By Erica Goldman
The boat's engine coughs once but does not turn over. David Kimmel tries the key in the ignition again. Still nothing. He checks the horn and the boat's winch device. What's the problem?
Everything else seems to be working fine. The boat has a brand new motor and, if he can get it started, Kimmel will be the first to take it for a test-drive.
Kimmel turns the key once more, and this time the engine roars to life. He shrugs his shoulders quizzically, red windbreaker almost reaching up to blue Yankees cap, and starts casting off lines. His first challenge: to maneuver out of the narrow boat harbor, here at the Horn Point Laboratory in Cambridge, Maryland, part of the University of Maryland Center for Environmental Science (UMCES) located on the Choptank River.
On this cool, clear day, Kimmel is heading out to the deepest spot in the river in search of zooplankton, tiny floating animals that form a key but often under-appreciated link down at the base of the Bay's food web. Subtle shifts in the who-eats-whom cycles down among these tiny, microscopic animals could be a sign of a huge, planet-wide change Ñ a change called global warming.
His boat is carrying gear that will let Kimmel "look" for zooplankton by listening for them. The key piece is an acoustic sensor that drops in the river and beeps out sound waves through the water column. When the sound bounces off the hard exoskeletons (carapaces) of tiny animals, Kimmel can look at the volume of the bounce-back responses and then estimate roughly which creatures inhabit the depths — oyster larvae, shrimp, copepods, and amphipods to name a few.
Kimmel spins the wheel several rounds to the right, and then quickly cuts back left. It takes a lot of effort just to keep the boat moving straight through the narrow channel. Once the boat clears the jetty, Kimmel slowly takes the engine up to 4500 rpm, then alternates between high and low speeds, carefully breaking in the new motor. The boat heads toward the Choptank's deep hole, where the river bottom lies more than 90 feet down.
The concept behind Kimmel's sampling scheme is simple. Find out where in the Bay different zooplankton live and pinpoint hot spots of abundance. Then try to understand how these hot spots behave, how they move around, how they appear and disappear. By comparing his current findings with findings from the past, he hopes to spot the early warning signs that show how global warming could change the Chesapeake.
The behavior of small creatures in the food web, — oyster and crab larvae, shrimp, copepods, and amphipods — may provide clues to the effects of climate change on the Chesapeake.
Clues to global warming are hard to come by in an estuary like the Chesapeake. Wet years or dry years, cold winters or warm winters, active hurricane seasons or quiet spells Ñ all alter the Chesapeake with some frequency. They raise or lower the volume of riverwater flowing into the estuary, shaping the boom and bust cycles of algae and other species in the Bay each year. All that year-to-year variability makes it hard to pick out the larger, long-term changes triggered by global warming. How do you separate the signal from the noise? How do you find evidence of climate warming among all that annual variability?
When Kimmel first came to Horn Point Laboratory in 2001 as a postdoctoral fellow, scientists did not yet have a firm understanding of how variation in temperature and rainfall affect the Bay's animal life on an annual basis. With his expertise in zooplankton, Kimmel decided to explore the effect of climate variability on this middle level of the food web. In the eat-or-be-eaten hierarchy, zooplankton appear on the menu between fish and algae — with algae forming the base of the food chain as primary producers that convert the sun's energy into food.
To sort out swings between wet years and dry years, Kimmel turned to a new tool called "synoptic climatology." He teamed up with David Miller, then a Ph.D. student at Horn Point working in the lab of oceanographer Larry Harding. Synoptic climatology helps to assess how climate varies on a regional scale, explains Miller. The approach uses data on daily sea level pressure to classify common atmospheric circulation patterns that influence the Chesapeake Bay region. By looking back through a time series of weather data, Miller can statistically sort weather patterns into categories and calculate the mean conditions that occur in a given time period. Examining as many as ten dominant patterns, he compares weather shifts to changes in flow in the Susquehanna River, and then to downstream changes in the Bay.
Kimmel is linking this climatological approach to the Bay's eat-or-be-eaten hierarchy by working with a zooplankton monitoring record maintained by the EPA Chesapeake Bay Program from 1984 to 2002. He and his colleagues are making connections between year-to-year changes in the Bay's climate patterns and its populations of zooplankton and fish. In a paper published in Estuaries and Coasts in July 2006, the team found that wet winters followed by high river flows in the spring produce conditions favorable to the zooplankton species Eurytemora affinis, a major food source for striped bass larvae. Closing the statistical loop, the group found that wetter winters could be positively linked to robust, healthy striped bass populations.
That's evidence this tool has some predictive power, but the response of the Chesapeake Bay food web to the influence of longer-term climate warming, not just year-to-year variability, remains largely unknown.
By aligning the climate record both with the historical zooplankton data and with data from his current acoustic sampling effort, Kimmel hopes to begin answering the question of how the system is responding to current environmental pressures. The next step, according to Kimmel, would be to try to anticipate what might happen in the future, as the climate changes.
