Media Contact: Lisa Young
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Coastal science gospel states
that eutrophication caused by elevated nutrient loadings has triggered
major alterations of coastal ecosystem structure and function. A recent
journal article in Estuaries and Coasts, 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.
Source: 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.
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