Effects of coarse particulate organic matter on fungal biomass and invertebrate density in the subsurface of a headwater stream

Links between groundwater invertebrates and their potential food resources were examined using biofilm development on fine wood. Little is known about biofilm development and organic matter content of lateral subsurface (i.e., parafluvial) environments and hyporheic habitats (upwelling and downwelling zones). Eighteen experimental baskets containing river rocks were paired by treatment in which 1 basket was supplemented with wood (six 7.2 cm × 12 cm strips of oak wood veneer). Nine pairs of baskets were buried 12 to 15 cm below the surface in Gallina Creek, a 1st-order mountain stream in northern New Mexico in late summer 1997. Three pairs were buried beneath the stream bank (i.e., parafluvial zone) and 6 pairs were buried in the hyporheic zone. Baskets were distributed along upwelling and downwelling reaches to assess the potential hydrologic influence of subsurface-surface exchange. Open baskets of wood veneer were placed on the streambed surface to compare fungal biomass on the surface with the subsurface. Wood in both hyporheic and parafluvial baskets was colonized by fungi, but fungal biomass was significantly greater on wood in surface water than in hyporheic and parafluvial zones. In addition, fungal biomass on hyporheic wood was significantly greater than on parafluvial wood. A similar pattern (i.e., surface > hyporheic > parafluvial) was observed for dissolved oxygen. In contrast, concentrations of retained particulate organic matter were significantly higher in the parafluvial than the hyporheic zone. Invertebrate densities were significantly greater in baskets supplemented with wood and were greater in the hyporheic zone than in the parafluvial zone. Our data suggest that wood and associated microbial biofilms represent an important food resource for interstitial invertebrate communities.