J. A. Percy, A. J. Evans, P. G. Wells and S. J. Rolston (Eds.). 2005. The Changing Bay of Fundy: Beyond 400 Years. Proceedings of the 6th Bay of Fundy Workshop, Cornwallis, Nova Scotia, September 29th - October 2nd, 2004. Environment Canada - Atlantic Region, Occasional Report No. 23. Dartmouth, NS and Sackville, NB.
The Changing Bay of Fundy—Beyond 400 Years
LATE 20TH CENTURY QUALITATIVE INTERTIDAL FAUNAL CHANGES IN COBSCOOK BAY, MAINE Thomas J. L. Trott R. S. Friedman Field Station, Biology Department, Suffolk University, Boston MA.
[email protected] Abstract Late 20th century changes in the intertidal distributions of macroinvertebrates within five sample sites in the Cobscook Bay, Maine, region were evaluated by comparisons with qualitative baselines, some as old as 35 years. These baselines were generated by the Maine State Planning Office Critical Areas Program (CAP) (1970–1987), which recognized the unique distributions of macroinvertebrates and high diversity of intertidal communities in Cobscook Bay that had attracted many zoologists, dating back to the early 1800s. The sample sites were Critical Invertebrate Areas registered by CAP between 1968 and 1976. None of the sites had been re-examined for at least 20 years, and all but one had been evaluated at least twice prior to this study. Many species, including those whose presence was used to designate habitats as critical, were common or abundant in original site descriptions but rare or absent in 2002. The dramatic change in community composition, away from species typical of hard bottoms to established mussel beds, suggests a faunal shift has occurred. The principal driving force that produced this change is proposed to be disturbance from increased sedimentation that altered intertidal habitats. Potential sources of this disturbance, and possible cascades that followed, are discussed. Introduction Cobscook Bay, Maine, is exceptional because of the extreme tidal range that characterizes this macrotidal estuary and creates an ecosystem with biodiversity comparable to that at lower latitudes (Trott and Larsen 2003). Postglacial changes in climate and ocean circulation, superimposed on the large tidal amplitude contributed to the assemblage of unique communities found there (Bousfield and Thomas 1975). However, compared to estuaries similar in size and commercial importance, such as Chesapeake Bay, general awareness of this ecosystem is poor in spite of its distinctiveness. Cobscook Bay, a rock-framed macrotidal estuary, is both the furthest east and the only boreal estuary on the eastern coast of the United States. Even though Cobscook Bay serves as the sink for two rivers, the amount of fresh water entering the bay is negligible (Campbell 2004), and differentiates it from many macrotidal estuaries, which often receive substantial river input (Gleizon et al. 2003). All the water entering the innermost region of the Bay, formed by confluence of Whiting and Dennys bays, must enter through a single narrow opening restricted by an island. All processes occurring in the inner Bay that are dependent on mixing with offshore waters rely on this single input. Cobscook Bay shares a history with a few locations in the United States, since Eastport, Maine, was where the United States Fish Commission established its second research station, after recognizing 324
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the decline of fisheries on the eastern US coast in the mid-1800s. For decades before this, the intertidal communities of Cobscook Bay had drawn zoologists to this remote region. The highly diverse macroinvertebrate fauna of Cobscook Bay is documented by a rich historical chronology that spans 162 years of collection records because of this attention (Trott 2004). Information of this kind is rare and useful in establishing species distributions and general knowledge of geographic ranges. It provides, however, no hint of relative abundance of the benthic community that inhabits intertidal rocky shores and mudflats. With the exception of Larsen and Gilfillan (2004), no quantitative information from systematic sampling in Cobscook Bay has been published, and even then their study sampled exclusively subtidal commuities. One focus of historical ecology is the examination of long-term changes in communities. These studies are difficult, since pre-existing databases are required to make time series comparisons. Surveys of marine benthic communities are few, especially in North America. Long-term changes have been best studied in northern Europe, where ecological investigations of marine communities have more commonly surveyed community composition (e.g., Buchanan 1963; Ford 1923; Kühne and Rachnor 1996). Subsequent studies have revealed interesting changes in community composition and biogeography (Bamber 1993; Shillabeer and Tapp 1989; Tyler and Shackley 1980). The rarity of benthic community surveys in North America makes them attractive for investigating community change and for comparison with studies from the northeastern Atlantic. Five specific locations in the Cobscook Bay region were the focus of qualitative, ecological assessments of intertidal macrobenthic invertebrates in the 1970s and 1980s, as were other noteworthy invertebrate communities along the coast of Maine (Maine State Archives). These evaluations were conducted as part of the Critical Areas Program (CAP) of the Maine State Planning Office that operated officially from 1970 to 1987, and aimed to identify rare and unique features throughout the state. Many publications resulting from CAP documented ecologically significant regions of the state of Maine and the fauna found within these special locations (Doggett et al. 1978; Gilbert 1977a, b; Speel 1978; Weiss 1980). Because of their qualitative nature, measuring changes using the databases produced by these studies is difficult, though highly attractive, since such location specific historical records are rarely available. Detailed site sketch maps, land owner information and tax maps, site descriptions, evaluations based on CAP criteria, species lists, and observations are available for most critical invertebrate areas. For some species, relative measures of their abundances, based on unit effort for particular localities, also exist. During the past six years of field studies in Cobscook Bay, many intertidal species once easily found, according to previous investigators and local knowledge, were rarely or never seen by the author and his colleagues who had sampled Cobscook Bay years before he began ecological studies there. The purpose of this investigation was to measure change, if any, in intertidal communities with assemblages of invertebrates that originally attracted attention to each Critical Invertebrate Area. These were a group of species selected by their inclusion in previous CAP records, which included, but was not limited to, the circumboreal prosobranch gastropod, Margarites helicinus, the Arctic bivalve, Mya truncata, and the northern Atlantic brachiopod, Terebratulina septentrionalis (see Table 1). Within Cobscook Bay, 325