The first thing to happen under global warming will be rising temperatures. Worldwide projections now forecast an increase of 2.5°F to 10.4°F by the end of the century. According to oceanographer Ray Najjar from Penn State University, who studies climate effects on the Chesapeake region, the Bay has never experienced years as warm as those predicted since we started keeping records. This means we don't have a clear sense of how the estuary will respond.
Najjar even questions whether temperature increases will bring more or less precipitation to the area. The response to temperature increases may vary depending on the month of the year, he explains. This frustrates scientists, because they have a better grasp on how the Bay responds to precipitation extremes than to temperature changes. "We know that it's going to get warm, but we don't know what the effects of that warmth will be. We don't really know whether it's going to get wetter or drier, but if we did know, then we'd really be able to say how the Bay would respond," Najjar says. "So we know half the story."
Kimmel has linked robust, healthy striped bass populations to wetter winters that bring an abundance of their major food source, the zooplankton Eurytemora affinis.
The second change global warming will bring is a rise in sea levels around the world, up to 50 cm by the year 2100. What will that mean for the Bay?
In the Chesapeake region, sea level rise could go twice as high (up to 1 meter by the end of the century). While the sea is rising worldwide, land in Bay country is subsiding (sinking), making the relative impact of sea level rise much greater, explains Thomas Cronin, a paleoclimatologist with the U.S. Geological Survey in Reston, Virginia. During the last ice age — when the great ice sheets pushed down to New York, New Jersey, and Pennsylvania — the ice was so heavy that it depressed the area underneath.
"It's like squeezing a rubber ball," says Cronin. "The area you squeeze goes down, but the area around it would go out." But when the ice began to melt, the area previously buried began to bounce back (rebound), while the land outside the ice sheet, like the Bay, began to sink. Low-lying regions like Smith Island and the Blackwater National Wildlife Refuge could eventually be underwater.
Sea level rise could also raise salinity in the Bay. Salt water will move in from the ocean and flow from the southern reaches of the Bay northward, similar to what happens in a drought year now, but on a sustained basis, explains Michael Kearney, a geographer at the University of Maryland College Park.
Evidence for the sea level-salinity hypothesis comes from recent modeling efforts. Using a historical data record for salinity over the past 50 years, mathematical modeler Ming Li from the UMCES Horn Point Lab is able to predict future salinity patterns. If the 1-meter rise in the level of the Bay comes to pass, Li's model predicts that the salinity in the southern part of the Bay could see an increase of up to 5 psu (practical salinity units) by the end of the century.
To further complicate matters, rainfall could also increase with global warming, and river flow from the tributaries may extend the reach of freshwater farther south. "This could cause a biological squeeze," says Victor Kennedy, a biologist at the UMCES Horn Point Laboratory and the first author of a 2002 report for the Pew Center on Global Climate Change on coastal and marine ecosystems. "You've got sea level increasing from one end and rainfall at the other end," he says. "If the estuary becomes constricted in this manner, it would have an effect on the organisms that cannot tolerate either higher or lower salinities."
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."
Fifteen minutes out from the Horn Point harbor, Kimmel checks the depth sounder and slows the boat. The depth sounder reads 70 feet, then 80. Over the Choptank's deep hole, he cuts the motor and drops anchor just shy of 90 feet.
"I hope the anchor's tied on," he says only half-jokingly and begins setting up the odd equipment that will help him locate and identify zooplankton species in the Choptank. On the flat surface above the steering wheel, he plants his computer data logger while Jamie Pierson, his post-doc fellow, and Horn Point microbiologist Byron Crump untangle the mess of wires at the winch. Then they clip the hook onto the metal cage that holds two instruments about to make their way to the Bay's bottom. Something they call TAPS is actually a Tracor Acoustic Profiling System and their CTD is a traditional meter for recording Conductivity, Temperature, and Depth.
Enclosed in their metal frames and tethered together with white line, the two instruments make a bulky package. Pierson and Crump position the instruments and Kimmel engages the winch. The rope sways precariously for a moment, but soon stabilizes as the winch lowers the instruments into the water. Kimmel turns on both devices just beneath the water's surface to get the air out and let them stabilize in their new surroundings.
Soon the TAPS system and the CTD meter are headed for the river's bottom. As they descend, the CTD meter sends Kimmel's computer a continuous stream of information. Water temperatures are actually warmer several meters down this time of year, typical of an early fall profile when the night air begins to cool. The water is well mixed and there are no signs of low oxygen (hypoxia).
The TAPS instrument uses six transducers to send out sound signals at six different frequencies. Each frequency measures a different size class of zooplankton, from 200 microns to 18 mm. That range spans the size of tiny copepod juveniles called nauplii, up to the size of a large shrimp, about as big as a seedless grape, at the top end of the range. The lower the frequency of the sound emitted, the bigger the animal the transducer can detect. As it descends, the CTD meter measures a continuous profile� of salinity, temperature, and depth.
