Needle in a haystack

Finding something smaller than a soda can in the ocean is a bit like finding a needle in a haystack. But in this case, finding the needle means recovering very valuable information.

We found a temperature logger deployed on the forereef!

The logger was attached to a buoy to make it easier to find.

In this picture, Tom has just recovered the old loggers and

is about to attach new loggers to the buoy. Photo by Yalan,

National Sun Yat-Sen University.

During our expedition to Dongsha last year, we deployed about two dozen temperature loggers around the atoll. Every fifteen minutes over the past year, these loggers have recorded the temperature of seawater. These temperature records are incredibly important for understanding how internal waves affect corals living on the atoll, as well as identifying warm events that may have caused coral bleaching (see post below on bleaching at Dongsha).

The tricky part is finding our loggers. Dongsha Atoll is about 500 square kilometers, and our loggers are not much bigger than a pen. To make matters worse, after a year in the ocean the loggers will be completely covered with marine life, which acts as camouflage. The key is taking accurate GPS coordinates when we deploy the loggers, and a little bit of luck.

A temperature logger deployed in June 2013 that we just 

recovered. After a year in the ocean, the logger is

completely covered with fleshy and coralline algae, and

even a coral!

So far we have found all the loggers that we left on the reef flat, which is the relatively calm and shallow part of the reef. We haven't had quite the same luck (yet) with loggers on the deeper, rougher forereef - we found 2 out of 8 on our first attempt. Actually, finding any forereef loggers is a huge success because the only way to find them is by SCUBA, and at 70 feet depth we only have 10-15 minutes to search before we need to return to the surface. And of course there is always the possibility that our moorings broke free during storms - one of the largest typhoons of the year passed nearby Dongsha. Though very pleased with the valuable information we have already recovered from the loggers, we are holding out hope that we will still find the rest!

- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution.