Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences
RESEARCH ARTICLE

Swan grazing on seagrass: abundance but not grazing pressure varies over an annual cycle in a temperate estuary

Gary E. Choney A C , Kathryn McMahon A , Paul S. Lavery A and Neil Collier B
+ Author Affiliations
- Author Affiliations

A School of Natural Sciences & Centre for Marine Ecosystems Research, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia.

B Faculty of Health, Engineering and Science, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia.

C Corresponding author. Email: gchoney@our.ecu.edu.au

Marine and Freshwater Research 65(8) 738-749 https://doi.org/10.1071/MF13126
Submitted: 19 May 2013  Accepted: 9 November 2013   Published: 16 June 2014

Abstract

Herbivorous waterfowl such as black swans are significant grazers in temperate waterbodies; their abundance, distribution and grazing rates vary over seasonal cycles. The present study examined spatial and temporal variation in the abundance and grazing rates of black swans in the Lower Swan River estuary, Western Australia, using visual surveys over 1 year (2009), and potential drivers of this variation, food sources and disturbance factors were assessed. We predicted that swan abundance and grazing pressure would be greater in summer and autumn when seasonal wetlands dry and the abundance of food sources would positively influence their distribution, whereas the level of disturbance would have a negative effect. Plant–grazer interactions are dynamic and complex; the present study revealed new findings on the seasonality of this relationship, where swan abundance but not grazing pressure varied over an annual cycle. Maximum swan abundance occurred in autumn (185) with minima in spring (53) but the swan grazing pressure did not vary between seasons, ranging from 6% to 25% of seagrass production consumed. Swan abundance was a function of season and the cover of seagrass. Key hot spots for swan abundance were identified where management efforts could be targeted by minimising human disturbances and protecting seagrass.

Additional keywords: black swan (Cygnus atratus), Halophila ovalis, Lower Swan River estuary, Western Australia.


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