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Drink Up Your Soft Drink – Another Critical Habitat in Decline

by Bill Chameides | July 9th, 2009
posted by Erica Rowell (Editor)

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Among other uses, seagrasses are a food source for endangered species such as the manatee, dugong, and green turtle. (NOAA)

A new paper in the Proceedings of the National Academy of Sciences takes stock of the global status of seagrass meadows. Seagrass meadows?

Of all the popularized marine habitats under assault in recent decades (such as coral reefs, mangroves, salt marshes — see “Are We Losing or Gaining Wetlands” section of our previous post), seagrass meadows have to be one of the least well-known, and yet their relative obscurity doesn’t jibe with their ecological and economic importance.

The 50 or so species of seagrasses that grow around the world evolved from land plants. One reason we know this is that unlike seaweed and algae, seagrasses have flowers, roots, and specialized cells that transport nutrients. Found in shallow coastal waters around the globe, they provide critical habitat and nurseries for a variety of fish and shellfish and support commercial, recreational and subsistence fisheries worldwide. There’s more. They also improve water quality by removing excess nutrients and sediment from coastal waters. Their economic value in terms of nutrient cycling alone has been estimated at almost $2 trillion a year.

Water Fight: Seagrasses Under Assault

Like many other aspects of ocean life, seagrasses appear to be under attack. Michelle Waycott of James Cook University and colleagues have just released the results of a comprehensive global assessment. Among their findings:

  • 29 percent of the known areal extent of seagrass has been lost since the late 1800s,
  • 58 percent of the sites in their survey declined in area, and
  • the rate of decline is increasing. Since 1990 the annual rate of loss has accelerated from about one percent per year to seven percent per year.

Possible Causes of Seagrass Decline

What could be causing the seagrass decline?

Disease could be factor. Because seagrass tends to come in monocultures, they are particularly vulnerable to precipitous die-offs. Such disease-related declines have occurred in the past, but they were always regional in nature. By contrast, the seagrass die-back documented by Waycott and colleagues shows up on both coasts of the Atlantic, the Western Pacific, and the Indian Oceans.

Pollution is almost certainly a factor. Most of the coastal areas with large seagrass losses are being developed at a rapid pace. Such development brings an oversupply of sediment and nutrients, which obscure the water column with silt and choke it with algae, leaving seagrass without an adequate supply of photosynthesizing sunlight and undermining its complex root system.

Less certain, but possible contributors include: the loss of natural predators from declines in fisheries, invasive species, a growing aquaculture industry, and climate change.

But all is not lost. Coastal communities have been able to recover seagrass by reducing nutrient and sediment loads. Tampa Bay is one success story. Since 1982 efforts to reduce point source pollution have successfully decreased nitrogen loads to the bay by 50 percent, improving water clarity by 50 percent and recovering about 25 percent of lost seagrass meadows.

At the other end of the spectrum are the coastal waters off the Mississippi Delta, where seagrass meadows are in widespread decline. Not very surprising since this area has one of the world’s most prominent dead zones — this year’s is expected to be as large as ever thanks to higher than normal spring flows delivering a large nutrient load to the gulf.

It would appear that dwindling seagrass meadows in the Gulf of Mexico are one more consequence of our nation’s farming and water quality policies that allow huge quantities of fertilizing nutrients to run off into our rivers and streams. Isn’t it good to know that every time you consume a little corn syrup in your soft drink, some fraction of what you paid, went to buy fertilizer that may have ended up choking some seagrass out of existence. Yum.

filed under: faculty, Planetary Watch, pollution
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