GEOMORPHIC DISTURBANCES IN KARST HYPORHEIC ZONES: LINKING HYDROLOGY AND ECOLOGY

The impact and frequency of thermal, substrate, and hydrologic disturbances has important implications for the biodiversity of karst lotic and hyporheic zones. Intermediate levels of disturbance are correlated with maximum taxonomic diversity in stream ecosystems because they maintain non-equilibrium conditions that allow a diverse assemblage of species to succeed. Thus, quantifying disturbance processes in the hyporheic zone provides a basis for evaluating the applicability of the intermediate disturbance hypothesis to karst hyporheic zone communities. Furthermore, the results should lend insight to the effectiveness of the hyporheic zone as a refuge from discharge-induced disturbances in the open channel.

Our results indicate that in humid climates with regular rainfall, hydrologic and sediment transport fluctuations will occur on a frequent basis. During base flow and moderate discharge conditions (recurrence intervals from monthly to several times a year), the hyporheic zone is a potential refuge from temperature and sediment transport disturbances in the overlying stream channel. However, larger discharge events, which occur with frequencies of between several months to 2 years, are capable of transporting significant (>75%) fractions of the coarse bedload. In this situation the mobility of the alluvial sediments which comprise the hyporheic zone would preclude it from serving as a refuge.

Determining the effect that disturbance events with these magnitudes and frequencies have on the karst hyporheos will require further study in collaboration with stream ecologists. The question that seems particularly germane is whether the frequency of large-scale (>75%) bedload transport reported here constitutes an intermediate level of disturbance.