2.4. Core sectioning and porewater chemistry

Sediment cores selected for immediate sectioning were processed within 1 week of retrieval from Jordan Lake, while those used in flux experiments were sectioned 1–3 days following the termination of the flux experiment. Prior to core sectioning, overlying water was carefully removed via syphon, and cores were extruded at 1-cm intervals for the top 3 cm, followed by 3-cm intervals for the remainder of the core. Extruded sediment was collected in clean 65-mL centrifuge tubes, cooled in a refrigerator (4 °C), then centrifuged at 4 °C to separate porewater and sediment. Porewater was passed through a 0.45-µm syringe filter and frozen until PO₄³⁻, NH₄⁺, and NO₃⁻ + NO₂⁻ analysis at WBAS. As we did not expect to find NO₃⁻ + NO₂⁻ below 3 cm, we analyzed NO₃⁻ + NO₂⁻ for the top 3 cm only. Care was taken throughout the process to minimize the introduction of oxygen prior to filtering, which could decrease the amount of PO₄³⁻ by forming particulate Fe(III) complexes. Samples for PO₄³⁻ were acidified prior to analysis, which would free any PO₄³⁻ adsorbed on Fe(III) in the post-filtered sample.