Species composition, abundance and distribution of zooplankton in a tropical eutrophic lake: Lake Catemaco, México

From April 1992 to May 1993, zooplankton samples were collected monthly by means of horizontal tows in nine sites of the lake. Prior to the towing, temperature of surface water, transparency (Secchi), pH and dissolved oxygen were evaluated. A total of 31 zooplankton forms, including 14 species of rotifers, three copepods, five cladocerans and one ostracod, as well as protozoans (mainly vorticellids and ciliates), were detected. Rotifers were the dominant organisms, mainly Brachionus havanaensis (27.6 ind l-1), B. angularis (6.9 ind l-1), Keratella cochlearis (4.9 ind l-1), Conochilus unicornis (10.8 ind l-1) and C. dossuarius (3.1 ind l-1). Within crustaceans, higher densities were shown by larvae (nauplii and copepodites) of calanoid (16.8 ind l-1) and cyclopoid (15.6 ind l-1) copepods, as well as Arctodiaptomus dorsalis (2 ind l-1), Mesocyclops edax (0.5 ind l-1), and the cladocerans Bosmina longirostris (1.6 ind l-1) and Diaphanosoma brachyurum (0.5 ind l-1). Densities were low, probably because of a high predation pressure imposed by fishes. A gradual increase in total zooplankton density related with a progressive diminution of transparency was observed throughout the sampling period. Zooplankton densities in the stations located at the central part of the lake were higher when compared with those at a more peripheral position. Time variation in rotifer's relative abundance was directly related to temperature fluctuations. The low density and diversity values, the small size of the zooplankters, the presence of an important number of indicator species, and the calanoid copepods: other planktonic crustaceans low ratio, are all indicators of eutrophy. Evidences suggest that the eutrophication process of Lake Catemaco is still progressing rapidly. Entre abril de 1992 y mayo de 1993, se realizaron mensualmente recolectas subsuperficiales de zooplancton, con red, en nueve localidades del lago, en donde también se determinaron la temperatura superficial, la visibilidad del disco de Secchi, el pH y la concentración de oxígeno disuelto. Entre las 31 formas de plancton detectadas en las distintas submuestras se registraron 14 especies de rotíferos, tres de copépodos, cinco de cladóceros y un ostrácodo, además de protozoarios, principalmente vorticélidos y ciliados. Los rotíferos fueron los organismos más abundantes; Brachionus havanaensis fue la especie dominante, seguida por Conochilus unicornis, Brachionus angularis, Keratella cochlearis y Conochilus dossuarius. Entre los copépodos destacaron Arctodiaptomus dorsalis y Mesocyclops edax, además de sus naupli