Influence of cultural eutrophication, climate, and landscape connectivity on 3 Kawartha lakes (Ontario, Canada) since the early 1800s

Abstract Laird KR, Li S, Gushulak CAC, Moir KE, Wang Y, Leavitt PR, Cumming BF. 2023. Influence of cultural eutrophication, climate, and landscape connectivity on 3 Kawartha lakes (Ontario, Canada) since the early 1800s . Lake Reserv Manage. 39:120–140. Paleolimnological analyses of 3 lakes within the Trent-Severn Waterway (TSW) were examined to evaluate the role of regional land-use practices (forestry and agriculture), climate change, and landscape position on cultural eutrophication and lake response over the past ∼200 yr. The lakes were selected to represent lake position along the chain of lakes of the TSW; these 3 lakes vary in lake-water conditions from a lower nutrient headwater lake (Cameron Lake) to more nutrient-rich sites (Pigeon and Stony lakes) downstream. Diatom-inferred total phosphorus (TP) concentrations estimate that all lakes had TP levels of ∼15 µg/L in the 1700s, which increased to ∼20–30 µg/L in the 1800s and 1900s. Significant changes in diatom and Cladocera assemblages circa the 1830s are consistent with increased water levels and elevated phosphorus conditions associated with the intensification of forest harvest, agriculture, and development of the TSW. Diatom-inferred TP levels varied among lakes since the early 1900s, declining in Cameron Lake, increasing in Pigeon Lake, and stabilizing in Stony Lake, reflective of landscape position, degree of water inflow from the northern forested regions, and responsiveness of basins to point-source nutrient reductions and climate change. Timing of changes in TP, and changes in planktonic community composition varied among lakes. Lake production (as pigments) increased coherently in 2 lakes over the past ∼20 yr, and modern algal composition (diatoms and pigments) are lake specific and unique in comparison to the last ∼200 yr, likely reflecting combined effects of climate change and changes in nutrient loading.