Estudio preliminar de las relaciones tróficas en el plancton de una laguna arreica de la provincia de Buenos Aires: experimentación «in situ»

Abstract

Shallow lakes can present two alternative stable states with or without submerged macrophytes that produce significant structural changes in plankton and nutrient dynamics. The aim of this study is to analyse the changes in the phytoplankton structure related to different conditions of zooplankton predation in an arreic lake. Four cylindrical pvc enclosures of 700 L of capacity were placed in the lake, two of them in free-vegetation waters and the other two in an area dominated by emerged and submerged macrophytes (Scirpus californicus and Potamogeton striatus respectively) with and without predation conditions on the grazer zooplankton. Samples were taken every eight hours for three days during the 2003/2004 summer period. Temperature, pH, dissolved oxygen, conductivity, turbidity and transparency were measured in situ. Water samples were taken to determine the different phosphorus and nitrogen forms and to analyse the phytoplankton. In all treatments, the conductivity oscillated between 1.520 and 1.550 μS.cm -1. Turbidity varied from 48 to 54 NTU and the pH was slightly alkaline (8,48-8,81). Chlorophytes and cyanophytes increased in free-vegetation-water enclosures with fish while no changes were found in the vegetated area. Preliminarily, we can say that the predation effect on zooplankton was more important in the enclosure placed in free-vegetation waters with fish. It was there that the biggest densities of eatable cholorophyte algae (Crucigenia quadrata, Dictyosphaerium pulchellum var. minor, Chlamydomonas sp. and Oocystis parva) were found towards the end of the experiment.