Isolated Waters Along the Coast of Maine
by Dave Grant
"I am the daughter of Earth and Water,
And the nursling of the Sky;
I pass through the pores of the oceans and shores;
I change, but I cannot die."
(Shelley - The Cloud)

Bass Harbor Light

The rugged coast of Maine offers the most dramatic scenery on the eastern seaboard. Rocky headlands and cobble beaches present numerous obstacles but great rewards to the persistent coast-walker. Rising abruptly from the cool, rich Gulf waters, the cliffs and steep-sloped shorelines provide a foothold for aquatic organisms that is scarce or nonexistent farther south.

Visitors and scientists alike are drawn to the shoreline and fascinated by the diverse and abundant intertidal life in the tidepools that are exposed at low water. While exploring the coast last year, I was intrigued by pools of rain and ocean water that collect above the high tide mark. These puddles represent the tiniest fractions of the vast amounts of water in circulation throughout the hydrosphere, and as happens with isolated waters everywhere in the world, they are quickly modified by environmental factors. They evaporate, are diluted by rain, and subjected to variations in temperature.

It's easy to overlook such minor features when you are surrounded by the grandeur of this coast, but the observant beachcomber always has a knack for spotting the unusual. Typically the puddles form in natural depressions in the granite or among basaltic dikes that weather faster than the surrounding rocks. Some collect salt spray, rainwater, or both; others momentarily trap runoff from the ground as it trickles back to the sea. Most are less than a foot deep and at best a few feet across. However, each of the scores that I have taken a closer look at is unique in some way - minute aquatic ecosystems that are isolated from but dependent upon the sea and sky.

The areas where I made repeated observations include Bass Harbor Light (the most photographed lighthouse in Maine). Wonderland, Otter Point, and Schoodic Point - all in Acadia National Park- and The Cranberry Isles which are a short ferry ride from Mt. Desert Island. By far the most interesting pools are at Schoodic and Otter Points, both popular scenic stops of notable geological interest. Two students from the Acadia Institute of Oceanography in Seal Harbor assisted me in this study, and their patience is greatly appreciated.

Maine's coast is bordered by a treeless strip of bare rock that, depending on exposure to waves and slope, varies in width from a few yards to over 100 feet. At the seaward edge the pools are flushed by the tides and contain dozens of intertidal algae and invertebrate species. The water conditions in them tend to vary only slightly from the adjacent Gulf of Maine waters which in mid-summer are around 50°F., have a salinity of 33-35-ppt, and a pH between 8.1 to 8.2. At the other extreme 10-feet or more above the tides at the tree line, depressions collect runoff. Here, upland vegetation such as stunted spruce, jack pines, and even freshwater bog plants survive.

The area in between, which might be washed by winter storm waves, but is hot and dry in the summer, is practically devoid of higher plants. However this is where the most peculiar waters and unique biological communities are found. Extremes of temperature and salinity are common among the pools in this strip. At Otter Point clear streams of forest runoff have typical freshwater inhabitants like tadpoles, amphipods, larval salamanders, and mosquito larvae. Cranberries and seaside plantain also thrive along the edges. These waters are slightly acidic and warm; over 80°F.

Adjacent to these spillways, isolated pools of ocean spray are much less inviting or biologically diverse. Many are inhabited only by bacteria or blue-green algae, and no wonder, since in the sunlight these pools become miniature solar collectors, which by July, can be over 100° F and have salinity levels four times ocean values. Closer to the tide mark where the sea waters mix occasionally with the Gulf, the more tolerant marine algae, Enteromorpha, as well as tiny stickleback fishes can be found.

Schoodic Point has an even more interesting complement of puddles. Some brine pools have salinities of over 150-ppt and dissolved oxygen levels of only one part per million - not very hospitable places. These pools are alkaline and can be identified by the rims of salt crystals and their coloration, either a reddish bacterial film or a dark blue-green algal slime.

Here spillways form some of the largest pools in great crevices of weathered basalt. Some of these pools have a startling concentration of life. Fed by forest and parking lot runoff they can support fringing rock gardens of iris, chamomile, yarrow, dock, willow herb, harebell and bog plants. The cliffs at Schoodic are some of the most picturesque and popular vistas on the coast, offering dramatic views of Mt. Desert Island, seals and seabirds. A legendary mob of well-groomed but aggressive herring gulls attends to those tourists who willingly or inadvertently share their lunches with the birds. ("Why I once saw a big gull fly off with a whole hoagie sandwich ...grabbed it right out of my boy's hand as he got out of the car!")

