Some species in the Chesapeake may already be feeling the heat. Eelgrass, blue crabs, and the oyster parasite Dermo have shown signs that they may be vulnerable to a climate-related shift in the places they call home (their geographic distribution, or range).

Eelgrass, an underwater grass that provides critical nursery habitat for juvenile blue crabs, experienced a major die-off in 2005. A one-year decline could be a blip on the radar, but seagrass ecologist Bob Orth, of the Virginia Institute of Marine Science, doesn't think so. Orth notes that five of the warmest years in the last century have occurred within the past decade. "Global warming has entered our conversation," he says. "In the last ten years we've seen changes in eelgrass populations that cannot be explained just by poor water quality."

Eelgrass grows best in cool, temperate areas with high salinity. It's already near the southernmost part of its range in the Chesapeake Bay, he explains. Within the estuary itself, eelgrass cannot shift its distribution northward because it cannot tolerate freshwater. If it fails in the southern reaches of the Bay, eelgrass will likely disappear from the Chesapeake.

Loss of eelgrass could have profound consequences for the ecosystem. Best known for its role as nursery habitat for juvenile crabs, eelgrass dominates in shallow water habitats, and its so-called "ecosystem services" may not be readily replaced by another grass species. The cascade of effects through the food web is "a tough one to predict," warns Orth.

As tough as the loss of eelgrass might be, blue crabs may face other profound effects, according to Tom Miller, a researcher at the UMCES Chesapeake Biological Laboratory who has just begun studying prospects for the Bay's largest and most valuable fishery in a warmer world. Miller suspects that blue crabs may be highly sensitive to an increase in global temperature because of their unique life history. In fact, says Miller, a long-term climate shift might lead to a population boom south of the Chesapeake Bay.

Here's how.

The blue crab's life cycle changes dramatically according to latitude, Miller explains. From South Carolina down to the Gulf of Mexico, crabs grow continuously and complete their life cycle within the year. From South Carolina northward, crabs must bury themselves in sediment to survive cold winter temperatures — which means that they won't reach maturity until the following year.

Whether or not crabs overwinter depends entirely on temperature. "There is a magical 11°C threshold," explains Miller. When water temperatures get below 11°C [52°F], crabs can no longer grow and molt. If a warming climate causes winter temperatures to rise in the Mid-Atlantic, the North Carolina blue crab fishery could quickly become more like the fishery in South Carolina — and more productive than the fishery in the Chesapeake Bay, creating an economic squeeze on the Bay's crab fishery. The southern states could replace the Bay as the blue crab capital of the country.

Oysters, of course, were once the Bay's most profitable fishery, and perhaps the best-documented example of a disastrous range shift is the spread of the Chesapeake's number one oyster-killer — Dermo disease. Beginning in 1990, scientists noticed a big explosion of Perkinsus marinus, the parasite that causes Dermo, in oysters in Delaware Bay. Susan Ford, a parasitologist at Rutgers University's Haskin Shellfish Research Lab, started searching up the coast in New Jersey and her team began finding it everywhere they looked. Then they started to get reports from growers in Long Island Sound. By the end of 1991, scientists found the parasite as far north as Cape Cod — a range extension of over 500 kilometers in just a couple of years. Formerly limited to Chesapeake and Delaware bays at its northern extreme, by 1996 Dermo was identified as far north as Maine.

When Ford looked at temperature records for this period, she realized that the range extension of the Dermo parasite corresponded with a clear warming trend, particularly one associated with warmer winter temperatures. A close look at historical patterns of disease incidence revealed that, following a period of high abundance after its initial introduction to Chesapeake Bay in the 1950s, the parasite virtually disappeared during the next decade, years dominated by cooler temperatures — in particular, cold winters. Publishing her findings in the September 15, 2006 online issue of Marine Biology, Ford hypothesizes that the parasite never really disappeared, but waited in some latent state for temperatures to warm.

Climate warming projections for the northern hemisphere point towards even warmer winters in the coming years, says Ford. Oysters further north could soon become susceptible to Dermo, still further changing the dynamics of the troubled East Coast oyster fishery.

Recently scientists have begun to unearth evidence for ecological and evolutionary changes linked to climate warming practically everywhere they look. Statistical analyses show that 41 percent of 1598 species studied — ranging from grasses to mollusks, butterflies to mammals — have responded to the global average warming of 0.6°C that has occurred in the last century, either by shifting their range or the timing of reproduction or development (called phenology). When Camille Parmesan, an ecologist at the University of Texas in Austin, published her first study on the effect of climate on the distribution of Edith's checkerspot butterfly in 1996, it was "a field of one or two," she says. Now journals have published a total of 866 peer-reviewed papers on the subject — 40 percent of them hitting the scientific literature between 2003-2006, Parmesan reports in a synthesis in the Annual Review of Ecology and Evolutionary Systematics published in December 2006.

Why the sudden surge of studies on species and climate change? It's a combination of factors, says Parmesan.

"Scientists are getting more interested in looking at their data sets with respect to climate change," she says. "But in almost every case I've looked at, the last five years have shown an enormously stronger response than I've seen over the last 30 years. People who might not have noticed anything in their system are suddenly saying, 'Gee...the last five years have been really weird.' That is prompting them to go ahead and publish."

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