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 < 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 (< 10.0 to 27.7 µg PO 4 liter −1 ). Pore‐water ammonia (NH 4 + ) concentrations were significantly lower at high groundwater discharge sites (<10.0 to 566.0 µg NH 4 + liter −1 ) than at low groundwater discharge and recharge sites (61.4–1464.9 µg NH 3 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), NO 3 ‐NO 2 , and O 2 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 (NH 3 ) 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.