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Preliminary results and recommendations for future deployments

During Expedition 329, the JOIDES Resolution Secchi discs were deployed in the world’s clearest marine water. The discs were deployed during the austral late spring, a time when chlorophyll abundance in the water is near its annual minimum value. The sun position (between 6 and 9 h following sunrise) and atmospheric conditions (clear to partly cloudy) of each deployment were nearly optimal. Deployment occurred on the JOIDES Resolution’s fantail while the bow faced east, thus allowing us to minimize glare by keeping the sun slightly behind us. At the time of the observations, the JOIDES Resolution was dynamically positioned over the center of the gyre, a location as far from sources of wind-blown, water-clouding clay as possible. Therefore, we expected the depth of the SDT to be the greatest possible, in excess of the transparencies reported for the Sargasso Sea by Boguslawski and Krümmel (1907) and Visser (1967).

Our results do not exceed those reported in the Sargasso Sea. The maximum confirmed depth of observation of our small (40 cm diameter) disc was 46.5 mbsl, 2 m shallower than Visser’s observation of 48.5 mbsl (acquired using a 30 cm diameter disc) but 5 m deeper than Krümmel’s observation using a larger, 50 cm diameter disc. The maximum observed depth of our large disc was 5 m shallower than Visser’s (1 m diameter disc) and approximately in the middle of Krümmel’s maximum observed depth range of 55.5–66.5 mbsl that he acquired using a 2 m diameter disc.

Several options possibly explain these results:

  1. The small amount of glare and chop we experienced was unique to our experiment. Our viewing conditions were somehow less ideal than those Krümmel and Visser experienced during their voyages.

  2. The instrument is true and the method is sound; thus, the water of the central South Pacific Gyre is less transparent than water in the Sargasso Sea.

  3. A Secchi disc is the quintessential scientific “blunt instrument” and is incapable of discerning the subtle differences that distinguish the world’s clearest marine water from the world’s second clearest marine water.

We believe option 3 is the most likely interpretation. Sufficient objective evidence exists to support the South Pacific Gyre’s claim of being the world’s most hyperoligotrophic water. We consider it unlikely that Visser, operating in the North Atlantic in February, enjoyed better observational conditions than we did. Thus, our conclusion is simple: the Secchi disc is not the right tool for investigating the clarity of water that allows light (PAR) to penetrate 100–150 mbsl.

The more important aspect of our work, we believe, is that the JOIDES Resolution’s Secchi discs can now serve as showcases of water monitoring efforts and marine scientists’ concern for the overall health of the world oceans. The best purpose of a Secchi disc is not to set records of greatest water transparency but rather to track changes in water quality that could be harbingers of lost water quality. For that purpose, the Secchi disc remains an important tool for scientists and citizens; therefore, we advocate the continued use of the JOIDES Resolution Secchi discs as a matter of routine shipboard operations and promotions during its journeys to all the world’s oceans.