Plankton are a diverse collection of microscopic organisms that float or drift in the world’s oceans and freshwater bodies. The term describes their lifestyle: being carried by water currents rather than swimming against them. These organisms are functionally divided into two main categories based on how they obtain energy: phytoplankton, which are plant-like primary producers, and zooplankton, which are animal-like consumers. Understanding the diet of these two groups reveals the fundamental energy flow that supports nearly all aquatic life and regulates global biogeochemical processes.
The Diet of Phytoplankton
Phytoplankton, meaning “plant drifters,” form the base of the marine food web by creating their own food through autotrophy. Similar to terrestrial plants, they capture light energy and transform it into chemical energy via photosynthesis. This process involves drawing in dissolved carbon dioxide (\(text{CO}_2\)) and water to synthesize carbohydrates and release oxygen as a byproduct.
Their growth requires a specific suite of non-organic nutrients dissolved in the seawater. Macronutrients, such as nitrogen (N) and phosphorus (P), are required in large amounts to build cell structures and genetic material. Diatoms, a major group of phytoplankton, also require silicon to construct their intricate, glass-like cell walls.
In many open ocean regions, the supply of certain trace elements, or micronutrients, can limit growth even when light and macronutrients are plentiful. Iron is the most common limiting micronutrient, especially in areas far from land where atmospheric dust deposition is low. Other trace metals, including zinc and cobalt, are also required as co-factors for the enzymes that catalyze essential cellular reactions.
The Diet of Zooplankton
Zooplankton are the primary consumers in aquatic environments, relying on organic matter created by the phytoplankton through heterotrophy. Their diet is highly varied, ranging from strictly herbivorous to carnivorous or detritivorous, depending on the species and its life stage. The most common food source for these “animal drifters” is the abundant phytoplankton, which they graze on in enormous quantities.
Beyond grazing on plant-like cells, zooplankton also function as micro-predators, consuming smaller organisms like bacterioplankton and other, even smaller, zooplankton. This internal predation within the zooplankton community helps regulate populations and recycles nutrients. Many species also consume non-living organic particles that constantly drift down from the surface waters, collectively known as marine snow.
Marine snow is a rain of detritus, including dead plankton, discarded exoskeletons, and fecal pellets from larger animals. By consuming this organic waste, zooplankton play a role in recycling nutrients before the material sinks into the deep ocean. This makes them a crucial link between the primary producers and larger marine life.
Specialized Feeding Methods
The microscopic size and dilute nature of their food supply have led to a range of specialized mechanisms zooplankton use. Filter feeding is one of the most widespread methods, particularly among crustaceans like copepods. These organisms use complex, hair-like appendages and cilia to create feeding currents that draw water and suspended food particles toward their mouthparts.
Other zooplankton employ predatory techniques, categorized as ambush feeding. Passive ambush feeders, such as some jellyfish, simply drift and wait for prey to bump into their sticky or stinging surfaces. Active ambush feeders, like certain copepods, can detect the hydrodynamic disturbance created by a nearby motile prey item and launch a quick, targeted attack.
For species that feed on marine snow, the feeding mechanism often involves colonizing the large, sinking aggregates of detritus. Some zooplankton also exhibit selective feeding based on size, shape, or chemical signature, rather than indiscriminately consuming everything in the water.
Plankton Feeding, Oxygen, and Carbon Cycles
Plankton feeding regulates the Earth’s atmosphere and climate. Phytoplankton photosynthesis is responsible for producing a large percentage of the oxygen we breathe. They take in \(text{CO}_2\) dissolved in the surface water, incorporating the carbon into their organic biomass.
This process initiates the biological carbon pump, which is the primary mechanism for sequestering atmospheric carbon in the deep ocean. When phytoplankton are consumed by zooplankton, the carbon is transferred up the food chain. When zooplankton excrete dense fecal pellets, the carbon-rich organic matter sinks.
This sinking material, part of the marine snow, acts as a downward conveyor belt. It locks carbon away in the deep ocean for hundreds to thousands of years. Without this constant transfer driven by plankton feeding, atmospheric \(text{CO}_2\) levels would be significantly higher.