VEGETATION-ENVIRONMENT RELATIONSHIPS IN IRRIGATION CHANNEL SYSTEMS OFSOUTHERN ARGENTINA

Using a multivariate approach, relationships were determined between f unctionally defined groups of aquatic plants and the disturbance and s tress pressures associated with ambient conditions and management regi me in artificial channel systems of two irrigation districts located i n the lower valleys of the Rio Colorado and Rio Negro, Argentina. Cano nical Correspondence Analysis (CCA) of 1993-1994 survey data showed th at the distribution and abundance of aquatic vegetation was closely re lated to water conductivity, winter drought, water depth, water moveme nt and management regime. Indices of disturbance and stress were const ructed fr om combined environmental data for each site. CCA showed tha t each variable (disturbance or stress) explained about half of the co nstrained variation. Four functional plant groups were identified usin g cluster analyses, occurring in different sets of disturbance and str ess conditions. Two groups (including Charophyta, Potamogeton pectinat us and Potamogeton pusillus) were well adapted to disturbance toleranc e, especially from weed control procedures, bur vulnerable to higher s aline-stress pressures. Ruppia maritima, Zannichellia palustris and fi lamentous algae characterised a third group found in shallow, high-sal inity, permanently filled channels. All plants of the fourth group (Po tamogeton illinoensis, Elodea callitrichoides, Ceratophyllum demersum, Myriophyllum aquaticum and the floating Azolla filiculoides) are comp etitive species, which occurred in channels with low environmental str ess conditions (low water conductivity and high light availability), b ut with component species differing in their tolerance of disturbance. (C) 1998 Elsevier Science B.V.