Floating to the surface. Sargassum, carrying the weight of the food web, floats in the Gulf of Mexico.

Marine Food Web Has Unexpected Base

VICTORIA, BRITISH COLUMBIA--Scientists long assumed that the Gulf of Mexico's booming ecosystem was powered by its most abundant component--the brown seaweed Sargassum. Now researchers suggest instead that free-floating microscopic organisms are carrying most of the weight of the food web, despite their relatively low abundance in the gulf.

The Gulf of Mexico supports a fishery of dolphin fish, wahoo, tuna, and king mackerel. Many fish, along with plankton and shrimp, live and feed within loose mats of Sargassum that float near the water surface. But on close examination, researchers at Texas A&M University didn't see any sign of grazing activity, such as scrapes, tears, or broken fronds, which suggested something other than seaweed had to be at the base of the food web.

To find out what eats what in this ecosystem, fisheries ecologists Jason Turner and Jay Rooker of Texas A&M University in Galveston first analyzed the composition of fatty acids in Sargassum, a green algae that grows on seaweed fronds, and phytoplankton--microscopic organisms that photosynthesize like plants. Turner and Rooker then looked for these fatty acid signatures in five species of invertebrates, such as zooplankton, shrimp, and crabs, and nine species of fish. To the researchers' surprise, as Turner reported here on 11 June at a meeting of the American Society of Limnology and Oceanography, the fatty acids were stamped with the phytoplankton signature. Carbon and nitrogen isotopes suggested that the fatty acids were funneled from phytoplankton to the larger zooplankton and shrimp, and finally into fish.

The study "challenges the dogma that the energy source with the biggest biomass is most important," says limnologist Michael Brett of the University of Washington, Seattle. He says using fatty acids to track energy flow through so many parts of the food web is an intriguing approach.

Related sites
Life history and ecology of brown algae
Home page of Jay Rooker