4. Discussion
The methods outlined in this study provide a future framework for classifying rocky and/or intertidal areas within ACAD and other National Park units, as well as other areas. The results demonstrate the effectiveness of the CMECS classification across a wide variety of geologic environments. The study also reiterates that integrating the Substrate and Geoform classes to define benthic geologic habitats (also referred to as depositional environments) can provide a greater ecosystem-level understanding of an area, following the approach used at Fire Island National Seashore [
35]. Additionally, while full-coverage mapping of ACAD at this scale is cost-prohibitive, understanding the distribution of geologic habitats within these mapped areas and other future areas of interest provides insights into the geology of other areas of the park.
While Biotopes ultimately could not be defined for ACAD in this study, working through the process to create statistically significant biotopes revealed the complexity of these study areas that may otherwise have been overlooked. In this way, applying the CMECS framework facilitated the interpretation of the data and a more holistic understanding of the study areas. Defining the map units using both the Substrate and Geoform Components, along with relative water depth, provided a suite of variables to be used (individually and in combination) that allowed for a more comprehensive statistical analysis to identify ecologically meaningful relationships with macrofaunal community dataset. The most significant variables at Thompson Island were the Geoform and relative water depth. Had the mapping only included the Substrate Component, our understanding of the area would have been more limited.
Statistically distinct macrofaunal communities were identified within the geologic habitat map units at Thompson Island; however, an assessment of the macrofaunal taxonomic data for each grab sample showed that the dominant species within a given map unit varies. This overall pattern of inconsistency creates challenges for assessing macrofaunal community structure and the relationship of those communities with their benthic geologic habitats. Defining a consistent macrofaunal community and dominant species across samples within a given geologic habitat type is critical to confidently extrapolate biotope classification across map units and the entirety of the study areas. This is because biotopes are, in part, defined by dominant species, following the methodology used at Fire Island National Seashore, which modified the CMECS framework to accommodate macrofaunal grab sample data [
35]. Because CMECS can accommodate various data types and resolutions, the framework provides flexibility in how information is communicated. Here, instead of biotopes, the resulting maps illustrate the Geoform as full coverage polygons across the study areas and the Biotic component at the point locations where grab samples were collected. These outputs combine datasets and highlight the commonality and variation within and among study areas. All four study sites exhibited a unique trend where a sample site was overwhelmingly dominated by one or a few species, suggesting “micro-scale” environments at these sites. This finding warrants further investigation by collecting additional macrofaunal and sediment samples, as well as other environmental parameters (e.g., water quality, organics) to help resolve biotic–abiotic relationships. Additional data may also allow for refined analysis and the development of biotopes.
While CMECS has been applied elsewhere in Maine and the adjacent Gulf of Maine [
8,
40,
41], this represents, to our knowledge, the first detailed application of the classification on rocky intertidal areas in the region. Large, regional projects that focused on Gulf of Maine scale mapping [
8] covered these areas; however, the map detail was substantially less than the maps presented here; examples of this include the Thompson Island study site that was mapped as a “Flat” with no additional information provided. Other mapping efforts provided similar units to the Geoform units interpreted here, albeit with less detail.
Figure 16 compares the mapping of Timson [
4] with the interpreted Geoform units for Thompson Island and Ship Harbor. Notable details missing on the older mapping include the isolated boulders at Thompson Island and boulder fields within Ship Harbor. The state-wide mapping of the coastal environments of Maine, mapped by Timson [
4] and later digitized [
5,
6], remains an excellent resource. The mapping presented here complements existing maps by offering a more detailed approach to delineating geologic habitats, biologic features (e.g., mussels), and other seafloor features. These higher-resolution data can also allow for finer-scale abiotic–biotic relationships to be assessed. An important note here is that the mapping effort undertaken in this study is appropriate for local-scale or site-specific when higher resolution data are needed; mapping the entire state of Maine at the scale used in this study would not be practical, and tradeoffs remain between mapping detail and efficiency.
Intertidal habitats represent particularly important habitats in Maine. Given the high tidal range and 8500 km (5300 mi) of coastline, Maine has 587 km
2 (145,000 acres) of intertidal habitat [
42]; 44% of the intertidal zone is mudflats and 25% is rocky [
42]. Given both the dynamic nature of these environments and the sensitivity of the organisms to changes in water temperature and sea level rise due to climate change [
43,
44], seafloor mapping the physical and biological communities to gain a holistic understanding of the various species, habitats and ecological function is vital to effectively managing these environments now and into the future. This sensitivity is especially important within the Gulf of Maine, which has undergone rapid warming in the 21st century [
45], including heatwaves that have led to changes in commercially important fish, shellfish, and macroalgae [
46,
47]. The Thompson Island site was dominated by intertidal and shallow subtidal flats, which are important for bloodworms (
Glycera dibranchiate) [
48] and documented mussel fisheries. Both species contribute substantially to the Maine fishing community, with landings of blue mussels (
Mytilus edulis) exceeding 430 metric tons (MT) in 2022 worth >
$4,000,000 [
49]. Bloodworm (
Glycera dibranchiate) landings in 2022 were >100 MT, with a value of >
$4,700,000 [
49]. The dredge trails mapped at Thompson Island ranged from distinct to faintly visible, suggesting some reworking of the seafloor here. Understanding the short- and long-term impacts of these harvesting activities is important for sustaining fishing activities and the natural ecosystem. For example, trails left from harvesting bloodworms (
Glycera dibranchiate) persisted for >5 months on Maine mudflats [
50]; and given this, it is likely dredge trails from hydraulic mussel harvesting, which are much larger, persist for longer.
