Biogeomorphic succession in a fluvial-lacustrine delta of the Middle Paraná River (Argentina): Feedbacks between vegetation and morphodynamics

Abstract

The Leyes' Delta lies at the Middle Paraná River, the second-largest fluvial system in South America, and it is being quickly formed into the Setúbal fluvial lake, Argentina. In the context of the Fluvial Biogeomorphic Succession theory, our aim was to better understand interactions between physical and biotic processes contributing to the formation of the Leyes' Delta. We studied the genesis, morphologic development, and vegetation in three groups of its deltaic islands with contrasting formation ages. Different methodologies were applied. Genesis and morphologic development were interpreted from time-series of satellite images and aerial photography obtained from 1974 to 2014. Then landforms and vegetation communities were mapped on the most current images, and they were corroborated and characterized during fieldwork. Our results evidence that the three island groups (Channel crevasse splay islands, Delta bar-plain islands, and Delta front islands) were formed via two different geomorphic processes, determining differences in the initial vegetation colonization. Eight fluvial-lacustrine landforms and eight vegetation communities, both differently represented among the island groups, currently characterize these islands. The species richness accumulation curves and beta diversity showed that the morphodynamic activity can be as important in the floristic composition of the deltaic islands as formation ages. The floristic composition differed statistically among island groups. After identifying three of the four phases of the FBS, we concluded that even though the morphodynamic activity under which islands were formed determined differences in the initial vegetation colonization, under similar subsequent levels of morphodynamics the resulting vegetation heterogeneity tends to be similar even under contrasting genesis. Among the vegetation types, Lotic prairies would play a key role in the morphologic evolution of the deltaic islands, therefore, their distribution and interactions with flow and sediments should be considered during the studies of social ecological systems as the Leyes' Delta, in Santa Fe, Argentina.