We reviewed a mass balance model devel-oped in 2001 that guided establishment of the phosphorus total maximum daily load (TMDL) for Upper Klamath and Agency Lakes, Oregon. The purpose of the review was to evaluate the strengths and weaknesses of the model and to de-termine whether improvements could be made using information derived from studies since the model was first developed. The new data have contributed to the understanding of processes in the lakes, particularly internal loading of phos-phorus from sediment, and include measurements of diffusive fluxes of phosphorus from the bottom sediments, groundwater advection, desorption from iron oxides at high pH in a laboratory set-ting, and estimates of fluxes of phosphorus bound to iron and aluminum oxides. None of these pro-cesses in isolation, however, is large enough to account for the episodically high values of whole-lake internal loading calculated from a mass bal-ance, which can range from 10 to 20 milligrams per square meter per day for short periods.
The possible role of benthic invertebrates in lake sediments in the internal loading of phospho-rus in the lake has become apparent since the development of the TMDL model. Benthic inver-tebrates can increase diffusive fluxes several-fold through bioturbation and biodiffusion, and, if the invertebrates are bottom feeders, they can recycle phosphorus to the water column through metabol-ic excretion. These organisms have high densities (1,822–62,178 individuals per square meter) in Upper Klamath Lake. Conversion of the mean density of tubificid worms (Oligochaeta) and chi-ronomid midges (Diptera), two of the dominant taxa, to an areal flux rate based on laboratory measurements of metabolic excretion of two abundant species suggested that excretion by ben-thic invertebrates is at least as important as any of the other identified processes for internal loading to the water column.Keyword Tags: