Contributions of sugar cane and invasive pasture grass to the aquatic food web of a tropical lowland stream

Abstract

In-stream ecosystem processes in a tropical lowland stream in far north Queensland were studied by measuring open-system community metabolism and analysing stable isotopes. The stream catchment, like many others in this region, has been extensively cleared for the cultivation of sugar cane, and in the absence of riparian shading, aquatic and semi-aquatic plants choke the stream channel. Stream community metabolism switched between autotrophy and heterotrophy, depending on the degree of cloud cover. Successive cloudy days may be sufficient to result in anoxia in the stream, particularly in the benthos, where limited oxygen penetration into the sediments was recorded. Stable-isotope data indicated that little of the primary production from sugar cane or other C4 plants was transferred into the aquatic food web. The only significant contribution of C4 carbon was to the diets of some larger predatory fish, which must be directly dependent on terrestrial prey. In the absence of significant riparian inputs of C3 carbon, it appears that in-stream primary production supported the aquatic invertebrate community. Restoration of disturbed stream systems such as this one must include the establishment of appropriate riparian species for the reduction of excessive in-stream primary production and the supply of detritus for the maintenance of aquatic food webs.