Plankton respond to warmer seas

A study of populations of North Sea plankton, published this week in Nature, suggests that annual phenological cycles—such as migration and blooming—are changing as the ocean warms. The study found that not all species are affected in the same way, so that peaks that once occurred together or in close succession are now moving apart—a breakdown of ecosystem synchronicity that could, warn the authors, have major consequences in years to come.Martin Edwards and Anthony Richardson, both based at the Sir Alister Hardy Foundation for Ocean Science, Plymouth, UK, examined the effects of increasing sea temperature on the annual productivity cycles of 66 plankton taxa, which they categorized into five functional groups: diatoms, dinoflagellates, copepods, non-copepod holozooplankton, and meroplankton.Using data collected from the central North Sea by the continuous plankton recording program every month from 1958 to 1992, the pair was able to form a picture of shifting seasonal peaks in marine production. Many species bloom earlier now than they did 4 decades ago. For example, taxa identified as belonging to the phylum Echinodermata have moved their peak production forward by an average of 47 days. However, not all production peaks have moved forward: in some species, there has been virtually no change at all.The authors attribute this down to differences in the factors that trigger blooming. "The response to climate varies across both trophic levels and functional groups, leading to a mismatch in timing between primary, secondary, and tertiary marine production," Edwards told The Scientist. "These changes are correlated with temperature, but not all taxa are driven primarily by temperature change. Many phytoplankton, for example, respond to changes in day length—a factor unaffected by climatic fluctuations. It is these differences that can lead to the decoupling of ecological relationships."Asynchrony in plankton phenology has the potential to impact on other marine life, Edwards said. This includes commercially important fish species, which depend on a bountiful supply of plankton at the right point in their own annual cycles. "Our results confirm the role of climate change in modifying ecosystems," he said. "However, the way that species interactions adapt in response to rapid climatic change remains a mystery."Gian-Reto Walther, a climate change ecologist at the University of Hanover who was not involved in the study, concurred. "Edwards and Richardson stress the ecological consequences of their findings, but we are only in the early stages of understanding the complex interactions that take place among species at different trophic levels and under changing environmental conditions," he said. "There are certainly many surprises ahead."Nonetheless, there could be severe consequences for the marine ecosystem. "The implications of trophic decoupling are enormous," said Terry Root, who is based at Stanford University's Center for Environmental Science and Policy. Root, who was not involved in the study, emphasized the particular impact on predator–prey relationships. "Predator populations could be devastated if alternative prey aren't available," he said. "Conversely, prey species could multiply unchecked in the absence of predators."Thorsten Blenckner, at the University of Uppsala, said that setting these results in a global context must be approached cautiously. "First, plankton were sampled from a fixed depth of approximately 10 meters," Blenckner, who was not involved in the study, told The Scientist. "However, the warmer upper layer might change more as a consequence of climate change, leading to differences in the way plankton distribute themselves in the water column." "Second, fishing practices over the duration of the study period have changed markedly, which might have altered the species composition anyway, even without climatic effects." Still, Blenckner said, "the study certainly suggests a tendency towards phenological change, but more work must be done to determine how widespread the effects are."Studies addressing the effects of climate change at the multi-species level are still rare. "Edwards' and Richardson's findings represent a valuable contribution to marine biometeorology," said Wulf Greve, at the German Center for Biodiversity Research, Hamburg. "Phenology is a marine ecological discipline that seems destined to change our traditional paradigms." Greve, who was not involved in the study, said, "these latest results give us a taste of future developments."

Plankton respond to warmer seas

Study suggests that climate change can decouple ecological relationships

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