Impacts of an irrigation and hydroelectric scheme in a stream with a high rate of diadromy (Guadeloupe, Lesser Antilles): Can downstream alterations affect upstream faunal assemblages?

In order to determine whether downstream alterations may affect upstream fa unal assemblages in streams dominated by diadromous species we sampled deca pods and fish by electrofishing before (1991-1992) and after (1994-1998) th e completion of an irrigation and hydroelectric scheme on the Grand-Carbet River, a typical fast-flowing stream of Guadeloupe, Lesser Antilles. Water abstraction occurred at a low-head dam (i.e. valley dam) at 230 m a.s.l. We investigated three study sites downstream of the dam at 15, 100 and 200 me tres a.s.l. (respectively Sites 1 to 3) and two sites upstream at 436 and 5 40 metres a.s.l. (respectively Sites 4 and 5). The completion of the irriga tion and hydroelectric scheme reduced the proportions of large-bodied carni vorous fish and decapod species at both downstream altered and upstream una ltered sites. Inversely the proportions of the small-bodied species signifi cantly increased. At downstream sites the faunal assemblages (guild structu re and taxon diversity) were more impacted at Site 2 where water depth decr eased most (-25 cm) than at Site 3 where water depth was least changed (-10 cm). An intermediate response was recorded at Site 1. At upstream sites th e faunal assemblages were more modified at Site 4, i.e. the site closest to the dam, than at Site 5, i.e. the highest site. Faunal changes were more o r less marked among sites, probably in relation to the degree of physical c hanges at downstream sites, and to distances from the dam at upstream sites .