Anyone who ever doubted that lots of little things could add up to a big problem should take a look at how one of the Bay's smallest species - algae - affect the nation's largest estuary.
Algae, after all, are the major target of the Bay cleanup effort. The Bay states have been working since 1987 to cut nitrogen and phosphorus entering the Chesapeake to eliminate the excess algae growth that fuels low oxygen conditions - so-called "dead zones" - in deep parts of the Bay.
But scientists and water quality managers increasingly recognize that too many nutrients can fuel a host of other ill effects tied to algae production.
Algae can blot out the light for important underwater grass beds, and blooms of certain harmful - sometimes toxic - algae species can threaten fish and humans. Too many nutrients can even shift the types of algae available, possibly shaking the entire food web from the base to the top.
The bottom line is that what may have been the "right" nutrient reduction goal to deal with the low oxygen problem may not be the same goal needed to protect grass beds or reduce the risk of pfiesteria outbreaks.
"We're starting to factor that in," said Rich Batiuk, associate director for science with the EPA's Bay Program Office. "In rivers, when you don't see a direct oxygen problem, that doesn't mean you don't have an algae problem."
That was recognized by the Executive Council - the governors of Maryland, Virginia and Pennsylvania, the mayor of the District of Columbia, the EPA administrator and the chairman of the Chesapeake Bay Commission (representing the state legislatures) - last October. It directed that nutrient reduction targets be re-examined to ensure "water quality that will support the living resources of the Bay and its tributaries."
That doesn't mean that concern about oxygen levels is diminished, Batiuk said, only that more issues will be factored into setting nutrient reduction goals.
In fact, the link between excess nutrients and the loss of oxygen in water - known as eutrophication - remains one of the best understood connections between nutrients and water quality because it has been studied in lakes for decades. It is a natural process, but can be accelerated when human activities dramatically increase nutrients.
The scenario goes like this: Excess nutrients fuel more algal growth than can be eaten by zooplankton, fish, oysters and other consumers. When that algae dies, it sinks to the bottom. Once there, it is food for bacteria which are consumed by other tiny microbes. This is known as the "microbial loop" - instead of feeding the food chain that leads to fish, crabs, herons and other large animals, the microbial food chain is mostly restricted to populations of single-celled protozoans and other microscopic creatures.
What's more, small organisms have faster metabolisms than larger organisms. As a rule of thumb, a pound of bacteria consumes far more oxygen than a pound of fish or something else. The result is that they drain some areas of oxygen needed by other creatures.
"If we're causing a shift of biomass in the Bay from phytoplankton toward other microbes - and smaller ones - we're basically going to be stimulating oxygen depletion," said Hugh Ducklow, a professor at the Virginia Institute of Marine Science.
The extent to which that has happened is uncertain because microbial studies only go back about 10 years, he said.
"We know that there is generally a high ratio of bacteria to phytoplankton in estuaries, and the Chesapeake Bay actually seems to be at the high end of the estuaries that we've looked at," he said. "So there are some indications that the Bay is already kind of rich in bacteria."
Growth of microbial communities can be promoted not only by excess algae, but by certain types of algae. The preferred food chain in the Bay goes something like this: Algae (phytoplankton) is consumed by fish larvae or tiny aquatic animals known as zooplankton, which in turn are consumed by larvae, fish or other larger organisms. But the mix of species that historically supported that food web can be altered by changing nutrient quantities.
"As you add nutrients to the system, they will encourage other forms of algae to develop that can outcompete those earlier forms," said Harold Marshall, a professor at Old Dominion University. "That is going to change your composition, which in turn is going to influence the kinds of herbivores and zooplanktors (algae consumers) that feed on them. This, in turn, would have a domino effect right up the food chain. "This won't take place overnight," he added, "but it is one of those successional patterns that we find in a lot of bodies of water that are subject to this kind of nutrient enrichment."
The things that "graze" the Bay's algae beds typically prefer large phytoplankton. In the classic picture of the Bay food web, those larger types of algae typically lead to things like oysters and striped bass.
Besides nutrients, some of the largest alga forms also require silicon for growth. But the supply of silicon in the Bay has not risen dramatically as have nutrients. That means there is plenty of excess nitrogen and phosphorus left over for the production of smaller algae.
"The larger zooplankton just can't deal with food particles that size," said Richard Lacouture, of the Academy of Natural Sciences' Benedict Estuarine Research Center. "They need something different."
It isn't just size that is important, it can also be the shape of algae. In recent years, Lacouture has observed increased numbers of a species of long, slender species of blue-green algae avoided by grazers. "It would be like you or I trying to eat a 2-foot sausage," he said.
While the small algae may not be suitable for larger grazers, they often are the right size to feed oxygen-depleting microbes. So an algae population change can have a twofold effect: restricting food for fish, while worsening the oxygen situation.
The degree to which such a switch has occurred is difficult to assess because the Bay Program's monitoring effort, which provides the most comprehensive Baywide look at algae, only dates back a little more than a decade.
But in examining that information in the mid part of the Bay, Lacouture has noticed a trend toward increased amounts of smaller algae and less large algae over the past six years.
In part, he said, that likely stems from several years of unusually high flows into the Bay. But, he said, the timing and amount of nutrients - as well as the ratio of phosphorus to nitrogen in the water - was likely another factor. "The whole dynamic of what's going on out there has to be changing with nutrients," he said. "We have noticed some very distinct changes over time. We do think there could be some consequences up the food chain as a result of some of the things that we are seeing at the bottom of the food chain."
Changing the types of algae that proliferate can not only change the food supply for fish; in some cases it could kill them outright. Nutrients - along with the right environmental conditions - can also fuel the growth of blooms that can be toxic to fish or other species.
Even increased incidences of non-toxic blooms contribute to the Bay's problems by fueling low oxygen conditions and clouding the water. Because of the lack of a long-term information, it is difficult to document the degree to which algae blooms have increased in the Bay, but such a trend - and the potential for harmful blooms - remains a worry.
"Blooms are very common in the summer, and they've been common in the Bay estuary now for years," said Marshall, who has worked with Virginia officials to begin a tracking program which samples all blooms in the state. "What we're concerned about is if we have more blooms of the nuisance species, and if the extent of the blooms is greater and their duration is increased, that's when we have problems."
Before European settlement, evidence from sediment core samples taken from the bottom of the Bay suggest that oxygen depletion in bottom portions of the Chesapeake was less common than is the case today, and that the algae makeup tended to favor larger species more strongly than is seen today.
The implication, said Tom Malone, director of the University of Maryland's Horn Point Laboratory, is that nutrient inputs to the Bay currently exceed its capacity to assimilate them into food chains leading directly to shellfish and finfish consumers. But, no one knows exactly what the "right" amount of nutrients for the Bay is. "One of the biggest challenges to the scientific community on the Bay today is to determine what that assimilation capacity is. That is, what factors govern the rate at which nutrients can be assimilated into food chains leading to large consumers vs. bacteria food chains that lead to eutrophication and how can this information be used to manage nutrient inputs to the Bay in cost-effective ways?" Malone said.
Nutrients are not the only factor that will affect the Bay's assimilation capacity, Malone noted. If there are large, healthy populations of oysters or algae-grazing fish, the Bay can probably handle more nutrients. "This is why it is important to coordinate fisheries management and environmental management," he said. "As you begin to fish down populations of large consumers, like oysters, you have a big impact on the capacity of that system to assimilate nutrients because you are removing a big chunk of biomass where those nutrients used to go."