Response of the biomass and the C:N:P composition of the first trophic level of a Pampean stream to the highest drought in the last 20 years: a case study

Abstract

Benthos, epiphyton, seston, free-floating filamentous algae and aquatic plants provide food and shelter to the rest of trophic levels of the stream. Here we studied the effect of an extreme drought on the biomass and elemental composition of La Choza stream in the Pampas by comparing a wet (2007, ~1200 mm/yr) and a dry year (2008, ~600mm or 40% less than the long term mean). Given that the abundance of these communities depends mainly on the mass transfer in the boundary layer at low speeds and the drift downstream, we postulate (H1) that a dry year creates unfavorable conditions for aquatic plants and seston but favors free-floating, epiphytic and benthic filamentous algae. As Pampean streams are rich in nutrients, we also postulate (H2) that there are no significant inter-annual changes in the elemental composition of biomass. Low flow rates of 2008 resulted in a higher conductivity, pH and available light, without appreciable changes in the typical high nutrient concentrations. The drought resulted in lower discharges (<50 L/s) and velocities (<9 cm/s), accompanied by declines in the amount of plants (2±0.3 g/m²) and seston (20±8 g/m²) and increases in free-floating but not in filamentous algae, partial supporting H1. Nutrient composition of the biomass did not change as proposed by H2, being highly variable in the case of seston and periphyton. The impact of a full-year drought on the Wet Pampa could affect animal communities as plants and seston biomass decreases in favor of increase of free-floating algae, compromising food and shelter.