Diatom responses to environmental changes in the Upper Paraná River floodplain (Brazil) during the last ~ 1000 years

the floodplain of the upper paraná river, brazil, is strongly influenced by hydrology, which in turn affects geomorphological and environmental conditions, and controls the form of islands in the river. such islands develop by deposition of river-borne sediment that creates small lateral sediment bars. geomorphological processes can produce a variety of aquatic environments on such islands, e.g. channels, backwaters, lakes, transitional areas, and swamps. our objective was to test whether subfossil diatoms preserved in the sediment on an island in the upper paraná river floodplain responded to changes in limnological conditions brought about by such geomorphological modifications. we hypothesized that the composition of diatom assemblages in the sediment shifted in response to past geomorphic, and hence limnological conditions. we analyzed diatom subfossils in a 2-m-long sediment core with a calibrated date near the base of 1047–1224 cal yr ad. absence of diatoms at the bottom of the sequence was associated with the channel phase, followed by appearance of diatoms 1229–1381 cal yr ad that were adapted to flow, in the backwater phase. after another 100–200 years, presence of eunotia species in the lake phase suggests a decrease in ph, phosphorus and nitrogen. replacement of eunotia spp. by diadesmis species, following a transition phase, suggests different environmental conditions, with reduced water depth. diatoms in surface deposits are distinct from assemblages in the other phases in the core and contain taxa that suggest a disturbed environment, with variations in water depth and flow. the data illustrate the importance of physical and hydrological factors in shaping diatom communities and show the utility of diatoms as bioindicators in this floodplain environment.