By studying three mid-coast Maine estuaries, researchers hope to provide clues to how estuaries operate in the Northeast.
Over the past three years, a research team of oceanographers from the Maine/New Hampshire Sea Grant Program has conducted a series of seasonal survey cruises along three adjacent estuaries in mid-coast Maine - the Damariscotta, Sheepscot, and Kennebec. The preliminary findings of this comparative study will have implications for aquaculturists and others interested in maintaining the health of the region's estuarine systems.
By: Kathleen Lignell
Maine/New Hampshire Sea Grant
North of Cape Cod, the coasts of New England are geologically young - created by the glaciers of the last ice age and constantly changing sea levels. The coastal region consists primarily of rock headlands, coves, bays, estuaries, islands, and sand beaches. Estuaries, the meeting places of fresh and salt waters, are common along the northern New England coast. Most, like the lower Kennebec River - the largest estuary on the south-central Maine coast - are river valleys "drowned" by the rising ocean.
To the northeast of the Kennebec, the Sheepscot, Damariscotta, Medomak, and St. George rivers form major estuaries characterized by salt marshes and mud flats. Compared to similar bodies of water nationwide, most of the bays and estuaries of northern New England are still relatively unpolluted. As the population in the region steadily rises, it has become increasingly important to find out how our complex estuarine systems function and to assess the environmental impacts from coastal development around these estuaries.
About four hundred years ago, the Popham Colony, a settlement of a small group of Englishmen, returned home in 1608 after barely surviving a harsh winter at Popham Colony at the mouth of the Kennebec River estuary. During their short occupation, the colonists constructed the 30-ton pinnace Virginia, the first sailing vessel built in the New World.
Only a short sail away from the historic site of Fort Popham, Boothbay Harbor now sits at the entrance to the mouth of the Sheepscot River estuary and is a bustling resort town attracting thousands of summer visitors. Boothbay is also one of the earliest-settled areas in Maine, renowned for over a century for fine boatbuilding. Today, Lincoln County - which encompasses the Damariscotta and the Sheepscot estuaries - is the second fastest growing county in the state of Maine.
Lincoln County's Pemaquid peninsula stretches into the sea below the town of Damariscotta and abounds with scenic ocean vistas and quiet villages. Near the tip of the peninsula is Christmas Cove, so named by Captain John Smith when he landed there on Christmas Day in 1614. If you turn back up Route 129 from Christmas Cove, and take the fork at Route 130 instead of heading back to Damariscotta, the road will lead to Walpole, a section of South Bristol that is home to the University of Maine's (UM) marine laboratory, the Darling Marine Center.
Since September 1993, the Darling Marine Center has been the hub of activity involving a modern-day group of explorers who have conducted six major cruises along the lengths of each of the three nearby estuaries - the Kennebec, Sheepscot, and Damariscotta. In order to compare the three adjacent estuaries in an efficient manner, the research team needed a large cadre of graduate and undergraduate students to assist on the numerous cruises. More than 15 cruise members have operated instruments, taken and processed water samples, and monitored computers on board the research ship R/V Gulf Challenger.
According to University of New Hampshire (UNH) chemical oceanographer Ted Loder, "When we'd get to a station, it was like a fire drill. With 10 to 12 people on board, we'd have only 15 minutes or so to get the maximum amount of information in the limited time it took to monitor the sites, going from the mouth to the head of the estuary during the ebbing tide."
The team of researchers and their students have conducted a plethora of experiments during their six cruises, including monitoring the development of the spring phytoplankton bloom in each estuary; measuring for salinity, temperature, chlorophyll fluorescence, and current velocities; and measuring concentrations of major plant nutrients and organic matter in suspended particulates. Of course, the real work of oceanographers who collect samples from the estuaries for chemical and biological analysis begins when the ship comes in. Back at the Darling Marine Center in Walpole and at the UNH Jackson Laboratory, the research team has been compiling an enormous data set of their preliminary findings on the effects the varying river inputs of fresh water have on the functioning of the three estuaries.
As their data bases begin to indicate patterns of salinity, circulation, and the distribution of phytoplankton in the estuaries, their findings are increasing our knowledge of the workings of hydrologic processes and expanding our understanding of estuarine systems. According to project leader and UM biogeochemical oceanographer Larry Mayer, "There's a remarkable difference among the etsuaries in converting nutrient input to planktonic biomass. The Damariscotta is very efficient at this conversion while the Sheepscot is quite inefficient. We don't know why yet, but suspect it to be due to the different mixing and light regimes."
