The effects of terrestrial dissolved organic matter on phytoplankton biomass and productivity in boreal lakes

Allochthonous dissolved organic matter (DOM) structures many physical, chemical, and biological properties of lakes including key variables that control productivity at the base of freshwater food webs. A growing number of studies have documented increasing DOM concentrations within lakes across Europe and North America, including boreal lakes. Such increases are associated with the recovery of catchment soils from acid rain and rising precipitation linked to climate change.We examined phytoplankton biomass, productivity, and their drivers across eight pristine boreal lakes with DOM ranging from 3.5 to 9.5 mg dissolved organic carbon/L. Physical and chemical properties were assessed using standard limnological methods. Phytoplankton biomass was assessed using both chlorophyll a (Chl-a) and via microscopy. Phytoplankton productivity was assessed using change in partial pressure of carbon dioxide within in vitro incubations.Increases in DOM were associated with significant increases in epilimnetic nitrogen, phosphorus and Chl-a concentrations suggesting that nutrients associated with DOM stimulated phytoplankton biomass and productivity. Such results were misleading; there was no significant relationship between Chl-a and phytoplankton biomass. Chl-a:biomass and Chl-a:carbon ratios indicated that increases in Chl-a with DOM were driven by photo-acclimation to declining light availability. Further, results presented as epilimnetic concentrations would not account for concurrent declines in thermocline depth or euphotic depth driven by DOM.Increases in DOM led to large declines in thermocline (c. 50%) and euphotic (c. 75%) depths, and depth-integrated phytoplankton biomass (c. 70%) and primary production (c. 70%).Our results indicate that DOM plays a structuring role for key physical, chemical, and biological properties of lakes, including productivity, at a landscape level. Further, practitioners should be cautious when using Chl-a as an indicator of phytoplankton biomass in studies using DOM or water clarity gradients, report results using depth integrated units due to changes in volumes of thermal layers induced by DOM, and incorporate estimates of sub-epilimnetic productivity when evaluating DOM effects.