Acadia intertidal zones

Category: Marine and Oceanic Biomes.

Geographic Location: North America.

Summary: This classic rocky intertidal ecosystem on the northeastern coast of the United States, though well protected by the National Park Service, still faces threats from climate change and invasive species.

Acadia National Park, founded in 1916, is a predominantly coastal reserve protected by the U.S. National Park Service, located in central coastal Maine. The park protects nearly 40 miles (65 kilometers) of rocky coastline, and its relatively high tidal range of greater than 10 feet (3 meters), combined with the geology of the bedrock, helps create an extensive intertidal ecosystem along the shores of the park.

This system is home to a wide range of marine plants, including rockweed, kelp, and algae (green, brown, and red); as well as a high diversity of marine invertebrates including snails, mussels, crabs, and urchins; and seabird species that typically function as top predators in the system. Acadia National Park is generally very well protected from human exploitation and isolated from large cities, so the health of this biome is generally considered to be fairly high, although it does face increasing threats from invasive species, increased tourist traffic, and climate change.

Acadia National Park constitutes roughly 73 square miles (189 square kilometers) of mostly rocky coastline. The majority of the park—about 47 square miles (122 square kilometers)—is located on Mount Desert Island. The island is also home to the largest nearby center of population: the town of Bar Harbor, with a permanent population of only about 5,500—although the national park plays host to more than 2 million visitors per year.

The remainder of the park consists of numerous smaller islands including Isle au Haut, Baker Island, and Bar Island, as well as approximately 3.5 square miles (9 square kilometers) on the nearby Schoodic Peninsula. Approximately three-quarters of the land managed by the park is owned by the National Park Service, including numerous private land donations, most notably about 15 square miles (39 square kilometers) donated by John D. Rockefeller. The remaining one-quarter is private land managed under conservation easements.

The rocky coastline can be attributed to several factors. Mount Desert Island is predominantly comprised of geologically young, hard, and chemically resistant granite that has been heavily scoured by recent glacial activity. Ice more than a half-mile (0.8 kilometer) thick covered the island during the last glaciation, roughly 20,000 years ago, creating a system of ridges and U-shaped valleys. This young hard rock also collides with the older stratified rocks of the Bar Harbor formation, creating shatter zones. The high rugosity of this coastline and a large tidal range of greater than 10 feet (3 meters) make for an extensive system of rocky habitats throughout the supralittoral fringe, the intertidal zone, and the shallow subtidal, all of which play host to a diverse assemblage of animal and plant life.

The rocky intertidal zone is an important natural resource and is somewhat biologically similar in temperate regions around the world, with congener species often filling the same ecological niches in different regions. Acadia is perhaps one of the best-known examples of this ecosystem.

The intertidal zone is almost universally defined as the zone between the highest high tide (Mean high high water or MHHW) and the lowest low tide (mean low low water or MLLW) Although the majority of this biome is regularly covered and exposed by the advance and retreat of the tides, it encompasses a broad range of extremes: very hot and very cold temperatures; broad ranges of salinity; and varying exposure to air, water, wind, waves, and predation.

Intertidal Plant and Animal Life

Rocky intertidal zones provide critical habitat and forage for many terrestrial and marine plant and animal species. Rocky crevasses, undersides of rocks, and tide pools in the intertidal zone support an array of hardy species that have adapted to withstand periodic exposure to air, subzero winter temperatures, and the force of pounding waves.

A broad range of organisms thrives in the dynamic area between the high and low tide marks, where the land meets the sea. Plants and animals growing in the rocky intertidal zone must cope with both aquatic and terrestrial environments to survive, and most species are adapted to a specific niche environment within the intertidal.

A combination of physical and biological factors determines where each species is found. Important physical factors that control distribution include light level, temperature, salinity, desiccation, and wave action. Biological factors that control distribution include competition for space, susceptibility to predation, and availability of prey. Often, the upper or lower range of a plant or animal species is determined by the temperature, salinity, or exposure tolerance of its principal predator or prey species.

Birds generally are the top predators in the intertidal ecosystem, preying on a wide variety of creatures that are exposed during low tide. In Acadia, a wide range of bird species are present, including gulls, terns, common eiders, cormorants, oystercatchers, and a variety of small shorebirds. Inside the park, 338 bird species have been recorded.

These predators rely on food sources such as small fish trapped in tide pools; grazers like sea urchins, snails, and limpets that feed on algae; filter feeders like barnacles and mussels, which typically stick to the rocks and feed on plankton washed in with the tide; and predators like crabs and starfish, which also prey on filter feeders and grazers.

The rocky coast of Acadia National Park also contains many kinds of marine algae and plants. In the splash zone and upper intertidal zone, spray from high tide moistens the algal mats of green and blue-green algae. The most common group throughout the intertidal is often rockweed, though more exposed areas or areas where rockweed cannot establish a foothold have a wide range of green, brown, and red algae. The lower intertidal zone and lower intertidal pools, which are rarely completely desiccated, contain many kinds of seaweed, including sea lettuce, kelp, and Irish moss.

Ecosystem Threats

Though human effects on this ecosystem are relatively low, due in part to strong protection and in part to relative isolation from large centers of human population, the Acadia intertidal system is not without threats from human influence. One major threat to this system comes from the impact of the roughly 2 million visitors to the national park each year. Visitors may affect bird behavior by disrupting nesting sites or feeding activity, and persistent foot traffic on rocky shorelines can affect barnacle and mussel colonization rates.

Another major threat comes from invasive species. In the late twentieth and early twenty-first century, two invasive crab species (green and Asian shore crabs), three species of tunicate (a type of sea squirt), and several plant and algae species have colonized the shoreline, competing with native species for space and food. Often, these invaders have few or no natural predators, and they may disturb the delicately balanced interspecies interactions that determine each organism's niche.

Temperate intertidal zones also face pressure from climate change. Although sea-level rise is generally considered to be slow enough that intertidal organisms can adapt over time, warming water and changing weather patterns expand the range of more temperate and subtropical species (such as the blue crab) and contract the range of boreal species (like the American lobster). Acadia National Park is on the border between a temperate and a boreal climate, and as such, it may be more susceptible to this type of shift.

With continued vigilance from federal, state, and local governments, research agencies, various nongovernmental organizations, and park visitors, researchers can continue to study and protect this beautiful and critically important transitional habitat and its inhabitants for generations to come.

Bibliography

Acadia National Park. http://www.nps.gov/acad/index.htm.

Cammen, Leon M. and Peter F. Larsen. An Ecological Characterization of Intertidal Resources of Acadia National Park: Macrofauna. Boston: National Park Service, North Atlantic Region, Office of Scientific Studies, 1992.

Kaiser, James. Acadia: The Complete Guide: Mount Desert Island & Acadia National Park. Ringgold, GA: Destination Press, 2010.

"Study of Algae in Acadia National Park Lakes Shows Recovery from Acidification." Science News, 2 Mar. 2022, www.sciencedaily.com/releases/2022/03/220302190006.htm. Accessed 14 Jul. 2022.

U.S. Geological Survey (USGS). “Nonindigenous Aquatic Species.” 2009. http://nas.er.usgs.gov/queries/SpeciesList.aspx?Group=&Sortby=1&state=ME.