Zooplankton Community Structure in Arctic Ponds: Shifts Related to Pond Size
A multi-year experiment in which zooplankton species were manipulated in 12 ponds indicated that the presence of the carnivorous copepod Heterocope septentrionalis eliminated Daphnia pulex whenever H. septentrionalis became established. In contrast, the congener D. middendorffiana was able to persist only in ponds where H. septentrionalis became established. The fact that Daphnia middendorffiana could not successfully colonize ponds that did not contain H. septentrionalis indicates that this predator was necessary for D. middendorffiana to thrive. These results suggest that D. pulex is able to outcompete D. middendorffiana when H. septentrionalis is absent. Pond size was a key determinant of colonization success, in that H. septentrionalis and D. middendorffiana were able to persist only in ponds with volumes greater than 10 m3. Results of these investigations support the pattern of zooplankton community composition observed in ponds in the western Nearctic and provide insight into the mechanisms responsible for these observations.