Christine Walder, who is continuing a project on the effects of rockweed harvesting from last summer, recently returned from a hiatus at the Bowdoin Coastal Studies Center, where she set up study sites to add a comparative component to her research. She used the same methods at the Coastal Studies Center as on Kent Island, so that she can see if there is geographic variation in ecosystemic reactions to rockweed harvesting. The Coastal Studies Center is considerably farther south than Kent Island, and is at the edge of rockweed’s range. Christine noticed that the rockweed at the CSC was shorter and higher density than on Kent Island, and supported a lower diversity of seaweeds. The animal diversity is very similar in both places, with the exception of the Asian Shore Crab, an invasive species introduced to New England by aquaculture boats. Christine set up twenty paired rockweed plots, each with an experimental plot in which she cut the rockweed to 16,’’ the harvest limit, and with an uncut control plot. These methods are the same ones that she uses on Kent Island, so that she can compare the two study sites to see if there is geographic variation in ecosystem responses to rockweed harvesting. Unlike Kent Island, where she is surrounded by her study sites and can walk to them in minutes, at Bowdoin she had to bike an hour to the Coastal Studies Center or drive for twenty minutes. When I asked her what it was like to be on campus after working on Kent Island for two weeks, she replied, “Lonely. Most other undergraduate scientists were working in labs, conducting someone else’s research, but on Kent Island you’re doing your own fieldwork and answering your own questions.” One of the best things about Kent Island is the freedom to pursue what interests you, though that comes with the caveat of having to figure things out on your own, and dealing with frustrations when projects go wrong.
I missed Christine, who is always up for a dip in the freezing ocean or a romp in the intertidal to look for nudibranchs, and was happy when she returned for our last two weeks on Kent Island to check up on her plots. From her own observation, she has found much fewer of the animals she surveys in her harvested plots than in the control plots, but not every difference is statistically significant, meaning there is a 95% chance that there is a difference. “My data’s not very sexy,” she lamented, pointing out that it showed trends but few significant differences between control and experimental plots. Her first look at her data doesn’t mean that the significant differences aren’t there, however. When she separates out her results by tidal height and substrate type, both of which have a large effect on ecology, the differences in species abundance between harvested and unharvested plots may become clearer. One important significant difference that she did see is a decrease in amphipods and isopods, small crustaceans that look like bugs, in her experimental plots. They are at the base of many food webs, including those of fish, crabs, and birds, and are therefore extremely ecologically important.
Ecology is complicated because everything is interdependent, and cause and effect can be difficult to untangle. Christine found an increase in large periwinkles in the beginning of the summer, most likely because of a sea lettuce bloom that the snails feasted on. The snail numbers have since declined as fucus, another algal type, has taken advantage of the increased light after the removal of the rockweed canopy. Season also matters, and she may have seen a decrease in anemones this year and not last year because she cut the rockweed a week earlier, which may have been during a critical time for juvenile settlement. While overall trends are similar, the extent to which they are significant varies from from year to year, such as a decrease in small periwinkles in her experimental plots that was greater this year than last year. It is clear from her data, however, that rockweed harvesting has lasting effects, not only on the biomass of the algae but also on the species that it supports. Most studies on Rockweed have been done within a small range, so Christine’s study is valuable because there are management implications if different places vary in their response to harvest.
Christine’s plots on Kent Island and at the Coastal Studies Center will be converted into long-term study sites, so that future scientists can further isolate the effects of rockweed harvesting. Setting up over forty plots, harvesting, drying and weighing the rockweed, and surveying the species in each plot was an enormous amount of work, and Christine uncomplainingly got up at four in the morning if the timing of low tide necessitated it. During her two summers on Kent Island, Christine has cut and hauled 3,000 pounds of wet rockweed, has counted over 18,000 snails, and once baked ten loaves of bread that we consumed in two days. In the fall, she will begin analyzing her data, paving the way for future rockweed researchers, who have a lot to live up to.