Zooplankton Community and Trophic State in Lake Chapala
Synopsis
Lake Chapala is the largest lake in Mexico, it supplies water for the city of Guadalajara and suffers a serious eutrophication problem. In the last 40 years, many indices related to trophic state, based on zooplankton abundance, have been developed for monitoring programs. In this study, we compared different trophic state indices (TSI) based on rotifers, cladocerans, and copepods at the end of the cold season during March 2023 at Lake Chapala, Mexico. We selected four sampling sites where limnological variables such as temperature, dissolved oxygen, pH, conductivity, depth, Secchi transparency, and zooplankton samples were obtained. The temperature was 25.6±1.6°C, dissolved oxygen 7.3±0.4 mg/l, oxygen saturation around 90 %, pH 9.1±0.06, conductivity 1012±5.3 μS/cm with a depth of 4.23±0.13 m and Secchi transparency of 0.46 m. The zooplankton community was composed of 19 species; 10 rotifers, 7 cladocerans, and 2 copepods. The dominant species were both copepods as well as Ceriodaphnia spp., Diaphanosoma cf. birgei, Chydorus brevilabris, Filinia opoliensis, F. longiseta, and Horaella thomassoni. Around 75 % of the total zooplankton abundance was represented by copepods, 13 % by rotifers, and 12 % by cladocerans. Carlson TSI based on Secchi transparency and TSIROT (BAC) indicate hypertrophic conditions, while TSIROT based on rotifer abundance results in mesotrophic conditions; for most TSI based on crustacean density, the eutrophic state was determined. The implications of calculations of TSI for zooplankton developed in temperate conditions may not completely match with observed conditions for Lake Chapala where heterotrophic bacteria and cyanobacteria dominance play a key factor in the zooplankton community structuration. Formulation of indices in tropical conditions is important for monitoring water assessment.
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