SPATIAL AND TEMPORAL VARIATION OF DISSOLVED OXYGEN AND ECOSYSTEM ENERGETICS IN DEVILS HOLE, NEVADA

Devils Hole, a unique ecosystem in the Mojave Desert, is home to a few dominant species of algae and invertebrates as well as the endangered Devils Hole pupfish, Cyprinodon diabolis. With consistently high water temperature (33.5 degrees C/93 degrees F) and low dissolved oxygen (O-2) concentration (about 2.5 mg O-2 . L-1), organisms are at the extremes of their physiological limits, and production of O-2 by microbial biofilms is essential to ecosystem stability. Water column O-2 concentrations were measured from July 2008 to March 2010 in the deep pool and shallow shelf habitats of Devils Hole to quantify variability in O-2 concentrations and ecosystem metabolism rates. Benthic O-2 dynamics were also measured in microbial biofilms using microelectrode surveys. Water column O-2 ranged from 2 to 6 mg O-2 . L-1 in summers and from 1.5 to 2.2 mg O-2 . L-1 in winter across the deep pool and shallow shelf habitats. Primary production ranged from 4 to 21 mg O-2 . L(-1)d(-1) with a significant decline over the study period, potentially due to a change in the microbial biofilm community. Respiration ranged front 1.5 to 9.7 mg O-2 . L(-1)d(-1) and showed a significant increase over time. Within microbial biofilms, O-2 ranged from 0 to 76 mg O-2 . L-1. Higher concentrations of O-2 produced by these microbial biofilms may be due to improved photosynthetic efficiency under limited sunlight exposure. Autotrophic biofilms had higher O-2 concentrations during direct light exposure than during indirect light exposure. In contrast, heterotrophic biofilms had similar O-2 concentrations regardless of light exposure. Because microbial biofilms are important components of this unique ecosystem, shifts in their composition or activity may threaten ecosystem stability by reducing background O-2 concentrations below the physiological limits of the endangered Devils Hole pupfish and the entire biotic community.