Aquatic macrophytes as indicators of catchment land-use and water quality in Wet Tropics streams

Abstract

Macrophyte assemblage structure in undisturbed streams and rivers varies naturally across gradients of resource availability and hydraulic/hydrologic disturbance (Biggs 1996). Forested headwater streams are generally characterised by low resource (nutrient and/or light) availability. These streams are devoid of macrophytes or are colonised by non-vascular macrophytes such as mosses and liverworts (Westlake and Dawson 1975; Sheath et al. 1986; Howard-Williams et al. 1987; Everitt and Burkholder 1991; Dawson et al. 1999; Scarlett and O’Hare 2006). Mosses and liverworts commonly have low light compensation points (the point at which photosynthesis balances respiration and net CO2 exchange is zero) and are morphologically suited to the relatively high hydraulic stresses (high stream gradients, high water velocities and coarse substrata) that occur in headwater streams (Biggs 1996; Suren et al. 2000). With increasing distance downstream, resource availability increases, streambed slopes decrease, stream substrata become finer and vascular macrophytes dominate (Holmes and Whitton 1977; Dawson 1988; French and Chambers 1996). In addition, a greater diversity of growth forms may occur when compared with forested headwater streams (Baattrup-Pedersen et al. 2006). Whilst greater resource availability may promote greater macrophyte growth in middle to lower reaches of streams and rivers, the distribution and abundance of macrophytes throughout any given stream reach may nonetheless vary considerably, reflecting habitat heterogeneity and the occurrence of disturbance events, such as floods and droughts, that periodically remove macrophytes (Bilby 1977; Sand-Jensen and Madsen 1992; Biggs 1996).