(Published in the Quoddy Tides on February 23, 2001)
Why do marine plants and animals grow so well in Cobscook Bay? The short answer is simple - because there is lots of food. However, a more complete answer requires a look at the nutrients that are found in the water.
Studies of the nutrients in the bay, where they come from, and how they flow throughout the bay, can reveal a lot about what makes the bay a healthy and productive place. In 1995, research by Chris Garside, of the Bigelow Laboratory for Ocean Sciences in Boothbay Harbor, provided some insight into nutrients in the Cobscook Bay system.
Garside found that the major source of nutrients in the bay is the deeper waters of the Gulf of Maine. In these off-shore waters, waste products and decaying organic matter from dead plants and animals accumulate on the bottom. Over time, this matter is broken down by bacteria and small marine organisms so that the nutrients they contain are released. The mixing action of the water brings these nutrients to the surface where sunlight gives plants the energy to take the nutrients in and convert them to plant tissue.
Cobscook's tremendous tides bring this nutrient rich water into the bay. During the spring especially, higher concentrations of nutrients in the salt water stimulate increased plant growth, most dramatically seen with spring blooms of phytoplankton. The amount of nutrients is directly related to the amount of marine plant growth that occurs.
This pulse of nutrients from the deep ocean is only part of the story, however. The nutrient levels are also enriched through a recycling process within the bay itself. The abundance of shellfish and other marine animals in Cobscook leads to a high amount of waste production. This waste, along with decaying material from dead organisms, is then recycled into an added source of nutrients for plant growth. This internal process is a key part of what makes Cobscook Bay so productive.
"One of the results of Garside's research that is especially interesting is the higher levels of ammonium found in the bay, which is a natural occurrence resulting from waste production," says David Phinney, also from Bigelow Laboratories and a contributor to this research. "This recycling process is what Cobscook Bay is all about, the clams and scallops and other filter feeders consuming phytoplankton and returning nutrients into the system."
A smaller percentage of nutrients enter the bay through runoff from human activities along the shoreline as well as from aquaculture pens.
Phinney comments, "Whenever you have an estuary system, with freshwater coming in and mixing with salt water, the estuary serves as a collection basin for anything that happens in the watershed and is carried in the water. However, there is not a tremendous amount of freshwater coming into Cobscook Bay when compared to the water brought in by the tides. This makes Cobscook different from most other estuaries along the coast."
He adds, "Based on our data collection in 1995, salmon aquaculture operations added the second largest amount of nutrients to the system, but that percentage of nutrients was relatively small compared to the nutrients brought in by the tide, in the neighborhood of 10 to 15 per cent."
A subsequent report by the Maine Department of Environmental Protection also found that nutrients resulting from aquaculture activity appear to have little impact on phytoplankton growth. It noted however that enhanced growth of larger attached seaweeds may occur, especially in areas where nutrients get trapped.
In a nutrient rich system like Cobscook, the question of balance is a critical one. Too many nutrients can lead to excessive and rapid plant growth, which depletes dissolved oxygen in the water, a process called eutrophication. This can lead to mass mortality of plants and animals. Too few nutrients, and not enough plant growth will occur to support the diversity and abundance of marine animals that we have come to expect in Cobscook Bay.
Fortunately, a combination of ecological processes keep these nutrients from tipping out of balance in Cobscook Bay: the filtering of the waters by shellfish and other marine organisms along with the tremendous flushing power found in the tides.
Bottom dwelling filter feeders, such as clams, mussels, and scallops, take in phytoplankton along with sediment particles and waste products that also float in the water. Urchins and periwinkles feed on the larger algaes. If the bay's populations of these plant eating creatures is severely reduced, this natural balance could be upset, resulting in significant ecological change.
Excess nutrients, resulting from animal waste production, periods of increased plant growth, or human activities, are flushed out of the bay fairly rapidly by the tremendous exchange of salt water that comes with each tidal cycle. It is only a matter of days before the main area of the bay is flushed out. But that flushing action is not uniform throughout the bay. There are areas that take longer, such as the upper arms of the bays, where green algal blooms have tended to occur.
Cobscook Bay is a place rich with life. The unseen nutrients form the foundation of this web of life. Understanding how these nutrients move through the system will help us do our part to keep that system in balance.
This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.
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