Acadia National Park - FLOW: Ponds

Acadia National Park and Mount Desert Island are famous for their granite coastal hills, but between those hills lie valleys with streams and ponds equally deserving of fame.

Ponds are stream- or spring-fed lowlands or valleys where the watertable (the level of water held in the soil) rises above the surface of the soil to form a body of relatively still water open to the air. In some places the word "pond" is reserved for shallow bodies of surface water and "lake" is used for deeper ones, but in Maine all surface waters are considered to be ponded and are referred to as ponds.

Just because water in ponds moves more slowly than it does in streams doesn"t mean there"s nothing happening in ponds. Changes in daily and seasonal temperatures of water near the surface of a pond sets up currents of flowing water within the pond itself, promoting a corresponding flow of nutrients. In both spring and fall, pond waters turn upside-down, bottom waters rising to the top, surface waters sinking to the bottom. The fall overturn is the result of falling temperatures that leave surface waters cooler than the depths, causing the warmer water on the bottom to rise to the top. In spring, equinoctial winds (winds near the equinox) stir up the surface waters of a pond, establishing currents that drive a second annual overturn.

Ponds are often full of plant and animal life, especially if their waters contain nitrogen (needed for plant growth) and phosphorus (needed for plant reproduction). Aquatic plants grow at various depths, some preferring the shallows near the edge of a pond, some thriving in moderate depths. Many freefloating plants and algae benefit from the bright sunlight at the surface and the waterborne nutrients in the pond itself. Two examples of shallow ponds in Acadia National Park are the Tarn (in the valley between Dorr Mountain and Huguenot Head south of Bar Harbor) and Aunt Betty"s Pond (north of Sargent Mountain). These two ponds are fast filling in with vegetation such as arrowhead, pond lily, and bayonnet grass and will be wetlands, not ponds at all, in the near future.

Acadia"s deeper ponds have such clear water and low nutrient levels that they are relatively free of plant life. Eagle Lake is one such pond and is the water supply for the town of Bar Harbor, as Jordan Pond is for Seal Harbor, and Long Pond is for Southwest Harbor. Pond clarity is measured by dropping a weighted target (Secchi disk) overboard from a boat and lowering it until it can no longer be seen. Secchi depths for many of the ponds on Mount Desert Island are contained in the righthand column in the table of ponds below.

Only two ponds on Mount Desert Island are acidic, Sargent Mountain Pond and Duck Pond, which are about one acre in size with a pH of 5.0 or less. The acidity of Sargent Mountain Pond is thought to be caused by atmospheric deposition on a small, granitic watershed with little soil; the acidity of Duck Pond results from its being fed by a naturally acidic wetland.

POND TROPHIC TYPES

OLIGOTROPHIC--Ponds lacking plant nutrients and plants, with high levels of dissolved oxygen (Eagle Lake and Jordan Pond) MESOTROPHIC--Ponds with moderate levels of nutrients, plants, and dissolved oxygen (Bubble Pond and Upper Hadlock Pond) EUTROPHIC--Ponds with abundant plant nutrients, algal blooms, with low levels of dissolved oxygen (The Tarn and Aunt Betty"s Pond) DYSTROPHIC--Acidic ponds with high levels of vegetable matter from a small variety of plants (Duck Pond)

The following table contains readings that show variations with depth of water temperature and dissolved oxygen content in Eagle Lake on September 23, 1998. At a depth between 19 and 20 meters, the temperature abruptly falls several degrees and the dissolved oxygen drops and then trails off. Cooler waters and decaying organic matter on the bottom markedly decrease the amount of oxygen available to living organisms. Those bottom nutrients are carried upward at the spring and fall overturns, making food available to organisms in the upper levels where dissolved oxygen is more plentiful. Generally speaking, in ponds what goes down must come up.