Wednesday, July 8, 2009

Seagrass meadows, which serve as essential nursing grounds and habitat for thousands of fish, birds and other marine species, and which also play a critical role in regulating water quality of shallow bays and estuaries, are declining worldwide at a rate comparable to coral reefs and rainforests.

That's the conclusion of a study published on June 29 in the Proceedings of the National Academy of Sciences.

In conducting the study, a team of 14 scientists synthesized data dating back to 1879 from 215 sites around the globe. They found that 58 percent of the world's seagrass meadows are in decline.

Causes of the decline are varied, ranging from pollution stemming from coastal development to propeller scars. Global warming is another factor. So is overfishing, which can result in the loss of an ecosystem's top predator and set off a cascade effect down the food web, eventually reaching herbivores that help keep seagrass clear of algae, the study says.

The authors also found that seagrass loss has accelerated over time, going from less than 1 percent per year before 1940 to 7 percent per year since 1990. Worldwide, 51,000 square kilometers of seagrass meadow have disappeared since 1879. A soccer field worth of seagrass disappears every 30 minutes, according to University of Maryland scientist William Dennison, one of the study's co-authors.

"Our report of mounting seagrass losses reveals a major global environmental crisis in coastal ecosystems, for which seagrasses are sentinels of change," the study concludes.

Despite, the overall declines, seagrass isn't going away everywhere, the study says. In a quarter of the sites reviewed by scientists, seagrass coverage has increased over the length of the data sets.

In Florida Bay, where seagrass serves as nursery or feeding ground for everything from manatee and fish, to lobster and jellyfish, results have been mixed in recent years.

A wide collapse of the dominant turtle grass population in the late 1980 and early '90s, especially in the western bay, turned 10,000 acres of seagrass meadow into bare bottom, said Michael Durako, a University of North Carolina Wilmington biologist who monitors Florida Bay seagrass health. Another 50,000 acres were damaged.

In addition to serving as food and nursery, Florida Bay's seagrass absorbs nutrients like nitrogen and phosphorus, thereby controlling algae growth and helping to give the bay its gin-clear appearance.

The collapse of two decades ago led to more extensive monitoring of Florida Bay's seagrass beginning in 1995. Since then, seagrass in portions of the bay has flourished. For example, in Johnson Key Basin in the western bay where the collapse hit hard, turtle grass cover went from approximately 5 percent in 1995 to more than 25 percent in 2008, data from the state-run Fish and Wildlife Research Institute in St. Petersburg shows. Populations of the less dominant shoal grass and manatee grass have also increased.

Other area haven't fared as well. Blackwater Sound, pressed against the coast of Key Largo in the eastern portion of the bay, saw a slight decrease in turtle grass coverage between 1995 and 2008. Since 2000 turtle grass coverage there has declined from more than 25 percent to approximately 5 percent.

Some of the drop, occurring since 2005, was likely the result of an algae bloom that took hold in the region, FWRI Seagrass Administrator Penny Hall said. Scientists blamed that bloom on phosphorus and nitrogen runoff, which entered the northeast bay as result of the harsh 2005 hurricane season and also because of the rebuild of the 18-Mile Stretch.

Hall said Florida Bay is different from most other bays throughout the world in that it is really a series of basins, separated by sandbar shelves. As a result, water quality and conditions can vary widely.

Florida Bay has also been a victim of propeller scar damage. A recent Everglades National Park study found that at least 8,000 acres of bay bottom are marred by prop scars.

But Hall said the bay has been lucky to avoid some of the nitrogen runoff afflicting other parts of the state. Nitrogen acts as a fertilizer for algae, leading to blooms that darken the water and choke out seagrass.

"I think that we are fortunate that Florida Bay is still in the condition that it is in given what has happened in Florida, around the nation and around the world," Hall said. "But with what we have seen recently in Blackwater Sound, we know that this system can be vulnerable to increased nutrients."

rsilk@keysnews.com