Long-term trends in zooplankton of Dorset, Ontario, lakes: the probable interactive effects of changes in pH, total phosphorus, dissolved organic carbon, and predators

Time trends in abundance, body size, species richness, and species composition indicate that crustaceanzooplankton communities of southern Canadian Shield lakes changed between 1980 and 2003. Total abundance did notdecline despite reductions in total phosphorus, but all other metrics changed. Species richness declined in Harp Lake(Ontario, Canada) following its Bythotrephes invasion, but richness increased in three other lakes. Average cladoceranbody length increased from 0.6 to 1.0 mm in seven of the lakes, as larger-bodied taxa replaced smaller ones. Correlationswith water quality and fish metrics suggest that cladoceran size increases were attributable to many factors: adecline in food availability following declining phosphorus levels increasing the competitive advantage of larger herbivores,a decline in acidity favouring the larger, acid-sensitive daphniids, and reduced risk of planktivory linked to a risein dissolved organic carbon levels and changes in predation regimes. Zooplankton communities on the Canadian Shieldare changing, and these changes are best viewed in a multiple-stressor context. Key anthropogenic stressors have alsochanged and may do so again if Ca concentrations continue to decline.