Diffusivity‐Based Flux of Phosphorus in Onondaga Lake

The upward transport of phosphorus through the lower portion of the metalimnion of hypereutrophic Onondaga Lake, mediated by vertical diffusivity, was estimated for a productive seven month period during 1980. This upward flux was simulated by Fickian diffusion, which incorporates estimates of the vertical diffusivity coefficient for heat and measurements of the vertical phosphorus gradient. The field and laboratory analytical program included thermal profiles and dissolved phosphorus fractions from depths bracketing the metalimnion. Samples were collected on 53 dates during the study period. Two different time segmentations, approximately semimonthly and monthly, were utilized in the determination of the vertical diffusivity coefficient. The turbulent component of vertical diffusivity demonstrated a strong seasonality during the stratification period, varying from $0.060{\mathrm{cm}}^2{\sec }^{-1}$ in early summer to $0.002{\mathrm{cm}}^2{\sec }^{-1}$ during the maximum stability period of late summer. The total dissolved phosphorus gradient increased during stratification to a maximum of $0.14\mathrm{mg}\cdot L^{-1}\cdot m^{-1}$ in late August and early September. The internal loading of phosphorus to the epilimnion from diffusivity‐based transport during the study was approximately 21% and 18%, respectively, of the external loading of dissolved and total phosphorus. The elimination of the present industrial ionic loading to the lake could result in increased internal loading of phosphorus through vertical transport by as much as a factor of 7.