Now before we explain HABs, we first need to understand what algae are. Although commonly thought to just be aquatic version of plants, in truth they are far from that.

Algae are an extremely diverse group of organisms and despite their similarities that have led scientists of times past to classify them together, not all of them share a common ancestor (referred to as a ‘polyphyletic group’ science-y words). For instance, one category of cyanobacteria, aren’t even eukaryotes like higher plants and animals. Which would mean that other categories such as green and red algae are actually more closely related to us than they are to cyanobacteria!

Despite their differences, their biggest shared trait is their ability to utilise light to fix carbon into more complex substances. This ability has allowed algae to thrive in many environments, from plankton and seaweed in the oceans to lichens in terrestrial environments. Even within a monophyletic group (i.e. sharing a common ancestor), algae forms are highly variable. Green algae can mean anything from unicellular Chlamydomonas to Ulvaceae (sea lettuce) which are edible resemble higher plants.

Without getting too caught up in the details, for the purposes of explaining pollution in this module, we can categorise algae into microalgae and macroalgae based on their size. And this is important because they have vastly different effects on the ecosystem. When microalgae decide to party, it’s a bad time for anyone or anything else nearby. However, macroalgae in an ecosystem may actually mitigate the effects of eutrophication by taking up excess nitrates and phosphates (spoiler alert: this will be important in later posts 😉).

Algal blooms are a phenomenon in which microalgae replicate quickly or accumulate in large enough numbers to cause changes drastic changes in colour of a water body. When they begin causing ecological damage, which may not necessarily just be through the production of toxins, they start being referred to as Harmful Algal Blooms (HABs). HABs are usually a result of excess nutrients in water, particularly in N and P. Despite their differences from higher plants they share similar nutritional needs to algae (figure 1.). This leads to a big problem when fertiliser and agricultural runoff enters water bodies.

 

The algae that are responsible for HABs typically exist naturally in water bodies prior to eutrophication, so how is more of them a bad thing? In simple terms, an ecosystem is usually kept in balance by the community (other plants, algae and animals) and environment (pH, nutrient concentrations etc.), to ensure that all niches are filled and no single species is too abundant. Under natural conditions, the algae are kept in check by nutrient limitations. The addition of excess nutrients tips the scale in the algae’s favour, allowing them to quickly out-compete anything else in the area.

Even in the case of non-toxic algae which don’t produce poisons, their large numbers, especially near the water surface, reduce light penetration into the water column, causing anything below them such as macrophytes and macroalgae which may support other members of the community to be shaded out. Additionally, the algae cause hypoxic conditions by using up oxygen within the water, which are also needed by plants and animals, which either die or move out of the area.

The trouble doesn’t end there unfortunately. As we all know, unless we’re talking about the expansion of the universe, exponential growth is not sustainable long term: As the algae keep rapidly reproducing, the Biological Oxygen Demand of the water increases as the algae themselves also require oxygen, the hypoxic conditions eventually end up killing them too. They are then decomposed by bacteria which further deplete oxygen and unlike algae, do not replenish oxygen through photosynthesis, resulting in hypoxic ‘dead zones’ and fish-kills. After which anaerobic decomposition may also occur, producing foul-smelling gases such as methane and hydrogen sulfide.

It gets worse when you realise that all of the impacts described above, happen excluding the effects of algal toxins, which further compound the issue! Join us the next part of bad HABits, where we’ll be exploring our toxic relationship with algal poisons!

 

References

Foree, E. G., & McCarty, P. L. (1970). Anaerobic Decomposition of Algae. Environmental Science and Technology, 4(10), 842–849. https://doi.org/10.1021/es60045a005

Jewell, W. J., & Mccarty, P. L. (1971). Aerobic Decomposition of Algae. Environmental Science and Technology, 5(10), 1023–1031. https://doi.org/10.1021/es60057a005

Vidyasagar, A. (2016). What Are Algae? | Live Science. Retrieved October 2, 2020, from https://www.livescience.com/54979-what-are-algae.html