Historical Eelgrass Abundance: Buttermilk Bay
Buttermilk Bay is a small embayment that for decades was dominated by eelgrass beds, particular on the flood delta in the center of the bay. Post wasting-disease, eelgrass appeared to peak in abundance in bay during the 1980s, but today eelgrass beds have died off, except for possible a few patchy areas in the channel entrance to the bay.
Eelgrass has died off in Buttermilk Bay, presumably largely the result of eutrophication. Mooring scars were visible in the eelgrass beds in the mooring field abutting Hideaway Village in some aerial photographs from past decades.
Previously Reported Summaries and Historical Trends
Buttermilk Bay is the only site in Buzzards Bay where re-colonization of eelgrass was mapped after the wasting disease (Stevens 1935, 1916, Stevens et al., 1950). Buttermilk and Little Buttermilk Bay was also extensively sampled and mapped by Costa (1988a), including the analysis of three sediment cores to document historical abundance prior to recorded information. With respect to the sediment cores and historical trends, Costa noted, “In Buttermilk Bay, eelgrass was widespread prior to the wasting disease (Stevens, 1935, 1936), and photographs show a broad recovery during the 1940’s and 1950’s. Eelgrass was somewhat less abundant near this core during the early 1960’s, but has expanded since then. Given these observations, and assuming rates of deposition are similar to Waquoit Bay, it appears that the wasting disease began at 27 cm. If sedimentation rates were similar prior to the wasting disease, the earlier decline occurred ~1903”
Other observations by Costa included, “During the 1960’s, eelgrass began to extensively colonize Little Buttermilk Bay, and grew deeper in Buttermilk Bay than during any other recent period (Fig. 14, 15 bottom). Total eelgrass cover in the central part of Buttermilk Ray in 1966 was unchanged from the 1950’s (Fig. 15 top) because of losses due to dredging and new declines in poorly flushed coves. For example, eelgrass was present in Hideaway Village Cove during the 1950’s, but largely disappeared by 1966. Today no eelgrass grows along the inner shore of this cove. Eelgrass continued decline in the deepest parts of the Ray during the 1970’s and 1980’s (Fig. above, bottom) but greatly expanded in Little Buttermilk Bay and other shallow areas.”
“The losses of eelgrass in the deep portions of the Bay and in some poorly flushed roves appear related to nutrient loading or increased turbidity. Today, eelgrass is absent from areas with the highest nutrients concentrations, depth of growth in Buttermilk Ray correlates with dissolved inorganic nitrogen content. of seawater (Costa, 1988).
Overall, Buttermilk Bay has not experienced the large declines observed in other highly developed bays. This is probably due to the high flushing rate, and because the Ray is so shallow, most beds are not at the lower depth limit of growth. The loss of some vegetation since the 1960’s: however, suggests that Buttermilk Bay may be affected by future increases in nutrient loading and sediment resuspension.”
Costa also sampled periphyton and nutrients along gradients in the bay. With respect to the nutrient and periphyton sampling, he noted “To identify what levels of nutrient loading cause these changes, concentrations and inputs of dissolved inorganic nitrogen (DIN) in Buttermilk Bay were measured. Periphyton on eelgrass leaves and plastic screen strips on floats correlated well to mean DIN. Experimental floats released nutrients and demonstrated that small increases in DIN significantly increase periphyton abundance. The depth of eelgrass growth in Buttermilk Bay decreased by 9 cm for every 1 increase in DIN. Periphyton abundance is more important than phytoplankton concentrations in limiting eelgrass growth in Buttermilk Bay, because water in this bay has a short residence time, and phytoplankton gradients are less prominent.”