Historical Eelgrass Abundance: Apponagansett Bay
On some nautical charts of the period 1890 to 1930 (prior to the wasting disease), in certain shallow bays where eelgrass is a particularly dominant feature, eelgrass is indicated by “grass” or “grs.” The figure above shows a portion of a National Ocean Survey Nautical chart from 1899 for Buzzards Bay, that clearly indicates eelgrass in Apponagansett Bay both near the mouth, and at the uppermost portion of the bay. Compare this map to the 1900 chart below.
Similar to the 1899 chart above, the figure above shows a portion of a National Ocean Survey Nautical chart from 1900 for New Bedford Harbor and approaches, that includes a portion of Apponagansett Bay, and indications of eelgrass.
For this assessment, only eelgrass habitat in inner Apponagansett Bay, the area north of the Apponagansett Bay Bridge, was evaluated. This embayment is broad, shallow, and protected, with bottom sediments in central areas composed of silts and muds. The dark muddy bottom of the inner bay is noted both in field reports, nautical charts (early charts note “STK” for sticky), and is evident in aerial photographs. These fine sediments are also easily resuspended on windy days and by boat traffic. Aerial photographs taken on windy days or after days of heavy rains are not interpretable for eelgrass mapping.
The Apponagansett bay watershed is small compared to the adjacent Slocums River and Acushnet River watersheds, but the watershed does include a portion of New Bedford. Much of the north and west portion of the Apponagansett Bay watershed remained farmland through the 1940s or later. The aerial photographic record show large numbers of homes built or expanded upon during the 1960s through 1990s. Most homes in the watershed were not sewered until the 1970s, and 1980s (the first were in Padanaram Village), and some much later.
Nautical charts prior to the 1931-1932 eelgrass wasting disease, and a sediment core taken by Costa suggest that eelgrass once dominated Apponagansett Bay north of the bridge. No eelgrass was found in the inner harbor by Costa during field visits in 1984 and 1985, although the sediment core and local anecdotal information suggested some patches of eelgrass were present perhaps as late as the 1960s or 1970s. Eelgrass was not observed in the inner Apponagansett Bay by DEP in 1995, nor has eelgrass ever been mapped or reported in the inner harbor south of Little Island (in the conditional shellfish area) in decades (Harbormaster Steve Melo, pers. comm.).
Costa (1988a) noted, “Some parts of the inner harbor are covered with a rich gelatinous ooze of mud and decaying algae that has been observed in other enriched embayments….” Notes from those surveys indicate there were large accumulations of the red algae Agardhiella and Gracilaria as well as sheets of the green alga Ulva (sea lettuce). This accumulated drift algae is another confounding factor when interpreting aerial photographs of Apponagansett Bay, and this drift algae now appears to have been present over a long portion of the aerial photographic record.
In this new analysis, the original aerial photographs used in the 1988 study were reevaluated with improved image contrast and better mapping and photo registration techniques. In addition, numerous more recent high resolution photographs and previously unevaluated historical photographs were interpreted and compared. Special attention was given to paired spring-summer recent aerial photographs, and features in the newest high resolution imagery was compared to features in the historical photographs. The new analysis presented here excludes certain previously questionable vegetation areas in the 1960s, 1970s and 1980s photographs.
Based on this new analysis it now appears that most of the benthic vegetation features in the September 1966 and April 1971 aerial photographs, and all the features in the October 1981 aerial photograph previously reported in Costa 1988 as eelgrass, were more likely to be drift algae. Even the areas identified as possible eelgrass as presented here, including the possible eelgrass beds identified on 1951 aerial photographs by DEP (classified as low confidence in identification in the GIS data set), may include or be composed largely of drift algae. Anecdotal information and the 1985 sediment core reported by Costa suggested eelgrass was present in some abundance after the wasting disease. Such high densities of eelgrass seed coats in the sediments are possible only if eelgrass were present in the inner harbor after the wasting disease because of how eelgrass seeds are dispersed.
Among the historic aerial photographs, the highest apparent eelgrass cover is found in a winter 1941 aerial photograph. The photograph was taken before the widespread use of chemical fertilizers on farms, and the number of homes in the watershed was still relatively low. In other photographs of this period, accumulations of drift algae in harbors are not apparent, so the vegetation features shown in this photograph could plausibly be eelgrass. Analysis of the photographs could be improved through interviewing senior lifelong residents and shellfishermen on the bay.
Despite the uncertainties in the interpretation of pre-1970 photographs, it is clear that whatever eelgrass may have been present between the 1950s and 1980s, the beds were small and insignificant compared to the widespread eelgrass cover prior to the wasting disease. The apparent widespread loss of most eelgrass by the 1960s or earlier, or the failure of eelgrass to return to pre-wasting disease abundance is consistent with the historic patterns of development in the watershed.
