Barnstable Harbor is a shallow, tidal coastal embayment, approximately 5.5 km long and 1.2 km wide, in the southern part of Cape Cod Bay. The salt marsh in Barnstable Harbor has long been a study object and currently occupies more surface area in the Harbor than open water, as it has in the past. Although much of the salt marsh was not mapped during the vessel-based survey, due to the objectives of this study, the approximate seaward edge of the salt marsh was mapped in many areas and therefore could be used to measure changes to salt marsh in the future.
Data were collected in two phases in 2018; phase 1 included vessel-based sonar surveys during the summer (May 30 – July 31) and phase 2 included grab samples for invertebrate and sediment analysis (October 1 – 4). Sampling stations were randomly stratified across 5 zones (subtidal creek, subtidal mouth, intertidal creek, intertidal, and aquaculture) as determined primarily by the acoustic data. Main findings include:
- Identification of 40,267 individuals belonging to 89 species were collected at 31 sample locations. The highest diversity in biotope 2 (65 species), where a combination of gravel and algae create a heterogeneous environment (varied bottom types) that provides cover and protection from predators for many smaller individuals. The most abundant stations can be found in biotope 7 where a high number of Amethyst Gem clams (Gemma gemma) were found.
- Although Massachusetts does not release an official list of invasive species, a literature review showed that the common periwinkle (Littorina littorea) is an established invader in New England and can be found at 12% of stations in Barnstable Harbor. Caprella penantis, a species of skeleton shrimp (present at 9% of stations) is actively monitored within the Marine Invader Monitoring and Information Collaborative (MIMIC) at the Massachusetts Office of Coastal Zone Management. Microdeutopus gryllotalpa, a small amphipod crustacean, and present at 29% of stations, is classified as a cryptogenic species, meaning its geographic origin is unknown.
- Compared to studies conducted in the 1960s and 1970s, we detected an increase in southern species more adapted to warmer waters (Table 9). Two southern species were found in Barnstable Harbor in 1962 and 1970 respectively, 5 species in 1977 and 14 species in 2018. Since sampling methods employed 50 years ago were different from the ones employed in the study presented here, these results are not directly comparable but are illustrative and could indicate a potential shift in species composition that could be monitored regularly.
- Based on abiotic factors such as grain size metrics and organic matter content of the sediment, we could explain 72.9% of species distribution in Barnstable Harbor. This number is comparably high when compared to previously published studies conducted around New England, which achieved 21 – 84% explanation. A high explanatory rate means that, as long as grain size metrics remain stable, changes in species composition should be minimal. Water column factors such as dissolved oxygen, pH and salinity also play and important role in species distribution, however, these factors are subject to change by season, tides, anthropogenic inputs and weather events and have to be monitored across seasons and years before potential inclusion as an explanatory factor. A well-established dataset of water column factors could increase the explanatory power of CMECS in Barnstable Harbor.
- Since species distribution is highly dependent on sediment characteristics (72.9%), future monitoring could be limited to yearly sampling and analyses of grain size metrics from the stations established by this study. Any significant changes in grain size could warrant further investigation into potential changes in species diversity. Regular monitoring of water column parameters (dissolved oxygen, pH, salinity) would be the first step towards establishing a baseline for future inclusion as an explanatory factor in terms of species distribution. The main tidal channel in Barnstable Harbor is very stable and does not need annual dredging, were this to change the impacts would likely be felt systemwide.
- Vessel-based acoustic data show a very dynamic harbor near the main inlet channel with rapid drop off of energy levels with distance from the harbor entrance. The Harbor does have marine debris in many places, though there does not seem to be a single source or sink of marine debris. The items range in size from small rectangular objects,likely aquaculture gear, to large unidentified items. None were shown to present a hazard to navigation at the time of mapping.
- The acoustic data was also useful in documenting the state of sub-tidal resources. Areas of salt marsh erosion were seen along the southern shore of Sandy Neck among other places. There were no signs of failure in the marina/inner harbor along the piers and pilings though this should not be used as an assessment for those areas, but rather a confirmation that failure was not imminent at the time of mapping.
- The data collected and maps created for this project will serve as a scientifically robust data set to establish baseline conditions within Barnstable Harbor but also to measure future change The entire project does not need to be repeated to do so, site-specific research can be done as needed, but similar methods to these should be employed at those locations.