Widespread 20th-century increases in Canadian lake primary production and the roles of climate warming, solar irradiance and human impacts

Lakes provide many vital ecosystem services but are highly sensitive to human impacts and climate change. While data needed to assess their long-term trajectories are generally unavailable, lake responses to such perturbations can be inferred from sediment archives, including records of chlorophyll, a proxy for primary production. However, few if any datasets exist to test hypotheses at large spatial scales about the drivers of long-term change. Here, we apply rapid hyperspectral imaging to produce high-resolution records of sediment chlorophyll (defined as the sum of chlorophyll a and b and their degradation products) in dated cores from 80 lakes across Canada and identify an overall increase since the mid-19th-century. Breakpoint analysis detected a shift whereby the average rate of chlorophyll increase was seven times greater after 1966 than before. We then compared spatiotemporal chlorophyll trends to climate variables, solar irradiance, and land-use data to assess their relationships to past changes in primary production. We identified robust correlations between chlorophyll and climate variables (temperature and ice cover) that were especially strong after 1966, suggesting continental-scale alterations to lake primary production in response to recent accelerated warming. We also detected significant associations with incident solar radiation and catchment human impacts, and show that both had the potential to influence past primary production at the continental scale. Strong correlation between increased chlorophyll and climate variables after 1966 suggests continental-scale alterations to lake primary production due to accelerated warming, solar irradiance, and human impacts, according to high-resolution imaging of dated cores from 80 lakes across Canada.