Our work in the Laurentian Great Lakes of North America indicates that significant fluxes of carbon and phosphorus can pass through the microbial food webs (MFW) of these lakes. Here we present a synthesis of our recent investigations conducted largely along a trophic axis from the heavily eutrophic coastal Sandusky Bay to offshore communities near the international boundary in the central basin of Lake Erie. We find that the significance of the MFW in transporting C and P to higher trophic levels differs along a trophic gradient. In relatively eutrophic nearshore communities, most C and P are fixed into phytoplankton, transport of materials is largely dependent on grazing by cladocerans, and transport through the MFW is relatively insignificant. In contrast, in relatively oligotrophic offshore communities bacterial biomass often exceeds phytoplankton biomass, the majority of P is fixed into bacteria, bacterivorous grazers (e.g. rotifers and protozoa) dominate, copepods are the dominant microcrustacean, and transport of C and P through the MFW represents a major pathway. We suggest that the management of large-lake ecosystems is largely based on relatively eutrophic “nearshore” views of the base of the food web and needs to be modified to include considerations of the MFW in the more oligotrophic offshore regions of these lakes.

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