Organic carbon and nutrients in shallow subterranean habitats

Abstract

Reduced food is often considered one of the major features of the cave habitat and an important selective constraint. Dissolved organic carbon (DOC) is the most frequently measured parameter. In common with caves, nearly all shallow subterranean habitats (SSHs) depend entirely on allochthonous carbon and nutrients. That is, SSHs require spatial subsidies. From experiments on detrital communities in shallow pits (technically SSHs), it was clear that the allochthonous input of carbon, and the nature of that organic matter, organized the community in terms of species composition and abundance. Different SSHs have different energy sources (percolating water, flowing water, etc.) but in general the amount of organic carbon is intermediate between that found on the surface and in deeper subterranean habitats. Typically, the amount of organic carbon declined, often rather sharply, with depth. Epikarst and the hypotelminorheic showed highly variable temporal patterns, with much of the DOC entering the system over short periods of time—rainfall events in the case of epikarst. The little data that are available suggest that caves may be carbon-limited, but that SSHs may be nutrient-limited, but much more work is needed. Guano is often cited as the counter-example to the evolution of troglomorphy and to food limitation in subterranean habitats. We argue for a re-examination of these communities, particularly with respect to the presence of troglomorphy, and provide an example of a troglomorphic species living on guano.