Copepods and diatoms represent one of the most fundamental interactions in the world’s oceans, forming the base of the marine food web. Copepods are tiny, ubiquitous crustaceans belonging to the zooplankton community. Diatoms are single-celled algae, classified as phytoplankton, that use photosynthesis to convert sunlight into energy. These two groups coexist in vast numbers across nearly all aquatic environments. Their interaction drives global nutrient cycles and sustains life across multiple trophic levels.
The Primary Food Source: Diatoms and Copepod Grazing
Copepods are major consumers of diatoms, which often form the largest component of their diet, especially during phytoplankton blooms. Diatoms are a high-value food source because they are abundant and contain energy-rich lipids, including omega-3 fatty acids, important for copepod growth and reproduction. However, the nutritional quality of diatoms is not always consistent and depends on the algae’s physiological state and nutrient availability.
The physical mechanism of feeding involves specialized mouthparts, such as the maxillae and maxillipeds, used for capturing and filtering food particles. Copepods employ a variety of feeding strategies, ranging from passive filter feeding to actively hunting individual cells or chains of diatoms. A single copepod can clear significant volumes of water and consume hundreds of thousands of phytoplankton cells daily.
The diatom’s defense mechanism, a rigid silica cell wall known as the frustule, presents a challenge to the grazing copepod. Diatom species with thicker frustules are often rejected or require longer handling times. This extended handling limits the copepod’s overall foraging efficiency and can reduce the consumption of other available food sources. Ingestion of highly silicified diatoms has also been shown to compromise copepod growth rates, egg production, and hatching success.
Some diatoms can also produce chemical deterrents, such as toxic aldehydes or inhibitory exudates, particularly when under stress. These compounds can negatively affect copepod reproduction or reduce their grazing rates. Despite these defense strategies, the sheer quantity of diatoms makes them the dominant energy source for many copepod species.
Beyond Diatoms: The Diverse Diet of Copepods
While diatoms are a primary food, copepods exhibit significant dietary plasticity and are not obligate specialists. Most copepod species are classified as omnivores, consuming both plant and animal matter. Their diet can shift dramatically based on environmental conditions and life stage, allowing them to thrive in various aquatic habitats.
Beyond diatoms, copepods consume other types of phytoplankton, such as dinoflagellates and green algae. They also feed on microzooplankton, including protozoa, ciliates, and bacteria. Some of the larger copepods are outright predators, actively hunting smaller zooplankton and even other copepod species.
Bottom-dwelling, or benthic, copepods have mouthparts adapted for scraping and biting, enabling them to consume non-living organic matter. This detritus, consisting of dead plant and animal material, forms a significant part of the diet for many species. By consuming detritus and associated bacteria, these copepods play an important role as scavengers, contributing to nutrient recycling in the ecosystem.
The Critical Role in Marine Ecosystems
The consumption of diatoms by copepods forms a fundamental connection known as trophic transfer, which fuels the majority of the marine food web. Copepods act as intermediaries, transferring the energy created by primary producers into a usable form for organisms at higher trophic levels. This conversion allows energy from microscopic algae to reach organisms like small fish, whales, and seabirds.
The health and abundance of copepods directly support commercial fisheries, as they are a major food source for the larvae of many commercially important fish species. Their ability to convert diatom lipids into highly nutritious packages makes them an important component of aquaculture and wild fish populations. Without this link, the energy flow from the base of the food web would be severely limited, impacting global fish stocks.
Copepods also play a major role in the biological carbon pump, a process that sequesters carbon from the surface ocean into the deep sea. When copepods consume diatoms, they package the carbon into dense fecal pellets that sink rapidly through the water column. This passive sinking of carbon-rich waste is one pathway by which atmospheric carbon dioxide is naturally removed from circulation.
A second mechanism involves the active transport of carbon through seasonal vertical migration, sometimes called the “lipid pump.” Certain copepod species, such as those in the genus Calanus, store significant amounts of fat after feeding on large diatom blooms. They then migrate to depths of several hundred meters to enter a state of hibernation, or diapause. This active movement transports carbon-rich lipids into the deep ocean where they are metabolized, effectively sequestering carbon far below the surface waters.