Résumé
Hyporheic exchange is important for nutrient cycling in rivers, but little is known about the magnitude of this process in karst systems or its influence on speleogenesis and the formation of river channels. We use pore-water depth profiles to assess nutrient and carbonate processing in the hyporheic zone of the Ichetucknee River (north-central, Florida). The Ichetucknee River is sourced from six major and numerous small springs which discharge from the karstic Floridan Aquifer. Order of magnitude increases in nitrate concentrations since the mid 20 (super th) century have been implicated in a recent proliferation of algae in the river. Nitrate concentrations decrease downstream and exhibit diel variations, along with specific conductivity and calcite saturation state. These patterns reflect in-stream processing, but hyporheic exchange should also influence the overall dynamics of nutrient and Ca fluxes in the river. Our two initial depth profiles extend through unconsolidated sediment to solid carbonate of the Floridan Aquifer at 116 and 121 cm below the river bed. The profiles were taken about 10 m from the stream banks in an approximately 100 m wide section of the river. Specific conductivity of the river water was approximately 320 mu S/cm and increased by 20 to 30 mu S/cm at depths around 60 cm below the river bed. Dissolved oxygen concentrations in the river water increased from 5.7 to 8.3 mg/L from the first to the second sampling time as a result of photosynthetic production, but decreased with depth to 1 mg/L or less at the base of the profiles as a result of organic carbon remineralization. This process likely caused the observed decrease in pH from 7.8 to 7.3. The lower pH should drive dissolution of the carbonate sediment, and thus could explain the increase in specific conductivity. Carbonate mineral diagenesis may also affect phosphate concentrations. The low oxygen concentrations could indicate denitrification occurs in the pore water, while organic carbon remineralization may produce excess N in the form of NH (sub 4) (super +) . The magnitude of nutrient cycling in the pore waters, and its link to nutrient cycling in the river, requires additional information on the magnitude of the nutrient concentrations, as well as the rate of exchange of water between the sediment and the river.