Geology Reference
In-Depth Information
Fig. 12.5.
Cozumel Island east of Yucatan Peninsula, Mexico.
A:
Map showing the narrow shelf region. Most coral reefs
are situated on the western shelf and at the shelf margin, only
a few on the near-coast eastern shelf.
B
: Schematic profile of
the western shelf of Cozumel Island. The reefs are located on
the morphological steps (after Muckelbauer 1990).
A
B
order to compare the post- and pre-hurricane situations
one year after Gilbert.
These drawbacks are avoided in the Cozumel study,
which is based on identical methods both in the field
and for the statistical evaluation of quantitative data.
The studied locations were grouped to represent mor-
phological sites of reef growth in different water depths
(reefs I, II and III) at the steps between terraces 1, 2,
and 3.
Patch reefs and the upper insular slope were also
studied. Using the photographic transect method
(Dodge et al. 1982), numerous photographs taken along
transects running parallel to the reef structures were
evaluated by point-counter analyses.
Corals were identified down to species, forms and
even morphotypes. Algae could be classified to the
generic or species level.
Altogether a total of 95 taxonomic categories was
distinguished, and the percentage of coverage was cal-
culated for each one. The resulting data were summa-
rized for the whole transect, thus obtaining the mean
coverage as well as the standard deviation for different
reefs. Coverage, diversity and species richness were
computed by statistical tests.
Location:
Cozumel is located approximately 18 km
offshore the northeastern coast of Yucatan Peninsula.
The narrow shelf (width 200 to 1500 m; Fig. 12.5A) is
organized into steps and terraces (Fig. 12.5B). The in-
sular slope drops down at very steep angles to depths
of 400 m and more than 1000 m. The steps are situated
at depths of about 4 m, 10 m, 20 m, and in places 30 m
and 50 m. The terraces between the steps vary in ex-
tension from a few meters to some hundreds of meters.
Most coral reefs occur on hard bottoms along the steps
and at the shelf edge on the western shelf. The terraces
are covered with calcareous sands and sea grass. Reef
distribution is controlled by the topography of the shelf,
which originated in the Pleistocene and was changed
during the Holocene, by the distribution of hardground
and softground substrates, and by strong ocean currents
(2 m/sec) from southern directions representing an off-
shoot of the Caribbean Current.
Pre- and post-hurricane comparisons:
Comparisons
of damages caused by hurricanes with the pre-hurri-
cane situation have been made in the Caribbean and
other regions (e.g. Endean 1976; Stoddard 1985;
Mergner 1985; Mah and Stearn 1986; Hubbard 1992;
Boss and Neuman 1993), but most reports are focused
only on certain organism groups, or use different quan-
tifying methods that can significantly affect the out-
come of pre- and post-hurricane comparisons.
Post-hurricane changes:
Hurricane Gilbert devas-
tated the shelf-edge reefs and many smaller reefs lo-
cated on the shallow shelf. The reef structures were
destroyed and eroded. Large areas were abraded by fine-
grained calcareous sand flushing over the newly cre-
ated bare hard bottoms. Broken corals and up to meter-
sized reef blocks were transported onshore as well as
offshore.
