Study shows importance of copepods in marine food webs

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Tiny crustaceans known as copepods may hold the clue to marine ecosystems a new study has revealed.

The study, undertaken by scientists at the University of Maryland Center for Environmental Science and the Virginia Institute of Marine Science (VIMS) has revealed that copepods – the most numerous multi-celled organism in the oceans – form an important link between phytoplankton and fish in marine food webs.

To understand how the copepods respond to environmental change the researchers looked at how many copepods were born and how many have died.

In the past studies have assumed that copepods died purely as a result of predation but this study found that samples of seawater taken in Chesapeake Bay showed significant numbers of dead copepods which was indicative of non-predatory mortality. In actual fact an average of 12-30 percent of the developmental stages of the abundant coastal copepod species Acartia tonsa were dead.

David Elliott, one of the principle researchers, said: "…copepod carcasses become less dense as they decompose, such that they can reach neutral buoyancy and float around for some time before reaching the sea floor." He believes that the organic matter from these remains is recycled into the seawater providing food for phytoplankton and microbes. This in turn will have a knock on effect on fish populations.

"The presence of copepod carcasses in the marine environment indicates the importance of non-predatory mortality factors," says Elliott. "It represents a diversion of energy from the traditional food chain that supports fish, to one that fuels microbes.

"A better understanding of the factors causing non-predatory mortality will improve predictability of the amount of copepod prey available to fish."

A separate study by the team has also found that turbulence from boat propellers can kill large numbers of copepods having a significant effect on the food chain.

For more information see: DT Elliott, KW Tang. Implications of carcasses for mortality estimates and population dynamics of Acartia tonsa. Marine Ecology Progress Series, 2011; DOI.