Biomass-size spectra and the prediction of fish biomass in lakes

Biomass-size spectra of planktonic communities offer a promising approach to predict fish biomass from simple measurements of phytoplankton or zooplankton biomass. Theoretical models based on the transfer of energy between size-classes have predicted fish biomass in the Great Lakes within two fold of measured biomass. To test the applicability of one group of biomass-size spectrum models (U. Borgmann 1987. Can. J. Fish. Aquat. Sci. 44(Suppl. 2): 136-140) to lakes in general, we gathered biomass and production data for phytoplankton, phytobenthos, zooplankton, zoobenthos, and fisk from 24 lakes and reservoirs studied during the International Biological Programme. In planktonic communities, the intercepts of biomass-size spectra are positively related to phyloplankton productivity and to lake mean depth, and the slopes become more negative with increasing lake size. Despite these regularities, fish biomass is predicted poorly from Borgmann's biomass-size spectrum models. Predictions of fish biomass were only weakly correlated to measured fish biomass (r(2) = 0.19, n = 24). Fish biomass was overestimated or underestimated by more than 50 kg . ha(2) in almost half the lakes where the models were tested. The inclusion of benthic algae and benthic invertebrates in the models does not improve the predictions of fish biomass.