Quarterdeck 4.1
"Bringing to light" anchialine cave ecology
One of the best kept secrets of the Yucatan Peninsula concerns the countless,
clear pools called "cenotes" that speckle the tropical landscape.
Cenotes are surface lesions formed by collapse of the ceilings of the labyrinthian,
submerged cave systems that meander through the Pleistocene bedrock of the
Yucatan. Aside from providing direct access to groundwater, cenotes serve
as gateways into the world's most extensive underwater cave systems. Caves
that connect to the cenotes are technically called anchialine (ank-ee-AY-leen)
caves, which means they are influenced by intruding coastal seawater. My
research provides the first description of ecological and biogeochemical
processes that drive the anchialine ecosystem.
Locals refer to the caves as "underground rivers" because they
drain fresh water from the riverless Yucatan Peninsula, but "underground
estuary" more accurately describes the caves due to the mixture of
fresh and sea waters found within them. Primary passages are oriented along
the dramatic and aggressive boundary between fresh and salt water, called
a halocline, that corroded the bedrock to form the caves. Many of the caves
are lavishly decorated by stalagtites and stalagmites that formed during
a low sea level stand over 18,000 years ago.
A unique group of organisms, cave-limited troglobites, is found in the caves.
Troglobitic bodies emphasize economy over specialty. They have essentially
reversed evolution to remove unneccessary body parts and functions. Simplified
yet specialized adaptations include loss of color and eyes, the presence
of long, spiny appendages, and reduced size. At present, nineteen species
of Crustacea and two species of fish are known to permanently inhabit anchialine
caves in the Mexican state of Quintana Roo.
As part of my research, trained cave-divers collected fauna and all possible
types of organic and inorganic carbon and nitrogen from the caves for stable-isotope
and quantitative analyses. The results, in conjunction with data generated
from samples collected in the cenote pool and forest, allowed me to describe
the biogeochemical cycling of carbon and nitrogen through the anchialine
system, define the hierarchy of the food web, identify several unique ecological
phenomena and present evidence for the occurrence of chemosynthesis (production
of organic material using reduced chemicals rather than light for energy)
by nitrifying bacteria.
Organic matter needed to support the two trophic levels of the troglobitic
community appears to come from three sources, two external and one internal.
External sources of organic matter are the cenote pool and forest soil,
while the internal source is chemosynthetic production occurring along the
halocline. An equally fascinating feature is that different species of crustaceans
occupying the lower trophic level, although they inhabit the same niche,
specialize in procuring organic matter from different sources. This pheno-menon
is known as niche partitioning and had not been previously reported for
any cave community, dry or aquatic.
Cenotes had tremendous spiritual and cultural significance for the ancient
Mayan civilizations and maintain their importance today as economic resources
and primary water supplies for many Mexican citizens. The welfare of the
people and the growing tourism industry in the Yucatan Peninsula depend
upon the sanctity of this environment. With the knowledge we now have about
the biogeochemical operation of anchialine systems, we can better assess
how to protect these environmentally sensitive resources in the future.
[Next | Previous | Contents
| Home]
Oceanography, Texas A&M University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD4.1/Grads/pohlman.html
Updated May 27, 1996