Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllum nodosum |
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Authors: | Carl Johan Svensson Henrik Pavia Gunilla B Toth |
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Institution: | 1. Department of Marine Ecology, G?teborg University, Box 461, 405 31, G?teborg, Sweden 2. Department of Marine Ecology, Tj?rn? Marine Biological Laboratory, G?teborg University, 452 96, Str?mstad, Sweden
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Abstract: | Plants have different strategies to cope with herbivory, including induction of chemical defences and compensatory growth.
The most favourable strategy for an individual plant may depend on the density at which the plants are growing and on the
availability of nutrients, but this has not been tested previously for marine plant–herbivore interactions. We investigated
the separate and interactive effects of plant density, nutrient availability, and herbivore grazing on the phlorotannin (polyphenolic)
production in the brown seaweed Ascophyllum nodosum. Seaweed plants grown at low or high densities were exposed either to nutrient enrichment, herbivorous littorinid gastropods
(Littorina obtusata), or a combination of nutrients and herbivores in an outdoor mesocosm experiment for 2 weeks. Seaweeds grown at a low density
tended to have higher tissue nitrogen content compared to plants grown at a high density when exposed to elevated nutrient
levels, indicating that there was a density dependent competition for nitrogen. Herbivore grazing induced a higher phlorotannin
content in plants grown under ambient, but not enriched, nutrient levels, indicting either that phlorotannin plasticity is
more costly when nutrients are abundant or that plants responded to herbivory by compensatory growth. However, there were
no significant interactive or main effects of plant density on the seaweed phlorotannin content. The results indicate that
plants in both high and low densities induce chemical defence, and that eutrophication may have indirect effects on marine
plant–herbivore interactions through alterations of plant chemical defence allocation. |
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