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Don Lewis, Massachusetts Audubon Society,
Fox Island Wildlife Management Area

Cape Cod Terrapins Active in Sub-Freezing
February Temperatures — 8 February 2003

What do the Florida Keys and Outer Cape Cod have in common, especially in mid-winter?  Give up?  Both have a diamondback terrapin population active all year-round.

Say what?  Do you mean that there are cold-blooded turtles awake and active in sub-freezing conditions at nearly 42 degrees latitude in February, just like their distant cousins in the sub-tropics?  You have got to be kidding!  And after today’s outing to Henson’s Cove, I wish I were.

Yesterday the Cape was buried in 12 inches of snow.  Today’s temperature nudged 30 degrees Fahrenheit with wind chills in the mid-teens.  A healthy sheet of ice closed in Henson’s Cove where Elizabeth Hogan had made her discovery of four turtles hunkering on the bottom in 32 degree Fahrenheit water temperatures on 8 January 2003 (See Mystery of Henson’s Cove).  As noted back then, we planned to revisit the cove with temperature probes to “shed light on the mystery.”  Since then, though, the Outer Cape has experienced its worst cold snap in 120 years.  In Wellfleet, ice floes 20 feet square by 4 feet deep have piled on the shore like driftwood.

Finally, with a slight break in conditions, Elizabeth returned to the cove on Thursday.  But instead of solving our puzzle, the mystery only deepened.  Since the cove was more than half iced over, she trudged overland to investigate what might have happened to those four turtles.  Under clear skies and bright sunshine, air temperature soared to 30 degrees Fahrenheit.  As she stared into the muck, a shocked expression froze on her face.  Out of the murk rose a diamondback terrapin that surfaced for a deep breath and then slowing disappeared back into the ooze.  If she hadn’t had her camera at the ready, she may not have believed her eyes.

She rushed back to her car for a thermometer and recorded a water temperature of –2° Celsius along the shore of the cove.  Clearly we needed to investigate these happenings, but the intervening snow storm thwarted our efforts.  The working hypothesis, as noted in the 8 January entry, is that a seep from the Outer Cape aquifer is leaking its fresh, warmer water into the cove, creating a thermal environment that keeps terrapins just above their brumation threshold.  Little else would explain this bizarre vaudevillian show “Turtles on Ice.”

This map of the Cape’s elbow gives you a sense of where Henson’s Cove is located at the northern end of Little Pleasant Bay in an area called the River in East Orleans.

We returned today, first hiking through snow drifts because an icecap prevented access by water until later in the day.  As illustrated in the panoramic view at the top of this story, the cove was locked in ice.  No turtles in sight.  But at the north end of the cove we spotted a circle of clear water in the midst of the icecap.  Closer examination showed that the water was actively bubbling, presumably above the hypothesized aquifer seep.  Unfortunately, the ice was too thin to support us and we couldn’t get close enough for a temperature/salinity probe.  Elizabeth observed that the turtle she had photographed on Thursday “was just feet away from the seep area, which was uniformly covered in ice.”

We waited until the tide turned and then mounted kayaks to see if we could force our way into the cove and to the location of the possible seep.  The River was free of ice, and we broke through narrow ice blocks to reach within about 200 feet of the circle . . . but no closer.  By then the tidal influx had forced the icecap over the seep and covered the entire northern half of the cove.  We managed to get readings at three locations, marked 1, 2, and 3, respectively, on the map below.  The star denotes the location of the possible seep.

At the opening of the cove to the River (#1), Elizabeth recorded a water temperature of –3.3° Celsius.  At the point marked #2, we got a temperature reading of +1.6° Celsius and a salinity of 20.1 ppt.  Breaking through a bit of ice and reaching within about 200 feet of the possible seep, we recorded +4.0° Celsius and a salinity of 20.5 ppt.  For perspective, the cove runs about 500 feet from north to south and about 200 feet at the north end from west to east.  Despite distortions I may have introduced in annotating the map above, the distance between each of these points (1, 2, 3, and the seep) are all about 200 feet.  So, temperatures rose nearly 5° C in the first 200 feet, then about another 2.5° C in the next 200 feet as we approached closer and closer to the target area.

Surprise!  The forecast calls for colder conditions the next few days.  But we’re planning to return to Henson’s Cove as soon as ice clears and tides permit, probably in another ten days or so.  Our goal is to get temperature and salinity readings directly at the possible seep location.  Also, we hope to net one or more of these active terrapins and obtain an internal body temperature.  While turtles are magical, we suspect they aren’t breaking the laws of nature.  So, unless our terrapins have slipped across the line from cold- to warm-blooded, we need to identify the thermal condition that permits this year-round activity among reptiles at the northern edge of their range.