Effects of Small-Scale Substratum Heterogeneity on Respiration Rates in Streams

Substratum heterogeneity is an important driver of nutrient transformations and organic carbon production in streams. Little information is available on small-scale (< 1 m) heterogeneity and its effects on larger scale stream functions such as whole-stream respiration. We used in situ incubations of small-scale (14.5 cm x 21.5 cm) containers broken into 12 compartments with each compartment containing small gravel or sand arranged in three levels of heterogeneity. We expected that more heterogeneous systems would have greater respiration rates due to closer proximity of oxic and anoxic zones. Oxygen microelectrodes were used to determine depth to anoxia within each section of the grids and whole-container respiration was determined by enclosure in a recirculating chamber in light and dark. There was no significant difference in respiration among heterogeneity levels. There was a significant difference in depth to anoxia between substrata types at each heterogeneity level, and average depth to anoxia in small gravel of high heterogeneity grids was 22 and 25 mm deeper than in medium and low heterogeneity grids, respectively. The differences in oxic/anoxic conditions between heterogeneity levels indicates that more heterogeneous systems are likely to house "hot spots" of microbial activity could be important to larger scale stream functions.