GUIDE TO INTERTIDAL AND SURROUNDING ECOLOGICAL ZONES OF
REID STATE PARK, MAINE
Text revised from a Reid State Park pamphlet by
Stuart Michener and Wendy Swallow
Department of Biology, Colby College
PHYSICAL FORCES
Before describing the intertidal zones themselves, it is necessary to understand the physical forces that
affect zone formation. Two major factors are the tides and the waves.
Waves are commonly caused by the friction of the wind pushing the ocean water into ripples and swells.
These swells travel through the water as a wave of motion. The water molecules only rise and fall as the
wave passes through them, they do not actually travel themselves. Only when these swells approach a
beach do they carry any water with them. The rising ocean floor catches the base of the swell, hurling
the water ahead. The wave rises higher, tips forward, crashes down upon sand, and rushes up the beach
until its energy is expended. The biggest breakers along the Maine coast have been known to exert a
force of 2 tons per square foot when they hit the shore. Clearly, their power is an important factor in the
constant shifting and breaking down of the coastline. Wave action can also cause undertows and
riptides along the shoreline.
Tides are the rising and falling of the ocean waters that result from the gravitational pull of the moon
and sun. When the sun, moon and earth all pull in a line, the high tide is at its maximal height and the
low tide at the lowest point. These are the spring tides. When the moon and sun are at right angles to
the earth they pull against each other. The tides that result are less extreme and are called neap tides.
The form of the coast and depth of the ocean influence how much the tides will rise and fall. Open
shores usually have tides that change two to 12 feet in amplitude. But sometimes in funnel-shaped
harbors, as in the Bay of Fundy (where the water may rises 50 feet), tides may be higher. Reid State
Park, located on the shore of Sheepscot Bay, has tides that rise. between 6 and 12 feet. This is due to the
funnel-like shape of the bay.
Tidal extremes (high and low tides) occur at 6 hour 12 minute intervals. Each day the high and low
points occur 50 minutes later than they did the day before.
ZONATION
Plants and animals of the Maine coast tend to live in horizontal bands of different tidal elevations.
Commonly called zones, these bands are formed when plant and animal species, each needing certain
conditions in which to live, move into the areas that fulfill their specific needs. The major requirement
for life is the correct balance of air, water, and food. As this balance varies for each species, so does the
zone with which they are compatible. For example, the common kelp (Laminaria agardhi) is best suited
for a region that has abundant water and relatively little exposure to air and sun. Due to these needs, it is
usually found in the lowest tidal zone, which is exposed to the air only during the lowest spring tides.
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Tidal action is the major factor in the formation of zones As one descends towards the sea, the
surroundings become wetter and darker, as the tides flood the lower regions for longer periods than they
cover the higher areas. The rocks high above the sea only feel the touch of the tides when the sun and
moon work together to create the extreme spring tides. At all other times only the spray from the
crashing breakers or an occasional rainfall wet the barren rocks.
Many of the plants and animals have interesting adaptations to survive the conditions existing in their
particular zones. Life in the upper zones has been forced to adapt to such environmental extremes as
prolonged exposure to the sun, changes in salinity (rain water vs. salt water) and changes in temperature.
In the lower zones, constant submersion and the beating of the waves are forces that all life must
contend with.
ZONES OF THE ROCKY SHORE
At Reid State Park, several seashore zones are found on Griffith Head and Todd's Point . The zones are
named for the dominant plants and animals found within them. The zones are 1) salt tolerant trees and
shrubs, 2) lichens, 3) blue-green algae, 4) barnacles, 5) rockweed, 6) Irish moss, and 7) kelp.
The Zone of Salt-Tolerant Trees and Shrubs
Above the rocks, where time has allowed soil to accumulate, and water to linger, lies a zone of bushy
vegetation consisting mostly of shrubs and a few isolated stands of evergreen trees . The soil near the
shore is thin and rocky, the wind strong and constant. Salt spray rises from the surf below and the sunlight is persistent and harsh. The evergreen trees found here are identical to their inland counterparts
save for their stunted and gnarled appearance, due to the severity of the seaside climate. Pitch pine
(Pinus rigida) is common and easily identified by its long needles (2-3") that grow in clumps of three
and by its closed cones, some of which only open after being burned. Also seen is white spruce (Picea
glauca) whose shorter needles (3/4") are arranged around the branches in a circular manner like a bottlebrush. The cones of the spruce are somewhat larger, papery and open at maturity.
