The most abundant zooplankton in Chilean freshwaters are calanoids situation that is different whereas in North American freshwaters, which are dominated daphnid cladocerans. The lower dominance of Daphnid cladocerans in Chilean freshwaters probably is caused by oligotrophy, and lower calcium and dissolved organic carbon concentrations. This hypothesis would be caused because the Daphnid cladocerans require higher chlorophyll concentrations as an energetic resource, and by calcium as structural resource essential for caparace, and dissolved organic carbon as protect against visual predation and ultraviolet radiation penetration in water.
Calanoids copepods domine and lower species richness is was observed in a very wide latitudinal gradient, confirmy the published antecedents in the specialized literature. In sites with fish, the calanoids copepods, had a low relative abundance. In according to experimental and descriptive studies realized in Torres del Paine National Park (51 south latitude) between 1989-1990, it was observed that the zooplanktivorous fishes have an important role as structurators of zooplanktonic assemblage, because, these fishes predate on calanoids copepods, and simultaneously the calanoids copepods have a key role as important grazers.
In sites with and without fishes, a direct association between chlorophyll concentration and relative abundance of Daphnid cladocerans, was observed. Also, a direct association was observed between maximun depth and relative abundance of Daphnid cladocerans, a situation related to protection against ultraviolet radiation and predation. In sites with fishes, similar relations were observed, and also, a direct association was observed between calcium concentration and relative abundance of Daphnid cladocerans. In sites without fishes, a direct association was observed between relative abundance Daphnid cladocerans and chlorophyll concentration, and an inverse relation was observed between relative abundance of Daphnid cladocerans and conductivity.
The main regulating mechanism of chlorophyll concentration was the mixing layer depth, wich was inversely related with chlorophyll concentration. Also, the nitrogen concentration was directly related with chlrophyll concentration, in Araucanian lakes (38 to 41 south latitude), and lakes and ponds in Torres del Paine National Park. A totally different situation was observed in North American lakes and ponds, with similar physical characteristics to Chilean lakes. In North American lakes, the relative abundance of Daphnid cladocerans was directly related with total phosphorous and chlorophyll concentrations, and simultaneously both variables were directly related.
This study confirmed earlier observations in Chilean lakes, showing that Daphnid cladocerans were sucessful in conditions with higher chlorophyll concentration, and with protection against ultraviolet radiation. Furthermore using pigmented zooplankton of fishless shallow ponds, this study showed that Daphnid cladocerans would have lower tolerance to ultraviolet radiation than calanoids copepods. A possible interpretation of these result, is that would be the possibility of generation of toxic substances generates by interaction between ultraviolet radiation and dissolved organic carbon, which cause damage to zooplankton.