4th Mar, 2013

Winter zooplankton

Over the past few months we have noticed that we have two species of Calanus in our samples from L4. While Calanus helgolandicus and Calanus finmarchicus appear very similar to each other under a microscope, with just a slightly different head-shape and differing leg structure to tell them apart, their spatial distribution and ecological niches are quite distinct. Usually we would expect to see Calanus helgolandicus, a warm water species, but lately we have also seen Calanus finmarchicus, a cold-water oceanic species. Although Calanus numbers are quite low at the moment we are finding that C. finmarchicus are making up a large proportion of adult females, reaching 75% this week. At pretty much the Southern limit of C. finmarchicus distribution here in the English Channel it will be interesting to see how long these copepods hang around, their range is thought to be shifting northwards in response to increasing temperatures. Their numbers have decreased dramatically over the last 40 years, with consequences on the recruitment of important fish species such as cod that eat these nutritious little beasts.

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