FOOD-WEB STRUCTURE AND PHOSPHORUS CYCLING IN LAKES

We assessed the effects of trophic structure on phosphorus (P) recycli ng by fishes, Chaoborus, and zooplankton in lakes with different food web configurations. Fishes in a piscivore-dominated system recycled li ttle P compared to that regenerated by zooplankton and Chaoborus. In a planktivore-dominated lake, excretion by fishes (especially young of year) dominated P recycling to algae; Chaoborus and zooplankton recycl ing was decreased due to intense predation by fishes. The sum of P inc orporated into primary production and lost to sedimentation in the pis civore-dominated lake was only 77% of that in the planktivore-dominate d lake. Our analyses suggest that the 30% greater primary production i n the planktivore-dominated system may be accounted for by the greater than 40% increase in estimated P recycling that was directly due to d ifferences in food web structure. Diel vertical migrations by Chaoboru s concentrated P excretion in the epilimnion during night hours. In th e planktivore-dominated system, Chaoborus migrations further affected spatial dynamics of P recycling by concentrating excreted P in the low er depths of the epilimnion. Diel vertical migrations by zooplankton h ad little effect on the spatial and temporal dynamics of P recycling e xcept to dampen the effects of the Chaoborus migrations.