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Red Tides and Black Water: A Deadly Duo for Coral Reefs
Hu, C., Hackett, K.E., Callahan, M.K., Andrefouet, S., Wheaton, J.L., Porter, J.W. and Muller-Karger, F.E.  2003.  The 2002 ocean color anomaly in the Florida Bight: A cause of local coral reef decline?  Geophysical Research Letters 30: 10.1029/2002GL016479.

What was done
The authors combined field observations with satellite ocean color data in an attempt to determine the cause of a dramatic die-off of corals that occurred at two sites north of Key West, Florida, USA, in early 2002.  The field data were derived from annual sampling conducted by the Florida Keys Coral Reef Monitoring Project that began in 1996, while the ocean color data were derived from the Sea-viewing Wide Field-of-view Sensor affectionately known as SeaWiFS.

What was learned
In the words of the authors, "percent cover of selected benthic community categories and stony coral species number at these sites were relatively stable from 1996 to 2001, but decreased by [greater than] 70% and [greater than] 40%, respectively, from 2001 to 2002."  In fact, they note that one of the Keys' primary framework building corals, Montastraea annularis, "declined from 8.4% cover in 2001 to 0% in 2002."  The authors also report that "satellite ocean color imagery showed the transit of water with abnormally low water-leaving radiance values between March and May 2002 over these sites."  At the same time, their field observations "showed that this 'black' water contained the red tide organism Karenia brevis."

What it means
The authors conclude that the red tide organism "may have caused the mass mortality on the west Florida shelf, as previously reported for a large red tide event in 1971 (Smith, 1975)," noting that a diatom bloom "that formed in response to decay of the red tide and new nutrients supplied by rivers, plus the red tide organisms themselves, represented significant stressors to the benthos at that time."

With respect to other coral declines that have occurred in the Florida Keys over the past two decades, the authors say several investigators have suggested that nutrification of reef waters, diseases and over-fishing have been involved (Cockey et al., 1996; Lapointe, 1997), together with the transoceanic transport of African dust (Shinn et al., 2000; Walsh and Steidinger, 2001).

But what about global warming and coral bleaching?  In the words of the authors, "the 1997-2002 Sea Surface Temperature time-series showed no apparent anomaly between 2001 and 2002, therefore, bleaching is not considered to be a contributing factor to the observed coral decline."

Cockey, E.M., Hallock, P.M. and Lidz, B.H.  1996.  Decadal-scale changes in benthic foraminiferal assemblages off Key Largo, Florida.  Coral Reefs 15: 237-248.

Lapointe, B.E.  1997.  Nutrient thresholds for bottom-up control of macroalgal blooms on coral reefs in Jamaica and southeast Florida.  Limnology and Oceanography 42: 1119-1131.

Shinn, E.A., Smith, G.W., Prospero, J.M., Betzer, P., Hayes, M.L., Garrison, V. and Barber, R.T.  2000.  African dust and the demise of Caribbean coral reefs.  Geophysical Research Letters 27: 3029-3032.

Smith, G.B.  1975.  The 1971 red tide and its impact on certain reef communities in the mid-eastern Gulf of Mexico.  Environmental Letters 9: 141-152.

Walsh, J.J. and Steidinger, K.A.  2001.  Saharan dust and Florida red tides: The cyanophyte connection.  Journal of Geophysical Research 106: 11,597-11,612.

Reviewed 10 September 2003