The response of Cladocera assemblages and size structure to multiple stressors in three Kawartha lakes (Ontario) over the last 200 years)
The Kawartha lakes region has experienced many impacts from human activities since the European settlement in early 1800s including damming, logging, agricultural activities, fisheries, urbanization, and the introduction of exotic species. The temperature has also increased with regional warming by over a degree Celsius. In this study I used the cores collected from a previous paleolimnological study that sought to understand how primary production changed in these lakes over the last ~200 years in three Kawartha region lakes (Cameron, Pigeon and Stoney lakes) using changes in diatoms and subfossil pigments. The previous study concluded that all three lakes experienced cultural eutrophication, beginning in the early 1800s, with only Cameron Lake potentially experiencing a less nutrient-rich conditions since c. 1940 according to the diatoms. This study assessed the cumulative impact of the multiple stressors on subfossil cladoceran over the last ~200 years by analysing the Cladocera assemblages and the changes in the size structure of Bosmina and Daphnia species. Major restructuring of cladoceran assemblages occurred c. 1830 in all three lakes with increases in the abundance of Chydorus brevilabris, suggesting that damming in c. 1830 was an important factor. Pigeon Lake remained stable in cladoceran assemblages after damming while Cameron and Stoney lakes exhibited increases of ~20-30% abundance of pelagic Daphnia spp. c. 1940-50s. The inconsistency in the timing of Daphnia spp. rising among lakes indicates potential interactions between environmental changes related to climate change, eutrophication, and other anthropogenic stressors. A recent return of Cladocera composition to pre-industrial conditions was seen in Cameron Lake. Changes in size of the dominant Bosmina and Daphnia spp. showed little consistency among lakes. These results support previous findings that nutrient enrichment related to multiple factors has resulted in large and significant changes to Kawartha region lakes. This information provides the context for setting realistic restoration goals.