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

A series of whole-lake manipulations of both food web structure and nutrient 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. Phosphorus transport to the sediments was examined by three independent methods: mass-balance budgets, sediment traps, and sediment cores. After 2 years of pretreatment study, manipulated lakes were fertilized for 5 years at rates 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 increased 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 measurements of the vertical P flux were significantly higher than the long-term sediment P retention measured by sediment cores and the seasonal sediment P retention calculated by mass balance. Using 210Pb data from the sediment cores, we corrected the sediment trap measurements of vertical P flux for focusing and brought them into better agreement with the seasonal sediment P retention calculated by mass balance.