There are more than a thousand moorings today in inner and outer Apponagansett Bay. In the shallow waters north of the Apponagansett Bay bridge, water skiing was a popular pastime in previous decades. In Costa (1988a), both nutrient loading and increased turbidity from sediment resuspension by boats were identified as likely contributing causes to eelgrass declines in Apponagansett Bay. Based on aerial photographs of comparable periods, he noted a ten-fold increase in recreational boats between the 1940s and 1970s. Mooring scars are clearly evident in the outer harbor area in a late summer 2009 aerial photograph below, but the disturbed area in this photograph may be covered with algae, and not eelgrass.
Costa (1988a+b) found no eelgrass in inner Apponagansett Bay during field visits in 1984 and 1985, but based on a sediment core in Apponagansett Bay, historical aerial photographs, and anecdotal information concluded, “eelgrass is abundant in the bay on nautical charts from the 19th century, eelgrass was destroyed in 1931-32, then showed recovery on aerial photographs during the 1950s and 60s, then disappeared again. (1988b),” and, “the most recent loss of eelgrass appears due to declining water quality from nutrient loading or increased turbidity from sediment resuspension by boats (Costa, 1988[a]).” There was uncertainty as to the identity of the vegetation in the 1981 photograph at the time, and Costa noted “Some vegetation appears along the banks at the head of the Bay in the 1981 photograph, but it was assumed to be largely composed of drift algae or Ruppia [widgeon grass].”
Based on the analysis of additional recent and historical aerial photographs, and a more detailed analysis of high resolution scans of the original study photographs, in this report it is concluded that all the vegetation in the October 1981 panel below, and possibly most of vegetation in the September 1966 and April 1971 panels, is in fact drift algae.
Data for the sediment core from Apponagansett Bay in Costa 1988a is shown below. It was taken 130 meters west of Little Island. Although eelgrass apparently disappeared from all of inner Apponagansett Bay when it was taken in 1985, it showed moderately high seed test counts in the top core segment (0-3 cm). This suggested the presence of eelgrass within the past decade or two, although perturbation of the sediments may blur the record here. No living eelgrass seeds were found in the top of the core, and occasionally one or two Ruppia seeds were observed in core segments. This sediment core had the highest seed density among the 5 sediment cores in the 1988 report.
Right: Sediment core data for Apponagansett Bay as reported in Costa, 1988.
Seasonality of Drift Algae in Apponagansett Bay
There is a strong seasonal pattern drift algae in inner Apponagansett Bay, with vegetation features more prominent in the fall, a pattern observable in both the spring-summer 2001 paired images, and the spring-fall 2009 images. In other embayments, such seasonality can be an indication of either annual eelgrass beds or increased growth of drift algae, but here the vegetation is drift algae. Annual eelgrass beds, tend to appear in very shallow waters, and tend to have a textured yellowish brown color (or lighter color in black and white photographs), whereas drift algae tends to have a dark flat appearance and are typically located in somewhat deeper areas and depressions. A striated pattern in a sheltered harbor is also typical of drift material. The dark circles at moorings (east side of photograph) are likely the result of drift algae accumulating in depressions caused by mooring chain scour.
Apponagansett Bay Historical Changes
No obvious eelgrass beds are evident in a December 1938 aerial photograph (below), the earliest aerial photograph available. However, this photograph has poor water transparency, and was taken just 6 years after the wasting diseases, and three months after the Hurricane of 1938 (September 21), so few conclusion can be drawn from this image. Elsewhere in Buzzards Bay, no obvious eelgrass beds were evident in the few aerial photographs obtained by Costa from the December 1938 survey for the 1988 eelgrass study, but in some Cape Cod embayments (like Waquoit Bay), eelgrass beds are distinct in the survey. One anecdotal report indicated eelgrass beds in outer Apponagansett Bay (off Bayview) were destroyed by the Hurricane of 1938 (H. Day, pers. comm.). It is unclear how much eelgrass had recovered by 1938 before the hurricane struck (eelgrass existed as just small patches in Buttermilk Bay about this time). Although it is difficult to discern in the 1938 photograph because the image was lightened and color stretched to better define submerged features in the bay, most of the upper watershed is dominated by farmland in this and photographs through the early 1950s.
The winter 1941 (probably January or February) photograph of Apponagansett Bay, although covered in ice in large areas, has some of the most convincing eelgrass bed features among any of the photographs. The sediment core was taken near Little Island, near the edge of this vegetation area. Subsequent photographs have features largely consistent with accumulated drift algae, although there are certain nearshore features that could be eelgrass beds, or a mix of eelgrass and drift algae, on the 1956 and 1966 photographs as indicated on the maps. Of note are the vegetation patches in the westernmost portion of West Cove in the 1966 color photograph that are brown colored. These features are consistent with heavily epiphitized eelgrass beds, but could also be accumulated algae in very shallow water. Analysis of the photographs could be improved through interviewing senior lifelong residents and shellfishermen on the bay, and by the analysis of new sediment cores.
An important feature observable in several photographs is the humic colored water plume arching toward the mouth of the bay from the Bayview village salt marsh. Water coloration, suspended sediments, and eutrophication likely all contributed to the apparent widespread loss of most eelgrass in Apponagansett Bay by the 1960s or earlier, or the failure of eelgrass to return to pre-wasting disease abundance.