The Shallow Back Reef Environment of Ofu, American Samoa

Writing as background for their work, the six-member research team of Koweek et al. (2015) cite several concerns about the future of Earth’s corals that have been projected to result from the so-called twin evils of global warming and ocean acidification, including “coral bleaching (Glynn, 1993; Hughes et al., 2003; van Hooidonk et al., 2013), increased dissolution and bioerosion (Andersson and Gledhill, 2013; Dove et al., 2013; Reyes-Nivia et al., 2013), decreased biodiversity (Fabricius et al., 2011), and shifts toward algal-dominated reefs (Hoegh-Guldberg et al., 2007; Kroeker et al., 2010; 2013).” However, despite these concerns, which have captured the attention of scientists and policy makers for more than two decades now, such worries may well be overestimated and overplayed.

The reason for such growing optimism has to do with the corals themselves, which along with other marine organisms appear to have an inherent ability “of controlling their own biogeochemical environments.” Such biologically-mediated controls, if they are of sufficient magnitude, could potentially offset future changes in the marine environment brought about by rising atmospheric CO₂ (projected ocean warming and pH decline). It is therefore of considerable importance for scientists to continue investigating these biological feedbacks in order to better ascertain the future of these precious marine species, for as noted by Koweek et al., “the paradigm of coral reefs as passive responders to their biogeochemical environments is rapidly changing.”

In further expanding the scientific knowledge on this important topic, the six American researchers set out to conduct a “short, high-resolution physical and biogeochemical pilot field study” on the back reefs of Ofu, American Samoa, where they measured a number of hydrodynamic and biogeochemical parameters there over a seven-day period in November, 2011. The specific study location was Pool 100 (14.185°S, 169.666°W), a shallow lagoon containing 85 coral species and various kinds of crustose coralline algae and non-calcifying algae. Koweek et al. selected Pool 100 because, as they state, shallow back reefs “commonly experience greater thermal and biogeochemical variability owing to a combination of coral community metabolism, environmental forcing, flow regime, and water depth.”

Results of their data collection and analysis revealed that temperatures within the shallow back reef environment were consistently 2-3°C warmer during the day than that observed in the offshore environment. In addition, and as expected, the ranges of the physical and biogeochemical parameters studied in Pool 100 greatly exceeded the variability observed in the open ocean. Inside Pool 100, the pH values fluctuated between a low of 7.80 and a high of 8.39 across the seven days of study, with daily ranges spanning between 0.5 and 0.6 of a unit (Figure 1). What is more, Koweek et al. report that the reef community in Pool 100 spent far more time outside of the offshore pH range than within it (pH values were between 8.0 and 8.2 during only 30 percent of the observational period, less than 8.0 for 34 percent of the time and greater than 8.2 for the remaining 36 percent of the observations). Additional measurements and calculations indicated that these fluctuations in pH were largely the product of community primary production and respiration, as well as tidal modulation and wave-driven flow.

Figure 1. Time series of pH_T (top panel) and pCO₂ (bottom panel) in Pool 100, Ofu, American Samoa from November 16-20, 2011. Vertical blue and orange lines show the occurrence of high and low tides, respectively. Gray vertical shading shows the period from sundown to sunrise. The different colored circles represent data that were collected from different locations in Pool 100 and the dashed horizontal black lines represent the mean value of each parameter in the offshore ocean. Adapted from Koweek et al. (2015).

Commenting on these and other of their findings, Koweek et al. write that “our measurements have provided insight into the physical–biogeochemical coupling on Ofu.” And that insight, they add, “suggests a significantly more nuanced view of the fate of coral reefs” than the demise of global reef systems that is traditionally forecast under the combined stresses of climate change and ocean acidification.

Indeed, if these ecosystems presently thrive under such variable (and more severe) environmental conditions than those predicted for the future—which conditions are largely derived and modulated by themselves—why wouldn’t they persist?

 

References

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