What are marine zooplankton?
Zooplankton are very small animals that live in the sea. They are not strong enough to swim against tides and currents and so drift along in the water. There are tens of thousands of species of zooplankton and they range in size from being smaller than a grain of rice up to giant jellyfish that can reach two meters in diameter. Zooplankton are divided into several categories based on their size but usually when scientists discuss zooplankton they are referring to a group sized between 0.2 mm to 2.0 cm known as the mesozooplankton. Marine Scotland Science (MSS) collects mesozooplankton at the Stonehaven and Loch Ewe sites as part of their Coastal Ecosystem Monitoring Programme. The data collected is being used to fulfil the requirements of European legislation relating to the marine environment such as the Marine Strategy Framework Directive as well as providing information on the status of the marine environment.
Different types of zooplankton
Zooplankton are divided into two main groups based on their life cycle. These are (1) the holoplankton – animals that spend their whole life drifting as part of the plankton and (2) the meroplankton – animals that spend only some of their lifecycle in the plankton.
(1) The holoplankton: The majority of animals in the holoplankton belong to a group of animals known as the crustaceans which include crabs and shrimps, as well as a group of tiny planktonic animals called copepods. There are many different species of copepod and they range in size from less than 1.0 mm to about 5.0 mm in length. Most copepods feed on phytoplankton (very small algae) and smaller zooplankton by straining them out of the water column with filter-like feeding appendages around their mouths. Other holoplanktonic animals include krill, water fleas, ostracods, amphipods, comb jellies and members of a group of animals called tunicates that are related to sea squirts.
(2) The meroplankton: With the exception of mammals such as whales and seals, almost all animals that live in the sea spend the early stage of their life as part of the zooplankton community as eggs and/or larvae. These stages then mature into animals such as fish that can swim against currents or animals that settle and live on the sea-floor such as worms, crabs, lobsters, sea urchins and starfish.
An exception to this rule are some jellyfish. They have an unusual life cycle as they spend most of their life as a colony of hydroids (sea firs) that are sedentary on the sea floor, often looking more like a delicate seaweed than an animal, before producing a swimming adult stage known as a medusa. This is the form we commonly call a jellyfish.
Why are zooplankton important?
Zooplankton form an important link in food webs. They accumulate food energy made by the phytoplankton when they graze on them. When zooplankton are eaten by fish larvae this energy passes up the food web. The availability of zooplankton at the right time and place to provide food to fish larvae is believed to be an important factor in determining the size of fish stocks. Any event causing a decline in the zooplankton population may have far-reaching effects on the ecosystem and the economy.
Most animals in the plankton have short lifecycles of less than a year. These cycles are largely controlled by temperature, so long-term monitoring of zooplankton is required to reveal the impacts of climate change.
Zooplankton also provide an important source of food to animals living on the sea floor as their waste sinks to the bottom of the sea and fertilises the seabed.
Krill and copepods are often added to the diets of salmon and trout grown in fish farms to improve the colour of their flesh. Some countries have commercial fisheries for krill and copepods which are used as human and animal food as well as a health supplement because they are very high in omega 3 oils.
Zooplankton studies at MSS
Marine Scotland Science has a committed group of scientists who are investigating different aspects of the zooplankton community in Scottish waters. A particular focus of its research is examining zooplankton abundance and diversity at the Coastal Ecosystem Monitoring Sites at Stonehaven (East Coast) and Loch Ewe (West Coast). Over the next few months we will present some of our findings from these investigations in more detail.
Did you know?
It is estimated that the weight of just one type of krill in the sea is double that of all the humans in the world.
During World War II, scientists researched the possibility of feeding theUKpopulation on zooplankton from Scottish sea lochs if food shortages became critical but, after trials, the scientists concluded it would be too difficult to catch enough plankton. Professor Geoffrey Moore from University Marine Biological Station in Millport discovered the plans in archives at the Scottish Association for Marine Science (SAMS) in Oban.
More information on plankton can be found at http://www.scotland.gov.uk/Topics/marine/marine-environment/species/plankton
More information on the coastal ecosystem monitoring programme can be found at
More information on the Marine Strategy Framework Directive can be found at
Article submitted by Kathryn Cook, Sheila Fraser and Eileen Bresnan