Water regimes and littoral plants in four weir pools of the River Murray, Australia

Abstract

The composition and distribution of littoral vegetation in four weir pools of the lower Murray were surveyed in summer 1994. Between-weir gradients in the amplitude of water level fluctuations were reflected in the typical distributions of plants, with a 4-6 m elevational range in upper-pool sites, where levels fluctuate most, and a 1-1.5 m band in the lower-pool sites, where levels are more stable. Forty-one of 48 species occurred across much of the longitudinal × elevational site matrix within this cone-shaped distribution, indicating considerable tolerance to flooding and exposure; this was especially apparent for Phragmites australis, Cyperus spp. and Centipeda spp. The 41 species were represented in seven of nine water-regime groups identified by cluster analysis. The remainder, found within ± 1 m of the water surface in lower-pool reaches, were aquatic macrophytes such as Vallisneria amcricana and Typha spp. and amphibious 'mudmats' such as Glossostigma elatinoides. Water regimes at given sites were measured by the number of days in 2 years flooded to any depth ( > 0 cm), or to 0-30 cm, and by days exposed by > 100 cm. Inter-pool differences in the median number of days flooded to > 0 cm and 0-30 cm were 3-30% and < 8%, respectively, for all species except Typha spp. but an order of magnitude for the number of days exposed by > 100 cm. However, eight of 14 common or representative species analysed showed significant inter-pool differences in the number of days flooded to > 0 cm, indicating that sufficient variation exists to necessitate considerable intra-pool replication to allow for the detection of statistical differences in a multi-pool experiment. The practice of maintaining stable weir pool levels limits vegetation processes, e.g. germination, recruitment, decomposition. An increase in the amplitude of river level fluctuations during low flows, from the current 10-20 cm range to 20-50 cm, would reinstate water regimes suitable to the majority of species surveyed. Copyright © 2000 John Wiley & Sons, Ltd.