The Changing Bay of Fundy—Beyond 400 Years
the ecological significance of the Critical Invertebrate Areas rested on the spread of many cold-water species from subtidal into the shallow intertidal zone. This expansion of vertical range is possible because of special physical oceanographic and meteorological conditions of this boreal macrotidal estuary that create cooler intertidal environments. The high species diversity, and the unusual southern distribution of Arctic species, made these Critical Invertebrate Areas located outside of Cobscook Bay ecologically significant. The qualitative baselines, generated for each Critical Invertebrate Area in Cobscook Bay and vicinity, served as reference points to measure changes in invertebrate distributions, community structure and diversity. This study marks another point in the timeline for each Critical Invertebrate Area in the Cobscook Bay region. Methods All five locations of this study were sampled in July to coincide within one month of the original historical reference data, marked by dates that ranged from June through August, on field evaluation forms obtained from the Maine State Archives. The duration of the quadrat sampling interval at each site ranged from 40 to 50 minutes. Each transect, later described, was sampled synoptically within a single low tide period by a team of three. Additional data recorded included tidal amplitude, air and seawater temperatures, weather conditions, and general observations of the sample site for evidence of disturbance and features identified in previous reports. Faunal Sampling Methods Location of Study Areas The five study areas selected for evaluation were all in the Cobscook Bay region (Figure 1). Three areas were situated within the inner bay complex: Crow Neck and Wilbur Neck both located in Dennys Bay, and Outer Birch Island located in Whiting Bay. The two remaining areas, Gleason Point and West Quoddy Head, are located north and south, respectively, of the entrances to Cobscook Bay. Gleason Point borders Passamaquoddy Bay, and West Quoddy Head projects into Grand Manan Channel. Sample Site Selection Reconnaissance surveys of each documented location were conducted as part of a pre-selection process for positioning sample transects. The primary goal was to locate one or more “target species” with distributions particular to each sample site (Table 1). Target species were not indicator species. Target species is a term created for this study. These species were considered ecologically significant among the set of those referenced from each site in Critical Area Program Field Evaluation Forms for any of three reasons: (1) a historical record more extensive than a listing in critical area field evaluations only, (2) a recognition of uniqueness in geographic distribution by the Critical Area Program that was used in the decision on registering an area, and (3) available estimates of the relative density of these species made at the time of site evaluation. A secondary goal was to construct a qualitative species 326
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list of all species encountered in the process of searching for target species. Each mission began an hour before low water, and its focus progressed from coarse, large-scale sweeps to fine, small-scale probing while moving towards the low water mark and following the falling tide. Maps contained in the original reports (Maine State Archives) were used for orientation to the specific designated regions within each specific location. These included navigational charts and both topographic and hand drawn maps. In all instances, hand drawn maps were used as the final reference to locate target species. They indicated landmarks, and other physiographic features, illustrated sufficiently to find the exact locations reported. Other supporting materials contained within each file, i.e. descriptions, correspondence and evaluation forms, also aided in this process. Upon completion of a reconnaissance survey, two outcomes were possible based on presence/ absence of target species: either (1) the target species was found and its distribution marked for assigning the boundaries of the sample transect, or (2) the target species was not discovered. In outcome 1, a sample transect was located within the boundaries of the Critical Invertebrate Area indicated by its large-scale delineation on topographic maps and navigational charts. In outcome 2, the boundaries of the sample transect were selected according to the distribution of non-target species found during reconnaissance that often corresponded to significant features of the physical environment. The rationale for following this selection method was based on the prediction that locations qualitatively characterized to have high species diversity would generate the best ecological inventory to compose a baseline update for an area. For example, at Gleason Point the gaper clam, Mya truncata, was not found within the boundaries of the archived, hand-drawn map from the Gleason Cove Critical Area file (Maine State Archives). A transect was, therefore, located within Gleason Cove in an area adjacent to Gleason Point that formed a natural boundary where the greatest numbers of different species were found during the reconnaissance survey. Faunal Transects Quadrat sample transects were oriented as a 50 m2 band (50 x 1 m) parallel to the low waterline with a width overlapping the low intertidal-high subtidal fringe exposed at low water. This stratum had the highest probability of coinciding with the greatest densities of target species. Approximately 30 minutes before the time of low water, the endpoints of the 50 m long transect were anchored with markers and the distance, divided into thirds, marked in turn. Approximately 20 min before the time of low water, a 1 m distance, perpendicular to the transect and extending in the direction of the falling tide was marked at each endpoint and each intra-transect marker. At Crow Neck, Wilbur Neck, and Gleason Point, sampling effort and tide permitted a second sample transect (50 x 1 m), that extended the initial one to give a total sample transect area of 100 m2. It was positioned slightly higher (
