Flow Modulated Metabolism: Connection with Coral Bleaching and Reef Oxygen Crises?
Principal Investigator: Dr. Mark Patterson, College of William and Mary

The condition of coral reefs in Florida and throughout the Caribbean is generally considered to be in serious decline. Regional problems related to pollution and overfishing are well documented as causes of coral reef degradation. However, global warming is considered by many to be the greatest threat to the long-term condition of coral reefs worldwide. The problem faced by coral reefs with global change is that corals are especially susceptible to warming; even a degree or two can make a difference during the summer when water temperature reaches its maximum. Surprisingly, average summer water temperatures are just a degree or two less than what totally stresses corals out, or even kills them. Coral bleaching is a known stress response exhibited by corals to increased temperature: corals turn pale in color, growth slows, reproductive output is reduced, and death is sometimes the result. But temperature is not the only factor that affects coral bleaching.

Careful monitoring of corals and coral reefs has documented that there is individual susceptibility to coral bleaching, where corals of the same species exhibit bleaching patterns that are difficult to explain. For example, one coral may bleach and another just meters away remains healthy, and sometimes only the tops or sides of corals bleach, which suggests that genetics or other environmental factors play important roles. This project uses innovative experiments and technology in an attempt to link small (at the level of individual corals) and large scale (whole reef) processes to understand how local currents, underwater topography (the "roughness" of the reef), and chemistry interact to affect the bleaching response of corals. Oxygen measurements are typically used to evaluate coral metabolism and ultimately condition, much as a physican will evaluate the condition of an athlete running on a treadmill. Interestingly, corals produce or consume oxygen depending on the time of day and the condition of symbiotic algae living in their tissues. Because corals are animals they need oxygen just as we do to support metabolism and our daily activities. But oxygen dissolved in water is much more variable in concentration than oxygen in our atmosphere. In fact, the delivery of oxygen and nutrients to corals from the surrounding water by currents and other mixing processes is fundamental to the survival of corals.

Previous projects conducted from Aquarius focused on fine scale measurements around individual corals, but no studies exist to evaluate how these results relate to processes that affect the whole reef. For example, anecdotal evidence suggests that oxygen concentrations on the reef can sometimes reach crisis levels during summer doldrums, resulting in fish kills and potentially serious consequences for corals. A goal of this project is to measure reef oxygen dynamics in three dimensions, seasonally and across the variable seascape of the reef bottom. The only way this is possible is to use a free-swimming underwater robot, specifically a Fetch-class Autonomous Underwater Vehicle (AUV).

There is a substantial body of work already completed to evaluate how water flowing around corals affects uptake of nutrients. The fundamental question addressed by these studies was to determine whether or not the low concentrations of nutrients provided to a reef are sufficient to support the high productivity of the reef ecosystem. This contrasts with an existing hypothesis that nutrient cycling and local regeneration of nutrients supports high reef productivity. Results suggest that corals can theoretically take up nutrients faster than they are provided by either water flow from surrounding ocean currents, or the ability of corals to disrupt flow and thus stir things up as water passes overhead and around. Corals can disrupt flow based on their different shapes, position on the reef, and even the structure and size of their polyps. These results are important because it means that processes occurring at the scale of the whole reef (measured in tens of meters and kilometers) can affect individual corals (measured at the scale of centimeters and fractions of a millimeter).

The basic idea is to follow a parcel of water as it moves across a reef, and at the same time measure chemical changes (such as oxygen concentration). Later in 2003 (September and November), a mobile sensor (the AUV) will survey Conch Reef, which will allow tests to evaluate how bottom roughness and flow interact at the landscape level to control productivity. The AUV is equipped with environmental and physical sensors and is preprogrammed to "fly" a pattern across and over the reef at multiple depths and locations, collecting data the whole time it's deployed - flights can last hours and cover tens of kilometers.

During the July 2003 saturation mission, aquanauts will make detailed measurements of individual corals to assess their metabolism and condition. One of the metabolic measures will include a specific protein, known as the heat shock protein (SHP) that is produced in response to various stressors - such as heat or low oxygen concentrations. During a bleaching event, corals that are exposed to a more favorable flow regime (such as lots of mixing due to local bathymetry) or a slightly lower temperature (depth related or affected by bathymetry) may be better able to synthesize HSPs to help maintain normal cellular functions, whereas other corals on the same reef may experience microenvironmental conditions that are too intense to produce HSPs and as a result will expel their zooxanthellae. The idea here is that HSPs require excess metabolic energy to produce, and at some point corals become too stressed and can't produce HSPs to protect themselves - and thus succumb to the deleterious affects of bleaching. We will use the habitat to provide power to flow chambers that will be gently heated to simulate conditions during an actual bleaching event. The chlorophyll fluorescence of parts of coral colonies exposed to different water motion will be measured using a special instrument; this will allow us to see how the photosynthetic machinery of the coral is affected by higher temperature and whether water motion affects how fast the bleaching processes occurs locally within a coral head. We will also use a handheld instrument to make detailed profiles of oxygen concentration over the coral reef, to complement studies made at a larger scale by the AUV later in the year. One of the surprises of the 2002 mission was how highly variable oxygen concentration can be over a coral reef, which is usually considered to be a well-oxygenated, well-mixed environment.

The specific questions addressed by this project include:

• Do corals bleach differentially within a colony in different patterns that can be explained by water flow at small scales around the colony?
• Do individual coral polyps express heat shock proteins (HSPs) within a based on patterns of stress related to water flow at small scales around the colony?
• Do bleached corals recover and if so, do they bleach in the same way in a subsequent year?
• Do different parts of the reef at Conch Reef show dramatically different oxygen dynamics when measured in three dimensions over daily, seasonal, and meteorological cycles?