Physical and ecologic features of the Sagadahoc Bay Tidal Flat, Georgetown, Maine

Abstract

Sagadahoc Bay is open to the ocean at the south and has no significant fresh-water stream entering it. The intertidal zone is roughly a mile long by half a mile wide; most of it is made up of medium to fine sand, but organic-rich mud characterizes the head of the flat and the protected coves. Refraction-seismograph surveys showed that the bedrock surface lies 30 to 200 feet below the surface of the tidal flat and that it is irregular and fluted longitudinally. Repeated surveys indicate that the intertidal flat builds up and cuts down but is apparently in equilibrium with the present sea level. The sediment that fills the bay came from the sea. Waves and tidal currents tend to move it landward; storms accelerate this, or reverse the direction of movement, depending on the characteristics of the storm. Tidal- and wave-generated currents 0.1 foot above the bottom range in velocity from 0.35 to 0.82 foot per second on incoming tides and from 0.20 to 0.58 foot per second on ebbing tides. Incoming tides float large quantities of sand landward; ebbing tides never carry floating sand. Two distinctive and extensive Mya shell-pavement layers were found at depths of roughly 2 and 3 feet below the present surface of the tidal flat. The lower layer is approximately 1,000 years old according to a radiocarbon age determination of its Mya shells. It is suggested that these shell layers formed by sluicing away of a layer of sand about 2 feet thick, which had been thrown into loose packing by an earthquake at high tide. The earth shock induced a submarine slide of the sand in the outer part of the bay, which oversteepened the profile of the sand headward nearly to the head of the bay. Mya and other shells settled through the layer of quicksand while the sand was running out seaward. Living in the intertidal zone is the usual assemblage of clams, gastropods, crustaceans, worms, and seaweeds found on most northern New England tidal flats. The Mya arenaria population is decreasing, but in general myas are more numerous in the muddy areas than in the sandy areas. They grow more rapidly in the sand, though in the past decade there has been no significant renewal of the Mya population in the sandy part of the flat. Macoma balthica inhabits the muddy areas, whereas Ensis, Spisula, and Arctica are restricted to the low-tide zone and the shallow water below. Gemma gemma grows in great abundance in the sandy part of the flat but is rare in the muddy parts. Small shrimp and green crabs are common. The calcareous shells of these animals are all potential fossils, but the shrimp and crab exoskeletons are not, for their tests are rapidly decomposed in this environment. Other potential fossils are wood and bark, acorn caps, conifer cones, leaves of deciduous trees, seeds, and occasionally even grass stems and pieces of eel grass. All these are reasonably well preserved in the constant reducing environment that prevails an inch or two below the surface. An inverse relationship exists between the abundances of Mya arenaria and Gemma gemma. Cores and test pits show that gemmas are more numerous on the Sagadahoc flat now than they have been in the recent past (estimated 10–100 years). Gemmas are the dominant mollusk in the sandy part of the flat now that the myas are so extremely rare. The speculation is that gemmas became dominant largely because the Mya population was greatly reduced by intensive digging during and just after the last war and through depredations by green crabs. Possibly the warming climate has favored the gemmas selectively. The writer infers that the gemmas are a serious competitor of the myas and that the gemmas now starve out Mya spat, which is known to be carried into the bay each spring and fall. Two recommendations are made: (1) determining under controlled laboratory conditions the food requirements of Gemma and the density of Gemma population that will permit survival of Mya larvae from set through a stage that will assure maturation; and (2) killing off a large percentage of the Gemma population and observing whether or not a natural set of Mya occurs. Gemmas can be killed quickly under a flame shield such as is used to soften asphalt-sand mixtures in street paving. Inasmuch as gemmas are ovoviviparous they should not repopulate the flat rapidly.