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Ecosystem respiration and photosynthesis by submerged aquatic plants result in diel (24-hr) cycles in dissolved oxygen (DO) concentrations, pH, redox conditions and mineral saturation states in streams, which in turn drive diel variations in elemental concentrations. Direct assimilatory uptake of micronutrients by aquatic plants may also contribute to diel elemental cycles. The objective of this study is to quantify the importance of biotic assimilation relative to abiotic controls on diel elemental cycling in streams by analyzing primary producer tissue stoichiometry, environmental element availability, and the magnitude and phase of diel variations. We deployed in-situ sensors and used ICP-MS to analyze elemental concentrations in the water and dominant algal and vascular plant species of the Ichetucknee River and its five main source springs. The entirely spring-fed Ichetucknee River, in north-central Florida, is a model system for distinguishing between the multiple drivers of diel chemical cycles due to its stable discharge, known input spring chemistry, and high primary productivity. Five kilometers downstream of the source springs, diel cycles were observed in DO, pH, NO3, and PO4, reflecting aquatic primary production. Thirteen major and trace elements exhibited diel cycling. Concentrations of Mg, K, Fe, Cu, As, U, Cr, V, and Co peaked in the afternoon, in phase with DO and pH, while Ca, Mn, Ba, and Sr concentrations peaked in the morning. Elemental concentrations in both plant species were similar to concentrations in terrestrial leaves but varied considerably between collection sites. Concentrations of V, Cr, Mn, Co, Ni, Sr, Ba, and U in plants from the springs correlated well (r2 > 0.6) with spring water concentrations. Differences between assimilation estimates and observed diel elemental cycles should reflect the magnitude and timing of abiotic drivers of diel elemental cycles and provide insight into the interactions between micronutrient availability and acquisition within aquatic ecosystems. |