Phytoplankton and periphyton interactions in turbid and clear shallow lakes from the Pampean Plain (Argentina)

SÁNCHEZ, MARÍA LAURA; PIZARRO, HAYDÉE; IZAGUIRRE, IRINA

Punta del Este

Congreso; Structure and function of world shallow lakes; 2008

Facultad de Ciencias, Universidad de la República Uruguay

The Pampean Plain is located in Buenos Aires Province, Argentina. This region contains a great number of permanent eutrophic shallow lakes; some are organic-turbid (due to high phytoplankton biomass) and others are clear (with high development of submerged aquatic macrophytes and very low phytoplankton densities). Many researches have been carried out in the framework of the “Alternative Equilibria Theory” (Scheffer et al., 1993) for shallow lakes, mainly focused on the interactions between submerged vegetation and phytoplankton. However, the role of periphyton and its relationship with phytoplankton has been comparatively less explored. The aim of this work was to analyse the competitive interactions by light between phytoplankton and periphyton. We postulated that the existence of these interactions modify the relative contribution of each community to the total algal biomass in shallow lakes with contrasting optical characteristics. An experimental study was carried out in two shallow lakes from the Pampean Plain: El Triunfo (a clear vegetated lake) and El Burro (an organic-turbid lake). Three equal microcosms were placed in each lake. Artificial substrates (polycarbonate strips) previously colonized by local periphyton were placed in the bottom of each microcosm (0.7 m deep).  Microcosms were filled with water from the lake previously filtered through a 55 mm-pore mesh net to exclude zooplankton. The experiment was conducted for one month. Weekly phytoplankton and periphyton biomass was estimated by mean of chlorophyll a concentration and different fractions of periphyton mass were analyzed by mean of dry weight and ash free dry weight. The periphyton was classified according to Lakatos (1989). In addition, physical and chemical variables were measured in each microcosm (pH, conductivity, dissolved oxygen, temperature) as well as the main nutrient concentrations (phosphorus, nitrate and ammonia). In the clear shallow lake (El Triunfo) periphyton biomass increased throughout the experiment in the substrates. Contrarily, phytoplankton chl a remained very low along the time, and even slightly decreased the last week. In this shallow lake chl a values of periphytic community were between 12 and 74 times higher than those of the phytoplankton community. According to the classification of Lakatos, El Triunfo showed a mainly organic periphyton, with ash contents less than 25% and in relation to the percentage of chl a this community was principally autotrophic. Conversely, in the turbid shallow lake (El Burro) phytoplanktonic chl a concentrations were always very high; the temporal pattern was relatively constant, although some variations were associated to environmental changes (storms). Periphytic chl a was low most of the times, exhibiting an increase the last week. In this lake phytoplanktonic chl a was between 7 and 117 times higher than the periphytic chlorophyll a. In relation to the Lakatos` classification periphyton varied among inorganic or organic, but in average the percentage of ashes was higher than in the clear shallow lake. With respect to the chl a percentage the periphyton of this lake was mainly heterotrophic