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

Abstract

We analyzed the succession of soil organisms and nutrient dynamics under lotic conditions. We simulated typical runoff and erosion conditions in the laboratory using lotic mesocosms. We then analyzed the effects of soil waterlogging under dissimilar times of ooding on environmental variables, water nutrients, the concentration of chlorophyll "a", and the succession of consumer organisms in two systems (one with plankton, the other with arti cial substrates). The soil samples used were collected from a sector of the Salado-River middle basin with agricultural use and a middle topography. The environmental variables that varied the most greatly were the temperature and conductivity, both increasing throughout the ooding time; whereas the turbidity attained a maximum value at the beginning, but decreased thereafter. The soluble reactive phosphorus remained at a low concentration throughout the experiment, but the nitrates plus nitrites increased over time. The chlorophyll "a" in the water passed through maximum values at the beginning, but in the presence of arti cial substrates the concentration increased with progressive ooding. Of the taxa assessed, 24 were recorded in the plankton system with 18 and 14 detected in Zones I and II, respectively, of the arti cial-substrate systems. The groups of organisms registered in both systems were ciliates, amoebas, rotifers, and nematodes with a dominance mainly by the ciliates. In both systems (plankton and substrates), the speci c diversity was low and the succession of the communities limited. The exposure of the soil to surface-runoff erosion along with the effects of current velocity, turbulence, and turbidity determined the nutrient dynamics plus the emergence and succession of organisms.