Sources and Sinks of Ions in a Soft Water, Acidic Lake in Florida

Abstract

Fluxes and storage of water and major ions were measured for McCloud Lake, a small, acidic soft water lake in north central Florida that receives 90% of its water input from direct precipitation. Wet precipitation had a volume‐weighted pH of 4.5 and accounted for a H+ flux of 420 eq/ha yr; deposition of SO2, NO2, and HNO3 contributed an additional 150–300 eq H+/ha yr. Although precipitation passing through the sandy soils and littoral sediment was neutralized by mineral weathering, sulfate reduction or adsorption, and nitrate immobilization, ground water was only 10% of the total water input to the lake and neutralized only about 30% of precipitation H+ input. Compared to atmospheric deposition, lake water shows little enrichment of cations but is depleted in SO42−, NO3− and NH4+, while >80% of precipitation H+ is neutralized. The major H+ neutralization processes within the lake are sulfate reduction (26–34 meq SO42−/m2 yr) and NO3− immobilization (17–20 meq NO3−/m2 yr), while NH4+ immobilization is a small internal source of H + . Sinks and sources of cations could not reliably be calculated because of uncertainties in groundwater inputs. Internal H+‐consuming processes are important in lakes with no surface inlets or outlets and which receive most of their water from direct precipitation. For these lakes, the rate and extent of acidification is largely determined by a balance between H+ inputs and H+ consumption by internal processes.