Calcium (Ca) concentrations are decreasing in softwater lakes across eastern North America and western Europe. Using long-term contemporary and palaeo-environmental field data, we show that this is precipitating a dramatic change in Canadian lakes: the replacement of previously dominant pelagic herbivores (Ca-rich Daphnia species) by Holopedium glacialis, a jelly-clad, Ca-poor competitor. In some lakes, this transformation is being facilitated by increases in macro-invertebrate predation, both from native (Chaoborus spp.) and introduced (Bythotrephes longimanus) zooplanktivores, to which Holopedium, with its jelly coat, is relatively invulnerable. Greater representation by Holopedium within cladoceran zooplankton communities will reduce nutrient transfer through food webs, given their lower phosphorus content relative to daphniids, and greater absolute abundances may pose long-term problems to water users. The dominance of jelly-clad zooplankton will likely persist while lakewater Ca levels remain low.
Holopedium Survey Data
Holopedium abundance data from the Ontario paleolimnological survey, the Nova Scotia paleolimnogical survey and the two (1980s and 2000s) Ontario limnological surveys.
Longitudinal data from 8 Ontario lakes
We tested the mechanisms underlying increases in the relative and absolute abundance of Holopedium glacialis in the planktonic cladoceran assemblage of 8 central-Ontario, Canada lakes that were sampled monthly for 30 years. Zooplankton were sampled during the ice-free season near the point of maximum depth by using a metered, 76 μm mesh, tow net. The net was deployed in 4 to 7 vertical hauls and its contents were combined to produce a composite that corrected for the diminution of lake stratum volume with depth. A minimum of 250 crustacean zooplankton were enumerated in each composite, subsampling to ensure that no taxon comprised more than 10% of the total count. Phytoplankton were sampled through the euphotic zone, and water chemistry samples were volume-weighted composites of all depths during isothermal periods, and of the mixed layers during periods of vernal and autumnal stratification. Dissolved oxygen profiles were generated at 1-2 m intervals from all depths. Finally, abundances of the zooplanktivore, Chaoborus spp. were quantified in samples from night-time vertical hauls at 10 stations visited during the autumn in all lakes in 1986 and in one lake from 1987-1994. In the laboratory, a minimum of 300 cells, colonies, or filaments of phytoplankton were counted within the Bacillariophyceae, Chlorophyceae, Cryptophyceae, Dinophyceae, and Euglenophyceae. Dimensions of all counted algae were measured, and summed to express values as an "edible" phytoplankton biovolume.
Longitudinal Data.csv
