Effects of Nutrient Enrichment and Large Predator Removal on Seagrass Nursery Habitats: An Experimental Assessment



Nutrient enrichment and overfishing are two of the most common man-induced perturbations of coastal systems. Eutrophication can produce many undesirable effects in coastal systems. Among them is a decline in submerged aquatic vegetation (SAV) through increased light attenuation and algal overgrowth of SAV leaves, which may outstrip the ability of their grazers to control them. Alternatively, reductions in the abundance and composition of predator populations can also produce profound effects in aquatic systems. A review of predator/prey interactions in SAV systems leads us to hypothesize that losses of top predators could also lead to the disappearance of SAV. Mechanistically, we expect that removing top predators would result in the following sequence of events: 1) increased small fish densities, with a subsequent decrease in their prey (i.e. epibenthic grazers such as amphipods and snails); 2) increased fouling on SAV after decreases in grazer populations; and 3) loss of macrophytes due to overgrowth by algal epiphytes. Therefore, the predicted effects of eliminating top consumers are identical to those of eutrophication: namely, a shift from a system dominated by rooted macrophytes to a plankton-dominated system. This "top down" alternative to the "bottom up" nutrient enrichment hypothesis could account for reductions in SAV biomass in heavily fished areas, but to date remains untested.

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