposure to toxins.

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