Changing nutrients, changing rivers

Abstract

Eutrophication—the excessive enrichment of a body of water with nutrients such as nitrogen (N) and phosphorus (P)—is Earth's most widespread problem for water quality (1, 2). Growing evidence suggests a global trend toward reversing eutrophication. However, in rivers and estuaries of developed countries and in lakes of emerging economies, the ongoing reduction in nutrient inputs—termed reoligotrophication—is much larger for P than for N (3, 4). Although the rapid emergence of this phenomenon has hindered detailed monitoring of the ecological effects, a few studies have documented an abrupt shift from green to clear waters and consequently from phytoplankton to macrophytes as dominant primary producers in response to reoligotrophication in rivers and estuaries (5–7). However, the improvement in water quality due to P decline does not imply a return to pristine ecological conditions, because high N:P ratios trigger undesirable changes in the ecosystem (8).