Whole Ecosystem Experiments

Large-scale, whole ecosystem experiments have contributed considerably to our understanding of ecosystem dynamics. With its beginnings in wholesale watershed experiments in the 1960s, ecosystems are now being studied experimentally and analyzed as system of interacting species processing nutrients and energy within the context of changing abiotic conditions. This is particularly relevant these days given the effects of anthropogenic climate forcing and pollution in both terrestrial and oceanic ecosystems.

A classic series of whole-lake nutrient addition experiments conducted in northwestern Ontario by David Schindler and his research group illustrated the role of phosphorus in temperate lake eutrophication. To separate the effects of phosphorus and nitrate, the researchers split a lake with a curtain and fertilized one side with carbon and nitrogen and the other with phosphorus, carbon, and nitrogen. Within 2 months, a highly visible algal bloom had developed in the basin in which phosphorus had been added providing experimental evidence that phosphorus is the limiting nutrient for phytoplankton production in freshwater lakes. Certainly, algae may show signs of nitrogen or carbon limitation when phosphorus is added to a lake; however, other processes often compensate for these deficiencies. For instance, CO2 is rarely limiting because physical factors such as water turbulence and gas exchange regulate its availiblity. Further, nitrogen can be fixed by blue-green algae. These species, which are favored when nitrogen is in short supply, increases the availability of nitrogen to algae, and the lake eventually returns to a state of phosphorus limitation. The practical significance of these results is that lake europhication can be prevented with management policies that control phosphorus input into lake and rivers.

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