Eutrophication and Restoration of Shallow Lakes from a Cold Temperate to a Warm Mediterranean and a (Sub)Tropical Climate

Abstract

Freshwaters are crucial to the establishment of any human community and to the conservation of all land-based wildlife. The majority of the world’s freshwater area is shallow, typically ≤3 m deep, and polymictic. Freshwaters are rather versatile in their use and have great economical and conservation value. They can be/are often dominated by littoral communities as the water depth is generally shallow enough to support the growth of submerged macrophytes, and the species richness of invertebrates, fish and waterfowl communities is more diverse than in deep lakes. Eutrophication or nutrient enrichment affects the entire aquatic system by altering trophic structure, biodiversity and biogeochemical cycles, as well as seasonal dynamics. In shallow lakes with low nutrient concentrations (<0.025 mg TP L–1), a clear state dominated by submerged plants often occurs as phytoplankton is nutrient limited. In north temperate shallow lakes, the TP range for coexistence of alternative states varies from ca. 0.025 to ca. 0.15 mg TP L–1. In these lakes, submerged plants stabilize the clear water state through several physico-chemical buffer mechanisms of which some have proven to be weak in warm lakes. An abrupt shift to a turbid state may occur after surpassing a lake-specific nutrient threshold. In warm lakes, internal nutrient cycling is relatively more important and external loading relatively less important than in cold temperate lakes. Warm lakes tend to be more productive than cold lakes with similar nutrient concentrations. With a diverse and abundant omnivorous fish community, the predation pressure on zooplankton is strong, top-down control becomes less important and nutrient control thus emerges as a decisive factor for the water clarity. For cold lakes, restoration methods encompass both bottom-up and top-down controls; however, for warm lakes bottom-up or nutrient control methods appear to be most significant for eutrophication control. As for climate change, warming is likely to exacerbate some symptoms of eutrophication in both cold and warm climates. The structure and functioning of cold temperate shallow lakes are expected to become more similar to those of (sub)tropical shallow lakes, as the temperature increase will enhance the top-down controls of omnivorous and benthivorous fish as well as the nutrient cycling.