Zooplankton (Cladocera and Copepoda) in Cajititlán Lagoon

Abstract

Cladocerans and copepods are the main zooplankton formers in freshwaters. The actual state of pollution faced by the lagoon of Cajititlán restricts the amount of species able to habit this water body, and with it, the functionality of the ecosystem due to their role as phytoplankton consumers and as the principal food source for fishes and other organisms. Until now, there was very little knowledge about these groups in the lagoon, which is because of the lack of specialists in the occidental region of Mexico and the inherent difficulties with respect to the manipulation of these organisms (their size round between 0.2 to 4 mm, besides, manual dissections must be done in order to observe taxonomic characters). In this study, we analyze the composition and abundance of the zooplankton in Cajititlán Lagoon focused in both groups (Cladocera and Copepoda). To achieve this objective, five sites within the lagoon were chosen based on their position with respect to the principal human populations. Samples were collected using a plankton net of 63 μm mesh light with one horizontal tow-in each site monthly. The analyzed period comprehends from May 2015 to April 2016, completing an annual cycle. Each sample was fixed using 4 % formaldehyde. Organisms were first identified the species level and their abundance was calculated with manual counts using a Bogorov chamber. Six species were recorded: Daphnia ambigua and Diaphanosoma birgei (Cladocera) and Arctodiaptomus dorsalis, Leptodiaptomus siciloides, Mesocyclops edax and Neoergasilus japonicus (Copepoda). The most abundant and frequent species was A. dorsalis, followed by M. edax and D. ambigua. Leptodiaptomus siciloides and N. japonicus appeared to be rare species, but L. siciloides was very abundant in March and April. The most abundant species was A. dorsalis, occurring nearly in 77 % of the samples and an average of 5 organisms per liter (up to 60 org/l), while the most frequent was M. edax, with up to 87 % of occurrences and an average of 3 org/l (up to 80 org/l). The general abundance of both groups in this water body was below and the species composition corresponds to the expected in an eutrophicated waterbody.