Sea Turtle Habitats and Water Temperatures: Using a Power Plant Closure to Measure Effects of Global Climate Change
Sheila V. Madrak
Joint Doctoral Program in Ecology
San Diego State University and the University of California, Davis
Global climate change and anthropogenic influences have already had dramatic effects on the world’s oceans. Rising sea levels, ocean acidification, and changes in water temperatures are altering marine environments on both global and local scales. With these trends expected to continue, understanding how marine animals will respond to these changes is critical to conservation and management of these species. Geographic ranges of marine species are limited by environmental characteristics and water temperature in particular influences behavior and habitat use.

Animals like sea turtles, sharks, marine mammals, and more use locations with specific thermal conditions to maintain physiological functions – a process called thermoregulation. Temperature boundaries, gradients, and seasonal changes often define and even constrain the movements of these species. Even slight changes in temperature may have significant impacts on the success and survival of affected marine animals. Laboratory studies have provided information about the environmental temperature needs of these species but may not be reflective of behavior in the natural environment. There are inherent logistical challenges in
Figure 1. Areal view of San Diego Bay. Inset, lower right: the power plant site. Inset, lower left: detail of the outflow area receiving the warm water discharge from the plant.
monitoring species response in natural habitats, so often laboratory studies provide the best available science.

Altering the water temperature in an ecosystem for the purposes of experimental study would be extremely difficult and obtaining permits for such a study would likely be impossible. In the absence of manipulating the water temperature in natural habitats, long-term in-situ monitoring studies have played a critical role in conservation and management. However scientists, conservation biologists, and fishers need to understand how marine animals respond to rapid changes in thermal conditions to better predict how these organisms will be affected in the future.

Power Plant Closures: Natural Experiments

While large-scale changes in water temperature can be attributed to shifts in the global climate regime, there are other human-mediated factors that influence local thermal conditions. One major anthropogenic influence on local marine environments is thermal effluent from fossil fuel power plants. According to the U.S. Energy Information Association in 2012, fossil fuels (coal, natural gas - dry, crude oil, and natural gas plant liquids) represent approximately 82% of energy production in the United States. The power stations that produce this energy generate waste heat, a by-product of the plant cooling process, which must be released into the environment (either via cooling towers or natural water source). Thus, fossil fuel power stations alter the thermal environment proximate to their locations. The effects of this thermal effluent on marine species vary across taxa and depend on thermoregulatory strategy (e.g. endotherm or “warm-blooded” versus ectotherm or “cold-blooded”).

Since the early 1970s, scientific studies have demonstrated physiological and behavioral changes across taxa in mobile aquatic organisms utilizing these areas of heated effluent. Across studies, there is evidence of certain common responses in a variety of animals: aggregative effects, declines in species richness, and changes in metabolic and growth rates and dispersal patterns. Changes in energy demands in the United States – namely a shift away from
Figure 2. The power plant at night, during normal operation and before the shut-down.
observed in the power plant outfall area since the 1960s. Researchers from NOAA’s Southwest Fisheries Science Center in La Jolla, California have suggested that these turtles used the SBPP outfall area to reduce metabolic costs, exhibiting higher growth rates than other populations of green turtles. On December 31, 2009, two of the plant’s four generators were permanently shut down; complete decommissioning of the plant (Figure 2) occurred on December 31, 2010. This power plant closure provided a rare experimental opportunity to assess how changes in water temperature over a short period of time will affect a resident marine turtle population in a coastal foraging area.
fossil-fuel power plants in favor of more renewable resources – have resulted in many fossil fuel power plants being decommissioned or scheduled for decommissioning in the near future. These closures provide an opportunity to explore how mobile marine vertebrates, like sea turtles or Florida manatees, respond behaviorally to rapid shifts in environmental temperature. Studying organismal responses to loss of thermal effluent, in the form of fossil fuel power plant closures, provides an opportunity to study these shifts in the form of a “natural” experiment.

Although local and global scale changes may be driven by different factors, changes in water temperature at the local level can provide a model system to study the effects of large-scale climate change. Taking advantage of these thermally-altered habitats near decommissioned fossil fuel power plants, allows for study of marine animal response before and after the loss of thermal effluent. Characterizing the responses of coastal fauna to rapid shifts in thermal conditions addresses a key gap in ecological knowledge – understanding how populations of long-lived marine vertebrates will be affected by a thermally dynamic environment that is changing at rapid rate.

San Diego Bay: A Case Study

The fossil fuel-based South Bay Power Plant (SBPP) in Chula Vista, California (just south of San Diego) was in operation from 1960 to 2010 and discharged warm-water effluent into southern San Diego Bay (Figure 1). East Pacific green sea turtles (Chelonia mydas), known residents of San Diego Bay since at least the 1890s, have been routinely
A collaborative study to monitor the habitat use of green turtles in San Diego Bay began in 2009 with researchers from San Diego State University, NOAA’s Southwest Fisheries Science Center, the United States Navy, and the Port of San Diego. Turtle distribution was monitored using acoustic telemetry before and after closure of the SBPP to determine how turtles utilize San Diego Bay, especially those thermally-affected areas adjacent to the SBPP’s outfall. Water temperature was monitored when tagged turtles were detected and throughout San Diego Bay. Because mobile marine animals, like sea turtles, utilize both the horizontal and vertical planes of their habitat, researchers also monitored dive behavior of these turtles using time-depth recorders (Figure 3).

Preliminary results from this study indicate that water temperature adjacent to the SBPP was significantly cooler in winter months (December, January, and February) after the decommissioning. Turtles were distributed in water temperatures that were, on average, significantly warmer than average ambient water temperatures both before and after the closure of a power plant in winter months. Additionally, turtles appear to contend with changes in water by altering their dive behavior and spend significantly longer periods of time resting when water temperatures are colder. Researchers are now in the process of completing this study and anticipate that full results will be published in the next year. However, this is just the beginning of what hopefully will be a long-term monitoring project to determine whether the turtles remain in the bay or leave,
whether the changes in water temperature affect their food supply, and how both temperature and a host of other “trickle-down” effects results in a change in turtle growth and health.

Understanding the relationship of mobile marine species with the water temperatures of their environments is critical given the ongoing shifts in local and global thermal conditions. Logistical constraints can make monitoring highly mobile species, with complex life histories, problematic. Human-induced changes to water temperature at the local level provide a relevant, model system to monitor behavioral and physiological responses to temperature shifts that can help predict likely responses to marine temperature shifts for long-lived, high mobile marine megafauna.
Figure 3. A very large green turtle captured in the bay is fitted with transmitters that will record its location, as well as its diving behavior. Once instrumented, Dr. Madrak (shown here) will return the turtle into the bay to document its movements and behavior.
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