A Blue Crab Moored in an Icy Sea
The Ross Sea is the one of the last places you'd expect to find the blue crab Calinectes sapidus. But there is one Calinectes here, as that's the name Walker Smith bestowed on an IVARS mooring in honor of Chesapeake Bay's renowned crustacean. The other IVARS mooring, Xiphias, honors the swordfish Xiphias gladius.
Moorings and transect stations serve different purposes—one temporal,
one spatial. The 24 stations in the IVARS sampling
transects give an in-depth view of a relatively broad swath of the Ross
Sea at a single point in time. The IVARS moorings capture data
at just two points, but over a relatively long period. Thus the moorings
track the spring bloom through time, while the transects
provide an early- and late-season snapshot of the bloom's distribution
in space (these snapshots can of course be compared from
year-to-year to add a temporal dimension).
The moorings are deployed each year during the initial IVARS cruise. This season, the IVARS crew released Calinectes on December 21, and Xiphias on December 23. The moorings remained in the water gathering data for 40 and 38 days respectively until we returned to retrieve them a few days ago. Dr. Vern Asper, an IVARS collaborator from the University of Southern Mississippi, spearheads the job of deploying and retrieving the IVARS moorings. He is aided by his graduate student Kevin Martin and other members of the IVARS team and Palmer crew.
Each mooring consists of 9 oceanographic sensors strung at 3-5 meter intervals along a 3/8" diameter chain of galvanized steel. The mooring runs from a surface float to a seafloor anchor. At the Calinectes site the seafloor is 600 meters down. The depth at Xiphias is 650 meters. Each mooring is anchored to the seafloor with two railroad wheels having a combined weight of 2,200 pounds. About 40-meters down the rig, an "S-tether" offers flexibility. The tether is a simple yet ingenious device consisting of two 120-meter lengths of rope: a nylon rope connected to the surface chain, and a polypropylene rope connected to the anchor wire. The nylon line is heavier than water and sinks. The polypro line is lighter than water and floats. This difference in buoyancy holds the connected lines in a slack, sinuous curve between the more rigid float and anchor segments of the chain, providing some give against strong currents and wind.
Our job during the last few days has been to find the moorings, release them from their anchors, bring them onboard, and begin downloading their sensor data. We were particularly eager to retrieve Calinectes, as it lay in the path of iceberg B15J. Two years ago, sea ice destroyed both IVARS moorings. This year we were luckier, and retrieved the moorings without incident.
Retreiving a mooring requires close coordination between scientists and crew. We're guided to the mooring by the Argos satellite system, which records radio signals transmitted by an antenna on the mooring's surface float. Once we've located the mooring and steamed within range, we send a series of coded sound signals into the water. These acoustic pulses trigger a release mechanism securing the mooring to the anchor. The ten glass floats on the mooring are then free to carry it to the surface.
The most physically challenging part of retrieving the mooring is bringing it onboard. This can be a difficult and dangerous job on a rolling ship. IVARS scientists don their "float coats," rubber gloves, insulated boots, hard hats, and other cold-weather and safety gear to join the Palmer's similarly clad marine technicians on the ship's cold, wet fantail. Those in the yellow zone nearest the stern must clip into a safety line. The captain uses the ship's thrusters to maneuver the stern close enough for a marine tech to snare the mooring with a lasso. We then winch the mooring onboard. We remove the sensors from the chain one-by-one and quickly carry them inside for a rinse and to begin downloading their data.
Sensors on the mooring perform many of the same tests that we've been doing in the lab. There are three fluorometers to measure chlorophyll concentrations; a silicate and nitrate analyzer to measure nutrient levels; a mini-CTD to record temperature, depth, and salinity; and current meters to measure the speed and direction of subsurface flows. These sensors have been recording and storing data for more than a month, and analyzing their data will keep IVARS scientists busy for months and even years to come