The microbes, algae and microscopic animals and jellyfish (more below) which live in mid-water in the oceans, are referred to as plankton. The larger ones can swim, but most of their swimming activity is directed to vertical movements, whilst their horizontal transport is dictated entirely by water currents. Hence the plankton are sometimes referred to as the 'drifters of the sea'. The plankton encompass a vast range of taxonomic groups and species. Some of the most abundant species on the planet are planktonic.
Plankton are usually categorised by their size, which reflects the mesh size needed to filter them out of the water:
- Viruses and baceteria smaller than 2 microns (1 micron is a thousandth of a millimetre) are referred to as ultraplankton
- Larger bacteria and single-celled flagellates up to 20 microns are referred to as nanoplankton
- Microzooplankton are between 20 and 200 microns in size
- Mesozooplankton are between 200 microns to 2 mm (2,000 microns)
- Macroplankton are between 2 and 20 mm
- Megaplankton are up to 1 metre in diameter, including jellyfish up to 1 metre in diameter
Meroplankton are temporary members of the plankton, for example the larval stages of benthic organisms.
Plankton can also be grouped according to the depth range were they live:
- The epipelagic species live in the top 150 metres of the ocean, which includes all of the continental shelf seas
- The mesopelagic species live between 150m and 1,000 metres
- The bathypelagic between 1,000m and 4,000 metres
- The abyssopelagic deeper than 4,000 metres.
Some species migrate between these zones on a daily basis, for example, some euphausiids (krill) migrate between the mesopelagic during the day and the epipelagic zone at night.
The most abundant mesoplankton species in the North Atlantic, the Calanus finmarchicus, lives between 150m and 1,000m in winter and in the top 150 m in summer. The variety of life forms in the plankton is truly astonishing. We find plankton which are:
- Autotrophs (photosynthetic)
- Chemotrophs (for example, sulphur respiring)
- Heterotrophs (omnivorous and carnivorous)
- Mixotrophs (combined autotrophic and heterotropic).
Some are essentially passive drifters which gently filter food out of the surrounding water, whilst others are voracious predators which hunt down their prey by a combination of chemo and pressure sensing.
Others produce fragile nets of mucus which they cast about themselves to capture prey. These are so delicate that they are instantly destroyed by contact with a sampling net. There are a wide range of colours, from the brilliant reds of some bathypelagic species, to the iridescent colours and glassy transparency of some epipelagic forms. Others are phosphorescent (produce light when agitated) or luminous.
The life cycles of plankton also vary enormously. In general, the smallest plankton have the shortest life cycles, so that bacteria and flagellates may multiply within a few hours to one day. The mesozooplankton may have life cycles of a few weeks, whilst the macro and megaplankton usually have a 1-2 year life cycle.
Life of the plankton is dominated by three main factors:
- The viscosity of water
To a plankton animal smaller than a grain of rice, water is a highly viscous medium. For a human, it would be like living in stiff treacle. This dominates the swimming and feeding habits of the mesoplankton and smaller organisms. The photosynthetic species need sunlight, and remaining in the surface metres of the water column is vital for them. Many plankton animals also migrate towards light sources whilst others avoid light and are adapted to live in the constant blackness of the deep ocean. Finally, being eaten by a predator is the most usual fate of a plankton organism - other than the large jellyfish. Plankton are the basis of the marine food web which supports the fisheries, whales and seabirds, and for this reason Marine Scotland Science has an interest in plankton monitoring and research.
Jellyfish belong to the Cnidaria phylum, with species characterised particularly by possession of cnidae (stinging cells), which inject complex toxins similar to snake venoms. The jellyfish are part of the taxonomic category hydroidomedusae. This in turn includes hydromedusae, scyphomedusae and siphonophores. There is also a separate phylum of Ctenophora.
The forms that live on the sea bed, along with hydroids, sea pens, sea anemones, corals and other bottom-dwelling cnidarians, also feed on passing plankton. The medusae form is the stage we mostly recognise as jellyfish. These jellyfish feed, grow and produce tiny swimming larvae that either settle to the seabed to form further colonies or develop once more into adult jellyfish. Jellyfish are almost entirely predatory, feeding on small crustaceans, larval fish and invertebrates and on each other, often forming dense swarms. There are many hundreds of species of jellyfish around European coasts and in all the world's seas.
Jellyfish as a nuisance species to aquaculture