What Mayer and his colleagues, oceanographers Neal Pettigrew and David Townsend in Maine, and Loder in New Hampshire, already knew before they began their study was that the way a river comes into an estuary has a major influence on the biology, geology, and chemistry of that estuarine system. What's not clear is how that influence operates.
The best way to understand estuaries - or anything else in nature - is to conduct an experiment. For example, if you want to find out the effect of rivers on estuarine systems, the obvious experiment would be to vary the river inflow and hold everything else constant. But, rejoins Mayer, there's the rub. "We know the river end of estuaries is important, but we haven't been able to run good experiments on how it works because there are too many other variables - such as light or temperature - that change along with river input."
Natural "experiments" are of two types. A temporal experiment might be a single estuary in which river input changes over time, such as spring to summer. However, factors such as light or nutrient concentrations likely also change over that time, so that the experiment isn't a good one to test for the effect of river flow on nutrient dynamics.
On the other hand, a spatial experiment could be run, comparing estuaries that are alike in every regard except for their river inflow. The problem here is that good natural experiments of this spatial type have not been possible before, because different estuaries usually vary also in more features than simply amount of river inflow.
The beauty of the mid-coast Maine estuaries is their similarity in nearly every regard except for river inflow. They consist of long, thin channels without large complications from side tributaries or human perturbations. And quite fortunately, these estuaries are located near the Darling Marine Center, which sets the stage for an experiment combining both spatial and temporal changes in river inflow with which to tease out the workings of this critical variable.
As predicted, the researchers have found that the three estuaries show varying influences from the rivers of differing size flowing into them. The Kennebec, which is a partially mixed estuary with a large amount of fresh water toward its head, has high river input coupled with high tidal flushing. This combination of high input and vigorous tidal exchange results in a well-flushed system with short water residence times - the average length of time it takes for the water in the estuary to replace itself - of a few days. The Sheepscot and Damariscotta act more like large tidal coves, with longer residence times due to lack of flushing by large volumes of river water.
The nutrient data set amassed by the research team is by far the most complete of any existing for Maine estuaries. Until now, the state of Maine - which is federally mandated to develop measures against nutrient-induced eutrophication in its coastal waters - has been hampered by lack of knowledge of existing nutrient regimes. In order to assist in planning these measures, two members of the Sea Grant project team have been engaged by the State Planning Office to provide nutrient and biological information.
A surprising potential source of nutrients was found in the lobster fishing activities. Several thousand lobster traps were counted in each of the Damariscotta and Sheepscot estuaries, with only a minor number in the Kennebec. After talking with local lobster fishermen to determine the amount of bait put into these traps and analyzing the nitrogen content of samples of bait being used, the researchers have found that the baiting of these traps likely introduces as much nitrogen as the amount resulting from other human activities. This finding has important implications for local community planning activities, which are often based on assumed nitrogen loading per capita of human population.
However, the scope of the Sea Grant project is much larger than assessing nutrient budgets. There are several interesting trends that have surprised the research team. First, there is the difference among the estuaries in converting nutrient input to planktonic biomass. Practical implications of these differences in nutrient conversion into planktonic biomass include management of the watershed for nonpoint source nutrients. Not surprisingly, these conversion patterns correspond to the siting of the most productive aquaculture lease sites in the Damariscotta, since plankton are a primary food source for farmed shellfish, such as oysters and mussels.
"We're providing aquaculturists with maps of where the food is," adds Mayer. So far, says Mayer, "our preliminary work indicates an intriguing complexity to these estuaries that has important implications for environmental management and resource utilization. Most important, the study has resulted in a 'health check' for the systems."
And just how healthy are Maine's mid-coast estuaries? To date, the project's preliminary findings have shown no major nutrient perturbations in the three estuarine systems in the study. Scientists close to home, as Mayer observes, are less likely to state that the three estuaries are pristine than other researchers from areas in southern New England where Maine's waters look pristine in comparison to their own.
"Ultimately," exlains Mayer, "the more we know about the hydrography and carrying capacity of our estuaries, the better we will be able to assess the environmental impacts from coastal development around estuaries."
Kathleen Lignell is Comunications Coordinator for Maine/New Hampshire Sea Grant.