Several species of shrubs that have adapted to life near the sea are common in this zone. The abundant
bayberry bush (Myrica pensylvanica) can grow to a height of 4 feet. The leaves of the bush are dark
green in color, slightly toothed at the ends and accompanied by blue-gray, waxy berries. The bayberry
minimizes water loss through evaporation by a waxy covering on the leaves. Beach plum (Prunus
maritima) occurs here as well. In this manner it has adapted to the large amounts of sunlight it receives.
Other common shrubs found in this zone are the Rugosa rose (Rosa rugosa), brought to this coast from
the orient (identified by red berries and a very thorny stem), the Virginia rose (Rosa virginiana) and the
lowbush blueberry (Vaccinium angustifolium) whose tiny-toothed and pointed leaves make it easy to
spot.
Below the evergreens and the shrubs, many ground covers and small flowering plants thrive in the ocean
climate. Of particular interest is the beach pea (Lathyrus maritimus), a delicate, vine-like plant whose
leaves curl up during prolonged exposure to the sun. This action reduces the surface area of the leaf,
moderating water loss through evaporation. The beach pea, along with the Americancan beach grass
(Ammophila breviligulata) is found in regions of sandy soils and on the dunes. Other plants found
among the shrubs are seaside goldenrod (Solidago sempervirens), chicory (Cichorium intybus), Queen
Anne's lace (Daucus carota), plantain (Plantago major) and white sweet clover (Melilotus alba).
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The Lichen Zone
Between the zone of the salt-tolerant trees and shrubs and the blue-green algae zone lies an expanse of
rock that appears to be devoid of life. Upon close examination, various colored encrustations are seen to
be growing on the rock. Called lichens, these hardy organisms are actually two different organisms
living mutualistically. The alga portion of the lichen produces food through photosynthetic whils the
fungus portion stores water in its thick cell walls. Separated, they would both die, but together they
spread their low form widely.
The lichens commonly seen at the park are the yellowish wall lichen (Caloplaca elegans), the thin
greenish rock tripe (Umbilicaria mammulata), and the black Verrucaria lichen (Verrucaria calciseda).
The Blue-Green Algae (Cyanobacteria) Zone
Intermixed with and beneath the barren expanse of the lichen zone, is a dark, blackened layer of rock
where the blue-green algae (Calothrix scopulorum) resides. This layer of algae consists of millions of
tiny filaments all intertangled together and covered by a sticky, jelly-like sheath. The sheath is a
protective covering which aids the algae in surviving changes in temperature and salinity. The
appearance given is one of a smooth rock that proves to be slippery when touched by the highest tides or
moistened by the spray. Several other green algae similar in appearance to Calothrix may be found in
this zone such as Enteromorpha (Enteromorpha intestinalis and other species) or Ulothrix (Ulothrix
flacca)
Although too dry for most life forms, a few animals can be found in the crevices of the rocks or slowly
moving across the slippery expanses of algae. The rough periwinkle (Littorina saxatilis) is a commonly
found snail that feeds upon the algae of this zone. With its radula (a rasp-like tongue) it creeps along,
scraping the algae filaments from the rock surface. Animals that feed in this manner are known as
grazers. The periwinkle's thick shell protects it from predators, and the organism often seeks refuge in
deep crevices where the environmental conditions are not as harsh. The periwinkle can attach itself
firmly to the rock with mucus to seal out the atmosphere, which prevents drying, and to secure its
position against heavy wave action. More a creature of the land than of the sea, the rough periwinkle
has developed the ability to bear its young as a juvenile stage rather than as an egg, a rarity among
snails.
