Early Shift: North Sea plankton and fish move out of sync

As ocean temperatures in the North Sea have warmed in recent decades, the life cycles of some species low in the food chain have accelerated significantly, sometimes setting off ecological havoc, new analyses suggest.

Many marine microorganisms bear chlorophyll, which lets them capture the sun’s radiation and use it to grow. Each spring, as the sun rises higher in the sky, these so-called phytoplankton experience a population explosion. Small crustaceans such as copepods graze on this bounty, and they, in turn, nourish larger organisms such as fish larvae.

The number of juvenile fish produced each year depends on the synchrony of these population booms, says Martin Edwards, a marine ecologist at the Sir Alister Hardy Foundation for Ocean Science at Plymouth, England.

Edwards and his colleague Anthony J. Richardson have analyzed more than

4 decades of census figures for 66 species that appear in the lowest three levels of the food chain in the central North Sea. Between 1958 and 2002, the timing of the spring bloom of diatoms—one group of phytoplankton—didn’t shift significantly.

However, peak populations of copepods, the next level up the food chain, occurred in recent years about 10 days earlier than they did in 1958. Overall numbers of fish larvae, which consume both phytoplankton and copepods, recently crested about 27 days earlier than they did when the census began, says Edwards.

The life cycles and growth rates of copepods and fish larvae are influenced by both daily sunlight and average water temperature, which in the central North Sea has risen about 0.9°C since 1958. The researchers report their findings in the Aug. 19 Nature.

If fish larvae don’t have a good supply of food when warmer water kicks their metabolism into high gear, fewer will grow into fry and make it to adulthood, says Keith M. Brander of the International Council for the Exploration of the Sea in Copenhagen. A mismatch between population booms for fish larvae and their prey doesn’t bode well for many fish species, he says.

Brander recently analyzed the same plankton-census data that Edwards and Richardson used. He suggests that climate change might explain the increase in cod populations during the 1960s and 1970s, despite a significant expansion of commercial fishing. In that period, he notes, the population booms at the base of the food chain were timed particularly well for young cod.

Since then, the timing has shifted and cod populations have plummeted. Today, the North Sea contains only one-third the number of cod considered a minimum to sustain a fishery.

Brander’s findings suggest that shifts in the timing of population peaks aren’t the only problems plaguing the North Sea fishery. For example, the overall abundance of diatoms and other plankton is dropping, and the species mix is shifting in favor of those having smaller body sizes.