Rocky Shore | Great Bay

The energy of waves and currents is very powerful, slowly but surely eroding shoreline outcrops, scouring river channels, and hauling sediments for miles before dropping them. If you’ve ever been knocked down and dragged out while playing in the surf, you get the idea. You also start to see what the creatures of the rocky shore are up against.

Many of them have streamlined bodies to minimize their exposure to surging water. They also share a common staying strategy: find a solid surface and cling to it, or hide under it. Though the inland shoreline is sheltered from the battering of ocean waves, the Piscataqua is still a notoriously rough-driving river. Fortunately, there are plenty of surfaces for settlement along its shores, including cobble beaches, bedrock ledges and dock pilings. Portions of Great Bay’s shoreline are covered with a coarse rock, called shingle.

Of all the habitats of the estuary, rocky shore is the easiest to explore. It is usually more accessible than marshes, eelgrass beds and channel bottoms. It is also free of the messy, stinky, sinking mud of the tidal flats. You’ll just want to remain vigilant about slippery rocks, and be very cautious of any currents or waves in the lower intertidal zone.

Flora & Fauna

The species of flora and fauna change from one end of the estuary to the other, reflecting changing water salinities. Water conditions by the ocean are relatively stable, with high salinities preferred by a great diversity of marine species. These are replaced by fewer kinds of estuarine species where the tides are diluted by freshwater flows in the upper Piscataqua River, Little Bay and Great Bay.

Marine Rocky Shore

Seaweed draped rocks speckled with barnacles are a common sight, particularly in the mid-intertidal zone. This is the length of shoreline midway between the low and high tide marks. The animals and algae here are submerged by tides about half the time. In some ways, it is the ideal zone.

The Upper Zone

Few species can tolerate the harsh physical environment of the upper intertidal zone, where the rocks are submerged only briefly at high tide. Some animals and algae take shelter in pools, which accumulate on the rocks when the tide goes out. These, too, can also be inhospitable. It is common for swinging temperatures, evaporation, or inundation by rainfall to cause drastic and intolerable changes in a tidal pool’s water chemistry.

rough periwinkles cling to algae-covered rocks Periwinkles tough out the upper zone by sealing their shells to the rocks to fend off dehydration. They graze on films of microscopic algae and bacteria growing in patches on the rocks. When moist, these films make footing treacherous, indeed. Barnacles are also found there, sealed inside their calcified plates waiting for the next submersion.

The Middle Zone

barnacles and red encrusting algae So, it is easy to see why the middle zone is the more desirable intertidal real estate. Animals and algae grow here in abundance since it is more often flooded with oxygen and nutrients. But there's a catch. Bigger populations lead to increased predation and competition for space and food—good incentives for enduring the upper zone.

Succession is the name of the game here. Clumps of seaweed tossing in the surf may prevent barnacle or mussel larvae from settling down. Then again, winter ice could tear that seaweed away, allowing a colony of barnacles to settle the following spring. Though barnacles literally cement themselves down, they are by no means permanent residents. northern sea star They can be uprooted by their own kind, or more likely smothered by blue mussels anchoring themselves overhead with their tough byssal threads. Mussels are considered one of the dominant species of the middle zone. Yet they too are vulnerable. Sea stars, green crabs and carnivorous dog whelk snails each have their own method of gaining entry to mussel shells and consuming the soft bodies inside. And so the competition for space and food continues.

Jonah crab in rockweed at New Castle Try (carefully) pulling back a clump of seaweed. Sometimes crabs, limpets, snails and other animals seek moist, shady refuge under this canopy. There can also be colorful seaweeds like tufted red weed, encrusting algae and perhaps a sponge growing on the rocks beneath. If there is standing water, you may also see a group of teeny, blue insects called springtail suspended on the water surface.

