Coe, Holly. 2001. Distribution patterns of hyporheic fauna in a floodplain riparian terrace. M.S.
Hydrologic linkages between rivers and their floodplains are essential for maintaining ecological structure and function in these dynamic ecosystems. In particular, hyporheic zones can regulate the transfer of nutrients from productive floodplains to oligotrophic surface habitats and provide habitat for aquatic insects, an important food source for resident fishes. Recently, attention has focused on the potential influences of invertebrates on the functional significance of hyporheic zones. Studies investigating hyporheic invertebrates have focused on communities within the active channel, either beneath the wetted channel or channel gravel bars. The floodplain hyporheic zone has received little attention in the literature but may be important in rivers with extensive floodplains. The objectives of this study were to describe hyporheic invertebrate community structure in the floodplain hyporheic zone and to relate spatial and temporal patterns in community structure to hydrology, organic matter and microbial biomass, and physico-chemical parameters. Piezometers in a floodplain riparian terrace on the Queets River were sampled during early summer 1999, late summer 1999, fall 1999, winter 2000, spring 2000, and later summer 2000. The invertebrate assemblage was consistent with cyclopoid copepods, copepod nauplii and rotifers dominating numerically throughout the study. Archiannelids, hydrachnidia and harpacticoid copepods were common but did not contribute substantially to overall invertebrate abundance. Aquatic insects such as chironomids, ceratopogonids, and plecoptera were surprisingly rare across sample dates. Overall invertebrate distribution across the terrace was characterized by high spatial and low temporal heterogeneity. At the terrace scale, spatial heterogeneity was related to wood, with high-wood piezometers representing temporally stable 'hotspots' of total invertebrate abundance. Within the terrace, along individual flowpaths, spatial heterogeneity appeared to be driven by a mosaic of overlying vegetation patch-types. Hyporheic invertebrate communities can provide unique insights into the linkage between rivers and their adjacent floodplains, and may ultimately play an important role as indicators of water quality and ecosystem health.