Hydrobiologia 489: 63–70, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 63 The interaction of water flow and nutrients on aquatic plant growth Mark N. Crossley 1 , William C. Dennison 2 , Richard R. Williams 1 & Alan H. Wearing 1 1 School of Agriculture and Horticulture, University of Queensland, Gatton, Qld 4343, Australia Tel: +07-54601183. Fax: +07-54601455. E-mail: mark.crossley@mailbox.uq.edu.au 2 Marine Botany Group, Department of Botany, University of Queensland, Brisbane, Qld 4072, Australia Received 1 February 2002; in revised form 2 October 2002; accepted 21 October 2002 Key words: Aponogeton, elongatus, flowing water, nutrition interaction Abstract A long-term experiment was conducted to compare the effects of flowing and still water on growth, and the relationship between water flow and nutrients, in Aponogeton elongatus, a submerged aquatic macrophyte. A. elongatus plants were grown for 23 weeks with three levels of nutrition (0, 0.5 and 1g Osmocote Plus  fertiliser pot -1 ) in aquaria containing stirred or unstirred water. Fertilized plants grew much better than non-fertilized. The highest fertilizer level produced 29% wider leaves and 58% higher total dry weight in stirred water. Stirred water increased leaf area by 40% and tuber size by 81%, but only with the highest level of nutrition. These results suggest that this plant depends on its roots for mineral uptake, rather than from the open water, and the major limitation to growth in still water is the supply of dissolved inorganic carbon. It was the combined effects of nutrient availability and stirring that produced the strongest response in plant growth, morphology and composition. This study provides some explanation for the observations of others that these plants grow best in creeks or river systems with permanently flowing water. Introduction The Australian native aquatic macrophyte Aponogeton elongatus F. Muell. ex Benth, grows predominantly submerged in still or gently flowing water in streams, creeks and rivers (Aston, 1973) although it also has been commonly observed in fast flowing waters (Sainty & Jacobs, 1994; Hellquist & Jacobs, 1998). It is found only in permanent fresh water, rooted in mud or silt (Aston, 1973; Sainty & Jacobs, 1981, 1994) or in mixed sediment of silt, sand and gravel. A. elong- atus is often found growing in water heavily shaded by overhanging trees as well as in full sun (Sainty & Jacobs, 1981; Hellquist & Jacobs, 1998). It pro- duces a crown of leaves from a tuber buried in the sediment. Most of the leaves remain submerged but some floating leaves are produced, usually when the plant is flowering (Sainty & Jacobs, 1981, 1994). Little is known about the physiology of A. elong- atus, particularly in relation to its growth habit, re- production and response to the environment but there have been some morphological and taxonomic stud- ies (Van-Bruggen, 1969; Aston, 1973; Hellquist & Jacobs, 1998). In other submerged aquatic macrophytes (SAM), water flow from 1.5–100 cm s -1 , depending on the species, increased net photosynthetic rate (Schwenke, 1971; Carpenter et al., 1991; Chambers et al., 1991; Losse & Wetzel, 1993). However, many of these stud- ies have been based on short-term measures of photo- synthetic rate or field measurements and observations. The significance of this study is that it measures long term growth rates of plants under controlled conditions in aquaria. Such long-term growth rates may reveal differences that are difficult to detect with short-term measures of photosynthetic rate (Titus et al., 1990). It is known that for SAM in general, moderate wa- ter flow improves leaf uptake of nutrients as well as dissolved inorganic carbon (DIC), and oxygen (Smith & Walker, 1980; Larkum et al., 1989; Stevens & Hurd, 1997). It is also well recognised that flowing water reduces the thickness of the unstirred (boundary) layer