A Test Of A Morphometric Index To Predict Vertical Phosphorus Transport in Lakes

Osgood (1988) proposed a morphometric index, mean depth over square root of surface area ([zbar] / √ A0), to estimate the probability of partial or complete mixing of lakes during summer storms. Such mixing could introduce phosphorus (P)-rich waters to the entire water column (polymictic lakes) or to the epilimnion, and could delay lake recovery following abatement of allochthonous P loading. Cooke et al. (1993) hypothesized that aluminum sulfate (alum) applications to control internal P loading would be most effective in lakes with an Osgood Index less than 6 and significant diversion of external loading. We tested the hypothesis that vertical P transport is higher in lakes with the lowest Osgood Index numbers by estimating transport in four northeastern Ohio lakes with Osgood numbers of 2.9, 7.4, 9.7, and 26.3 during Summer 1992. The hypothesis was rejected. Although the lake with the lowest number most often had the highest vertical P transport rate, transport was not significantly different between lakes. The qualitative trends among lakes corresponded with their relative differences in P concentration gradients. Lakes with a high Osgood Index could have significant vertical P transport if there is a steep P gradient in the water column. The results suggest that alum applications have the potential to decrease trophic state in lakes with a high Osgood Index value.