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

Abstract

Time trends in abundance, body size, species richness, and species composition indicate that crustacean zooplankton communities of southern Canadian Shield lakes changed between 1980 and 2003. Total abundance did not decline 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 cladoceran body length increased from 0.6 to 1.0 mm in seven of the lakes, as larger-bodied taxa replaced smaller ones. Correlations with water quality and fish metrics suggest that cladoceran size increases were attributable to many factors: a decline 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 rise in dissolved organic carbon levels and changes in predation regimes. Zooplankton communities on the Canadian Shield are changing, and these changes are best viewed in a multiple-stressor context. Key anthropogenic stressors have also changed and may do so again if Ca concentrations continue to decline.

Linking mean body size of pelagic Cladocera to environmental variables in Precambrian Shield lakes: A paleolimnological approach

“…When nutrient levels are higher, competition pressures decrease and smaller individuals can proliferate. TP levels are currently declining in many Ontario lakes (Dillon & Molot 2005) and, consistent with our observations, a trend towards larger mean cladoceran body size with decreasing TP has been observed for Crosson, Dickie, Harp, and Plastic lakes for the time period of 1980-2003 (Yan et al 2008b).…”

“…For example, it can impair the visual ability of size-selective predators (Brooks & Dodson 1965). This suggests that larger Cladocera species should predominate in high DOC lakes due to decreased fish predation (Yan et al 2008b). However, our study demonstrated an opposing pattern where, on average, larger cladocerans occurred in lakes with lower DOC concentrations.…”

“…In addition to TP, DOC has been shown to be important in shaping cladoceran communities in these study lakes (DeSellas et al 2008;Yan et al 2008b). DOC can influence cladoceran size in a number of ways.…”

“…We explore changes in cladoceran size at a genus or grouplevel, and at a community level. These predictor variables we chose were based on previously identified factors that are important determinants of cladoceran size (Yan et al 2008b). Based on the results of previous limnological studies, we hypothesized that the mean size of cladoceran groups, and the community index, will be higher under low total phosphorus (TP; Brooks & Dodson 1965;Burns & Rigler 1967;Gliwicz 1990), and lower with declining dissolved organic carbon (DOC; Yan et al 2008b).…”

“…These predictor variables we chose were based on previously identified factors that are important determinants of cladoceran size (Yan et al 2008b). Based on the results of previous limnological studies, we hypothesized that the mean size of cladoceran groups, and the community index, will be higher under low total phosphorus (TP; Brooks & Dodson 1965;Burns & Rigler 1967;Gliwicz 1990), and lower with declining dissolved organic carbon (DOC; Yan et al 2008b). Calcium (Ca) is a key structural component of cladoceran carapaces, in particular Daphnia (Jeziorski & Yan 2006), and, as a result, low Ca concentrations in lakewater could potentially display lower mean cladoceran body sizes.…”