Fish swimming along the North Carolina coast this spring may be especially well fed, thanks to New York, Pennsylvania, West Virginia and other far-flung reaches of the Chesapeake watershed.
In an unusual event, scientists this year have observed huge amounts of algae flowing out of the Bay and onto the coastal shelf. It is a sign, they say, that chlorophyll concentrations - a measure of algae production - are among the highest ever observed in the Chesapeake. More algae was produced than could be consumed within the Bay.
"Normally, the Chesapeake Bay is pretty selfish," said Mike Roman, a scientist at the University of Maryland's Horn Point Environmental Laboratory. "There's a lot of recycling, and not much of this production goes out on the shelf waters."
But this year, he said, "if you had a satellite that saw chlorophyll, you'd probably see a green river coming from Chesapeake Bay and going down the coast all the way to North Carolina."
While no final analysis is complete, this spring's algae blooms rival or surpass those observed in the previous high years of 1987 and 1990.
When Lawrence Harding, a scientist at Horn Point and U-Md.'s Sea Grant College who monitors the algae blooms during air surveys, made his first flights this spring, he observed massive expanses of red tides in the upper Bay while more southern areas were filled with golden-brown water - a situation that persisted for weeks.
The red tides were caused by large blooms of dinoflagellates, a type of algae that is capable of swimming in the water, while the golden brown water was caused by blooms of diatoms, a larger, more dense algae that usually sinks to the bottom more rapidly.
"There's definitely a lot more biomass," Harding said. This year's blooms, he said, "seem to be more widespread, lasting longer, and extending farther toward the mouth of the Bay."
The blooms are far more extensive than those observed during 1993 and 1994, both of which had unusually large flows of fresh water - or "freshets" - into the Bay.
The reason is timing. Freshets carry with them large amounts of nutrients that can trigger algae blooms. While the spring freshets of 1993 and 1994 carried lots of nutrients into the Bay, the blooms were limited because the water was clouded by sediment, which was also pouring into the Chesapeake, blocking sunlight that algae need to grow.
This year, snowmelt and rain triggered floods throughout the watershed in January, resulting in the highest water and nutrient flows into the Bay in more than two decades.
But most types of algae don't grow in January because it's too cold. By the time it warmed up in March and April, the sediment that had been carried into the Bay with the flood waters had settled to the bottom. As a result, the water had cleared significantly, and there were plenty of nutrients to feed the algae.
While that alone would have resulted in a large bloom, other factors contributed as well, Harding said.
In late April, when blooms are usually declining, storms with winds of more than 60 knots stirred the Bay's water. Diatoms - which had been sinking to the bottom - were mixed back to the surface layer, where they were again exposed to sunlight and therefore continued to grow. The accompanying storms, meanwhile, delivered more nutrients into the Bay.
The blooms continued through much of May - weeks longer than is typical. They filled the southernmost part of the Bay and extended out onto the continental shelf.
That's unusual because algae normally consume most of the nutrients before they reach the mouth of the Bay. Typically, springtime concentrations near the Bay's mouth are only 1 to 3 milligrams per cubic meter. This year, they approached 30. "Which is very rare," Harding said. "I've never seen it."
That algae will become food for copepods - a type of zooplankton - which in turn can be eaten by larvae and small fish which, in turn, may be eaten by larger fish. At least conceptually, if the Bay's excess algae are transformed into fish production, fishermen along the coast this year may be thanking the Chesapeake, Roman noted.
But what about the Bay itself?
One of the Chesapeake's main water quality problems is that in any given year, too many nutrients - from sewage treatment plants and runoff from croplands, suburban lawns and other areas - fill the Bay. That phosphorus and nitrogen fuels algae blooms. When there are more algae than predators in the Bay can consume, the excess typically sink to the bottom where they decompose in a process that depletes water of oxygen needed by other organisms.
That is the basis for the Bay Program's 40 percent nutrient reduction goal. Computer models indicate that amount of reduction in phosphorus and nitrogen, in a typical year, should reduce algae production enough to improve oxygen levels at the bottom of the Bay.
But a wide array of factors - such as size, temperature and timing of flows into the Bay - affect how much algae is produced and what happens to it. In any given year, the amount of algae that ends up at the bottom of the Bay, and the amount that is passed up the food chain, can vary dramatically.
Just as the blooms washing out of the Bay could enhance fish production along the coast, they may boost production in the Bay. This year's extreme bloom is giving scientists a chance to study how such an event plays out.
"Basic biological theory would tell you that if you have an increase in primary production, that you might expect to see some increases in secondary production up along the food chain," said Ed Houde, a scientist at U-Md.'s Chesapeake Biological Laboratory. "There's more food. It's that simple."