Common periwinkles are practically everywhere in the middle zone, moving along on their big foot. They scrape algal particles into their digestive track using their radula. limpet Smooth periwinkles hide in knotted wrack, mimicking the shape of this seaweed’s bladders. Lift up some rocks and you’ll probably find a limpet on the underside of one. This mollusk looks like a flattened cone the size of a nickel. (If you remove it, leave it face down since it can’t right itself.) Tiny crustaceans called amphipods may scurry from underneath.

pinkish coral crust at the bottom of a tidal pool In many tidal pools, what look like blobs of bubble gum carelessly tossed aside are actually coralline crust algae. They tend to coat small rocks and shells. These calciferous crusts are safe from the periwinkles, which avoid them to prevent damage to their radulae. Limpets have these crusts all to themselves, since their radulae have such strong teeth. The crusts benefit from the scraping, since it removes debris that blocks the sunlight necessary for photosynthetic food production.

The Lower Zone

Irish sea moss On the rocks exposed only briefly at low tide, there is an even greater diversity of species. Again, competition and predation ensure that none are in particularly great abundance. The rocks are decked with colorful algae like Irish moss, sea lettuce, tufted red weed, coral weed, coralline algae and fleshy red crustose algae.

green sea urchin Little shell-less gastropods, called rough-mantled nudibranchs (also called sea slugs), search for barnacles to eat. Red-gilled nudibranchs hunt for anemones and hydroids. Anemones, hydroids and sea vases cling to lower rocks, feeding on suspended phytoplankton and detrital particles in the water. Green sea urchins munch on the algae in the lower zone.

green crab found behind rockweed Crabs have an easier time hunting around rocky shores within the estuary, sheltered from the crashing waves of the outer shore. Rock crabs and Jonah crabs are fun to see. Of course there is the seemingly omnipresent green crab, a fairly recent immigrant to the area (late 1800's), which threatens to supplant native crab species. All these crabs are common in the lower and middle intertidal zones of the more salty waters.

Brackish Rocky Shore

The diversity of species wanes as the water becomes more brackish in and beyond Little Bay. The narrow cobble and shale beaches here are flanked in many places by mudflats at the water’s edge, and marshes or woods at the upland edge.

knotted wrack Though there is no big line marking the transition of marine to estuarine species, researchers have found that changes are typical of certain places. Little Bay has some of the species common to more saline waters: common and smooth periwinkles, northern rock barnacles, blue mussels and green crabs. But by the time you get to Great Bay, which is fed by three rivers, the estuarine species dominate. Ribbed mussels replace blue mussels, acorn barnacles replace northern rock barnacles and green crabs are less abundant. Rockweed and knotted wrack cover the rocks that are either loose, or wedged in the peat or mud. Just beyond the low tide mark in some places, eastern oysters are clumped together in beds. They must contend with a carnivorous snail called the oyster drill. Little hermit crabs move in with the tide, housed in shells once occupied by periwinkles or dog whelk snails.

References for further exploration

Bertness, Mark D. 1999. The Ecology of Atlantic Shorelines. Sinauer Associates, Inc. Sunderland, MA.

Carson, Rachel. 1955. The Edge of the Sea. Houghton Mifflin Company, Boston.

Coulombe, Deborah. 1984. The Seaside Naturalist, A Guide to Study at the Seashore. Simon & Schuster, New York.

Day, Cherie Hunter. 1987. Life on Intertidal Rocks, A Guide to Marine Life of the Rocky North Atlantic Coast. Nature Study Guild, Rochester, NY.

Gosner, K.L. 1978. A field guide to the Atlantic seashore: from the Bay of Fundy to Cape Hatteras. Peterson Guide Series. Houghton Mifflin Company, Boston, MA.

Hardwick-Witman, Morgan and Mathieson, Arthur C. 1981. Intertidal Macroalgae and Macroinvertebrates: Seasonal and Spatial Abundance Patterns Along an Estuarine Gradient. Estuarine, Coastal and Shelf Science (1983.) 16, 113-129.

Miller, Steve; Sherfeseee, Lou; Caprio, Christine; Miller, Patricia. 2000. SSC Teacher Guide #1: The Rocky Shore. Seacoast Science Center, Rye, NH.

Short, F.T. 1992. (ed.) The Ecology of the Great Bay Estuary, New Hampshire and Maine: An Estuarine Profile and Bibliography. NOAA - Coastal Ocean Program Publ. 222 pp.