Bleaching, nutrients and recovery

[Gregor Hodgson]

Debbie,

I bet you are correct that nutrients will be more important than suspected so far
in the bleaching/mortality/recovery story, but maybe not in the bleaching end as
you first suggested. A search for an important role of nutrients might be more
likely to bear fruit if it were aimed at the post-bleaching mortality and/or
recovery processes and  focused on the interactions among nutrient availability,
bacterial activity on dying corals, hypoxia, algal recruitment and growth, the
ability of damaged coral tissue to recover and new larvae to settle.  Michael
Risk and colleagues have made interesting discoveries about the synergistic
effects of nutrients and sediment.

Since 1997, the results of Reef Check have clearly confirmed on a pan-tropical
basis what John Munro suspected in the late 1950s in Jamaica and Tim McClanahan
showed experimentally in the 1990s in Kenya --  that overfishing on reefs (and
not just of fish)  is one of the most serious threats to their health.

On the other hand,  observations of bleaching events around the world suggest
that the "fish poop theory" will not be supported the data. To confirm this, the
results of Reef Check and other datasets such as the AIMS long-term monitoring
program could be used to show that reefs with high populations of various guilds
of reef fish were often more heavily bleached than reefs with lower populations
-- as noted by Alina and others. Good examples for this lack of correlation may
be found by comparing well-managed, no-take MPAs such as reefs at Orpheus Island,
Australia with fished reefs nearby which bleached equally or less badly in 1998.

The pattern of bleaching follows a consistent trend globally that suggests that
following bleaching events, reefs located in areas with less water column mixing
are usually the worst affected. Typically these are inshore reefs where both
nutrient flux and absolute levels of nutrients are likely higher than outer
reefs. Obvious large scale examples are Australia (GBR), Fiji, Okinawa. One
reason that the Acropora go first is simply that the water usually heats from
shallow to deep, thus it is the common shallow water corals that get nailed first
-- such as Acropora. Experiments in the early 20th century in Australia, Hawaii
and Japan also have documented species-specific tolerance levels for a variety of
physical stressors including hot water.

No doubt if more people looked at the starving basking shark story at the bottom
of your homepage http://www.fisherycrisis.com, they would better understand your
views.
Greg

McClanahan, T. R. 1995. A coral reef ecosystem-fisheries model: Impacts of
fishing intensity and catch selection on reef structure and processes. Ecological
Modeling 80(1):1.

Hodgson, G. 1999. A global assessment of human effects on coral reefs. Marine
Pollution Bulletin. 38 (5) 345-355.

Edinger, E. N., Limmon, G. V., Jompa, J. Widjatmoko, W., Risk, M. J.  The Janus
effect: are coral growth rates good indicators of healthy coral reefs?  Coral
Reefs.  (in press)
--
Gregor Hodgson, PhD
Director, Reef Check Foundation
Professor (Visiting), Institute of the Environment
1652 Hershey Hall 149607
University of California at Los Angeles
Los Angeles, CA 90095-1496 USA

Office Tel: 310-794-4985  Fax: 310-825-0758 or 310-825-9663
Email: gregorh at ucla.edu  Web: www.ReefCheck.org


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