Decoupling the Effects of Mass Transfer, Water Motion and Temperature on Reef Health.
Principal Investigator: David Wethey, University of South Carolina

The October mission was canceled due to equipment damaged by water. Repairs included replacement of a breaker box and other electical components. Dr. Wethey's team worked topside after modifying their science plan. All repairs were completed in time to conduct the November mission with Dr. Patterson.

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Decoupling the Effects of Mass Transfer, Water Motion, and Temperature on Reef Health
David Wethey (PI), Christopher Finelli, Brian Helmuth, Dean Pentcheff
This project is a continuation of work in Aquarius by this team during the 1999 and 2000 mission years.

In recent years, numerous studies have begun to elucidate the important role that water motion plays in determining the physiological performance, health and survival of corals and other reef biota. Like other benthic organisms, corals rely critically on the movement of water for the overall exchange of dissolved substances (such as oxygen, carbon dioxide, nitrogen and phosphorous) between the water column and the benthic environment. For example, studies have demonstrated that photosynthetic rates by several species of coral are limited by the rate at which carbon dioxide (in the form of bicarbonate) is delivered to their surfaces, and others have suggested that bleaching can occur if oxygen removal rates are not sufficiently high. Other studies have suggested that reef hydrodynamics may also play a significant role in modulating the effects of eutrophication on coral reef health, by determining the rates at which nutrients are taken up by the corals from the surrounding water column.

In all of these cases, the transfer of nutrients and gases from the water column to the living reef is driven by the rate at which water is refreshed at those surfaces from the overlying water, and cannot be predicted simply from knowledge of the concentrations of these substances in the surrounding water. Importantly, the rate at which water is mixed at a coral's surface is determined to a large extent by the effect of its own size and shape on the water moving over its surface. Thus, the transfer of dissolved solutes such as nitrogen, phosphorous, bicarbonate and oxygen from the overlying water column to the benthic reef biota requires a careful analysis of the role of reef hydrodynamics in driving these exchanges, and knowledge of the effects of organism size, behavior and morphology on these processes. Previous studies have made significant progress in understanding the transfer of nutrients at the scale of entire reefs. With a few exceptions, however, we have yet to thoroughly investigate how mass transfer limitation differs between coral species, nor do we fully understand the potential role of water movement in contributing to coral bleaching. These shortcomings limit our ability to predict how changes in the physical environment are likely to affect the community composition and ecological dynamics of coral reef ecosystems, and furthermore reduces our ability to explore how alterations of the flow environment by one species of coral can affect the survival and performance of other neighboring species.

This proposal has the interrelated objectives of

1. experimentally separating the effects of bicarbonate (CO2) and O2 transfer limitation on corals in field conditions,
2. addressing the potential role of water temperature in modifying these processes, and
3. quantifying patterns in Benthic Boundary Layer and Momentum Boundary Layer formation under natural field conditions.

The approach for this project is manipulative and experimental, and builds directly upon our previous Aquarius missions. In two separate experiments the physical and chemical environment of individual corals will be altered by pumping experimental solutions directly to the coral surface. In both experiments chlorophyll fluorescence techniques will be used to measure photosynthetic response of corals exposed to experimental treatments. In the first experiment, the flow regime and concentration of oxygen or carbon dioxide will be varied to test the hypothesis that mass flux to the coral directly controls photosynthesis. In the second experiment, flow and temperature will be altered while maintaining the chemical environment, thus decoupling the physiological effects of these variables. In all cases, microsensor technology will be employed to make measurements of gas concentration, water velocity, and temperature right at the coral surface.

These rigorous field experiments are critical to our understanding of coral physiology, including mechanisms of coral disease and recovery. Without such manipulative experiments conducted in the natural environment, the development of predictive models of coral response to anthropogenic influences will be severely limited.