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Sampling plankton in Plymouth

Published: 15 October 2015

Night samplingBy MERP scientist Dr Martin Lilley


As part of MERP’s module 2 I have been involved in a series of visits to Plymouth Marine Laboratory from my base at Queen Mary University of London. Regular 6-weekly sampling trips are being made to the L4 station, offshore from Plymouth, to collect plankton samples and develop our understanding of the size spectrum present throughout the year in the plankton.

These samples supplement the weekly L4 sampling already conducted by PML, with the addition of a larger 500µm ‘jelly-net’ alongside the regular L4 200µm and 63µm nets. Unlike the regular sampling we have also been comparing day and night samples to understand any daily variability which may be present in the plankton. A typical sampling trip on Plymouth Quest requires 8-10 hours depending on the tides and time of sunset, taking place in conditions up to a ‘moderate’ sea. Efficient teamwork between scientists and an enthusiastic crew has shortened the sampling time and makes for enjoyable working conditions; this is especially important when the last samples may come aboard after midnight. Each trip involves a little over two hours of sampling in the afternoon to collect and preserve 12 nets (4 replicates of the three mesh sizes), and then repeating the work after nightfall (restarting at 22:30 in midsummer).

Plankton samples are immediately filtered when they arrive on deck, to concentrate the samples, and then are preserved in ethanol, formalin or snap-frozen to allow a variety of analytical techniques and several MERP objectives to be fulfilled.

Formalin preserved samples are analysed on return to the laboratory to quantify the main species groups and measure the size range of individuals. Some common species will act as a benchmark to validate the jelly nets against the regular L4 WP2 nets. To date five sampling trips have taken place (in February, April, June, August and September 2015) with the next planned for November 2015 and further sampling into 2016.

My particular focus is on the gelatinous zooplankton. From sample replicates that arrive on deck we are picking out the obvious species, measuring them before preservation, and freezing them to analyse their stomach contents. This adds to the size spectrum work and should improve our understanding of the trophic interactions between species. We are planning to use molecular techniques to identify prey within the guts of the jellyfish, thereby including prey items which may have already been digested or are harder to identify in microscope samples. Most of the jellyfish caught during sampling so far have been small hydrozoan and ctenophore species, but there have been a few larger scyphozoan jellyfish.

One of the scyphozoan species known to occur is the Barrel Jellyfish Rhizostoma octopus, which has been widely reported in SW England this year. Despite the abundance it has not been caught in our nets so far, although several fish-sampling nets off Plymouth have caught individuals. We have observed this species at the mouth of Plymouth Sound on the June sampling, observing about 3 individuals a minute as we steamed along at 10 knots. Calculations estimated that these jellyfish were only at densities of 0.5-1.2 individuals per 1000m2 surveyed, but being 10-20kg each would still contribute significantly to any ecosystem they were present in. It has also been very interesting to observe an increase in the sea gooseberry Pleurobrachia pileus, followed by an increase in its predator Beroe gracilis, with both species declining in turn. These sporadic increases will pose challenges in translating the gelatinous biomass present in the area to simple numbers for modelling of the data on a longer timescale, but are fascinating for an ecologist such as myself to witness.


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