Food web structure and nutrient enrichment: effects on sediment phosphorusretention in whole-lake experiments

A series of whole-lake manipulations of both food web structure and nutrien t loading rate revealed that the downward vertical P flux was significantly affected by food web structure. Food webs and nutrient input rates of two lakes were manipulated while a third lake served as a reference system. Pho sphorus transport to the sediments was examined by three independent method s: mass-balance budgets, sediment traps, and sediment cores. After 2 years of pretreatment study, manipulated lakes were fertilized for 5 years at rat es that were similar within each year but varied among years from 0.97 to 6 .0 mg P.m(-2).day(-1). Increased vertical P flux was associated with the in creased abundance of large-bodied zooplankton grazers such as Daphnia pulex . Vertical P flux increased with P input rates, but the increase was often insufficient to prevent P accumulation in the water column. Sediment trap m easurements of the vertical P flux were significantly higher than the long- term sediment P retention measured by sediment cores and the seasonal sedim ent P retention calculated by mass balance. Using Pb-210 data from the sedi ment cores, we corrected the sediment trap measurements of vertical P flux for focusing and brought them into better agreement with the seasonal sedim ent P retention calculated by mass balance.