But exactly what that means for fish production, no one knows.
As a rule of thumb, the food chain is highly inefficient. Only 10 percent of the biomass at one level is transferred to a higher level. That is, 10 percent of the energy contained in algae is transferred to zooplankton, the next rung up the food chain. In turn, about 10 percent of the zooplankton energy is transferred to fish larvae and small fish. About 10 percent of the biomass of those small fish is transferred to larger fish, such as striped bass.
But, said Houde, "if you double the amount of chlorophyll, which might have happened this year, I don't think you're going to see a doubling of copepods and a doubling of fish," Houde said. "It's going to dissipate somehow."
The reason, again, is timing. Algae begin grow within a matter of days when a wave of nutrients fills the Bay. Zooplankton take longer - maybe several weeks or longer.
A sudden burst of nutrients can, therefore, cause an algae bloom that would grow, die and sink to the bottom before the zooplankton can consume any. But if climatic conditions spur a longer-lasting bloom - one fed by a continued flow of nutrients - the zooplankton population gets a chance to expand and consume larger amounts of phytoplankton.
This is critical. Being longer-lived, zooplankton can stabilize the nutrients contained in the algae and "keep them in the ballpark, or the playing field," Roman said. "If there were not zooplankton, or fewer zooplankton, more of that phytoplankton would just grow up, sink, rot and use up the oxygen."
And preliminary indications are that this year will, in fact, have high zooplankton production, Roman said. The next critical step is whether the species that consume zooplankton - fish larvae and small fish such as bay anchovy - are present in adequate numbers to consume the zooplankton crop before it, too, dies and sinks to the bottom.
"The hope is to have as much of the nutrients as possible transferred into long-lived species, such as fish," Houde said. "Such species could transport nutrients out of the Bay by migration, maybe be caught in a fishery, or sequester the nutrients at least for a number of years rather than just recycling them and perhaps just being the stimulus for another bloom during the summer, which, of course, is the foundation for lower oxygen in the Bay."
But this year's picture is far from clear at this point. Bay anchovy are usually the most abundant zooplankton-eating fish, Houde said. But this year, while extremely high numbers of zooplankton were counted in a prime production area north of the Bay Bridge, there were no bay anchovy to eat them.
"Part of the reason is that the Bay is so fresh that the conditions are not really conducive for the fish [bay anchovy] to be up there," Houde said. "There's plenty of food up there for it, but it's not up there. Last year it was."
Houde and colleagues will try to determining whether bay anchovy return in time to eat the copepods. If they don't, what will? Another fish such as white perch? If so, will it be as efficient as bay anchovy in converting nutrients to more fish production? Or will the nutrients be consumed by something like short-lived jellyfish which may die and return the nutrients to the water in a matter of weeks?
The answers are important for the Bay's production. Whether fish spawn in time to take advantage of such an abundant zooplankton crop can make a big difference in how many will survive. "The average fish larvae dies in Chesapeake Bay," Roman said. "Something like 99 percent of them die. But there is a big difference between the amount of striped bass or bluefish you catch if the survival rate is 1 percent or 2 percent."
Because of the way nutrients dissipate with each step up the food chain, the scientists cautioned, tracking their movement becomes difficult to monitor after the zooplankton stage.
Still, Harding, Roman, Houde and a team of other Bay region scientists are tracking how this year's algae blooms to gain new insights about how they ultimately are used within the Chesapeake. Their work, is part of a six-year research project known as TIES - Trophic Interactions in Estuarine Systems. This is the second year of the effort, which is aimed at better understanding biological relationships within the Chesapeake, and how - and why - they vary from year to year. The research is supported by the National Science Foundation.
Ultimately, this type of information will help scientists and managers better understand how certain actions can impact the Bay - and what is beyond their control. For instance, people will have little ability to control natural events like this year's floods, an event so dramatic it overshadows nutrient-reduction efforts. "We have to be willing to accept some setbacks when we have weather controlling what goes on," Harding said.
At the same time, better understanding of Bay interactions may some day open the door to new management possibilities. In theory, for example, Roman said it might be possible to manipulate the amount and timing of flows past the Conowingo Dam on the Susquehanna River in ways that could enhance fish production - something that is done to increase production elsewhere, such as San Francisco Bay. But the knowledge that will allow such actions is still in the future.
"We all would like to be able to sit at our computers and predict how many striped bass are going to be produced this year, or, if you cut down the nitrogen a little more than you would have less anoxia [low oxygen conditions] and more fish," Roman said. "But there are certainly a lot of elements of the puzzle that we really don't understand yet."