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The planktonic communities composed of rotifers and crustaceans of two tropical karstic lakes in the Yucatan Peninsula, Mexico, with similar origin but different trophic state were analyzed. Both systems, the meso-eutrophic (El Padre) and oligotrophic (Minicenote) were considered as monomictic lakes. The abundance, temporal distribution of species, richness and diversity were measured and discussed in relation to the influence of abiotic factors and the presence of a natural predator. These analyses indicated that these tropical systems were not homogenous for several of these measures at both spatial scales and during the annual cycle analyzed, despite their reduced temperature variation compared to temperate lakes. The results indicated that the dynamics in the zooplankton community were related to the environmental factors, the opportunistic species (with maximum densities related to resource maxima), and the exploitative competition between the dominant herbivorous species. Independently of the trophic condition, the abundance, richness, and diversity were lower during the mixing period. During the annual cycle studied in the meso-eutrophic system, the key environmental parameters related to plankton abundance were pH and temperature whereas in the oligotrophic system, oxygen concentration was important. In both lakes, richness and diversity were related to conductivity, although in the oligotrophic lake the relation was negative whereas in the meso-eutrophic lake it was positive. No relationship was found between the abundance of the studied predator (Chaoborus sp.) and the changes in the abundance, diversity and richness of zooplankton in the systems.

Two new species of the freshwater cyclopoid genera Eucyclops are described, Eucyclops tziscao sp. n. and E. angeli sp. n. Both species belong to the serrulatus-group defined by morphological features such as: the presence of distal spinules or hair-like setae (groups N1 and N2) on frontal surface of antennal basis; the fourth leg coxa with a strong inner spine that bears dense setules on inner side, yet proximally naked (large gap) on outer side; and a 12-segmented antennule with smooth hyaline membrane on the three distalmost segments. Eucyclops tziscao sp. n. is morphologically similar to E. bondi and E. conrowae but differs from these species ina unique combination of characters, including a caudal ramus 4.05±0.25 times as long as wide, lateral seta of Enp3P4 modified as a strong, sclerotized blunt seta, coxal spine of fourth leg with inner spinule-like setules distally, and sixth leg of males bearing a strong and long inner spine 2.3 times longer than median seta. Eucyclops angeli sp. n. can be distinguished by an unique combination of morphological features: the short caudal ramus; the long spine on the sixth antennular segment of A1; the presence of one additional group of spinules (N12’) on the caudal surface of A2; the presence of long setae in females, or short spinules in males on the lateral margin of fourth prosomite; the strong ornamentation of the intercoxal sclerite of P4, specially group I modified as long denticles; the distal modified setae of Exp3P3 and Exp3P4 in females and males; and the short lateral seta of P5. Finally, we report on a new record of E. festivus in México, and add data on morphology of the species.