Background
With respect to the recent decline in coral reef health and cover at many sites around the world, the author notes that "while there has been much analysis and discussion over physical and biological stress contributing to this decline, comparatively little has been given to chemical stress, acting directly or in combination with biological and physical stress." In this regard, he further notes that a chemical stress of particular importance to reef habitats is that produced by agricultural pesticides, especially herbicides, which are employed more than any other class of pesticides throughout the tropics.

What was done
Jones "review[s] and evaluate[s] the known ecotoxicological effects of one of the main classes of herbicides, the Photosystem II (PSII) herbicides (which include diuron and Irgarol 1051) on scleractinian corals." Why? Because (1) these herbicides have recently been found to be contaminating the tropical marine environment, and (2) they "appear able to readily penetrate the coral tissues and rapidly (within minutes) reduce the photochemical efficiency of the intracellular algal symbionts."

What was learned
Jones' review indicates that algal symbiont photosynthesis "can be affected at exceptionally low concentrations (i.e., in the part per trillion range)," and that "at these levels and over short exposure periods, the effects are often reversible." However, he finds that "under elevated light levels, higher herbicide concentrations and over longer exposure periods, the binding of PSII herbicides will result in a long-term sustained reduction of the quantum yield of the algal symbionts in the host (coral) tissues, symptomatic of chronic photoinhibition" that "seems to act as a cue for the corals to expel their algal symbionts" or bleach, after which "it may take many months for corals to recover their full algal complement to normal levels."

What it means
In view of these observations - as well as the fact that Jones reports that (1) "currently, nowhere are pesticides more valued than in developing countries, particularly in tropical regions seeking to enter the global economy by providing off-season fresh fruits and vegetables to counties in more temperate climates (Ecobichon, 2001)," (2) "food production in such areas, and the production of two or even three crops per year, cannot be achieved without the increased use of pesticides (Forget et al., 1993)," and (3) "global sales figures for pesticide use in the tropics shows greater use of herbicides than any other class of pesticide (Aspelin et al., 1992; Ecobichon, 2000)" - there is good reason to believe that PSII herbicide-fouling of tropical waters, even at low concentrations, may well be predisposing the bulk of earth's corals to bleach in response to much lower levels of physical stress (such as periodic high temperature excursions) than was characteristic of times past.

References
Aspelin, A.L., Grube, A.H. and Torla, R. 1992. Pesticide industry sales and usage, 1990 and 1991. Market estimates. U.S. Environmental Protection Agency Report, Washington, DC, pp. 1-7.

Ecobichon, D.J. 2000. Our changing perspectives on benefit and risks of pesticides: a historical overview. Neuro Toxicology 21: 211-218.

Ecobichon, D.J. 2001. Pesticide use in developing countries. Toxicology 160: 27-33.

Forget, G., Goodman, T. and deVilliers, A. (Eds.). 1993. Impact of pesticide use on health in developing countries. Proceedings of a symposium held in Ottawa, Canada, 17-20 September, 1990. International Development and Research Centre.