The Paradox of Enrichment: A Real-Life Example

In this post, I will introduce one of my favorite biological systems that can help demonstrate multiple interesting phenomena and act as empirical support for such theories as top-down trophic control, alternative stable states, criticallity theory and, of course, the paradox of enrichment.

Flipping Lakes

In an article in Ecology & Society, Professor Martin Scheffer gave a wonderful account of the phenomenon of "flipping" lakes. These lakes, which are usually shallow, are found in two states: clear water with lots of underwater vegetation or turbid water with a thick layer of green algae at the water surface, giving the lake a greenish tinge. The clear lakes have a large diversity of animals, such as ducks, various kinds of fish as well as a myriad of zooplankton. The turbid lakes have greatly reduced animal and plant diversity, as the majority of the energy and nutrients of the system are stuck inside the algae layer.

So what does this have to do with the paradox of enrichment? Well, one can obtain turbid lakes by enriching the clear one (eutrophication). Human beings have been enriching lake ecosystems for years by dumping detergents rich in nutrients (such as nitrogen) in the lakes, causing the various fish kills that are associated with these algae "blooms". By reducing the diversity of species and trophic levels in the lake, eutrophication empirically validates the paradox of enrichment. Of course, the effects of eutrophication are not as simple nor as clear-cut as in Rosenzweig & MacArthur's model, but those complexities of the natural system interact with the enrichment to cause a cascade of effects that result in reduction of diversity in the ecosystem.