Limnol. Oceanogr., 45(3), 2000, 677–685

Tissue biomass (ash-free dry weight) and symbiotic dinoflagellates (density, chlorophyll a cell21 or cm22 of coral surface area) of five species of reef-building corals were monitored seasonally for up to 4 yr at three different depths in the Bahamas. The lowest values of all tissue biomass and algal symbiont parameters occurred during the late summer–fall sample periods. In contrast, the highest densities and pigment content of symbionts usually occurred during the winter, whereas tissue biomass peaked most often in the spring, the time lag implying a functional relationship between these variables. Corals living in shallow water often (but not always) had higher levels of all parameters measured compared to deeper corals, except chlorophyll a content, which usually displayed the opposite trend. The results show that corals from all depths exhibited bleaching (loss of symbiotic dinoflagellates and/or their pigments) every year, regardless of whether they appeared white, tan, or mottled to the human eye. We speculate that these patterns are driven by seasonal changes in light and temperature on algal and animal physiology. Furthermore, we hypothesize that all tropical reef-building corals, world-wide, exhibit similar predictable cycles in their tissue biomass and symbiotic algae. The health of reef corals has been an issue worldwide for several years, made more prominent by incidences of bleaching, where loss of symbiotic dinoflagellates (also known by the generic term zooxanthellae), the algal pigments, or both has been linked to heat stress (Jokiel and Coles 1990; Brown 1997). Although a proposed relationship to global warming has not been firmly established (Atwood et al. 1992; Brown 1997), the prevalent concept of bleaching is that something terribly wrong is occurring during the warm season of socalled ‘‘bleaching years’’ that is not happening in other years. Although extreme environmental events have been linked to regional observations of bleaching in the past, including osmotic stress related to storms and tidal flow (Goreau 1964; Lang et al. 1988; Van Woesik et al. 1995), photobleaching in relation to high ultraviolet (UV) light exposure (Gleason and Wellington 1993) and elevated seawater temperatures associated with El Niño (Glynn 1993), the mechanisms of widespread seasonal thermal bleaching are apparently much more subtle. Research projects on thermal bleaching events during the last 25 yr have historically begun after the first observations 1 Corresponding author ([email protected]). Acknowledgments This research was funded by the NOAA National Undersea Research Program through both the Caribbean Marine Research Center on Lee Stocking Island in the Bahamas and the Florida Program run by the University of North Carolina at Wilmington. Bob Wickland, George Dennis, Steve Miller, and Tom Bailey have been instrumental in supporting the project, and numerous people working at Lee Stocking Island in the Bahamas have helped in its implementation, including Joe Acaba, Brian Kakuk, Kerlean Lloyde, Nick Mehta, Ray Mojica, Eilean Smith, and Judy and Jim Tryzbiak. Additional financial support came from the National Science Foundation (9203327, 9702032, 9906976) and funds from the Office of Naval Research (N0001492J1734). We appreciate constructive editorial comments from Robbie Smith and one anonymous reviewer. of discoloration of corals, making it easier to categorize these events as unusual disasters rather than in the context of the normal life of a coral (e.g., Jaap 1979, 1985; Porter et al. 1989; Szmant and Gassman 1990). Although laboratory experiments have documented decreased densities of symbiotic dinoflagellate and pigment content in corals correlated with acute increases in temperature (Coles and Jokiel 1978; Hoegh-Guldberg and Smith 1989; Fitt and Warner 1995), their relevance to the more gradual temperature changes found in nature have been difficult to establish. Two notable exceptions include Jokiel and Coles’ (1977) precocious study detailing the sublethal effects of long-term exposure to temperatures only 1 or 28C higher than the ambient mean (e.g., 28–308C) on corals in Hawaii. In a second seminal thermal study, Glynn and D’Croz (1990) provided definitive data showing steady decreases in densities of symbiotic dinoflagellates cm22 from Pocillopora damicornis in the Eastern Pacific at 28, 30, and 328C over a 12-week period, with the rate of decrease being directly proportional to the increase in temperature. Even with the results of these studies providing a framework for what might be happening to corals in the field, it is interesting to note that except for the monitoring of the Caribbean 1987 bleaching event (Porter et al. 1989), recovery of corals on a Florida reef over the course of the next 2 yr (Szmant and Gassman 1990; Fitt et al. 1993), and Stimson’s (1997) aggregation of data over an 8-yr period documenting high and low periods of symbiont densities in the Pacific coral P, damicornis, there have been few studies designed to investigate what might actually be happening to corals on the reef throughout the year. With this goal in mind, we established a long-term coral tissue monitoring program for five species of reef corals in the Bahamas and in the Florida Keys. Results from 4 yr of seasonal sampling of coral tissue biomass and zooxanthellae densities and their pigment content from the Bahamian sites are presented here. A brief summary of this work was first reported in Science (Fitt et al. 1997; Pennisi 1998).