Finally the conditions are perfect for a game of Boccia! There is only a soft breeze, and we are in open water surrounded by a broad band of sea ice which keeps the swell from the open ocean from hitting us. The Polarstern is fairly stable in the water, so we started our boccia game at 6 a.m.
We play with four copper balls, the smallest of which is about as big as a hazelnut, and the largest the size of a tomato. Now in our game the goal is not to bring the balls together as closely as possible, but rather to get them into a target area under the ship. This area, 15 m under the keel at 11 m below the surface, 30 m behind the bow of the 120-m-long ship and exactly in the middle of the 25-m-wide Polarstern, has a diameter of 1.5 m.
Our balls are very special in that they have clearly defined acoustic properties. That means, they reflect and disperse sound of a given frequency in a defined way. The target area under the keel of Polarstern is positioned in the sound cone of the zooplankton echosounder. Rather than measuring the time between the pulse of sound and the strongest echo and thereby calculating the distance to the sea floor at a known travelling speed of sound waves like a regular echosounder, the plankton echosounder measures the strength of the echos returning from different distances. In principle it is possible to determine how many sound-dispersing objects there are in a certain water layer and how much the objects are dispersing sound waves.
In order to calibrate the zooplankton echosounder, the balls must be kept in the sound cone. Calibration tells you something about the particular properties of an instrument, which is necessary because each instrument is different regarding not only the transformation of electrical energy into sound, but also the transformation of the intensity of the echo into the actual signal. And the four different balls are used because the zooplankton echosounder works with four different frequencies. Such balls with well-defined acoustic properties are used for calibration on research vessels worldwide to make comparisons of acoustic measurements from different vessels possible.
The calibration process requires, aside from technical understanding, a certain athletic competitiveness. The balls have to be guided into the target area by a system of lines and ropes. Gallows are fastened to the board wall at approximately equal distances. The points are located on the ship according to drawings, two on the starboard side and one on the port side. Ropes are running through the gallows straight into the water, held taught by weights. Rings are fastened to the ropes one meter apart, and through these rings a fishing line is running. The fishing lines are tied together on the free ends, and exactly at this knot the copper ball is sitting.
As we do not have divers on board to tie the knot under water, this has to happen on deck before the ball is put into the water. The first challenge is to put a loop of line into the water at the bow and then pull it astern underneath the ship’s hull. This exercise has to be repeated for each ball. Once the ball is in the sound cone, one can see on a computer screen how the ball is moved through the cone by paying out line in 10-cm steps on one side and hauling it in on the other. For a solid calibration the ball must be guided into each of the four sectors of the sound cone and exposed to sound with different parameters ranges of the zooplankton echosounder.
The zooplankton echosounder helps to obtain important scientific data on the distribution of planktic animals down to 600 m at high resolution, temporally as well as vertically and, during travel, horizontally.The use of four different frequencies allows to distinguish among four size classes of zooplankton and consequently helps to extrapolate the species composition.
Data gained with the zooplankton echosounder form the base of the Lazarev Sea krill study (LAKRIS). They also contribute substantially to the IPY project SCACE (Synoptic Circum-Antarctic Climate-Processes and Ecosystem Study). As SCACE is linked closely with ANDEEP-SYSTCO (ANtarctic benthic DEEP-sea biodiversity: colonisation history and recent community patterns – SYSTem COupling) to find out more about the flux of biogenic particles and therefore carbon flux to the sea floor, ANDEEP-SYSTCO can utilise measurements from the zooplankton echosounder as well.
It is not one team of scientists playing against the other in this game of Boccia. The common enemy is movements of the ship relative to the water under the ship, which cannot be avoided completely. All scientists are playing together, hand in hand with the bridge and crew. They have to. The current game will not be over before tomorrow morning.
Volker Strass, Alfred Wegener Institute
Photo: B. Ebbe, Senckenberg