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University of Georgia


Biogenic and simulated nutrient enrichment increased levels of various ecological components of Okefenokee Swamp marshes. Fertilization by wading birds and an artificial experimental source increased stores of phosphorus in diverse links of the food web. Simulation modeling lent support to the hypothesis that birds mediated such changes that persisted after they abandoned this ecosystem.

To simulate nutrient enrichment from birds, I placed pots of enriched agar inside clear enclosures. In the laboratory, pots released a mean of 45 mg NH$\sb4$-N d$\sp{-1}$ and 11 mg PO$\sb4$-P d$\sp{-1}$ into water. In the marsh, mesocosms containing pots had higher stocks of zooplankton (primarily Diaphanosoma brachyurum) than controls. These results demonstrate that simulation of enrichment remaining after birds left can cause significant effects on zooplankton primary consumers in the marshes.

Evidence was presented for residual enrichment by a biotic component of the ecosystem itself, the birds. One to two years after they abandoned the marsh, it showed continued nutrient effects. Elevated available phosphorus in sediments contributed to enhanced levels of phytoplankton. Planktivorous fish were greater in biomass than at reference sites. These results and the zooplankton data suggest residual enrichment effects of birds on sediments, and indirectly on plankton and fish.

I measured the trophic transfer to Leptolucania ommata and Gambusia affinis, in situ. Both fishes ate mainly insect larvae (such as Chironomidae) and Cladocera. Consumption by both species in summer (26.31 mg m$\sp{-2}$ d$\sp{-1}$) supports a hypothesis that invertebrate prey production must be substantial in such blackwater marshes.

Trophic model dynamics were checked by seasonal data before, during and after simulated nutrient input from birds. The model reasonably estimates effects of enrichment on benthic detritus, aquatic macrophytes and phytoplankton, and conservatively estimates the increase of zooplankton and fish. Simulated levels decreased after simulated abandonment, and the decreases generally followed field trends. The model indicates that nesting birds can have considerable effects on aspects of an ecosystem not normally associated with them.

On the applied side, wetlands have been suggested for natural sewage treatment. Results described here indicate a sizeable capacity to buffer pulses of sewage nutrients, by time-delayed release.