Plankton are diverse microscopic organisms that drift in oceans, seas, and freshwater bodies. These tiny life forms cannot swim against currents, moving wherever the water takes them. Despite their small size, plankton exist in immense numbers, forming a fundamental and pervasive presence across nearly all aquatic environments on Earth, found from the surface waters down to considerable depths. They play an important role in aquatic ecosystems globally.
What Plankton Consume
Plankton exhibit diverse feeding strategies. Phytoplankton, which are plant-like, are autotrophs; they produce their own food through photosynthesis. This process involves capturing energy from sunlight and converting carbon dioxide and dissolved inorganic nutrients, such as nitrates, phosphates, and silicates, into organic matter. This makes phytoplankton the primary producers in most aquatic food webs, similar to plants on land.
In contrast, zooplankton are animal-like and heterotrophs, meaning they must consume other organisms or organic material for energy. Many zooplankton species graze directly on phytoplankton. Other zooplankton are carnivorous, preying on smaller zooplankton or the larvae of other marine animals. Some zooplankton are omnivores, consuming a mixed diet of both phytoplankton and detritus, which is dead organic matter sinking through the water column.
Organisms That Feed on Plankton
Plankton form the foundational layer of the aquatic food web, providing sustenance for many organisms. Small crustaceans, such as copepods and krill, feed directly on phytoplankton and smaller zooplankton. These small plankton-eaters, in turn, become a food source for larger animals. Larval fish and juvenile stages of many marine invertebrates rely heavily on plankton for their initial growth and development.
Moving up the food chain, larger filter-feeding organisms have evolved specialized mechanisms to consume plankton. Baleen whales, like blue whales and humpbacks, are examples, sifting plankton from large volumes of seawater using their baleen plates. Manta rays and whale sharks also employ filter-feeding strategies, swimming with open mouths to strain plankton and small fish from the water. Various bivalves, including clams, mussels, and oysters, are stationary filter feeders that draw water through their bodies to extract plankton and other suspended organic particles.
Plankton’s Wider Ecological Significance
Beyond their role as a food source, plankton contribute to global ecological processes. Phytoplankton, through photosynthesis, produce a substantial portion of Earth’s atmospheric oxygen. Estimates suggest they generate between 50% and 80% of the planet’s oxygen, making them comparable to terrestrial forests in their oxygen output. Their health and abundance are directly linked to the air we breathe.
Plankton also play a central role in the global carbon cycle. During photosynthesis, phytoplankton absorb carbon dioxide from the atmosphere and dissolved in ocean waters. When plankton die, their organic matter, including absorbed carbon, sinks to the ocean floor, effectively sequestering carbon in marine sediments. This process helps regulate Earth’s climate by removing carbon dioxide, a major greenhouse gas, from circulation. Plankton also facilitate nutrient cycling, moving dissolved nutrients from surface waters into the food web and transporting them vertically through the water column as they sink or are consumed.
Environmental Influences on Plankton
Plankton populations are sensitive to environmental factors, and changes can have widespread effects on aquatic ecosystems. Ocean temperature is a primary influence, as many plankton species have specific temperature ranges. Rising ocean temperatures can alter plankton distribution, shift species composition, and impact their metabolic rates and reproductive cycles. Ocean acidification, resulting from increased absorption of atmospheric carbon dioxide by seawater, reduces ocean pH. This can impair shell-forming plankton, like coccolithophores, from building their calcium carbonate shells.
Nutrient availability also influences plankton growth and abundance. Upwelling, where deep, nutrient-rich water rises to the surface, often leads to large plankton blooms. Conversely, nutrient depletion can limit plankton populations. Pollution, including plastic microparticles and chemical contaminants, poses a threat by directly harming plankton or disrupting their physiological processes. These environmental stressors affect plankton health, distribution, and numbers, impacting the aquatic food web.