As much as oceanographers dread these studies because of all the work they entail, it is nonetheless quite important in nature to study how the physics, chemistry and biology of the biosphere vary with daily cycles of light and darkness. This is true no matter what sunlight-driven ecosystem is under study: some organisms come alive at night and “sleep” during the day while others adopt the opposite strategy. The chemical fingerprint of the environment changes as organisms’ activities change. Likewise solar warming may heat up the water to produce a daily thermal mixed layer near the sea surface that goes away at night. Daily heating of the atmosphere can also produce turbulence and winds which will affect for example gas fluxes out of and into the oceans and atmospheric aerosol formation from bursting bubbles and breaking waves at the sea surface; and of course sunlight produces ultraviolet radiation that can affect microorganisms in the photic zone. We do not know in many ways what to expect with respect to day night cycles, especially as they may apply to coral reefs. It is well known that coral reefs come alive at night, and we therefore expect to see day-night differences, especially with respect to aspects of the coral reef that we are studying–microorganisms’ activities, volatile gas production, or DMSP and acrylate biological consumption.
We’ve already completed two day-night studies separated by a week from each other. The first study was done in the back reef (BR) and the second in the open ocean at the station we designated as “OO” — see my sampling map from an earlier blog. Both studies were conducted over a 30-hour period, with some of the experiments continuing on for another 12 hours. For each study, the first sampling started at 0400, about one-and-a-half hours before sunrise, and sampling continued every six hours thereafter. We made the same complement of measurements that we made during our transect sampling, except that no light measurements were made at night for obvious reasons. I focused on how quickly acrylate and DMSP were consumed by the microbes (see one of my earlier blogs for more details on these two compounds), but I won’t know any of the results until the samples are analyzed back in Syracuse, NY. I expect that rates of consumption will be higher in the reef compared to the open ocean and that rates will be higher at night compared to the daytime due to sunlight-driven photoinhibition of microbial rates during the day, primarily as a result of extended exposure of the microbes to damaging ultraviolet (UV) radiation from the sun.
We get very tired from doing this work. My family know my sleeping habits all too well. Here’s a photo of me taken by Celia after a long night:
….but by the next day we’ve recovered: