Comparing the correlation between DOC and cladoceran assemblage trends in reference lakes from northwest Ontario and the Adirondacks (NY, USA): why are there contrasting cladoceran assemblage shifts between northwest Ontario and the Adirondacks?

Zooplankton assemblages vary based on a variety of environmental factors. Cladocera are planktonic freshwater crustaceans whose clades prefer unique environmental conditions, allowing cladoceran assemblage shifts in reference systems to often reflect climate change. Despite similar atmospheric warming, however, reference lakes in the Adirondacks (NY, USA) and the Experimental Lakes Area (ELA) in northwestern Ontario exhibit contrasting cladoceran assemblage shifts since preindustrial time. Specifically, large-bodied daphnids increased in relative abundance in ELA but decreased in the Adirondacks. As this difference suggests opposing size selection, we hypothesised that contrasting change in visual predation of large-bodied cladocera by planktivorous fish due to different change in dissolved organic carbon (DOC) concentration, therefore water transparency, could explain the shifts. Prolonged sulphate deposition in the Adirondacks likely caused larger decreases in DOC than did the minimal deposition in ELA, meaning the Adirondacks needs more time to recover to or beyond preindustrial levels. Our study investigates the change in DOC from preindustrial to modern time in reference lakes from the Adirondacks and ELA. We hypothesised that DOC would have increased in ELA but decreased in the Adirondacks, and the magnitude of the change would be greater in the Adirondacks. Following a top-bottom approach, we quantified the DOC in preindustrial and modern lake sediments whose cladoceran assemblages were previously analysed. Absorption spectra were recorded for each sample using near-infrared spectroscopy and DOC was spectrally-inferred using a partial least squares regression (PLSR) calibration model. Our preliminary results demonstrate DOC increases since preindustrial time in both regions, with the Adirondacks exhibiting a greater mean increase. Further analysis will compare the DOC increase of each region to the changes in cladoceran assemblage to assess the role of DOC in the assemblage shifts. Other environmental variables will also be analysed to investigate the relationship between regional climate change and cladoceran assemblage shifts.