Causes and consequences of soil carbon mobilization and lake brownification in northern forested landscapes

Global changes are fundamentally changing terrestrial and aquatic ecosystems. Some of the highest rates of global change are in the north, where they are leading to the faster destabilization of forest soil carbon and its mobilization as dissolved organic matter and causing the brownification of lakes. This thesis investigated the causes of soil carbon destabilization and consequences of the mobilized soil carbon to lake food webs. The first finding was that global change increased carbon export from catchments. Increased temperatures and changes in hydrologic connectivity interacted with catchment topography to modulate the timing, magnitude, and fate of soil carbon export. Increased temperatures led to hydrologic disconnectivity that favoured export of soil carbon from carbon-rich wetlands to the atmosphere. However, extreme precipitation events saturated the soils and increased the frequency of periods of hydrologic connectivity from the catchment to the drainage network that led to higher export of carbon to streams, rivers, and lakes. The second finding was that increased carbon content in lakes with an associated shift towards more refractory carbon resulted in lower light availability and larger nutrient pools in lakes. Brownification of clear oligotrophic lakes increased pelagic primary productivity, but favoured cyanobacteria that could adapt to the browner conditions. The third finding was that changes in the biomass and composition of phytoplankton communities altered the carbon transfer and efficiency of lake food webs. The brownification-driven shift towards cyanobacteria prevalence was associated with a decline in phytoplankton quality due to cyanobacteria having a lower content of essential fatty acids. The decline in phytoplankton quality did not impact the essential fatty acid content of primary consumers, but it shifted their reliance from essential fatty acids transferred from phytoplankton, to essential fatty acids transferred through the less-efficient bacterial driven microbial loop. As global change proceeds, further destabilization of soil carbon is likely to stop having a stimulatory effect on lake production by alleviating nutrient limitation, to having an inhibitory effect by creating light limitation. Once lakes pass this threshold, the declines in the productivity and transfer of essential fatty acids to higher trophic levels will place food webs at great risk