The concentration of these birds, a result of human activities, has a dramatic effect on the biological communities in the large pools. Guano fertilizes the water and consequently these pools have much higher nutrient levels than any other area we studied. The eutrophic waters support exceedingly dense populations of freshwater invertebrates. They are not diverse which is not surprising for such a stressed environment, but the numbers present must be very close to the maximum carrying capacity.

Only five types of invertebrates were identified and in order of abundance are: daphnia and seed shrimp (by far the most abundant), Psychoda or "moth" flies, midges, and water boatman. No vertebrates seem to use these pools. Daphnia (water fleas) and seed shrimp are omnivorous crustaceans found in a wide range of freshwater environments ranging from lakes to clogged rain gutters. They are able to survive in almost any circumstances. In the lab and field we found they were active between 38°F and 88° F, not an unusual summer temperature range for Maine, and they tolerated dilute seawater for short periods. Also their activities were not inhibited by low oxygen levels at night. These creatures survive winter by producing a "resting" egg that is light enough to be transported by wind to a favorable location. This ability, akin to an aerial planktonic stage, also accounts for their wide distribution. Many species are parthenogenic (females don't need males to fertilize their eggs) which simplifies reproduction. Their combined populations exceeded 2000 individuals per liter in some pools.

(Daphnia, Psychoda fly pupae and water boatman)

The largest of the daphnia can grow to almost ¼-inch and make these pools teem with life. Psychoda flies are common foul-water residents, and because they feed on decaying matter, can be a nuisance around sewer plant trickling filters. Campers sometimes encounter the tiny adults clinging to walls of out-houses and wilderness latrines. Their sarcophagus-like pupae made up less than one percent of our samples.

(Ostracod or "seed shrimp" and bloodworm)

Midge larvae also were rare, although this complex group has many more marine representatives than any other aquatic insect family, and adults are common around decaying algae. Their hemoglobin is not encapsulated like ours, a feature that apparently helps them tolerate almost anaerobic conditions and that accounts for their common name, blood worms. Boatmen are the largest and most active pool residents. They are the most widespread of all aquatic bugs, but they were rare in our samples. Like most of the other inhabitants they feed while swimming, sweeping the water for algae, protozoa, or detritus. They over-winter as adults or nymphs and in some places are even active under the ice. They tolerate stagnant water by carrying a film of air beneath their wing covers, and if conditions don't suit them, they simply fly elsewhere.

With the exception of only a few groups of invertebrates, notably insects and certain snails that originated on land, most fresh water invertebrates seem to have sprung from marine ancestors. Possible evolutionary routes include tide pools as well as rivers and estuaries. Areas like this provide a picture of the difficulties the ancestors of these creatures must have encountered in making the transition from the stable conditions of the sea to the variable and often harsh environment outside the sea.

Few groups have made the reverse trip and there is a notable scarcity of insects in the sea (Presumably because of the paucity of flowering plants). Many marine invertebrates tend to be isotonic with their environment - that is, they have internal salt concentrations similar to the 3.5% salinity of the sea. Freshwater invertebrates have internal fluids that are three to 40 times as salty as their surroundings and must therefore work harder to maintain the correct salt and water balance against a much stronger gradient. Other distinctions between fresh and salt water invertebrates include reproduction, distribution, size, coloration, and the ability to withstand environmental stress. Many freshwater forms produce only a few eggs with large yolks, few larval stages, and rare or short planktonic stages, which is quite different from most marine life. A number also have internal fertilization and the habit of carrying their eggs until they hatch, a feature that is less common in the sea.

Freshwater invertebrates tend to be much smaller and less colorful than their marine cousins and rarely exhibit bioluminescence, which tends to make them a less glamorous to us, but they excel in their adaptability to stress. Creatures living in a small puddle must endure greater extremes of temperature, salinity, pH, and dissolved oxygen. The threat of the pool flooding, or worse, drying out, hangs forever like the sword of Damocles over their small world.

These fascinating creatures are tied to ephemeral water worlds that are much less forgiving environments than the nearby ocean. They endure droughts and floods, heat and cold, and yet they make do and even thrive using survival techniques that are often more impressive than those found in the sea. They are some of the world's most successful opportunists, able to invade distant puddles, isolated ponds, and almost any body of fresh water. Take a closer look at the next
puddle you step in at the shore or anywhere else, and you'll likely find some of these animals and begin to appreciate their remarkable and tenuous existence.

NOTE: Society Advisory Council member and chief naturalist, Grant leads a Society trip to
Mt. Desert Island every summer.
Vol. 17, No. 4