Author

Jorge Cortés

Date of Award

8-1981

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Supervisor

Dr. M. J. Risk

Abstract

The coral reef at Parque Nacional Cahuita, Limón, Costa Rica, is a reef under stress due to siltation. The amount of suspended sediments is high: 7.4 mg/l (0.1 to 212.7 mg/l), resuspension of bottom sediments is also high: 30 to 360 mg/cm²/day. As a consequence, growth rates of corals are low: 5.3 mm/year (3.7±0.8 to 7.7±0.4 mm/year), live coral coverage is low: 40% (4 to 80%), and coral diversity is low: H'=1.443 (0.038 to 1.602). Coral colonies are generally larger than in other areas studied, and recruitment of coral planulae seems to be low. Most of the corals present are good at rejecting sediments. Morphologies of some corals change to resist better the sediments (vertical fronds of Agaricia agaricites) or to receive more light (shingles of Montastrea annularis and Porites astreoides).

Analysis of the currents and the type of minerals present in the non-carbonate fraction (illite, montmorillonite, feldspar and kaolinite) of the sediments at the reef, point to Río La Estrella as the source of sediments. The amount of sediments carried by this river has probably increased recently as a result of watershed deforestation. The problem of siltation in reef environments is bound to increase as new areas in the tropics are being developed.

Sediments affect both the individual coral and the coral community. It can be determined if a reef is (or was) under stress due to siltation, by analysing the following: growth rates of the corals, amount of trapped sediments in the coral skeletons, live coral coverage, species composition and diversity, and coral morphologies.

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