Is Nutrient Loading A Smaller Problem Than We Think?

Coastal science gospel states that eutrophication caused by elevated nutrient loadings has triggered major alterations of coastal ecosystem structure and function. The article, authored by Dauphin Island Sea Lab scientists Dr. Ken Heck and Dr. John Valentine, turns this conventional wisdom on its head, making the case that the cause of these problems can be found at the top, rather than the bottom, of the food web.

The authors assert that rather than nutrient loadings, the more likely culprit is the depletion of top-level consumers in coastal and estuarine ecosystems. Indirect effects of the removal of large consumers are often indistinguishable from effects of nutrient loading, they argue, and they present evidence gathered from more than 100 studies of coral reefs, rocky intertidal areas, and sea grass beds to support the claim.

For example, the authors report that studies evaluating the relative effects of consumers and nutrient supplies on algal biomass have often concluded that consumer (top-down) effects are greater or equal to those of nutrients.

One example they cite takes on the classic model of loss of estuarine seagrass. While common understanding holds that nutrient enrichment leads to epiphytic growth on seagrass, killing the plants by blocking sunlight, cascading trophic effects are likely have just as much influence. Epiphytic abundance is also controlled by grazers, the absence of which would have the same overgrowth effect as nutrient enrichment.

This paradigm could have major repercussions for management of coastal ecosystems, considering the research and management emphasis of recent decades on nutrient control. Especially if upper trophic levels have been altered, nutrient reduction is unlikely to help restore benthic habitats, note the authors.

Reference: Heck, K. L. Jr. and J. F. Valentine. 2007. The primacy of top-down effects in shallow benthic ecosystems. Estuaries and Coasts 30(3): 371-381.