The effect of impoundments on the structure and function of fish fauna in a highly regulated dry tropics estuary

Ross River flows through the Townsville/Thuringowa urban area in north Queensland, Australia, which has a dry tropical climate characterized by high inter-annual rainfall variation. Unregulated rivers in the Ross catchment basin deliver freshwater flows to their estuaries during both strong and weak wet seasons. The construction of a series of dams and weirs on Ross River means the wet-dry cycle is accentuated, leading to constant marine salinities throughout the estuary becoming the norm, with a lack of freshwater flow for five or more years at a time. The fish fauna of Ross River estuary was sampled in the post wet and dry seasons during an extremely dry climatic period (1994) and extremely wet climatic period (2000) using a small mesh (6 mm) pocket seine net. The fish fauna seemed to reflect seasonal differences. Catches from 1994 (dry period) were comprised entirely of 88 marine and euryhaline species, while the 69 species captured in 2000 (wet period) included 13 freshwater species. However, the freshwater species in the upper estuary were individuals washed over the weir, rather than part of a functional faunal gradient. During 1994 faunal composition was related more to site identity than to the position of the site along an upstream gradient. In contrast, during 2000 there were clear upstream faunal gradients with compositions in upstream sites heavily influenced by freshwater species, and marine and euryhaline species dominating downstream sites. Patterns of species dominance also varied between years. In contrast, trophic composition showed consistent shifts in both years, from high proportions of herbivores, carnivores and benthoplanktivores in May towards high proportions of benthivores in August. Not only do faunal composition, seasonal faunal change and ecological connectivity seem to be impaired, but ecological processes in the estuary that rely on seasonal freshwater flows are likely to be unable to operate normally in most years. The extreme seasonality in Ross River may serve as a model for many of the changes that will be experienced in dry tropics estuaries under global climate change scenarios of more extreme seasonality.