By Matt Kaplan

Nitrogenous fertilizers and detergents have long been known to cause algal blooms that block sunlight and strangle ecosystems, but a study now reveals that overfishing of large predatory fish is also playing a key part.

Britas Klemens Eriksson at the University of Groningen in the Netherlands noticed that populations of predatory fish in the Baltic Sea seemed to be declining in areas where algal blooms subsequently tended to form. Curious as to whether there was a connection, Eriksson and a team of colleagues from the Swedish Board of Fisheries in Öregrund set up an investigation.

The team reviewed a year's worth of field data on predatory pike (Esox lucius) and perch (Perca fluviatilis) populations from nine areas covering 700 kilometres of coastline in the Baltic Sea. They then compared this information with information collected during the same period on smaller fish and algal populations in the region. They found some intriguing patterns.

"In areas where there were strong declines in perch and pike there were massive increases in smaller fish and large blooms of algae," comments Eriksson. Where perch and pike populations were intact, the surrounding waters had a 10% chance of experiencing an algal bloom; in areas where their populations had been substantially reduced, the chances of an algal bloom were 50%.

Intrigued by these trends, the researchers ran small-scale field experiments for 2 years in unpolluted waters to investigate the forces responsible for controlling algal growth. They manipulated the environmental conditions in these experiments by: sometimes excluding large predatory fish through the use of cages; sometimes adding nitrogenous fertilizer pellets; sometimes applying both techniques; and sometimes leaving areas as untouched controls.

As expected, the nitrogenous pellets increased algal growth. But surprisingly, when predatory fish were prevented from accessing a given area, algae in that area became much more prevalent. The effect even proved to be true when nitrogenous pellets were not added to the system.

"This is the first study to show that top predators are linked to the formation of macroalgal blooms," says marine biologist Heike Lotze, at Dalhousie University in Nova Scotia, Canada.

In tiers

Eriksson speculates that the effect results from the disruption in the food chain caused by excluding the large predator fish. Top-predatory fish feed on mid-level predatory fish, which in turn feed on invertebrate herbivores such as snails and crustaceans. These are the animals that control the algal community. Knock out the top predators, and mid-level predators develop huge populations which, in turn, reduce the numbers of algal-eating species, allowing blooms to grow unchecked, explains Eriksson.

The team report in Ecological Applications1 that, on the basis of their findings, fighting algal blooms by more tightly controlling nitrogenous materials in waste water and agricultural run-off is not the best approach. "If we want to manage algal blooms effectively, we need to start by taking an ecosystem perspective ... we have to restore depleted fish communities," says Eriksson.

"That they are showing effects over four trophic [feeding] levels is really impressive," says Lotze. "We've tried to experimentally explore these sorts of interactions before, but with so many levels there is often too much noise to see trends. That they've managed to get clear results is exciting."

Yet even with these results, nitrogenous material must not be ignored. "When we added nitrogen and removed predators we saw blooms that were two times larger than those created by predator depletion alone. The two certainly appear to be connected," says Eriksson.

This is an important finding, because environmental management programmes currently handle fish population conservation and algal-bloom control as separate entities. "Environmental practice has to change based upon these results," says experimental ecologist Birte Matthiessen at the Leibniz Institute of Marine Sciences in Kiel, Germany. "Eutrophication and fisheries management need to be combined."