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Volume 5, Number 3
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Global Warming
and the Bay

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Two copopods. Photograph by Adam Frederick
Leptocheirus (amphopod) larvae
Stage 8 crab larvae (zoea). Photograph by Andrea Allman
oyster larvae
Searching for Signals

Fifteen minutes out from the Horn Point harbor, Kimmel checks the depth sounder and slows the boat. The depth sounder reads 70 feet, then 80. Over the Choptank's deep hole, he cuts the motor and drops anchor just shy of 90 feet.

"I hope the anchor's tied on," he says only half-jokingly and begins setting up the odd equipment that will help him locate and identify zooplankton species in the Choptank. On the flat surface above the steering wheel, he plants his computer data logger while Jamie Pierson, his post-doc fellow, and Horn Point microbiologist Byron Crump untangle the mess of wires at the winch. Then they clip the hook onto the metal cage that holds two instruments about to make their way to the Bay's bottom. Something they call TAPS is actually a Tracor Acoustic Profiling System and their CTD is a traditional meter for recording Conductivity, Temperature, and Depth.

Enclosed in their metal frames and tethered together with white line, the two instruments make a bulky package. Pierson and Crump position the instruments and Kimmel engages the winch. The rope sways precariously for a moment, but soon stabilizes as the winch lowers the instruments into the water. Kimmel turns on both devices just beneath the water's surface to get the air out and let them stabilize in their new surroundings.

Soon the TAPS system and the CTD meter are headed for the river's bottom. As they descend, the CTD meter sends Kimmel's computer a continuous stream of information. Water temperatures are actually warmer several meters down this time of year, typical of an early fall profile when the night air begins to cool. The water is well mixed and there are no signs of low oxygen (hypoxia).

The TAPS instrument uses six transducers to send out sound signals at six different frequencies. Each frequency measures a different size class of zooplankton, from 200 microns to 18 mm. That range spans the size of tiny copepod juveniles called nauplii, up to the size of a large shrimp, about as big as a seedless grape, at the top end of the range. The lower the frequency of the sound emitted, the bigger the animal the transducer can detect. As it descends, the CTD meter measures a continuous profileĀ of salinity, temperature, and depth.

Kimmel stops the instruments at different depths and records continuously for several minutes at a series of heights off the river's bottom. Data come in fast and furious but the acoustic pinging of the TAPS device registers no signal on Kimmel's laptop. All those data are logged into the instrument itself, to be downloaded and analyzed at a later date. To figure out which organisms inhabit these depths will require a lengthy analysis process back in the lab.

Whether or not Kimmel has hit upon a hotspot of zooplankton with this measurement remains to be seen. This day will register as only one in a long series, this sampling station one of many that line the length and width of the entire Bay — one data point in a vast array. It's the giant zooplankton record as a whole that will offer clues about the impact of global warming on the Chesapeake's food web.

Reaching out to stabilize the instruments' bulky frames, post-doctoral fellow Jamie Pierson helps to position the sensors to begin their descent into the Choptank River's deep hole (photograph by Sandy Rodgers). The instrument on top sends out sound waves to count tiny zooplankton in the water column. The instrument below delivers a continuous profile of salinity, temperature, and depth back to researcher David Kimmel's laptop aboard the boat. Among the zooplankton counted are copepods, amphipods, and crab and oyster larvae (shown at above, from top, copepods photo by Adam Frederick and crab larvae photo by Andrea Allman).
Jamie Pierson helps to position the sensors. PHotograph by Sandy Rodgers.

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