Eutrophication of aquatic ecosystems: bistability and soil phosphorus

Abstract

Eutrophication (the overenrichment of aquatic ecosystems with nutrients leading to algal blooms and anoxic events) is a persistent condition of surface waters and a widespread environmental problem. Some lakes have recovered after sources of nutrients were reduced. In others, recycling of phosphorus from sediments enriched by years of high nutrient inputs causes lakes to remain eutrophic even after external inputs of phosphorus are decreased. Slow flux of phosphorus from overfertilized soils may be even more important for maintaining eutrophication of lakes in agricultural regions. This type of eutrophication is not reversible unless there are substantial changes in soil management. Technologies for rapidly reducing phosphorus content of overenriched soils, or reducing erosion rates, are needed to improve water quality.

Fig. 1.

Rate of change of phosphorus in lake water (g·m^-2·y^-1) versus density of phosphorus in lake water (g·m^-2) in hypothetical situations with no recycling (A) or recycling faster than losses over some range of phosphorus in lake water (B).

Fig. 2.

Equilibria of the model with nominal parameter values versus net rate of phosphorus input to soil W + F - H. Solid lines are stable points, dashed lines are repellors. (A) P density in water (g·m^-2). (B) P density in surface sediment (g·m^-2).

Fig. 3.

Phosphorus density (g·m^-2) in soil (brown), sediment (red), and water (green), and phosphorus input rate to soil (black, g·m^-2·y^-1) versus time in two simulations. Attractors (blue) are shown for water phosphorus density. Note log scale of the y axes, and different x axis scales of the two plots. (A) Simulation 1. (B) Simulation 2.