Diel aqueous chemistry and biogeochemical processes in streams of karst areas

Study on diel cycling of stream hydrochemistry can help to reveal relatively rapid biogeochemical processes in natural water (processes of in stream flows)and discriminate drainage basin processes in re-charge areas.Existing research shows that biological processes(photosynthesis and respiration),geochemical processes(bicarbonate equilibrium,and calcite precipitation)are the main controlling factors on diel varia-tions of pH values,specific conductivity(SpC),concentrations of Ca2 + and HCO -3 in streams.Furthermore, stream orders and types and even microenvironments of the riverbed all have remarkable influence on diel a-queous chemistry.The pH value and dissolved oxygen(DO)are mainly controlled by photosynthesis which is closely related to air temperature.In high-alkalinity and calcium-rich streams,representing carbonate-rich basins,calcification and acid secretion of organisms may play an important role in aquatic plant photosynthe-sis,thus resulting in diel hydrochemical cycling with daytime decrease(up to a 20% to 30% decline)and nighttime increase of concentrations of Ca2 + and HCO -3 .Diel DIC cycling downstream caused by photosyn-thesis and its changes along the stream flow indicate that the stream is losing inorganic carbon along its flow path.It converts to organic carbon,such that inorganic C storage in streambeds will be an important net DIC sink in small productive streams.The effect of diel cycling of biogeochemistry on interpretation of carbon cy-cling,sink and source,especially on clarification of karst carbon sink stability and net carbon sink estimation trends becomes increasingly important in karst aquifer systems.Diel variability has implications for the de-sign of long-term surface water monitoring programs and interpretation of water quality trends.