Groundwater flow influences the biomass and nutrient ratios of epibenthic algae in a north temperate seepage lake
Groundwater flow influenced epibenthic algal biomass and N:P ratios at a seepage lake (Sparkling Lake, Wisconsin). During seasonal studies, biomass and seepage flux were positively associated (r = 0.453; P \textless 0.001). Pore-water soluble reactive phosphorus (SRP) concentrations (29.2–110.7 µg PO, liter-1), SRP fluxes, and algal biomass were significantly higher at high groundwater discharge sites than at low flow sites (\textless 10.0 to 27.7 µg PO4 liter-1). Pore-water ammonia (NH4+) concentrations were significantly lower at high groundwater discharge sites (\textless10.0 to 566.0 µg NH4+ liter-1) than at low groundwater discharge and recharge sites (61.4–1464.9 µg NH3 liter-1). The coupling between pore-water nutrient concentrations and local groundwater flow dynamics suggests a mechanism for the observed spatial patterns in biomass. In situ experimental chambers evaluated coupling between epibenthic algal biomass, N:P ratios, and groundwater flow patterns. Biomass responded rapidly in chambers, reaching ambient levels within 1.5 months of initiation. Free-flow chambers in discharge regions had consistently higher soluble reactive phosphorus (SRP), NO3-NO2, and O2 concentrations, higher phosphate and nitrate-nitrite fluxes, higher algal biomass, and lower N:P ratios in the developing mat. Free-flow chambers in recharge regions had high ammonia (NH3) concentrations, lower algal biomass, and higher N:P ratios. These results confirm that groundwater-related nutrient fluxes influence the local physicochemical environment and affect epibenthic algal biomass.