Long-term trends in calcifying plankton and pH in the North Sea

Anthropogenic carbon dioxide emissions are changing ocean chemistry at an unprecedented rate. As the ocean absorbs carbon dioxide, the pH of marine waters is decreasing; this process is known as ‘ocean acidification’. It is currently unclear how ocean acidification is affecting the plankton. We know that there are large differences between responses of organisms to increasing levels of CO₂ in seawater, even between strains of the same species. Calcifying taxa are widely predicted to be adversely affected, since ongoing acidification is rapidly lowering the calcium carbonate saturation state of surface waters. Most investigations into the effects of decreasing pH on planktonic organisms have taken place in short-term laboratory, or mesocosm, experiments, with a focus on the physiological effects of pH change. Very little information is available about the impacts that ocean acidification is having, or might have, on the abundance of calcifying plankton. Uniquely, the Continuous Plankton Recorder (CPR) survey data offers a long term (80 year) database of the abundance of plankton, which can be explored in relation to environmental factors including pH, temperature and nutrients.

Relationships between the time-series of six calcifying plankton groups (foraminifera, coccolithophores, echinoderm larvae, bivalve larvae, Clione limacina, and Limacina helicina) routinely found on CPR samples and pH were explored in a highly biologically productive and data-rich area of the central North Sea. The long-term trends show that abundances of foraminifera, coccolithophores, and echinoderm larvae have risen over the last few decades while the abundances of bivalves and pteropods have declined. pH appears to have been declining since the mid 1990s but there was no statistical connection between the abundance of the calcifying plankton and the pH trends. If there are any effects of pH on calcifying plankton in the North Sea they appear to be masked by the combined effects of other climatic (e.g. temperature), chemical (nutrient concentrations) and biotic (predation) drivers at this time.

Monitoring programmes such as the CPR are crucial for establishing baselines and recognising futures changes in the plankton which may be linked with ocean acidification. Complementary reliable datasets on environmental parameters are also needed in order to understand ecological responses to climate- and anthropogenic- driven changes in the sea.

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Beare, D., McQuatters-Gollop, A., Hall-Spencer, J., van der Hammen, T., Machiels, M. and Teoh, S.J., (2013). Long-term trends in calcifying plankton and pH in the North Sea. PlosOne, 8: e61175.