Intertidal and Subtidal Zones

Two NPS staff members conducting rocky intertidal monitoring on Calf Island. Two park staff stand on shoreline rocks densely covered with seaweed and barnacles which appear orange-green and fuzzy. To the left, open water is visible, streaked with white foam with islands rising above the horizon.

NPS Photo/Vincent

Intertidal zones are the range of surfaces that exist between high and low tides—and with more than thirty miles of coastline, there is no shortage of intertidal habitat to explore within the Boston Harbor and upon the Boston Harbor Islands. Within these areas, the water depth in the harbor can be more than ten feet deeper at high tide than at low tide. This “tide range” varies from month to month and day to day depending on the combined gravitational forces of the sun, the moon, and the earth.

Intertidal zones are cyclically submerged under salt water and exposed to the open air, and depending upon the type of geologic materials and degree of sloping, they support a huge variety of different habitats, such as salt marshes, sandy or gravel beaches, and rocky tidepools. On rocky shores and dock pilings specifically, keep your eyes open for barnacles, the little, round white inhabitants of the intertidal zone. Barnacles filter their food underwater when below the tide, and then must clamp closed to protect against drying out when the tides ebb.

 

A rocky shoreline on Outer Brewster Island at low tide. Rows of grey-brown rocks, broken up into blocks, have stratified lines of pale yellow, dark orange and brown. The lowest lines are patterned with pale brown seaweed. The Graves Light is visible in the distance against a grey, cloudy sky.

NPS Photo/Vincent

The subtidal zone remains submerged except during particularly low tides and is often inhabited by species of seaweeds and crustaceans. Seaweeds like kelp and knotted wrack can cover large areas of the sea floor. Many subtidal animals including lobsters and various crab species are sensitive to open air, lack of moisture, and temperature extremes, and take shelter under the moist seaweed or on the open sea floor.

 

Low-tide at the northernmost point of Thompson Island (Perera Point), looking towards Spectacle Island and the outer harbor. A park staff member in a brimmed hat stands under a tree and looks out across a flat brown-grey expanse of rocky tidal flat. The blue harbor water spans beyond the shoreline. Across the water to the left is the yellow profile of Spectacle Island. Distant islands form small blurs in the horizon, with low white clouds hugging the horizon beneath a clear, blue sky.

NPS Photo/Lampley