The Barnacle Zone
The region below the blue-green algae zone is suddenly a territory of the sea. Visited twice daily by the
high tides, exposed to the sun and air during the low tides, and constantly withholding the onslaught of
the waves, lies the barnacle zone. Found here, firmly attached to the rocks, are dense populations of
barnacles (Semibalanus balanoides). Inside a tubular shell of calcium carbonate, and secured to the rock
by a cement-like substance, the barnacle remains forever stationary, dependent on the tides to bring the
necessities of water and food. On the top of its shell are two overlapping plates which are closed during
low tide, enabling the barnacle to survive the drying effect of the air and sun. The barnacle is a good
example of a suspension feeder. When covered by water during high tide, it opens its shell and extends
feather-like appendages which sweep plankton, and other organisms floating on the tide, into its mouth.
By virtue of their firm attachment to the rocks, barnacles can survive even in areas of heavy, pounding
surf.
Often accompanying the barnacles are blue mussels (Mytilus edulis). Like the barnacle, the blue mussel
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lives in a shell which is firmly attached to the rocks by means of long protein filaments called byssal
threads. Due to their fixed position in this zone, mussels depend on the tides for their water and food.
The mussel is called a bivalve mollusk due to its hinged, two-piece shell. When the water recedes, it
closes its shell by means of two powerful muscles. But when the tide returns, the mussel opens its shell,
and filters food from the water.
Common in this zone is the dog whelk (Nucella lapillus), a small snail with a spiraled shell. A predator
of blue mussels and barnacles, the dog whelk feeds on these victims by drilling through the shell with its
radula, and then extracting the contents.
The Rockweed Zone
Looking like a carpet thrown over the rocks and lying below the barnacles is an area of dense, brown
seaweed called the rockweed zone. Found here are great jungles of rockweeds, and many animals which
can only survive the drying effect of the sun during low tide by burrowing into the wetness held by the
rockweeds. In sheltered areas, the rockweed called the knotted wrack (Ascophyllum nodosum) is
predominant as its relatively weak stalk prevents it from inhabiting areas of heavy surf. The bladder
wrack (Fucus vesiculosus) is sturdier and freely covers the less hospitable areas of this zone. However,
its slower growth allows Ascophyllum to dominate protected areas. Both rockweeds are equipped with
air bladders which help the weeds float to the surface of the water so that, even during the highest tides,
the sunlight can reach them. The rockweeds attach themselves firmly to the rocks by means of a discshaped base called a holdfast. Occasionally, tufts of a very fine, red alga are found growing on the
knotted wrack. Called Polysiphonia (Polysiphonia lanosa), this red alga is a small epiphyte, or a plant
that lives on another plant for means of support. Its presence is not harmful to the rockweed and only by
attaching itself there can it exist in the region of the waves.
Crawling through the rockweed, depending on the water they hold, is the common periwinkle (Littorina
littorea). Although related to the rough periwinkle, the common periwinkle, has a less prominent point
on its shell and cannot survive long periods of dryness. The limpet (Acmacea testudinalis), a snail with
a shell like a flattened cone, also needs a moist environment. Pressing itself firmly against the rocks, the
limpet can form a strong seal, locking in the moisture and keeping out the sun. The smooth periwinkle
(Littorina obtusata) is also commonly found in this zone as well as in the Irish Moss Zone. Numerous
amphipod crustaceans can be found crawling in the moist mats of rockweed.
The Irish Moss Zone
The zone inhabited by the red algae, Irish moss, is underwater except for the few short hours when the
tide is at its lowest point. The short, dense, flat-bladed seaweed, Irish moss (Chondrus crispus), forms a
great mat of finely branched vegetation capable of holding water more efficiently than a sponge.
Accompanying the Irish moss is its close relative, mastocarpus (Mastocarpus stellata), a similar plant
except for its slightly finer structure and what looks like tiny, warty growths on the blades. Often seen in
this zone is the bright green alga, sea lettuce (Ulva lactuca). Thin and sheet-like in structure, it is only
two cell layers thick. Protected by the dense, kinky mats from the crashing surf and the drying sun of the
low tides are many small animals such as the green crab (Carcinus maenas), rock crab (Cancer
borealis), several species of sea stars (Asterias forbesi and Henricia sanguinolenta and the green sea
urchin (Strongylocentrotus droebachiensis) Some of these may also be found in small protected "coves"
in the rockweed zone. Also found in this area are some amphipods and isopods.
