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Article << Previous     |         Contents Vol 60(12)

Effects of multiple disturbances in seagrass meadows: shading decreases resilience to grazing

Johan S. Eklöf A C D, Kathryn McMahon B, Paul S. Lavery B

A Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden.
B School of Natural Sciences & Centre for Marine Ecosystems Research, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia.
C Present address: Department of Marine Benthic Ecology and Evolution, Biological Centre, University of Groningen, Kerklaan 30, PO Box 14, 9750 AA Haren, The Netherlands.
D Corresponding author. Email: j.eklof@rug.nl
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Ecosystem shifts are often associated with multiple disturbances, but limited knowledge on the mechanisms involved hampers management. This study investigated how short-term shading affected the resilience of the seagrass Halophila ovalis to grazing by black swans (Cygnus atratus) – a historically dominant grazer currently recovering from hunting and habitat loss – in south-western Australian estuaries, using field surveys and a grazing-shading experiment. Black swans were heterogeneously distributed and, in a high-density site, consumed 23% of seagrass production. Seagrasses recovered rapidly from a single disturbance (i.e. short-term shading or grazing), even though shading alone halved carbohydrate content. When seagrasses were exposed to both disturbances, recovery depended on the type of grazing; where grazing was confined to leaves, leaf densities recovered within 3 weeks, but where grazing was on both leaves and rhizomes there was no recovery. Shading increased the frequency of apex initiation, as did rhizome grazing, but only if the plants had not been shaded. This indicates that shading alters the flow of energy needed to produce apices and leaves following disturbance on rhizomes. Based on the historical swan densities and continuing recurring phytoplankton blooms, management actions reintroducing swans without controlling algal blooms could have an on impact seagrass resilience and associated organisms.

Keywords: black swan, carbohydrates, Cygnus atratus, Halophila ovalis, recovery, Western Australia.

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