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Water Transparency:
The Neglected Factor in Coral Decline

Volume 6, Number 11: 12 March 2003

"Coral health and percent coral cover," according to Yentsch et al. (2002), are "decreasing worldwide (Ginsburg, 1993) and at an alarming rate in the Florida Keys (Porter and Meier, 1992; Dustan, 1993; Dustan et al., 2000)."  Competing potential causes of this decimation of some of the world's most magnificent and biologically-diverse ecosystems include, in their words, "removal of grazers (Hughes, 1994; Hughes et al., 1999), nutrient enrichment (Lapointe and Clark, 1992; Lapointe et al., 1997), both resulting in overgrowth by macroalgae and disease, and increase in temperature (Glynn, 1990) and/or excess light/ultraviolet exposure (Schick et al. 1996) which results in coral bleaching," the latter of which phenomena (global warming-induced coral bleaching) has become an effective poster child for the world's anti-CO2 climate-alarmists.

Apparently unwilling to accept perceived wisdom in this matter, Yentsch et al. throw yet another hat into the ring of potential reef-decimating factors - insufficient sunlight and water transparency - arguing that the world's corals, and those of Florida in particular, "are strongly stressed by seasonal change in solar input plus decreasing water transparency."

To illustrate the effects of seasonal change in solar input, the researchers resurrect the work of Ryther (1956) and, building upon it, demonstrate the importance of light intensity and the daily duration of illumination for keeping the photosynthesis/respiration ratio of zooxanthellae high enough for sufficiently long periods of each day to sustain the long-term growth of the zooxanthellae-coral association.  Based on laboratory measurements of photosynthesis by zooanthellae isolated from cultured Cnidarian colonies, which were made at a variety of irradiance values, plus daily measurements of photosynthetically available radiation (PAR) at a water depth of one meter, which were made at the Dry Tortugas (24°38'N, 82°55'W), they determined there is enough PAR for coral growth to occur at one-meter depth at that site for 12 to 13 hours per day in summer, but only enough for about three hours of daily growth in winter.

To illustrate the effects of decreasing water transparency, which is what their paper is mostly about, Yentsch et al. use data from a number of sources to demonstrate that both percent coral cover - as shown by Tomascik et al. (1993) - and compensation depth ("the depth of 'no growth' by corals") - as shown by themselves - decline in essentially identical fashion with increases in water PAR attenuation coefficient [K (m-1) = (ln Eo - ln Edepth)/depth, where E is irradiance].  These observations explain how it is and why it is that coral cover in excess of 40% can be maintained in water of the highest transparency (K = 0.05-0.10 m-1) but then decline dramatically with relatively small increases in K, dropping to values on the order of 10% at K = 0.15 but then declining more slowly to reach values on the order of 2% at K = 0.6.

Derived from data obtained in both nature and the laboratory, these findings suggest, in the words of Yentsch et al., that "as water transparency decreases, corals are forced to grow in more shallow waters," where they "are caught between two jaws of a vice: one, they cannot grow any deeper because of light limitation; two, in shallow regions, the high wave energy limits colonization of these organisms [due to mechanical destruction of their skeletons]."

So why do more corals appear to be dying these days?  Yentsch et al. note that "as anthropogenic effects increase (e.g. dredging, beach erosion, eutrophication), the trend will be reduction of water transparency and corals will be some of the first organisms to be influenced (Veron, 1995)."  With respect to this situation generally, but speaking specifically of Florida - where they say their field and laboratory work demonstrates that "corals of the Florida reefs are functioning close to the compensation point where respiration (of coral polyp plus zooxanthellae) consumes the products of photosynthesis of the zooxanthellae, with little if any remaining for growth" - they rightly and urgently ask, "Why is this [problem, i.e., anthropogenic-induced decreases in water transparency] not being critically examined at the same degree as for other environmental factors?"

Probably no one can say for sure what the answer to this question is; but one thing is clear: continued climate-alarmist claims that anthropogenic CO2 emissions are to blame for the declining growth, and even death, of corals around the world does little to encourage remedial actions where they really count, i.e., in the immediate vicinities of where corals are struggling - and failing - to survive.  The vast majority, if not all, of the problems faced by corals today are locally-caused and can only be solved by local actions.  To cry great crocodile tears over their impending demise, while bemoaning rising anthropogenic CO2 emissions as the cause of their dieback and lobbying for reductions in fossil fuel usage, does no one, nor any corals, any good.  In fact, it actually hurts corals, convincing people who tilt at ethereal windmills made of CO2 that they are doing their part to help preserve earth's "tropical forests of the sea," when they in fact are doing nothing of the sort, and by their consequent inattention to the real problems faced by coral reefs are only helping to hasten their disappearance from the earth.

Sherwood, Keith and Craig Idso

Dustan, P.  1993.  Developing methods for assessing coral reef vitality: a tale of two scales.  In: Ginsburg, R.N. (Ed.), Global Aspects of Coral Reefs.  University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, FL, USA.

Dustan, P., Porter, J.W. and Japp, W.  2000.  Florida Keys Water Quality Protection Program: Coral Reef Monitoring, TAC 2000 Report, unpublished.

Ginsburg, R.N., Ed.  1993.  Global Aspects of Coral Reefs.  University of Miami Rosenstiel School of Marine and Atmospheric Science, Miami, FL, USA.

Glynn, P.W.  1990.  Global ecological consequences of the 1982-83 El Niņo-southern oscillation.  Elsevier Oceanography Series 52.

Hughes, T.  1994.  Catastrophic phase shifts and large-scale degradation of a Caribbean coral reef.  Science 265: 1547-1551.

Hughes, T.A., Szmant, A.M., Steneck, R., Carpenter, R. and Miller, S.  1999.  Algal blooms on coral reefs: what are the causes?  Limnology and Oceanography 44: 1583-1586.

Lapointe, B.E. and Clark, M.W.  1992.  Nutrient inputs from the watershed and coastal eutrophication in the Florida Keys.  Estuaries 15: 465-476.

Lapointe, B.E., Littler, M.M. and Littler, D.S.  1997.  Macroalgal overgrowth of fringing coral reefs at Discovery Bay, Jamaica: bottom up versus top down control.  Proceedings of the 8th International Coral Reef Symposium 1: 927-932.

Porter, J.W. and Meier, Q.W.  1992.  Quantification of loss and change in Floridian reef coral populations.  American Zoologist 32: 625-640.

Ryther, J.H.  1956.  Photosynthesis in the ocean as a function of light intensity.  Limnology and Oceanography 1: 61-70.

Shick, J.M., Lesser, M.P. and Jokiel, P.L.  1996.  Effects of ultraviolet radiation on corals and other coral reef organisms.  Global Change Biology 2: 527-545.

Tomascik, T., Suharsono, T. and Mah, A.  1993.  Case histories: A historical perspective of the natural and anthropogenic impacts in the Indonesian archipelago with a focus on the Kepulauan Seribu, Java Sea.  In: Ginsburg, R.N., Ed.  Global Aspects of Coral Reefs.  University of Miami Rosenstiel School of Marine and Atmospheric Sciences, Miami, FL, USA, pp. J26-J32.

Veron, J.E.N.  1995.  Corals in Space and Time: The Biogeography and Evolution of the Scleractinia.  Cornell University Press, Ithaca, NY, USA.

Yentsch, C.S., Yentsch, C.M., Cullen, J.J., Lapointe, B., Phinney, D.A. and Yentsch, S.W.  2002.  Sunlight and water transparency: cornerstones in coral research.  Journal of Experimental Marine Biology and Ecology 268: 171-183.