Fevold, Karen. 1998. Sub-surface controls on the distribution of benthic algae in floodplain back channel habitats of the Queets River. M.S.
Availability of nutrients (often phosphorus and nitrogen) for uptake by benthic algae often controls the rate of primary production by periphyton in streams. The hyporheic zone is a site of active biogeochemical processing and its interactions with the riparian zone can control nutrient transformations and nutrient fluxes to the surface aquatic ecosystem. Water upwelling from the hyporheic zone can deliver a subsidy of nutrients necessary for primary production into aquatic ecosystems.
The objectives of this study were to 1) determine the influence of hyporheic upwellings on benthic algae biomass and distribution and 2) determine the extent of this phenomena in the low gradient floodplain of the Queets River and 3) determine the mechanisms controlling the increased algal biomass at hyporheic upwellings. Benthic algal biomass, vertical hydraulic gradient, and surface and sub-surface nutrient concentrations and DO were measured in two floodplain back channels, throughout the summer to determine mechanisms controlling the hyporheic-benthic algae association.
In back channels adjacent to alder terraces, benthic algal chlorophyll a standing stocks were significantly higher at sites with hyporheic upwelling, than at sites without upwelling (82.3 vs. 21.7mg/m2). Chlorophyll a standing crops were similarly distributed in the cobble bar embedded back channels; benthic algal biomass was higher at upwelling sites than non-upwelling sites (136.0 mg/m2 vs. 21.0 mg/m2). Chlorophyll a concentrations were positively correlated with VHG in all back channel habitats.
The pattern of nutrient concentrations in surface hyporheic waters was markedly different in the two different back channels. In the alder terrace back channel (ATBC), SRP concentrations were similar in surface and hyporheic waters. SRP concentrations in the abandoned mainstem back channel (AMBC) were significantly higher in hyporheic water than in surface water. The mean concentration of dissolved inorganic nitrogen (DIN) in hyporheic water were significantly different than surface water in each back channel, however, ATBC hyporheic water DIN concentrations were higher than surface water concentrations and DIN concentrations in hyporheic water of the AMBC were lower than in surface water.
Results of this study reveal a complex pattern of surface and sub-surface interactions that influence algal biomass distribution within floodplain back channel habitats of the Queets River. Although nutrient subsidies from upwelling hyporheic water are important controls on surface benthic algae dynamics, the nature of those controls varies with the type of back channel in which the upwelling is located. In highly active floodplain habitats, back channels are continuously recovering from fluvial disturbances such as frequent channel altering floods, and major shifts in main channel location. The two channel types we monitored in this study represent end members of this disturbance-recovery continuum.