Historical Ecology and Intertidal Algae at Acadia

by Chris Petersen P.h.d, Emily and Mitchell Rales Chair in Ecology, Director of the Graduate Program, College of the Atlantic

Historic document marking survey original algae survey location.

Courtesy of Chris Petersen

Every fall I bring my College of the Atlantic marine biology class to Otter Point to observe the dramatic intertidal on an exposed shoreline. Starting in 2013, after doing some work with William Procter’s 1930s surveys of marine invertebrates in Frenchman’s Bay, I became interested in how we might be able to use historical data to look at changes in marine intertidal communities, and I also started looking at a set of papers from the 1920s describing intertidal algae at Otter Point. With a drawing of the area, including one very large tide pool, and an x on a map, a field assistant and I were able to find the same spot by exploring the general area during a few low tides.

In the set of three papers, published in 1928 in the journal Ecology, Duncan Johnson and then graduate student Alexander Skutch (who later went on to become a famous natural historian and ornithologist in Central America) gave complete species lists, locations, and a lot of detail on possible factors affecting algal species distribution in and around tide pools at Otter Point. How might algal species composition have shifted in the past 90 years? Are there patterns that could be consistent with causes due to climate change? With warming seas (and warming air temperatures), more northern, cold-adapted species might be driven northward out of our area while more warm-tolerant species from the south might move in. I was also curious if introduced species, in particular green crabs (Carcinus maenus) or common periwinkles (Littorina littorea) might be changing algal communities through herbivory.

Students conducting algae species surveys along the coast of Acadia National Park. This is a continuing project changing algae numbers over time. Continuing research projects in the park like this are important in helping park staff make management decisions throughout the park.

Courtesy of Chris Peteresen

Over the next five years, students from my fall marine biology class censused the area during the term. They gained experience in identifying species, determining geographic ranges from the literature, calculating relative abundance, and making photographic comparisons with pictures published in the original papers. One of the students from the 2015 marine biology class, Heather Sieger, picked up the work as her senior project, and over 2018-19 focused on sampling at other times of the year, adding spring and summer samples to our fall work. Heather gained more confidence in her identification skills as she worked with two COA alumnae, Kipp Quinby from Sedgwick (where she works in her family’s business, Ocean Resources) and Jordan Chalfant (who is currently working on a book of Maine marine algae with the Maine Natural History Observatory). Once Heather completed her sampling, she visited Dr. Art Mathieson at the University of New Hampshire, who was a great help in clearing up some confusing species.

Adding winter surveys into the yearly process. This will help researchers identify what might be impacting the green algae populations.

Courtesy of Chris Petersen

When Heather compared the current data with the historical data, she didn’t find support for either of our predicted patterns. The loss and gain of species in our surveys seemed to be random with respect to species geographic range. However, we noticed we had many fewer green algae than in the 1920’s. This may be a real pattern, but it could also be a reflection of our sampling schedule. Until this year, we had done no sampling from December – March, and green algae are typically more abundant in winter and early spring before being grazed by more active herbivores as the season progresses and the weather warms up. Additional winter sampling will clarify whether the green algae are truly less abundant now than they were in the 1920’s.

We did find one trend that we think may be due to climate change. Examining the pictures taken by Johnson and Skutch, it appears that there is much more brown algae in the intertidal now than then. Local ecologists have pointed out that ice scour is now much less common, making it easier for rockweeds to dominate the mid-intertidal zone.

So as I move forward with new students where are we going to take this work? In addition to continued sampling in the winter and spring, two things seem to be important next steps. First, we have identified several introduced algal species that were not listed in the 1920’s or by Mathieson and his colleagues in the 1990’s. We would like to track the abundance of these species over several years to see if they are becoming more dominant, or if, as they currently appear to be doing, they coexist with the other intertidal algae. We are also now interested in looking at the algae growing on the nearby exposed (emergent) intertidal rocks, to see if the same patterns of species diversity and abundance of introduced species that exists in the tide pools is true of that habitat.