The Chesapeake's declining nettle population is a symptom of bigger forces at work, explains Breitburg. Nettles have followed a downward trajectory that closely mirrors the downward spiral of the native oyster, Crassostrea virginica, which began its unabated freefall in the early 1980s as the result of the cumulative effects of overfishing and the diseases MSX and Dermo.

Like oysters, "sea nettle densities in the Patuxent are now more than an order of magnitude lower than in the mid 1980s," says Breitburg. And these parallels are hardly a coincidence.

Oyster shells provide a hard surface for sea nettle polyps, the sedentary, bottom-dwelling stage of the jellyfish's life cycle, to settle upon, explains Breitburg. Without enough hard surfaces available, sea nettles cannot complete their reproductive cycle. Breitburg suspects that the Bay reached a "threshold" level of hard surface availability in the Patuxent right around 1985, beyond which the nettle population could not sustain a constant level (see graph above). In addition, since the decline of nettles has also led to a rise in the population of comb jellies and since comb jellies feast upon free-swimming larvae, oyster larvae face higher and higher predation rates. So the food web's sea nettle-oyster link could now be stuck in a rut, explains Breitburg — fewer oysters mean fewer nettles, fewer nettles mean many comb jellies, many comb jellies mean fewer oyster larvae, fewer oyster larvae mean fewer oysters — and so on.

A Bay with fewer oysters, of course, heralds a whole slew of problems. Oysters achieved ecological fame for their ability to filter algae from water and to help maintain water clarity by locking up nutrients (nitrogen and phosphorus). Historically, they've helped the Bay to absorb natural and human-caused insults without a change in "ecological state."

"Oysters helped to make the Bay resilient, providing a buffer to the whole ecosystem," says Breitburg.

The Chesapeake's troubled tale of shifting states and resilience lost has been widely told by now. First came changes in land use, the loss of oysters and underwater grasses, overfishing and the spread of low oxygen zones. Then in 1972 came the onslaught of a monster tropical storm called Agnes that pounded the Bay with water, nutrients, and sediment loads. This cascade of catastrophes proved more than the ecosystem could handle.

"Something fundamentally changed in the 1970s," says sediment biogeochemist Jeff Cornwell at UMCES Horn Point Laboratory in Cambridge, Maryland. The shallow regions reached a threshold, which basically caused a shutdown of coastal processes. "It is somewhat of a chicken and egg problem, though, because many of these changes occurred at the same time," he says. Reduced light penetration, the growth of small plants (epiphytes) on underwater grasses, oyster mortality, and increasing anoxic conditions caused a shift from an ecosystem driven by photosynthetic bottom processes (dominated by underwater grasses), to one in which phytoplankton in the water column carry out the lion's share of the Bay's photosynthesis (primary production).

Many scientists suggest that the Bay's dramatic regime change occurred because of a combination of factors that limited its ability to bounce back. In his book, Turning the Tide, journalist Tom Horton eloquently characterized the lost buffering capacity of the Chesapeake, citing the highly interdependent environmental problems caused by the conversion of forests, wetland and shoreline areas to impervious surfaces, like pavement. Forests and wetlands trap sediments and help to slow the flow of pollutants into the Bay from agricultural runoff higher in the watershed. Their loss, coupled with the decline of Bay grasses and oysters in the 1970s and 1980s, caused the Bay to lose much of its resilience, its ability to recover from disturbances without undergoing a fundamental change, making it more and more sensitive to events that could push it over the edge, such as storms like Agnes.

Efforts to "Save the Bay" are usually synonymous with bringing it back to a stable state that had clear water, underwater grasses, bountiful fish, crabs and oysters. But the current state of the estuary lacks many of the buffers, such as oysters and grasses, which helped sustain that state in the first place. The question remains unanswered: Can we restore some of the Bay's resilience and, by doing so, jump-start its ability to rebound the rest of the way back to a bottom-driven system? If so, will we see a Bay similar to the 1970s? If not, what will this "restored" Bay look like?

Gelatinous powerhouses of the Bay's food web, both sea nettles (Chrysaora quinquecirrha) (top) and comb jellies (Mnemiopsis leidyi) (bottom) are simple in form, yet profoundly influential in the Bay's ecosystem.

Ecological resilience is a slippery concept — it is relatively straightforward in theory, but it remains very difficult to measure. In its most general definition, ecological resilience provides a measure of the amount of disturbance that an ecosystem can withstand without shifting into an "alternate stable state" (see sidebar, "The Language of Resilience"). For the Chesapeake, the shift from a food web dynamic driven by benthic processes — such as underwater grasses and oysters — to one driven by phytoplankton in the water column is a classic example of what some ecologists call a regime shift, a shift between stable states.

What is less clear, however, is what weight to give multiple factors that caused such a transition to take place.

"Ecological systems are idiosyncratic," says ecologist Lance Gunderson from Emory University in Atlanta, Georgia. "Some systems are very resilient to a wide range of perturbations; some are not. It often takes a lot of work to see what is involved in state transitions."

Gunderson is an expert on the Florida Everglades and one of the founding scientists of a group called the Resilience Alliance, a research organization of scientists and practitioners from many disciplines who collaborate to explore the dynamics of social-ecological systems. Their goal is to understand how different systems function, mostly through case studies, to learn how to effectively influence their resilience and adaptability.

The Alliance has its conceptual roots in a framework that is more than 30 years old. In a classic 1973 paper theoretical ecologist Crawford (Buzz) Holling first introduced the concept of resilience to the ecological literature as a way to help understand non-linear dynamics observed in ecosystems — such as an unexpected major change following a storm. Today, the appeal of this approach is growing, as a way to think about how humans interact with their environment and how they may move toward a workable framework for management (see sidebar, "Towards Adaptive Management").

"Resilience offers a satisfying way of thinking about ecological changes," says Gunderson. "Humans have preferences about the desirable states of ecological systems and want to know how to either maintain the current state or what to do if a system is not in a desired state."

But before resilience can inform management decisions, researchers must develop a systematic way to anticipate when a system is getting close to a threshold or tipping point and prevent it from going over the edge (see sidebar, "Identifying Thresholds"). They must also learn how to turn around systems that, like the Bay, have arguably already shifted into an undesirable stable state — one that may be resilient in its own right.

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