Spatial variation in the distribution and abundance of submersed macrophytes in an Australian subtropical river Stephen J. Mackay a,∗ , Angela H. Arthington a,b , Mark J. Kennard a , Bradley J. Pusey b a Cooperative Research Centre for Freshwater Ecology, Centre for Freshwater Ecology, Faculty of Environmental Sciences, Griffith University, 170 Kessels Road Nathan 4111 Qld, Australia b Cooperative Research Centre for Tropical Rainforest Ecology and Management, Centre for Riverine Landscapes, Faculty of Environmental Sciences, Griffith University, 170 Kessels Road Nathan 4111 Qld, Australia Received 24 September 2001; received in revised form 23 April 2003; accepted 23 June 2003 Abstract Spatial variation in the distribution and abundance of submersed macrophytes in the Mary River, a subtropical Australian river, was examined at 29 sites on four occasions (116 samples) over a 1 year period. Thirteen submersed macrophyte taxa representing seven families were recorded during the study period. Submersed macrophyte cover was generally patchy and mean quadrat cover per sample was below 7% for every recorded taxon. Classification and ordination identified four distinct groups characterised by differences in submersed macrophyte abundance and associated environmental variables. Three of the four groups were characterised by different abundances of three core taxa, Myriophyllum verrucosum, Vallisneria nana and Potamogeton crispus. The distribution of the four sample groups within the Mary River catchment was associated with two environmental gradients, the first gradient representing discharge intensity, discharge variability and total Kjeldahl nitrogen (TKN) concentration and the second gradient representing discharge intensity, substrate composition, riparian canopy cover and total phosphorus (TP) concentration. Both environmental gradients were constrained by geomorphology at the catchment as well as the reach scale. Our findings are consistent with a general conceptual model that highlights the importance of major environmental gradients in structuring submersed macrophyte assemblages. © 2003 Elsevier B.V. All rights reserved. Keywords: Submersed macrophytes; Distribution; Abundance; Spatial variation; Environmental gradients; Conceptual model ∗ Corresponding author. Tel.: +61-7-3875-7101; fax: +61-7-3875-7615. E-mail address: s.mackay@mailbox.gu.edu.au (S.J. Mackay). 0304-3770/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-3770(03)00103-7