Darkening waters: climate-induced shifts in precipitation and DOM reduced phytoplankton but not zooplankton in dimictic boreal lakes

This study examines the extent to which temporal (1980–2019) increases in precipitation drove dissolved organic matter (DOM) loads and lake concentrations, and evaluates implications to physical, chemical, and biological variables within dimictic boreal lakes at the IISD Experimental Lakes Area, Canada. Long-term increases in precipitation (c. +27%) were associated with increased DOM loads and lake concentrations (c. +9%). Increased DOM within dimictic lakes over time was associated with shallower thermocline depths (c. −17%) and euphotic depths (c. −13%), reduced mean light intensity in the water column (c. −20%), reduced depth-integrated phytoplankton biomass (c. −32%) but increased epilimnetic chlorophyll a concentration (c. + 27%), and we detected no significant temporal declines in crustacean zooplankton. Our results confirm earlier observations that caution should be exercised when using epilimnetic chlorophyll a as an indicator of phytoplankton responses to DOM. However, once corrected for photoadaptation depth-integrated chlorophyll a was a useful indicator of depth-integrated biomass and its response to DOM.