Decline in large predatory fish promotes bloom in algae

Spirogyra, a comon filamentous green algae found in the Baltic Sea. Image credit, Wikipedia.

A new study finds evidence that declines in large predatory fish are promoting macroalgal blooms in the Baltic Sea. Past attention to the problem has focused mostly on nutrient enrichment from agricultural runoff and other anthropogenic sources.

With the pressing threats of fishery declines and eutrophication, Britas Klemens Eriksson and fellow researchers set out to examine the importance of higher trophic level consumers in controlling ephemeral and bloom-forming algae.

The researchers conducted one large-scale study utilizing historical records of perch and pike abundance and algae cover, as well as two field experiments. The field experiments isolated piscivorous fish, added agriculture fertilizers to the water, and manipulated canopy cover. The purpose of the experiments were to determine the effect of each variable on algal recruitment, and small fish and invertebrate abundance and composition.

The large-scale study found a strong negative correlation between large fish abundance and algal blooms. The field experiments provided further evidence that “top-down forcing” may be occurring in coastal systems in which large predatory fish exert control over primary production.

Once large fish were excluded, nutrients added, and canopy cover reduced three shifts took place indicating top-down forcing:

• Increase in small-bodied, herbivorous fish

• Shift in composition and size distribution of grazers, with a significant decrease in smaller gastropod scrapers.

• Increase in algae cover and recruitment

The field experiments also demonstrated cascading effects through all trophic levels. Exclusion of large predatory fish only promoted algal recruitment when grazers were present; without grazers no predator effect occurred.

Previous studies from the Baltic Sea have indicated the importance of top-down and bottom-up control on algal production, yet eutrophication management programs are still focusing most of their efforts on reducing nutrient loads.

This study suggests that wetland restoration should be the next big step in battling eutrophication in the Baltic Sea. Wetlands can naturally reduce nutrient loads, while also providing recruitment areas for piscivorous fish. The authors conclude the Ecological Applications article with a strong message,

“Local declines of larger predatory fish had complementary effects to nutrient enrichment, by promoting the production of benthic algae through loss of grazer control. This indicates that failures to restore eutrophication impacted coastal areas by reducing nutrient loads may depend on failures to acknowledge top-down effects from degenerated predatory fish communities.”

--Reviewed by Evyan Borgnis

Eriksson, B., Ljunggren, L., Sandström, A., Johansson, G., Mattila, J., Rubach, A., Råberg, S., & Snickars, M. (2009). Declines in predatory fish promote bloom-forming macroalgae Ecological Applications, 19 (8), 1975-1988 DOI: 10.1890/08-0964.1