title: Experimental evidence for enhanced top-down control of freshwater macrophytes with nutrient enrichment.
abstract: The abundance of primary producers is controlled by bottom-up and top-down forces. Despite the fact that there is consensus that the abundance of freshwater macrophytes is strongly influenced by the availability of resources for plant growth, the importance of top-down control by vertebrate consumers is debated, because field studies yield contrasting results. We hypothesized that these bottom-up and top-down forces may interact, and that consumer impact on macrophyte abundance depends on the nutrient status of the water body. To test this hypothesis, experimental ponds with submerged vegetation containing a mixture of species were subjected to a fertilization treatment and we introduced consumers (mallard ducks, for 8 days) on half of the ponds in a full factorial design. Over the whole 66-day experiment fertilized ponds became dominated by Elodea nuttallii and ponds without extra nutrients by Chara globularis. Nutrient addition significantly increased plant N and P concentrations. There was a strong interactive effect of duck presence and pond nutrient status: macrophyte biomass was reduced (by 50%) after the presence of the ducks on fertilized ponds, but not in the unfertilized ponds. We conclude that nutrient availability interacts with top-down control of submerged vegetation. This may be explained by higher plant palatability at higher nutrient levels, either by a higher plant nutrient concentration or by a shift towards dominance of more palatable plant species, resulting in higher consumer pressure. Including nutrient availability may offer a framework to explain part of the contrasting field observations of consumer control of macrophyte abundance.
authors.affiliation: Department of Aquatic Ecology, Netherlands Institute of Ecology, Droevendaalsesteeg 10, 6700 AB, Wageningen, The Netherlands, l.bakker@nioo.knaw.nl.
authors.forename: Elisabeth S
authors.initials: ES
authors.lastname: Bakker
authors.forename: Bart A
authors.initials: BA
authors.lastname: Nolet
mesh_terms: D000818:Animals; D018533:Biomass; D044784:Chara / Q000254:growth & development*; D004372:Ducks / Q000502:physiology*; D005068:Eutrophication; D060434:Herbivory; D031321:Hydrocharitaceae / Q000254:growth & development*; D009426:Netherlands; D061005:Ponds; D011897:Random Allocation; D013997:Time Factors