Phytoplankton is a really small algae. They are similar to plants that grow on land because they also have chlorophyll and they also need sunlight to grow. Most of the phytoplankton grows on the upper part of the water, where it can get the most sunlight. Phytoplankton also needs some nutrients, for example, nitrates, phosphates, and sulphur. They make these nutrients into fats, proteins, and carbohydrates. There are two main classes of phytoplankton: dinoflagellates and diatoms. Dinoflagellates use a whip-like tail, also called a flagella, to move in water. Their bodies are covered with shells. Diatoms don’t use flagella to move in water. They use ocean currents to travel in oceans. Diatoms bodies are also covered with shells, but these shells are made with different materials, and their structure is stiff and made up of joined parts. A lot of different creatures feed on phytoplankton. For example, shrimp, snails, and jellyfish. If there are too many nutrients in the water, then phytoplankton starts to grow a lot and forms toxic algal blooms. These blooms produce toxins that are harmful for fish, shellfish, mammals, birds, and also for people.
Zooplankton are the very little, almost microscopic animals in the water. They are usually found in the upper part of the water. Similarly to phytoplankton, zooplankton is also a weak swimmer, and they mostly use currents to move in the water. Zooplankton is divided into two bigger groups: holoplankton (they are permanent members) and meroplankton (temporary members). Temporary members mean that at one phase in their lives, they are the same size as zooplankton, for example, sea urchins, sea stars, crustaceans, most of the fish, and so on. Holoplankton remain plankton for their entire lives. Similarly to phytoplankton, zooplankton is the base component of aquatic food webs.
Most of the time, when talking about phytoplankton and zooplankton, they are referred to just as plankton. Plankton are the most economically important organism, and they are in every aquatic food web. Phytoplankton are primary producers. They either produce food by chemosynthesis or photosynthesis. Phytoplankton produces a large amount of oxygen and is also an indicator of the health of the ocean. Zooplankton feeds on phytoplankton. Plankton is also part of the Earth’s carbon cycle. If phytoplankton dies, then it sinks to the bottom of the ocean, and with that, the carbon that it absorbed in its lifetime also sinks along with it.

There are hundreds of different species of plankton, so it’s unlikely that all of the species will be affected by climate change. And also, there are not enough researchers on this area of how climate change will affect the plankton. Climate change will affect the ocean currents, and plankton depends on the currents to move. It also lowers the amount of nutrients that are brought up from the bottom of the oceans. This will result in a smaller amount of phytoplankton. This leads to the ocean’s ability to absorb less carbon dioxide from the atmosphere, which worsens climate change.
There is a study that shows warming temperatures will change the phytoplankton from carbon sequesters to carbon emitters. Some of the phytoplankton are mixotrophs, which means that they can produce their own energy from the sun or they eat other organisms. If they eat other organisms, then they end up emitting more carbon than they absorb.

The biggest issue when it comes to plankton and climate change is the large number of phytoplankton blooms. There are several reasons why phytoplankton starts to bloom. For example, more nutrients come from the fertilizers that are used on the fields, the upwelling, warmer water, still water, and low turbidity. A recent study showed that between 2003 and 2020, coastal marine blooms increased by 13% and were also 59% more frequent. Algal blooms are toxic to marine life.
It is still unclear if climate change is the reason why there are more algal blooms or not. But global warming creates the conditions that make the algae bloom. For example, if there is a long drought and it is fallow by rainfall, it causes run-off from agricultural lands. This transports more nutrients to the water, which makes the algae grow. Also, because of the droughts, water moves and changes less, which causes the water temperature to rise. Algae prefers warmer temperatures, so it grows faster. Algae, especially toxic blue-green algae, need CO2 to grow, and since there is more CO2 in the atmosphere, it means better conditions for algae.
Toxic algae blooms cost millions of dollars every year to the U.S. because of the loss of fisheries and tourism, undrinkable water, clean-up costs, and hospital visits. For example, cyanobacteria can cause neurological damage, liver damage, different skin problems, and respiratory issues.
Not only are the algae blooms toxic, but after they die, they become great ‘dead zones’. They are called ‘Dead Zones’, because when the algae starts to decompose, it uses oxygen in the water, and because of this, the other organisms suffocate and die as well. The decomposing process also produces harmful gases like methane and hydrogen sulphide.

As a result of climate change, we might see changes in plankton growth. Climate change probably leads to a change in species type, the consequences of which for the food web are unknown. The changes in plankton growth are likely to affect other species because they are the base of the food webs.

Sources:
https://oceanservice.noaa.gov/facts/phyto.html
https://www.marinebio.org/creatures/zooplankton/
https://byjus.com/neet/difference-between-phytoplankton-and-zooplankton/
https://news.climate.columbia.edu/2023/08/23/plankton-are-central-to-life-on-earth-how-is-climate-change-affecting-them/
https://coast.noaa.gov/data/estuaries/pdf/estuary-food-pyramid-climate-change-white-paper.pdf