Underwater video imagery was a vital component of the mapping efforts. Traditional sediment grab sampling was not useful in rocky habitats, as the sampler “bounced” off of the rocky substrate and did not return a representative sample. The underwater imagery provides important geologic content, even when a sample was recovered, and was useful for indicating the heterogeneity of the seafloor at these sites. The videos also provided a qualitative assessment of species distribution; of note, this included blue mussels (
Mytilus edulis),
rock crabs (
Cancer irroratus), and sand dollars (
Echinarachnius parma). European green crabs (
Carcinus maenas) were also identified at the Thompson Island and Ship Harbor study areas. These crabs are an invasive species that have had negative impacts on native species, particularly soft-shelled clams
(Mya arenaria) and rock crabs (
Cancer irroratus), which are both valuable commercial species in Maine [
51,
52,
53]. European green crabs have also been linked to the decline in eelgrass (
Zostera marina) [
54] and erosion of salt marshes [
55,
56] elsewhere in Maine. Identification of this species further documents their extent and persistence in Maine tidal waters.
Overall, the current CMECS terminology was suitable for classifying rocky and intertidal environments. The only term that was lacking from the Geoform component was the inclusion of a term for sorted bedforms (aka Rippled-scour repressions) [
39,
57]. These features were mapped as “Ripples” in the Geoform; however, sorted bedforms are ubiquitous along shorefaces around the world, including in the various national seashores and coastal national parks [
58,
59,
60] and should be included within future revisions of CMECS. These sorted bedforms have also been mapped elsewhere in the Gulf of Maine [
2,
61]. Future revisions to CMECS should consider the scale of the Geoforms; many of the terms used to map the geomorphology here were considered “level 1” Geoforms, with a scale >1 km
2. While users of CMECS are encouraged to apply Geoform Component map units as appropriate to the scale of the mapping [
1], more flexibility should be considered in future reiterations of CMECS.
The mapping effort presented here supports the strong need for continued mapping and monitoring into the future to continue to understand these study areas and how they are being impacted by both climate change and anthropogenic activities. The importance of periodically mapping submerged environments, especially in dynamic near-shore areas, is hard to overstate. These mapping efforts allow management intervention to be properly considered and science-based decisions to be made. Similar mapping completed prior to the 2012 and 2016 heatwaves would have quantified changes in macrofaunal species and community distribution. Similarly, areas could be reassessed for changes to the physical environment following a storm event, such as the series of storms that occurred in January 2024, which resulted in substantial erosion and sediment overwash at ACAD and undoubtedly impacted submerged habitats and species. Elsewhere, repeat surveys of NPS areas have offered insight into changes to the seafloor following storm events [
35,
62]. Repeat mapping also facilitates changes assessment related to anthropogenic impacts on the seafloor, both new disturbances as well as recovery of prior disturbances. For example, future mapping efforts at Thompson Island would determine the persistence of the hydraulic dredge trails from commercial shellfish harvesting.
The high degree of geologic heterogeneity of these areas is related to the complex geologic history of the region, specifically the Late Wisconsinan rapid sea level rise during the late Pleistocene and Holocene and resulting deposition in a variety of coarse and fine-grained glacial environments. Ultimately, the lack of stratigraphic data (e.g., sediment cores and seismic reflection profiles) limits a more detailed discussion of the geologic formation of these areas and is beyond the scope of this paper. However, inferences can be made on the general controls of the depositional processes in these areas. The shallow subtidal and intertidal geomorphology of the region is largely controlled by the combination of the bedrock composition and morphology, distribution of glacial sediment, and modern coastal processes (e.g., waves, tidal currents) [
63]. The bedrock topography creates patterns of wave sheltering, while the glacial deposits provide a source of erodible sediment [
64]. The glacial deposits adjacent to both Compass Harbor and Thompson Island largely consist of Presumpscot formation [
16] which provides an abundant source for fine-grained sediment (largely silty clay) while it was eroded during marine transgression. However, the substantial difference in energy controls the deposition at these two sites. Fine-grained sediment dominates the relatively sheltered Thompson Island, except where boulders erode from the glacial till to form fringing deposits along the shoreline [
63] or crop out within other habitats. Conversely, deposition of the fine-grained sediment (e.g., mud) eroded from the glacial deposits at Compass Harbor is inhibited by the relatively high wave-energy. These observations largely match the geomorphic zones identified for coastal Maine [
63], where more protected sites (i.e., Thompson Island, Inner Ship Harbor (
Figure 11 and
Figure 12)) are dominated by mud flats and marshes, while more exposed areas (e.g., Compass harbor and outer portions of Ship Harbor (
Figure 9 and
Figure 11)) are dominated by bedrock outcrop and gravelly coastal deposits. Frazer Creek (
Figure 10), with the complicated bedrock outcrop and various energy levels, has characteristics of both sheltered and more exposed areas. The features around Ship Island and portions of Frazer Creek are similar to those discussed elsewhere in the region [
65,
66]. The distribution of geologic habitats at these study sites contributes to the evolving literature on estuarine facies within the rock record [
66,
67,
68,
69,
70].