ARE NUTRIENTS AFFECTED BY DIEL CYCLES IN STREAMS? STUDY OF A LOW-DISCHARGE STREAM IN MIDDLE TN

A 48-hour water sampling campaign was conducted in Big East Fork Creek in Franklin, TN in October 2020 to determine whether the nutrients P and N are affected by diel cycles, which can be caused by daily variations in solar radiation, gas solubility, biological activity, streamflow/transpiration, and sorption. This open canopy low-discharge stream occurs in limestone bedrock in an area dominated by temperate forests with minor organic agriculture. Every four hours measurements were made and water samples collected and filtered to 0.45 μm. Daytime cloud cover ranged from 0-3% (October 2-3) up to 52 % (October 4) during the sampling campaign. The water was calcium bicarbonate type with low average specific conductivity of 201 mS/cm. Field measurements showed diel cycles in temperature and pH but not salinity. Calculated parameters P_CO2 and saturation indices of calcite and dolomite also showed diel variations, as did dissolved concentrations of Ca, Mg, Se and Fe measured by ICP, Cl and SO₄ measured by IC, DIC, and δ¹³C measured using an IRMS. Selenium had the highest magnitude of increase, 96%, over the stream campaign, and 7 of the 12 samples had Se concentrations greater than the maximum EPA WQC of 3.1 ug/L for lotic aquatic systems. All water samples were undersaturated in calcite and oversaturated in CO₂ relative to the atmosphere, which we hypothesized was caused by mixing of calcite-saturated waters with high P_CO2 (groundwater) and low P_CO2 (surface water). However, the entire mixing trend falls well below the calcite solubility curve and cannot be explained by reasonable P_CO2 values. Cl and S increase from mid-day through the evening, possibly due to high evapotranspiration in the riparian zone, and then decrease through the night. Potassium declined continuously throughout the measurements, while P and N did not show diel cycles, even though they are incorporated into organic matter during photosynthesis. Oversaturation in hydroxyapatite likely determined dissolved P concentrations. More study is needed to identify the processes that control the nitrogen species nitrate, nitrite, and ammonia.