GEOCHEMICAL AND BIOLOGICAL CONTROLS ON DIEL (24-HR) ELEMENT CYCLING IN A CARBONATE-DOMINATED RIVER

Respiration and photosynthesis by submerged aquatic plants produce in-stream diel cycles in dissolved oxygen (DO), pH, redox conditions and mineral saturation states which can indirectly drive diel cycling in major and trace element concentrations. Diel element cycling may also result from direct assimilatory uptake by submerged plants. We combined high-resolution measurements of the magnitude and timing of diel element cycles with analysis of submerged plant stoichiometry in the Ichetucknee River (north-central Florida) to assess the geochemical and biological processes controlling element cycling at the ecosystem scale. The river is entirely spring-fed with stable discharge (6-9 m³/s) and source-spring chemistry, and high primary productivity, making it a model system for distinguishing between processes controlling element cycles. 5 km downstream of the source springs, diel cycles were observed in DO, pH and NO₃, reflecting submerged plant metabolism. Daytime calcite precipitation, induced by the diel cycle in pH, is evidenced by diel cycles in SI_calcite, Ca, DIC and δ¹³C concentrations. Fe, Cu, U, As, V, Cr, Mn, Ba, and Sr concentrations all exhibited statistically significant diel cycles but with varying phases, suggesting multiple controls. Calcite co-precipitation can explain 30-100% of the Mn cycle but has little impact on Sr or Ba cycles due to differences in ionic radii from Ca. Temperature- and pH-dependent adsorption could also explain the inverse timing of cationic Mn and Ba relative to pH and temperature, as well as the in-phase cycle in anionic As. Redox changes resulting from daytime oxygen production could drive the diel cycles of U, V, Cr, and As, which are soluble in oxidized form, but not Fe, which is insoluble as Fe(III). Photo-reduction of Fe could explain the observed cycle and might be possible under high pH conditions due to the high clarity of the water. Direct assimilatory uptake by submerged aquatic plants appears to control about 30% of the diel cycles of Mn and Fe, but only 1-2% of Ba, Sr, and Cr, based on measured plant tissue stoichiometry and estimates of net carbon fixation. These results demonstrate how the complex interplay between biological and geochemical processes, directly and indirectly the result of plant metabolism, controls elemental concentrations in streams.