Quarterdeck 3.3
Part 2
Down under . . . out yonder
by Stephen
R. Gittings
. . .Continued from Part 1
Fish populations are also under study. Less than
150 reef fish species inhabit the Flower Gardens' reefs, a low number compared
to reefs farther south. Yet censuses show that a full complement of feeding
types exists, including herbivores, carnivores, and omnivores. This is significant
because it assures the reef's resilience to most natural environmental catastrophes
and some human-induced changes.
In the 1980s, the reefs experienced the mass mortality of a sea urchin species,
one that happened to be a dominant grazer. The sea urchins fed on potentially
aggressive algae, allowing corals to thrive. Without the sea urchins, the
algae spread to cover 14% of the seabed, up from virtually zero percent.
During the next several years, however, a species of herbivorous parrotfish
became abundant and consumed most of the algae that had grown. This biological
control mech-anism is not available on many reefs, due to overfishing of
grazers and other reef fish. On those reefs, coral cover has been decreasing
steadily, from 50% to less than 5% in one case.
The fish study illustrates a significant issue. We humans have finally tinkered
with the oceans and their resources to the point of obvious abuse, particularly
in coastal and nearshore environments and on many coral reefs. Aldo Leopold,
a naturalist and pioneer in the application of ecological principles to
natural resource management, spoke volumes when he stated that the first
step in "intelligent tinkering" is keeping all the parts. In many
environments of the world, the removal of critical species has contributed
to the ecosystem's premature demise. Likewise, on many coral reefs, indiscriminate
removal of fish has decimated the populations of herbivores. This has led
directly to increases in algae cover at the expense of corals. The outcome
is what some call "algal reefs," which have low coral cover, low
diversity, and lack reef-building capacity.
The Flower Gardens' corals themselves are also under study, and they are
revealing the secrets of the past. Dr. Niall Slowey, of Texas A&M, is
using coral cores up to five feet long to study historical atmospheric and
oceanographic conditions (See "Coral
cores from the Flower Gardens," this issue). Corals secrete seven
to eight millimeters of limestone each year, analogous to trees adding annual
rings. In each annual layer the corals leave chemical records of water quality.
Water chemistry varies with changing temperature, salinity, and elemental
composition, all of which reflect changes in climate. Dr. Slowey's cores
contain several hundred years of data that should reveal climate conditions
in times before humans kept records. These findings may help scientists
and others predict future conditions and improve our resource management
abilities.
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Oceanography, Texas A&M
University
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URL=http://oceanography.tamu.edu/Quarterdeck/QD3.3/Gittings/gittings-b.html
Updated December 20, 1995