A stable isotope study of organic cycling and the ecology of an anchialine cave ecosystem

Stable carbon and nitrogen isotope data, complemented with other geochemical parameters, were used to identify the sources of organic matter that support the food web of an anchialine cave ecosystem in the northeastern Yucatan Peninsula, Mexico. Anchialine caves, common along tropical karstic and volcanic coastlines, are completely or partially inundated by highly stratified layers of fresh and marine waters. Stable isotope data from the cave fauna, the particulate organic matter (POM) from the cenote pool and from the cave, the forest soil and the cave sediments indicated that at least 3 sources of nutritive organics could support the anchialine food web. These sources were: (1) soil from the overlying forest; (2) freshwater algae from adjoining open water pools; and (3) chemoautotrophic nitrifying bacteria living in the cave. Production of nitrate and a decrease in O-2 along the halocline provided geochemical evidence of nitrification. Stable nitrogen isotope data defined 2 to 2.5 trophic levels in the food web. Furthermore, it was found that troglobitic (cave-limited) species residing in the water column are capable of preferentially feeding on specific organic reservoirs. This study presents the first extensive description of the ecological and biogeochemical relationships of the anchialine cave ecosystem.