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doi:10.2204/iodp.proc.329.111.2011

Secchi disc construction

We constructed two Secchi discs similar to those used previously by investigators in the North Atlantic Ocean (Fig. F1). As reported by Visser (1967), the Royal Netherlands Navy’s sweeping survey of the North Atlantic in 1964 and 1965 utilized one small-diameter (1 ft) and one large-diameter (1 m) disc. Our discs mimicked this 1:3 size ratio but used the currently more widely employed 40 cm diameter disc as our small-diameter instrument and a 1.2 m diameter disc as our larger instrument.

The smaller Secchi disc was custom built but essentially replicated the design parameters of discs that are commonly available through environmental monitoring equipment retailers. It was cut from ⅜ inch high-density plastic and painted in the standard black and white quadrant pattern. The black quadrants were covered with matte black spray paint (Nippo) and the white quadrants with glossy white spray paint (Rustoleum). An eyebolt was installed in the center of the disc as the mounting point for weights and line. Two strands of high tensile–strength tarred marlin line were tied to the eyebolt and used for lowering and hoisting the disc. A ~5 kg shackle was threaded through the eyebolt and provided the weight needed to draw the rope taught during deployment.

Our second Secchi disc was constructed with the purpose of improving our ability to recognize a small object at a great distance. That is, our 40 cm Secchi disc at an anticipated depth (distance) of over 60 m would represent an object no larger than 8.5 mm in diameter to one of our observers. Given this small target size and anticipated scattering caused by surface disturbances (and that the objective of the exercise is to assess water transparency and not the keenness of the observers’ vision), we selected a diameter of 1.2 m for our second disc. This size is not without precedent: Boguslawski and Krümmel (1907) used a 2 m diameter Secchi disc for his observations of water transparency in the Sargasso Sea. Our disc also required additional rigging to maintain the horizontal surface needed to optimize visual identification of a distinct, reflective surface (Tyler, 1968). Our larger disc was cut from similar plastic and painted in the same manner as our smaller disc. Twenty-eight 5.08 cm diameter holes were cut through the black quadrants to facilitate vertical motion through the water and reduce pitch, roll, and yaw of the disc that could result from current flow across its broad surface. An additional measure incorporated into the disc design involved rigging the disc from two sets of antipodal points rather than one central point. The four-part harness was constructed of ¼ inch cables secured to eyebolts installed just inboard of the disc’s perimeter. An eyebolt was installed in the center as an anchor for weights. A total of 11 kg of weight was attached to this anchor bolt. A ⅜ inch polypropylene rope was fitted with a shackle and marked with permanent marker and cable ties in 10 m lengths. The interval between 60 and 70 m was marked in additional 1 m lengths. A total of 140 m of this line was installed on a hydraulic winch located next to the crane.

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