Kimmel stops the instruments at different depths and records continuously for several minutes at a series of heights off the river's bottom. Data come in fast and furious but the acoustic pinging of the TAPS device registers no signal on Kimmel's laptop. All those data are logged into the instrument itself, to be downloaded and analyzed at a later date. To figure out which organisms inhabit these depths will require a lengthy analysis process back in the lab.
Whether or not Kimmel has hit upon a hotspot of zooplankton with this measurement remains to be seen. This day will register as only one in a long series, this sampling station one of many that line the length and width of the entire Bay — one data point in a vast array. It's the giant zooplankton record as a whole that will offer clues about the impact of global warming on the Chesapeake's food web.
Despite studies like those by Kimmel, David Miller, Tom Miller, Harding, and Orth, the Bay region has lagged behind other ecosystems in the U.S. and around the world in evaluating the current and future impacts of climate on its flora and fauna. "The Bay community is becoming interested in climate issues, but we are not yet at the tipping point where there is a significant amount of funding available," says Kimmel. "It's kind of a shame because this is one of the best studied estuaries, especially from a historical perspective."
"How do you get people to understand what it could be like in the future, and what if we are wrong?"
Elsewhere the impact of global warming on plants and animals has definitely reached a threshold that has caused the scientific community to sit up and take notice. Will changes like these — the northward drift of a butterfly or even the loss of eelgrass in the Chesapeake — be sufficient to rally broad public and political will?
Kimmel has his doubts. The political will in the Chesapeake Bay region may be close to maxed out, he suggests. "Unless we demonstrate that there is going to be a large-scale fisheries change, people might not care. Even then, they might not care," he says. "This is going to be the hardest sell of all, because it's a prediction. We've been making these predictions for fisheries, like blue crab, for some time. But climate is even a tougher sell because many people don't believe in global warming."
Part of the problem, he says, is that climate change does not have an immediate impact on people's lives. The only way that we get a taste of what it might be like is with events like the heavy rains in June 2006, he says. But even that was temporary. "How do you get people to understand what it could be like in the future, and what if we are wrong?"
Rallying people around the issue of climate change in the Chesapeake Bay may indeed prove difficult. The long-term effects of nutrient pollution in the region have taken a toll on public and political will and consumed the lion's share of resources directed at the watershed. People have only just started to consider the possibility that global warming may offset the positive impacts of the tremendous nutrient reduction effort in this region.
"Climate predictions may tell us that if we stay on this trajectory with greenhouse emissions, even if we spend $15 billion in nutrient reduction, climate change could just increase nutrient levels in the Bay and that money would be wasted," says Kimmel.
Does this mean that climate warming will doom restoration efforts to failure? While we can't know the answer with certainty, the outlook is likely not as bleak as all that. Managers have already begun to incorporate scenarios for climate warming into their next round of model predictions for 2030, which will amend the nutrient reduction strategy for the period after 2010 — the court-ordered deadline for getting the Bay off the federal list of impaired waters (see Model Forecasts for a Warming Watershed). Researchers have already begun to work more closely with city planners to incorporate predictions for sea level rise with community growth objectives (see Bad Storms on the Rise). While not a guarantee, proactive efforts to anticipate the impact of climate change on the region should help safeguard against unexpected surprises.
Anticipating changes in the estuary itself, shifts in the composition of plants and animals, and the who-eats-whom interactions of the food web, may mean revising our assumptions that a Bay in the warming world of the future will closely resemble the Bay of the past. Restoration of eelgrass in the southern Bay, for example, may prove difficult or impossible if the region gets warmer, promising little return on our financial investment. But this does not necessarily mean that ecological services provided by other species of underwater grasses, if healthy and abundant once again, would not perform most, if not all the necessary functions — providing sufficient food and habitat for waterfowl, fish, and shellfish, filtering and trapping sediment, absorbing excess nutrients, and inhibiting shoreline-eroding wave action.
Meanwhile, the alarming current trajectory for global warming might yet be temporary. While rising temperatures are a fact of the near-term future, because current levels of greenhouse gases in the atmosphere cannot be immediately reversed, future policy decisions on local, national, and global scales may buy us time — time for a further course correction for the Chesapeake watershed.
Kimmel flips the switch on the winch and the acoustic zooplankton counter and CTD instrument slowly make their way to the surface, wending up from a depth of 87 feet. He keeps an eye on the screen of his laptop to keep track of the depth so that the instruments do not break the surface too fast.
He looks at his laptop and then back out toward the water. The screen reads 1.5 meters (5 feet) and still the two large instruments, encased in their bulky metal frames, are nowhere to be seen. The waters of the Choptank River, murky and turbid, completely conceal them from view, even at that shallow depth.
Kimmel glances at Jamie Pierson, his post-doc who is a recent transplant from Seattle and the clearer waters of Puget Sound. Pierson is eyeing the water incredulously, astounded that he still can't see the instruments. "Welcome to the Chesapeake Bay," Kimmel says ironically. Once he has the high-tech package back aboard, he turns the boat around and starts heading for shore.