Modern and historical variation in aquatic macrophyte cover of billabongs associated with catchment development

Abstract

Visual estimates of macrophyte cover in Murray and Ovens River billabongs were unrelated to farming and regulation activities, but were positively related to the ratio of Chydoridae to total Cladocera in assemblages of skeletal remains from surface sediments. This ratio (%CHYD) was thus used as a surrogate for macrophyte cover in a palaeoecological analysis. Values of %CHYD were lower in sediments deposited after European settlement than in the pre-settlement parts of sediment cores from three relatively large deep billabongs, and two moderately-sized billabongs, reflecting an historical contraction of the littoral zone in these billabongs. No evidence of a contraction in the littoral macrophyte zone was found in historical records from the two smallest billabongs, although the records were of poorer quality. Visual estimates of macrophyte cover confirmed that macrophytes were generally found in small or shallow billabongs, and that large deep billabongs were macrophyte poor. The timing and spatial pattern of the historical decline in macrophytes implicates an early farming activity as the cause. ‘Control’ billabongs, at present remote from farming centres, have evidence of historical farm impacts which may explain why visual estimates of macrophyte cover and farming are unrelated. Use of the floodplain by farmers may have declined recently, and the historical pattern of macrophyte cover suggests that a naturally stable state of algal dominance has prevented macrophytes from recolonizing large deep billabongs. Evaluations of the role of billabongs in river–floodplain functioning and river–floodplain management strategies must take into account the historical impacts of farming on macrophyte cover. Methods for assessing billabong health that rely on comparisons of affected and non-affected sites (cf. RIVPACS; Wright, 1995) will not work in this region. Attempts to rehabilitate rivers through environmental flows will fail unless steps are taken to remedy farm impacts. Copyright © 2000 John Wiley & Sons, Ltd.