Plankton refers to a diverse collection of organisms that drift in water, unable to swim effectively against currents. Macroplankton represent a specific subset of these drifting organisms, distinguished primarily by their size. They are found in both marine and freshwater environments around the globe.
Defining Macroplankton
Macroplankton are defined by their size, generally ranging from 2 centimeters to 20 centimeters in length. For instance, microplankton are much smaller, typically between 20 to 200 micrometers, while mesoplankton fall between 200 micrometers and 2 centimeters. Megaplankton, on the other hand, are larger than macroplankton, exceeding 20 centimeters in size and including organisms like large jellyfish.
Their ability to swim is limited, often only allowing for weak or vertical movements within the water column rather than sustained propulsion against strong flows. Their body forms are diverse, including both animal and plant forms, though animal macroplankton (macrozooplankton) are more commonly discussed in this size range.
Life in the Ocean
Macroplankton inhabit various zones within the ocean, from the sunlit surface waters to considerable depths. They are present in the euphotic zone, where sunlight penetrates and primary production occurs, as well as in the deeper disphotic and aphotic zones where light is scarce or absent. Their global distribution spans both coastal areas and the vast open ocean.
Ocean currents significantly influence the distribution of macroplankton. While most are abundant in surface waters, some macrozooplankton species can be found throughout the water column. The availability of light and nutrients are factors that affect their local abundance.
Ecological Significance
Macroplankton hold a significant position within marine food webs, linking smaller plankton and larger marine animals. Many macroplankton species are primary consumers, feeding on phytoplankton, which are microscopic photosynthetic organisms forming the base of the food web. Other macroplankton are secondary consumers, preying on smaller zooplankton.
These organisms also serve as a food source for a wide array of larger marine animals. Fish, whales, and seabirds depend on macroplankton, such as krill. Beyond their role in energy transfer, macroplankton contribute to nutrient cycling in the ocean. They play a part in the biological pump, a process that transfers carbon from the surface waters to the deep ocean, influencing global carbon cycles. This occurs partly through the production of fecal pellets, which sink and carry organic carbon to deeper waters.
Diverse Forms of Macroplankton
The macroplankton group encompasses a variety of fascinating organisms, each with unique characteristics. Krill, for example, are small crustaceans (euphausiids) that are particularly abundant in polar waters, forming dense swarms that serve as a primary food source for baleen whales, seals, and penguins. Krill are capable swimmers, a notable trait among plankton, making them difficult to capture with standard nets.
Jellyfish, including various Medusae and ctenophores (comb jellies), are also prominent macroplankton. Many jellyfish are predatory, using stinging cells called nematocysts to capture prey, while comb jellies, though similar in appearance, lack these stinging cells. Salps are barrel-shaped tunicates that filter feed on phytoplankton and can form long, sometimes colonial, chains. Arrow worms, or chaetognaths, are transparent, predatory worms known for their rapid movements and ability to ambush and consume copepods and other small zooplankton.