The short version
In his 1988 Boston University dissertation, Joseph Costa pieced together the history of eelgrass in Buzzards Bay and nearby waters, including Waquoit Bay. Using old aerial photos, nautical charts, first-hand accounts and mud cores that preserve eelgrass seeds, he showed that eelgrass once grew even in the deepest parts of Waquoit Bay. It recovered after a disease outbreak in the 1930s, then began disappearing after 1965. By the 1980s it survived only near the bay’s mouth, at less than 10% of its peak. Costa linked this latest loss to nutrient pollution.
What the researchers did
Costa’s dissertation, supervised by Ivan Valiela, combines several lines of evidence to reconstruct how eelgrass changed over the 20th century and why.
- Mapping: He mapped present-day eelgrass in Buzzards Bay from aerial photos and field surveys, and measured how fast it grows.
- History: He gathered aerial photographs back to the 1930s, nautical charts and first-hand accounts, and wrote detailed histories for a dozen bays.
- Mud cores: In Waquoit Bay he took four sediment cores, three of them in a line across the deep central bay. Layers of eelgrass seed coats and plant fragments, along with shells, record when eelgrass grew there.
- Nutrient experiments: In Buttermilk Bay, at the head of Buzzards Bay, he measured nitrogen levels and the algae growing on eelgrass leaves to see how nutrients limit eelgrass.
What they found
- Eelgrass once filled Waquoit Bay. The cores show eelgrass grew even in the deepest parts of the bay before the “wasting disease” of 1931–32 wiped it out across the region.
- It came back, then collapsed. Photos show eelgrass recolonizing the central bay in the 1940s and becoming abundant by the late 1950s. After 1965 it disappeared from the deepest areas first, and by the mid-1970s it was gone from the bay’s edges as well.
- Only a remnant remained. By the late 1980s, sizable eelgrass beds grew only on the sand delta near the bay’s mouth, covering less than 10% of their 1950s–60s peak.
- Seaweed took over. Layers of drift seaweed up to 70 cm (about 28 inches) thick had built up in the bay. Costa noted that seedlings cannot survive under such dense mats.
- Small nitrogen increases matter. In Buttermilk Bay, small increases in dissolved nitrogen significantly increased the algae coating eelgrass leaves. The deepest eelgrass grew 9 cm shallower for every 1 µM increase in dissolved inorganic nitrogen.
- Scallops followed eelgrass. Costa cites an estimate that Waquoit Bay once supplied 80% of Falmouth’s scallop fishery.
What it means for Waquoit Bay today
This is one of the earliest detailed records of what Waquoit Bay lost. It shows that eelgrass filled the bay within living memory and bounced back once before, after the 1930s disease. The difference after 1965 was nutrient pollution from a fast-developing watershed. Costa concluded that the pattern of decline in Waquoit Bay suggested it was related to nutrient loading
(Costa, 1988). Later studies confirmed the link (see why Waquoit Bay lost its eelgrass).
The thesis also explains why recovery won’t be quick. Eelgrass seeds mostly land close to the beds that produce them, and thick seaweed mats smother seedlings. Once nitrogen comes down, the bay may need time, and perhaps help, before eelgrass spreads back from the few places it survives. By 2007, state mapping found no eelgrass left in the main bay at all (see our summary of the statewide mapping).
Read the thesis
Costa, J. E. (1988). Distribution, Production, and Historical Changes in Abundance of Eelgrass (Zostera marina L.) in Southeastern Massachusetts. Ph.D. dissertation, Boston University. No DOI. Online copy: Buzzards Bay National Estuary Program.
License: © Joseph E. Costa. The dissertation is not openly licensed, so we link to the copy posted by the Buzzards Bay National Estuary Program rather than hosting it here.
Drafted with AI assistance from the linked source.
