Limnological Characteristics of the Manantial de La Aleta:
A Preliminary Assessment
A Preliminary Assessment
by: Bill Jones, School of Public & Environmental Affairs, Indiana University, Bloomington, IN
Click on the smaller, side figures for a larger view of the information.
In his famous Treatise on Limnology, G.E. Hutchinson differentiates 76 different lake types based on their genesis. One of these lake types is a sinkhole or solution lake. Sinkholes are formed when percolating rainwater dissolves limestone bedrock and forms a basin. In the United States, sinkholes are most common in the karst regions of Indiana, Kentucky and Florida. The Caribbean area, particularly the Yucatan Peninsula of Mexico, is also well-known for having many sinkholes where they are referred to as cenotes. The Manantial de la Aleta within East National Park of the Dominican Republic is a rather spectacular example of one of these cenotes.
We believed that the Aleta is the sinkhole referred to by Spanish missionary Bartolome de
Las Casas in 1503. Las Casas described a large and important sinkhole, or cenote, surrounded by
ceremonial plazas. This sinkhole was 40 brazas deep (67 meters) and contained drinkable water
to a depth of 8 brazas (13 meters). Preliminary dives by Indiana University teams during the Fall
of 1996 and Spring of 1997 provided preliminary descriptions of water characteristics of the
Aleta and these descriptions matched those of Las Casas. These observations were confirmed
during our July 1997 investigations.
Nine small openings,
the smallest would stop a basketball and the largest is only two by
three meters in size, provide the only access to the Aleta. The water surface lies 15.5 meters
below the ground (Figure 1). At the surface, the
water is 47 meters in diameter and the
maximum water depth reaches 76 meters. A cap rock, formed when the roof collapsed, lies 36
meters below the surface, at the middle of the sinkhole. A mass of tree roots extends from the
surface down to the water level. The only light penetrating the water comes from several narrow
beams of sunlight that play across the water's surface once each day as the sun passes overhead.
Because space for scientific gear was limited, we relied on a HydroLab DataSonde 4 as our primary instrument. With it, we measured a vertical series of depth, temperature, dissolved oxygen, pH, redox, turbidity, conductivity, and photosynthetically available light (PAR). We also collected water samples for nutrient, isotope, and plankton study.
The vertical temperature profile was a constant 24oC from top to bottom, not because the water circulates but rather because the subterranean setting is shielded from solar heating (Figure 2). pH was a fairly uniform 7.0. Dissolved oxygen was about five ppm (60% saturation) down to seven meters, whereupon it dropped to and remained at zero from eleven meters to the bottom.
While there
is no evidence of thermal stratification, the water column is chemically
stratified. The profile of specific conductance (Figure 3) illustrates this. Conductivities ranged
from 1,500 µmhos/cm at the surface to over 52,000 µmhos/cm at the
bottom. This is nearly identical to the conductivity of the seawater
offshore from East National Park. The steep salinity gradient between 7
and 20 meters
(chemocline) isolates the dense, ion-laden water below 20 meters (monimolimnion). In the Aleta,
this isolation is most likely permanent. The surface water layer (mixolimnion) may circulate
weakly, because there is little density change but there is no wind at the water surface to provide
vigorous mixing energy.
Redox potential, a
measure of electron activity, changes in accordance to changes in
dissolved oxygen (Figure 4). Above 10 meters, the
Aleta's waters are oxidizing (+ millivolts). A
strong gradient exists just below 10 meters where redox changes from oxidizing to reducing (-
millivolts). The reducing environment below 13 meters can explain the presence of the hydrogen
sulfide (a reduced form of sulfur) taste that the divers reported. We suspect that sulfur-reducing
bacteria are using dissolved sulfate from the limestone walls as an energy source as they reduce it
to sulfide. With only limited light available in the Aleta, this chemosynthesis could be an
important source of organic material for a limited biotic food chain. The turbidity we measured
at 13 meters could be the bacteria themselves (Figure
5)
The water characteristics that we've been able to describe thus far are generally ideal for the preservation of organic artifacts. These include: low oxygen, constant temperature, low light, limited circulation, and high ionic content.
We also observed 20 small fish (Limia perugiae, identified by Carlos Rodriguez of the Museo Nacional De Historia Natural) swimming at the water's surface. Two divers saw a 2-inch long, white shrimp on the bottom at 130 feet. How did these organisms get there? What do they feed on?
There is much more work to be done at the Manantial de La Aleta in future years. Additional limnological investigations can help clarify our limited understanding of this fascinating system.