Chemical Fluxes and Dynamics in River and Stream Ecosystems

Abstract

Streams and rivers have four contrasting biogeochemical zones: channel, channel surface, channel sediments below the surface (hyporheic zone), and backwaters. The channel is dominated by mass transport; metabolic processes occur, but at relatively low rates. The channel surface is covered with bacteria, protozoans, and fungi, along with nonliving organic and inorganic matter, the combination of which constitutes a biofilm. Invertebrates live within or around biofilm, upon which they feed. Oxic metabolic processes (photosynthesis and oxic respiration) are dominant. Biofilms are also found in the hyporheic zone, but do not involve photosynthesis and sometimes are anoxic. Anoxic respiration, which can be conducted by microbes, involves the use of chemical substances other than oxygen as electronic acceptors for respiration. It generates chemically reduced byproducts such as reduced nitrogen (N2), reduced iron, and reduced manganese, sulfide, and methane. Backwaters may support considerable accumulation of autotrophs, invertebrates, and small fishes that cannot withstand high currents. Biomass accumulation and oxic metabolism are typical of backwaters. Changes in stream flow, reflecting precipitation or snowmelt, affect mass transport and concentration of individual chemical constituents. Peak flows coincide with the highest mass transport, but effects on concentrations are varied. Major contributors to dissolved solids typically show a decline in concentration with increasing flow, whereas dissolved organic matter and particles of all kinds increase in concentration with increasing flow. Peak flows also disturb the channel surface, where much of the oxic metabolism occurs; a regeneration process follows peak flows. The movement of elements through streams can be viewed as a combination of a longitudinal flow with immobilization periods when a given substance combines with a biofilm or a fixed particle. The combination of longitudinal motion and interruption of motion often is viewed as a spiral. Substances that are in high demand biologically (labile substances) have tighter spirals than elements less subject to biological transformation (conservative substances).