Water quality tests at Great Rock Bight in Chilmark have improved significantly this week, though earlier spikes in enterococci bacterial levels have led the Martha’s Vineyard Land Bank Commission to reaffirm their swim-at-your-own risk advisory. The commission issued the advisory on July 27 after finding water conditions below acceptable levels per the State of Massachusetts and internal standards.

Earlier Gazette reporting incorrectly stated that MVLBC had lifted the swim advisory.

Great Rock Bight is state-tested weekly from June through August, and on July 25 tests found enterococci bacteria levels of 166 cfu/100 ml, exceeding the single-test state standard of 104 cfu/100 ml. Two days later, levels had jumped to 216 cfu/100 ml on July 27. Levels had been at just 20 cfu/100 ml on July 18. Levels dropped to 41 cfu/100 ml this August 1 and further to 10 cfu/100 ml on August 2. While that level alone is well below the state’s trigger for a swim advisory, beaches that have repeatedly tested above the cutoff are reopened when a geometric mean of the five most recent non-storm tests falls below a 35 cfu/ml coastal standard.

According to the Massachusetts Department of Public Health, enterococci are a group of bacterial species within the Streptococcus genus, some of which (e.g. Streptococcus faecalis) are typically found in human and animal intestines and are therefore present in sewage. These bacteria are also referred to as indicator organisms.

Chilmark public health agent Marina Lent told the last week that back-to-back tests exceeding state safety limits suggested that there was “fecal contamination of that water.”

Land Bank land superintendent Harrison Kisiel called the high tests an “anomaly” and provided a number of potential explanations, including a high avian presence on the beach. He said that seabirds can flood beach waters with fecal matter. Mr. Kisiel further suggested that an overgrowth of seaweed might be trapping the enterococci close to the shore.

Both conditions, Mr. Kisiel stressed, were “not indicative of the Vineyard Sound.” Other Island beaches did not experience similar elevated bacteria tests.

State records show that July 2015 was the last time Great Rock Bight test results exceeded state standards.