Every week on Monday or Tuesday a group of eight Falmouth residents gets up with the sun and fans out in two teams to 15 Falmouth estuaries from Megansett Harbor in North Falmouth to Waquoit Bay on the Mashpee town line.

Each team drops the sensor end of a YSI water quality meter into the water off docks at specific, repeatable locations. These electronic hand-held devices—the instruments are named for the town of Yellow Springs, Ohio, where they are made—measure the temperature, salinity and dissolved oxygen of the estuarine water. They also collect a bottle of water to measure turbidity back in the laboratory.

These early risers are volunteers in the Falmouth Water Stewards Pondwatch program. They assemble every week in spring, summer and fall and—showing true dedication—every two weeks even in the dead of winter.

This year marks the 10-year anniversary of the first Pondwatch measurements. The data go to the Waquoit Bay National Estuarine Research Reserve, where WBNERR research associate Jordan Mora oversees the turbidity measurements, checks and archives the numbers and keeps the meters repaired and calibrated. The Pondwatcher data now provide a rich record of the health of the largest estuaries along Falmouth’s 68 miles of coastline.

Jennie Rheuban, a research associate at the Woods Hole Oceanographic Institution, recently took a detailed look at the data to determine how our estuaries are changing. She found that water temperatures increased, especially during the months of May and September. In 10 of the 15 estuaries, the average water temperature rose significantly in one or both of those months. This is interesting because in the business of discerning climate trends, 10 years is not a particularly long time to be able to sort out long-term trends from year-to-year variations—so these changes are actually quite large.

Where they occurred, faster rates of change occurred in May, but more sites showed warming in September. These increases in temperature were consistent with, or slightly higher than, increases in temperature of about 1.4°F per decade that Rheuban and her colleagues found for estuaries around Buzzards Bay using more than 20 years of data from the Buzzards Bay Coalition’s Baywatchers program.

These temperature changes are important and interact with the biggest cause of water quality decline—excess nitrogen. Excess nitrogen from wastewater and other sources fuels algae growth. Warmer water accelerates the rate of algal decay, which lowers dissolved oxygen, particularly in the summer. Because oxygen is less soluble in warmer water, higher temperatures means less oxygen is available to fin- and shellfish.

Rheuban found exactly that pattern in the Pondwatch data—a trend of declining dissolved oxygen concentrations in nine of 15 estuaries. Because increased temperatures occurred in May and September, the growing season for estuarine organisms—and the period over which low oxygen is more likely—appears to be getting longer.

These higher temperatures are consistent with projections for climate change for the northeastern US recently released by the University of Massachusetts Amherst’s Northeast Climate Science Center. Summer and winter temperatures in Massachusetts during the next 50 to 60 years are expected to rise 6°F. The hottest summers of 1950 to 2000—a time when most current Falmouth residents had their first contact with our coastal waters—will become the coolest summers of the future.

Rheuban found other interesting things in the Pondwatch data. Since the start of the Little Pond oyster-farming project in 2013, average annual water clarity increased by 22 percent, or by about one foot. This lends early support to the notion that shellfish can improve water clarity by filtering.

In addition, almost all of the estuaries showed improved overall water quality during the last four or five years. This improvement followed a steep decline in water clarity between 2009 and 2011. Rheuban and Mora used additional data from the Waquoit Bay National Estuarine Research Reserve to uncover the cause. The low water clarity aligned with a major spike in dissolved organic nitrogen, phosphate, and silicate in 2010. We know that all of these nutrients are associated with inputs from surface water. A record-breaking rainfall in March 2010 caused considerable flooding throughout New England. Rheuban and Mora’s working theory is that these inputs combined with a heat wave in July 2010 to decrease water clarity and quality. They think the estuaries then recovered over the next several years.

These analyses show the value of regular and consistent measurements that capture the events and conditions that control water quality in Falmouth’s estuaries. New technologies in the future will improve our ability to both record data and detect trends. And the Pondwatch program aims to add nitrogen to its suite of measurements. But for the foreseeable future, the willingness of volunteers to get up early and carefully record water quality measurements will continue to contribute to our understanding of how our estuaries are changing—and ultimately how we can take actions to help them.

Dr. Neill is a senior scientist at the Woods Hole Research Center and a member of the Falmouth Water Stewards board of directors.

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