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

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 di stributions 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, wh ere levels are more stable. Forty-one of 48 species occurred across much of the longitudinal x elevational site matrix within this cone-shaped distrib ution, indicating considerable tolerance to flooding and exposure; this was especially apparent for Phragmites australis, Cyperus spp. and Centipeda s pp. The 41 species were represented in seven of nine water-regime groups id entified by cluster analysis. The remainder, found within +/- 1 m of the wa ter surface in lower-pool reaches, were aquatic macrophytes such as Vallisn eria americana and Typha spp, and amphibious 'mudmats' such as Glossostigma elatinoides. Water regimes at given sites were measured by the number of d ays in 2 years flooded to any depth (> 0 cm), or to 0-30 cm, and by days ex posed by > 100 cm. Inter-pool differences in the median number of days floo ded to > 0 cm and 0-30 cm were 3-30% and < 8%, respectively, for all specie s except Typha spp. but an order of magnitude for the number of days expose d by > 100 cm. However, eight of 14 common or representative species analys ed showed significant inter-pool differences in the number of days flooded to > 0 cm, indicating that sufficient variation exists to necessitate consi derable intra-pool replication to allow for the detection of statistical di fferences in a multi-pool experiment. The practice of maintaining stable we ir pool levels limits vegetation processes, e.g. germination, recruitment, decomposition. An increase in the amplitude of river level fluctuations dur ing 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 (C) 2000 John Wiley & Sons, Ltd.