Latitudinal patterns in the size distribution and seasonal dynamics of newworld, freshwater cladocerans

Analysis of zooplankton communities in over 1,100 water bodies spanning the western Hemisphere (77 degrees S, 81 degrees N latitude) revealed latitudi nal patterns in cladoceran body size and Daphnia seasonal dynamics. The mea n body length of cladoceran species occurring at different latitudinal inte rvals was greatest in north temperate regions, declining in size toward the poles and equator. Thus, mean cladoceran size was greatest in regions wher e the mean annual surface temperatures of lakes ranged from 6 to 8 degrees C in both the Northern and Southern Hemispheres. The date of maximum Daphni a abundance and the period of minimum Daphnia abundance in lakes of North A merica was positively, linearly related to the latitudinal position of the lakes. The date of maximum abundance increased with latitude such that Daph nia peaked during nearly all months of the year depending on latitude, but usually within the same, narrow temperature range (15-20 degrees C). The pe riod of minimum Daphnia abundance decreased linearly with latitude such tha t abundance generally was minimal when water temperatures exceeded 20 degre es C.