A phosphorus (P) budget was created for Grand Lake St Marys (GLSM), a hypertrophic lake in Ohio with a highly agricultural watershed. Inputs totaled 71,200 ± 8,400 kg P, with tributaries contributing the majority of P inputs at 60,100 ± 4,500 kg P (84%). Other inputs included benthic flux at 9% (internal loading), point-source discharges into streams at 5%, and atmospheric deposition at 1%. Rainfall in 2011 was greater than average, which may affect results when comparing this P budget to years with average rainfall. Transport of P by two rivers draining GLSM was approximately three times greater than benthic deposition.
Addition of alum to remedy internal loading in GLSM did not appear to inhibit benthic flux of P. Residence time of 113 days suggests the ability of the system to decrease P concentrations in lake water if inputs are significantly decreased by the implementation of best land management practices.