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Algal Blooms

Author: Joshua Payne

Editors: Hwi-On Lee, Ethan Tai, Serena Tsao

Artist: Helen Gong


Algal blooms—rapid growths of algae and/or cyanobacteria in water—are crucial parts of ecosystems. They occur in all types of natural bodies of water, including freshwater, saltwater, and brackish environments. Specifically, they are prominent in lakes, reservoirs, rivers, ponds, bays, and coastal waters. 

Several factors can contribute to the growth of algal blooms. For example, algal blooms grow in response to excess nutrients, such as phosphorus and nitrogen, which are often components of fertilizer, sewage, poop, and runoff from cities and industrial buildings: this is called eutrophication, the process by which lakes and ponds receive excess nutrients from the land, causing plants and algae to grow and die quickly due to their shorter life spans. Harmful algal blooms are more likely to grow in warm, low, and slow-moving bodies of water, such as during a drought. Thus, they occur more frequently in the warmer months of summer or fall, but can occur at any time of year. 

Other water conditions—such as turbidity and pH—also significantly impact algal bloom growth. Water with low turbidity allows greater light penetration, aiding the growth of algal blooms. Changes in local ecology—the interactions among living things, with the environment, and among themselves—can also affect the growth of algal blooms. Climate change can also promote algal growth, extending their range and increasing the frequency and duration of their appearances, thus intensifying their severity. 

Algal growth is an essential component of the environment, but unchecked proliferation can be detrimental to the local ecosystem. Algal blooms can harm the environment and ecosystem by producing toxins and harmful gases, increasing density, and depleting oxygen. People and animals are harmed by toxins in water or food, or by gases such as methane and hydrogen sulfide. 

Additionally, algal blooms can become so dense that it prevents plants and animals in the water from getting the sunlight needed to survive. These dense blooms can also clog the gills of fish, shellfish, and other animals, preventing them from breathing. Even more, when a bloom dies off, the decay process may deplete all the oxygen in the water, suffocating other living beings. This is called a dead zone—a body of water with very low oxygen levels, rendering it uninhabitable for most marine life. 

Luckily, one solution to eliminate—or at least mitigate—the growth of harmful algal blooms is by limiting nutrient pollution in water. Even nutrients from fertilizer in a home and yard can make their way into lakes, rivers, and oceans during heavy rainstorms. To do this, the recommended amount of fertilizer should be used (as indicated by the product’s instructions). Another solution is to maintain septic systems—wastewater treatment systems that are not connected to municipal sewage systems. 

In the Baltic Sea, increasingly large algal blooms are mainly caused by eutrophication, which means that large amounts of nutrients are released into the waters surrounding the Baltic Sea or directly into it, and eventually flow into the Baltic Sea. The Baltic Sea is eutrophic—a term used to describe a nutrient-rich body of water supporting a dense plant population that can cause animal death by depriving them of oxygen when it decomposes—due to emissions from land-based activities. 

Additionally, climate change, driven by both human activity and natural processes, has intensified algal blooms and disrupted the Baltic Sea’s ecosystem. For instance, in Sweden, agriculture is the largest source of nutrient leakage, but wastewater treatment plants, industries, and individual sewage systems also contribute. In the Baltic Sea, nitrogen emissions are unfortunately increasing up to the Åland archipelago. The poor water quality on the coast and in the archipelago is mainly due to Swedish emissions. 

The spread of algae depends on sea currents, light, water temperature, winds, and nutrient supply. With climate change, algal blooms are expected to occur more frequently, become more intense, and spread to more areas in the Baltic Sea. Nutrient leakage has decreased in recent decades, but it takes time before we see the positive effects of reduced emissions into the sea. Much of the nutrients are stored in the seabed sediments from past emissions. The Baltic Sea is an inland sea with slow water exchange. It takes more than 30 years to replace all the water. 

Cyanobacteria are classified as bacteria but resemble algae. They thrive best when nitrogen in the water is depleted after the spring bloom and when phosphorus remains in the water. High water temperatures and sunny, calm weather make blooms extra large and easy to identify. The algal blooms in the Baltic Sea are seasonal, occurring twice per year: once in spring and once late into summer. Blue-green algae growing on the water’s surface is usually the cause of summer algal blooms.

Algae thrive when ocean waters are warm, and winds are calm, which makes it seasonal in the Baltic Sea. Algal blooms are mainly formed by Nodularia spumigena and Aphanizomenon, which were described and published between 1887 and 1938. Long-term information on possible changes in cyanobacterial blooms in the Baltic Sea, formed mainly by Nodularia spumigena and Aphanizomenon sp., was sought in published records from historical (1887–1938) and modern (1974–1998) phytoplankton datasets. Old and new sampling methods and fixatives were tested to improve the comparison of data collected and analyzed in different ways. 

A hundred years ago, plankton was mainly of interest as a source of fish food; eutrophication problems were only locally reported along the coast, mainly in the southern half and the receiving waters of larger cities. There were few recordings of open-sea blooms before World War II. Abundances of Nodularia spumigena and Aphanizomenon sp. were low in the old material, and 137 summer samples from 1887–1938 showed no peak abundance. High abundances are typical in the new material, and the range of the numbers of both taxa has increased markedly relative to the old material. Since the 1960s, cyanobacterial blooms have been common in the open sea of both the Baltic proper and the Gulf of Finland, indicating a high nutrient availability.

Some algae are beneficial to marine life because they provide a food source. Cyanobacteria fix nitrogen, which naturally adds nutrients to the ecosystem. Generally speaking, algae don’t offer many benefits beyond the sources of food they provide for the environment in bits and pieces. In fact, large amounts of algae block sunlight from the plants beneath them, preventing them from continuing photosynthesis. This also reduces biodiversity, so the area doesn’t have as many nutrients as it should. When algae start to die off near the end of their life cycle, they decompose, creating a dead zone with low oxygen levels that sea animals can’t survive in. Not only are these adverse effects, but cyanobacteria also release harmful toxins.

Some proposed ideas for human intervention include reducing nutrient input, as much of the excess waste ends up in the ocean, causing greenhouse gas levels to rise. If proper fertilizer practices were implemented, with excess fertilizer recycled or cut off, it would lower emissions. People have also looked into natural filters, such as filter ponds, to place in areas with high nutrient levels to capture some of those nutrients before they reach the ocean.

Citations:

Centers for Disease Control and Prevention. “Harmful Algal Blooms: Contributing Factors and

“Use Floating Leaf Disks to Study Photosynthesis.” ScienceBuddies,

WWF Baltic. “Algae Blooms Have Arrived in the Baltic Sea.” WWF Baltic,

Accessed 5 Dec. 2025.

Finni, Terttu, et al. “The History of Cyanobacterial Blooms in the Baltic Sea.” AMBIO: A Journal of

the Human Environment, vol. 30, no. 4, 2001, pp. 172–178, https://bioone.org/journals/ambio-a-

“Summer Bloom in the Baltic Sea.” NASA Earth Observatory, 5 July 2005,

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