Epiphyte grazing enhances productivity of remnant seagrass patches

Authors

  • MIRELLA P. C. VERHOEVEN,

    Corresponding author
    1. Department of Environmental Sciences, University of Technology Sydney
      Present addresses: Department of Biological Sciences and Climate Risk CoRE, Macquarie University, North Ryde, NSW 2109, Australia, and Sinclair Knight Merz Pty Ltd, PO Box 164, St Leonards, NSW 1590, Australia (Email: mirella.verhoeven@mq.edu.au)
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  • BRENDAN P. KELAHER,

    1. Department of Environmental Sciences, University of Technology Sydney
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  • MELANIE J. BISHOP,

    1. Department of Biological Sciences and Climate Risk CoRE, Macquarie University, Sydney, New South Wales, Australia
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  • PETER J. RALPH

    1. Department of Environmental Sciences, University of Technology Sydney
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Present addresses: Department of Biological Sciences and Climate Risk CoRE, Macquarie University, North Ryde, NSW 2109, Australia, and Sinclair Knight Merz Pty Ltd, PO Box 164, St Leonards, NSW 1590, Australia (Email: mirella.verhoeven@mq.edu.au)

Abstract

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above-ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.

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