The Ships Harbor site extends along the intertidal zone pictured in the foreground and onto the point in the background is also monitored © S. Delheimer.
At the top of the Ships Harbor Nature trail loop, the path emerges from the dense evergreen forest full of melodious warblers to a rocky coast swarming with raucous gulls. The strip of shoreline that is covered and uncovered as the tide advances and retreats – the intertidal zone – is caked with white adult barnacles and their fluorescent orange eggs. Crabs, mollusks, and mussels feed on barnacles and other non-mobile intertidal organisms, such as algae. Visitors squeal with fascinated delight as the predatory gulls that had been circling above, eyeing the rocks for tasty treats, suddenly dive and artfully pluck a crab, mollusk, mussel, or sea urchin from a rock’s surface. Children scramble across the rocks, mouths agape as they discover odd-looking creatures in tide pools and scattered along the rocks. As more visitors swarm the intertidal zone, the gulls gather on small islands of rock just off the coast, waiting for their hunting grounds to clear out.
Barnacles are filter feeders that cover the rocks in the intertidal zone © S. Delheimer.
Gulls eat sea urchins found in the rocky intertidal zone and leave the spiny shells behind © S. Delheimer.
A Herring gull forages in the rocky intertidal zone © S. Delheimer.
Bird predation is a natural part of the intertidal ecosystem; however, park visitors may be disrupting bird use of the rocky intertidal zone. Park visitors can disturb intertidal communities by handling, trampling, removing, or killing organisms that birds depend on for food, or scaring the birds away from the intertidal zone. If park visitors are preventing birds from using the intertidal zone for foraging, this could have a cascading effect on the entire food web.
Park visitors explore the intertidal zone and are often surprised to find such a wide array of species living in the zone © S. Delheimer.
Researchers and park staff at Acadia National Park began a pilot study in 2008 to establish a protocol and gather baseline information for a long-term rocky intertidal zone monitoring program. As part of the pilot study, a site at Ships Harbor has been monitored to determine which species live in the intertidal zone and how visitor use of the intertidal zone may be affecting the number and type of species present.
Specifically the project measures the impacts of tidal, bird, and human disturbances on the rocky intertidal zone by monitoring the rate of recolonization of barnacles (a lower trophic level organism and crucial food source for the species that birds prey on), the intensity of bird predation, and tidal flow. The pilot study correlates this data with visual observations of bird and human activity in the intertidal zone to provide baseline information and highlight trends in species composition and variation due to disturbance in Acadia NP’s rocky intertidal zone.
Elliot setting up the study site © S. Delheimer.
Last week I went out in the field to take a look at the Ships Harbor site and participate in a human and bird activity survey. John Williams, a wildlife biological technician, picked me up at Acadia NP park headquarters and we drove out to the quiet side of Mount Desert Island, where we met Elliot amongst the rocks and the fog in Ships Harbor. Elliot was already hard at work, sleeves pushed up and hands gloved, gluing washers onto the rock surface at various plots across the site. Once hardened to the rocks with epoxy, the washers will serve as mounts for the various contraptions that will measure bird predation and tidal flow.
The Ship Harbor monitoring site has 65-75 predation mount plots. At these plots, we glued a single washer to the surface of the intertidal rock using waterproof epoxy. A mussel is hot glued to the frayed end of a tether, which is then tied onto the washer. The mussels attach to the rock and are left as food for intertidal birds. Researchers will return to the site to check these tethers to see if and when the mussel is eaten. This monitoring data will provide information about how much bird predation occurs in the intertidal community and whether humans are affecting the amount of bird predation.
Fifty flow mount plots have also been constructed at the Ship Harbor site to establish the impact of tidal disturbance on the intertidal zone. At these plots, we glued four washers to the rock surface and then zip-tied a mesh bag containing a plaster puck, coated with polyurethane on three sides, to the washers. As the tide washes over the pucks, the side that is not coated with polyurethane will dissolve and flow can be measured. Flow mount monitoring data will clarify whether changes in the species composition of intertidal zones can be attributed to “natural” or human disturbances.
The plaster puck inside the mesh bag is coated with polyurethane on three sides. Noting how quickly the uncoated side dissolves, researchers can measure tidal flow © S. Delheimer
At each plot, barnacles are scraped off the rocks and arrays of washers are glued to the rocks © S. Delheimer.
Human exposure plots, which are used to determine the intensity of human disturbance by monitoring barnacle colonization, were set up in mid April. A series of small square areas of rock in the high and low tide zones were scraped clean. Half of these areas were covered with stainless steel wire mesh cages to minimize the impacts of human disturbance (the cage protects the recolonizing barnacles from human footsteps, etc). Researchers record how quickly each area of rock is re-colonized by barnacles.
Half the plots are exposed to disturbance, like the one above © S. Delheimer.
Half the plots are covered with steel cages to minimize disturbance © S. Delheimer.
Once each washer had been mounted to the rocks, we switched tasks to monitor bird and human activity in the intertidal zone. During a two-hour time span, beginning one hour before low tide, we observed the intertidal zone and documented anything that entered the zone. Collected daily, these records of activity can be correlated with the information provided by the barnacle colonization monitoring to help researchers determine the nature and extent of tidal, bird, and human disturbances.
Although the wind managed to find me no matter how close I shrugged up against our large boulder perch, I enjoyed the bird and human surveying. I was forced to pay close attention to the behavior of all of the zone’s animal and human visitors. I watched intently as sea gulls swooped in perfectly parabolic form to nab a juicy sea urchin or crab and laughed at their clever technique of cracking the shells by dropping the crustaceans onto the hard, jagged rocks below. I was enthralled by the sea gull soap operas that unfolded as adults stole lunch from the juveniles and black-backed gulls used their size to take over a herring gull’s feast. Out of nowhere, 20-30 common eider, including two or three chicks, popped up near the coast after a long dive under the surface. The cormorants made frequent passes by, wings furiously flapping, but never landed in the intertidal zone. Although earlier the fog had been of pea soup consistency, by the end of our survey it had lifted, making it easier for us to spot birds in the air and sea. At the end of our two hour observation period, we had tallied numerous gulls, diving birds and ducks (within 20 meters of the shore), and humans despite the bleak weather.
Acadia NP scientists and volunteers will continue to track changes in species composition and bird use of the intertidal zone throughout the summer to determine how park visitors influence these measurements. Results will inform better management of the rocky intertidal zone. For example, if data show that the zone is sensitive to human disturbance during a particular time of year, resource managers can close the area to visitors.
