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Phytoplankton and Their Role in Oceanic Ecosystems

Updated: May 6, 2023


Living by the Southern California coast, you may have heard about plankton, the tiny, microscopic creatures that drift about at sea. Different species of plankton can be divided into two general groups: plant-like and animal-like plankton. Today, we will be discussing the plant-like category of plankton called phytoplankton. They get their name, “phyto,” meaning “plant,” due to their plant-like characteristics. We’ll dive deep into their significance to oceanic ecosystems and how we can protect these microscopic organisms.

Diatoms under a microscope. Photo courtesy of ​​Gordon T. Taylor, Stony Brook University.

Similarly to plants, phytoplankton are primary producers, meaning they create their own food through photosynthesis. However, they differ from plants in that they are protists. They typically float along the surface of the water, soaking in as much sun as they can. They play a key role in ecosystems and are an essential food source for a variety of marine species such as zooplankton, crustaceans, and small fish. There are two main types of phytoplankton: diatoms which have hard shells made with interlocking parts and dinoflagellates which have a shell and a whip-like tail called a flagella that allows them to move around in the water. Despite their small size, phytoplankton play a significant role in the production of the world’s oxygen. Though they make up less than 1% of the world’s photosynthetic biomass, they produce more than 50% of the world’s oxygen, even more than all of the terrestrial plants on Earth!

Phytoplankton are delicate creatures, which makes them an important indicator of water health. We are able to detect how healthy our oceans are by looking at populations of plankton. They are most comfortable in warm, alkaline waters with a pH at around 8.2. The Redfield ratio provides an ideal element proportion of 106 carbon:16 nitrogen:1 phosphorus. This gives an idea of the nutrient levels that are most comfortable for phytoplankton in the ocean. In these ideal conditions, their populations will maintain themselves while other conditions can cause their populations to fluctuate. Changes in temperature, acidity, and nutrient availability all have an effect on plankton populations. Although plankton are important to our ecosystems, sometimes they can grow out of control through what are called harmful algal blooms (HABs).

Algal blooms can be seen on satellite images as swirls of blue and green in the ocean. Photo courtesy of NASA Earth Observatory.

HABs can be caused by a number of factors: a nutrient increase from fertilizers and water runoff, an increase in temperature, and even some of the effects caused by climate change like rising carbon dioxide levels in the atmosphere. HABs are detrimental to our oceans because they produce harmful toxins and they can create what are called “dead zones” in the water. These are formed when a thick layer of algae forms at the surface of the water, dies off and sinks to the bottom of the ocean. This dead organic matter is then broken down by bacteria through decomposition, a process that involves the intake of oxygen and the release of carbon dioxide. Although this is a natural process, the excess amount of organic matter that needs to be broken down uses more oxygen in the water than normal. As a result, these low oxygen levels suffocate surrounding marine life and a dead zone is created. A major example of this is located in the Gulf of Mexico. According to the National Oceanic and Atmospheric Administration, nutrient runoff from human activities near the Mississippi watershed has significantly increased algae populations, leading to a dead zone in the area that occurs every summer (NOAA).

It's important to protect phytoplankton because if they were to die off, we would lose many of our photosynthesizing organisms. This would lead to an increase in the carbon dioxide in our atmosphere, accelerating the effects of climate change. This would also significantly reduce the amount of oxygen present on Earth which would likely lead to mass deaths of both animals and humans. Lastly, because every creature in the ocean either eats plankton or eats other organisms that eat plankton, a significant amount of life in the ocean would die off as well (INSH). This leads to less food for us and less jobs for people who depend on them for their livelihood. If phytoplankton bloom and become too numerous, they will produce toxins and kill off surrounding life, leading to a similar issue with having less food available for other organisms.

Choosing organic helps to prevent harmful algal blooms by reducing synthetic fertilizer runoff. Photo courtesy of Markus Spiske.

Although changes in phytoplankton populations will always be natural processes, there are ways we can help to mitigate them. One major way we can help is by reducing or eliminating our use of synthetic fertilizers in our gardens and using compost instead. This would significantly reduce the amount of excess nutrients that are washed away and end up in the ocean. Another way we can help is by taking our cars to the car wash rather than washing them ourselves in the driveway or on the street. Many detergents contain nitrogen and phosphorus which can lead to nutrient overload and pollution if they are dumped into local waterways. Commercial car washes are required to dispose of their wastewater properly so you don't have to worry about your suds being washed into the ocean (EPA). Lastly, a great way to help them is to reduce our carbon emissions by driving our cars less often, whenever possible. Excess carbon dioxide gets absorbed by the ocean which negatively affects marine life. As of now, it appears that phytoplankton levels have been decreasing over time along our coast. Being mindful of these organisms and taking steps to protect them ensures that they are able to maintain their populations at a healthy level.

Environmental stewardship and advocacy is important and OC Habitats as well as other organizations such as OC Coastkeepers, Surfrider, Laguna Ocean Foundation and the Ocean Defenders Alliance are here to help. Become involved by volunteering at one of these organizations or donating much needed funds. OC Habitats hosts a monthly restoration event where you learn more about our local ecosystems and take part in giving these sensitive habitats the care they need.

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