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The Kelp Zone
The lower edge of the Irish moss marks the low water line. Below that point are the kelp beds. Even the
lowest spring tides seldom expose these kelp. The plants and animals of this region do not have to
undergo great changes in environmental conditions such as temperature and salinity. Dominant here are
two species of kelp (Laminaria saccharina) and (Alaria esculenta). The seaweeds are actually thick
brown algae possessing long flat blades that yield to the motions of the waves, thus preventing damage
to the plant. Laminaria has been found to grow up to 30 feet in length. They attach themselves to the sea
bottom by means of a holdfast. Within the many branches of the holdfast are communities of creatures
such as horse mussels, sea squirts, and starfish. Lobsters (Homarus americanus), fish, and even
occasional harbor seals are seen within the kelp beds. The horse mussel (Modiolus modiolus) is often
abundant in this zone.
COMMON BIRDS AND MAMMALS OF REID STATE PARK
Probably the most frequently sighted bird along the Maine coast is the herring gull (Larus argentatus).
An important predator in the intetidal zones, it often feeds on sea urchins and crabs. The adult gull is
identified by its snowy head and underside, and grayish back while the immature gull is mottled brown
in color. Gulls usually nest on offshore islands that provide an undisturbed place for rearing young.
The great blue heron (Ardea herodias) is an easily recognized bird that frequents marshes and quiet
inlets. Possessing long legs and an arched neck, it is most commonly seen wading in regions of shallow
water, feeding on the small fish that dwell there. An adult bird can attain a wing spread of six feet.
Many waterbirds seek their prey by diving underwater and swimming with their large webbed feet. The
double-crested cormorant (Phalacrocorax auritus) is one of the many birds in the park that hunt this
way. Its thin, streamlined body, and long bill aid in obtaining quarry underwater.
A common mammal sighted in the park is the harbor seal (Phoca vitulina). The seal feeds upon fish and
crustaceans and may grow to five feet in length and weigh up to 300 pounds.
RECENT REFERENCES
Robbins, S.F., and C.M. Yentsch. 1973. The Sea is All About Us. The Peabody Museum of Salem,
Mass.
Hillson, C.J. 1977. Seaweeds. Pennsylvania State Univ. Press, University Park.
Smith, M.M. 1974. Protecting Your Coastal Wetlands. Maine Department of Environmental
Protection, State House, Augusta.
BIBLIOGRAPHY
Abbott, R.T., 1968, Seashells of North America: a Golden Field Guide. Golden Press, New York 280
pages
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Carson, R., 1955, The Edge of the Sea. Houghton Mifflin Co., Boston 276 pages
Carson, R., 1955, The Rocky Coast. The McCall Publishing Co., New Yorkk 118 pages
Edey, M., 1972, The Northeast Coast. Time-Life Books, New York 184 pages
Hay, J. and Farb, P., 1966, The Atlantic Shore. Harper and Row, Publishers, Inc., New York 246 pages
Hay, J., 1968, The Sandy Shore. The Chatham Press, Inc., Chatham, Mass. 63 pages
Kingsbury, J.M., 1970, The Rocky Shore. Chatham Press, Inc. (or Viking Press), Old Greenwich, Conn.
77 pages
Norman, M.G., 1963, Treasures of the Shore. The Chatham Conservation Foundation Inc., West
Harwich, Mass. 135 pages
Sharpe, G.W., 1968, A Guide to Acadia National Park and the Nearby Coast of Maine. The Golden
Press, New York 80 pages
Zim, H.S., 1955, Seashores. The Golden Press, New York 160 pages
Zottoli, R., 1978, Introduction to Marine Environments. 2nd ed., The C.V. Mosby Co. St. Louis 252
pages
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