Water chemistry, landscape, and spatial controls of δ13C and δ15N of zooplankton taxa in boreal lakes: One size does not fit all

Carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios of zooplankton are potentially good indicators of energy and nutrient fluxes, and trophic interactions in lake food-webs, yet, are poorly understood. Based on a synoptic survey of 233 boreal lakes, we evaluated the relationships of water chemistry, hydromorphological, and land cover variables (lake/catchment-specific factors), and the spatial position of lakes in the landscape (representing potential regional factors) with the delta C-13 and delta N-15 of nine meso-zooplankton taxa. The delta C-13 variation of most taxa was negatively related to water chemistry variables associated with allochthonous inputs (colour/dissolved organic carbon), and positively correlated with nutrient concentrations (together explaining 12-69% of variation). Most of the delta C-13 variation explained by significant hydromorphological and land cover variables (% peat area, drainage basin area:lake area, shoreline development index, and lake area, explaining 26-47% of variation) was shared by significant water chemistry variables. Together, this suggests that delta C-13 variation of zooplankton reflects different environmental influences on delta C-13 of lake primary producers. The delta N-15 variation of most taxa was significantly related to pH, total phosphorous concentration (water chemistry variables, explaining 8-64% of variation), and water retention time (hydromorphological variable, explaining 13-87% of variation). These relationships are probably reflective of the association of zooplankton delta N-15 with terrestrial organic matter flux, environmentally-induced biogeochemical nitrogen transformations and phytoplankton N-15 fractionation. Specific water chemistry, hydromorphological, and land cover predictors of delta C-13 and delta N-15 and the direction of their effects (i.e. positive or negative) were largely similar among taxa. However, their degree of importance varied among taxa particularly for delta C-13, probably due to contrasting feeding selectivity and resulting differences in allochthony. This suggests that delta C-13 of different taxa may respond differently to changes in limnological gradients due to environmental perturbations affecting boreal regions. The lake/catchment-specific factors were more important than regional factors (lithology, soil properties and atmospheric nitrogen deposition; which are represented by the spatial position of lakes) in explaining delta C-13 and delta N-15 variation of zooplankton. Hence, before inferring regional effects in stable isotope studies, the influence of lake/catchment-specific factors needs to be explicitly quantified.