Effects of surface runoff from agricultural soils on the succession of microconsumer organisms in a lotic mesocosm

In the present work, we studied the succession of organisms and nutrient dynamics in the laboratory through the use of an artificial lotic system, simulating typical conditions of runoff and soil erosion. We analyzed the effects of the soil at different flooding times on environmental variables, water nutrients, and chlorophyll a concentration and on the succession of the consumer organisms of two communities: the plankton (in the water) and the periphyton (on artificial substrates). Soil samples were collected from a sector of the middle basin of the Salado River (Buenos Aires), an area with intense agricultural activity. The soil samples were removed from a medium topography, where flooding is infrequent. The environmental parameters that varied the most were the temperature and the conductivity, with both increasing over the flooding time; while turbidity reached a maximum value at the beginning, but decreased thereafter. The soluble reactive phosphorus remained at a low concentration throughout the experiment, but nitrates plus nitrites increased over time. The chlorophyll a in the water reached maximum values immediately after flooding, while the artificial-substrate concentration increased with flooding time. Of the evaluated taxa, 24 were recorded in the plankton system and an average of 16 in the artificial substrates. The groups of organisms in both systems were ciliates, amoebas, rotifers, and nematodes, with a predominance mainly of ciliates. In both communities the specific diversity was low and the succession limited. Our results demonstrated that the exposure of the soil to the proposed lotic conditions determined the dynamics of the nutrients, the emergence of soil inocula, and the development and succession of organisms, with the of the same groups of organisms being present throughout the trial.