Gathering Information on Plankton
The Continuous Plankton Recorder (CPR) surveys, operated by the Sir Alister Hardy Foundation for Ocean Science, provide us with the most comprehensive information on species diversity and distribution of mesozooplankton.
The survey operates mainly in the North Atlantic using a specially designed net which is towed each month at a fixed depth of approximately 7 metres behind cargo vessels and ferries plying a number of regular shipping routes. The catch of plankton can be divided up into 10 nautical mile segments along each tow.
The CPR has been operating since the mid-1940s and there is a substantial literature on the geographical distribution of taxa, and especially on trends in abundance and diversity in relation to climate indices. Recent publications describe the changes in composition of the plankton in the northeast Atlantic. Briefly, the plankton of the northwest European shelf, including the North Sea, can be grouped into a number of categories of species including:
- Sub-arctic species
- Temperate species
- Indigenous shelf species
The main sub-Arctic species is Calanus finmarchicus which has its main centre of distribution in the Norwegian Sea, but extends south into the northern North Sea.
The main representative of the temperate species group is the copepod Calanus helgolandicus which is most abundant in the Bay of Biscay and southwest approaches to the UK, but extends into the North Sea from the south and west.
Indigenous species such as Temora and Acartia are small copepod taxa which are most abundant in the shallow southern North Sea. Since the mid-1960s the CPR data show a northward retreat of the subarctic species and advance of the temperate species, in parallel with a progressive warming of sea temperatures and changes in ocean currents connected with global scale changes in climate.
In the 1960s, the abundance of Calanus finmarchicus in the northern North Sea was about 10 times that of Calanus helgolandicus. Now the two have approximately the same abundance.
What the Continuous Plankton Recorder Can Tell Us
Annual average biomass of late copepodite stages of Calanus finmarchicus and Calanus helgolandicus in the northern North Sea. Drawn from data supplied by the Sir Alister hardy Foundation for Ocean Science.
The CPR surveys only collect data from the upper 10 metres of the water column, and with a relatively coarse mesh net (0.35 mm mesh). Also, the analysis of the samples, whilst comprehensive with respect to species, does not provide great detail of the relative abundance of the various life-stage of each species. The latter point is very important for understanding of the population dynamics of species and their links to fish populations. Fish eat different life-stages of zooplankton as they themselves develop from larvae to adults. Hence, Marine Scotland Science has an active programme of zooplankton monitoring and research to provide answers to questions about:
- Links between climate and particular species of zooplankton
- The role of zooplankton in the ecosystem
- The relationships between zooplankton and fisheries