Long-term changes in precipitation drive DOC and phytoplankton dynamics in oligotrophic boreal lakes

Dissolved organic carbon (DOC) influences a wide range limnological properties and processes, including potentially large effects on productivity at the base of lake food webs.  This study evaluated the effects of DOC on phytoplankton communities and primary production using long-term observations and space-for-time substitutions for a series of unmanipulated lakes at the IISD-Experimental Lakes Area, Canada.  Spatially, elevated DOC corresponded with substantial declines (c. -70%) in phytoplankton biomass and productivity among lakes.  Temporally, long-term changes in precipitation were correlated with increased DOC loads and lake concentrations.  Consistent with spatial surveys, increased DOC within lakes over time was associated with reduced water clarity and large declines (c. -18-30 %) in thermocline and euphotic depths and phytoplankton biomass (c. -40 %) within deeper stratified lakes.  However, within a shallow polymictic lake increased DOC appeared to drive a shift in alternate stable states resulting in large increase (c. +300 %) in phytoplankton biomass.   The use of chlorophyll a (chl) as an indicator of phytoplankton biomass across gradients of DOC and water clarity was highly misleading due to the capacity for algae to alter their pigment concentrations in response to light conditions.  Ratios of chl:carbon and chl:biomass systematically increased with DOC.  Moreover, relying on volumetric units of epilimnetic biomass or productivity was also misleading.  Given that epilimnetic and euphotic zone volumes systematically declined with increases in DOC, employing areal or volume weighted units was more appropriate for evaluating ecosystem-level effects of DOC. 

Primary Presenter: Scott Higgins, IISD Experimental Lakes Area (shiggins@iisd-ela.org)

Authors:

Scott Higgins, IISD Experimental Lakes Area  (shiggins@iisd-ela.org)

Bryanna Sherbo, Fisheries and Oceans Canada  (bsherbo6@gmail.com)

Michael Paterson, IISD Experimental Lakes Area  (mpatterson@iisd-ela.org)

Sonya Havens, IISD Experimental Lakes Area  (shavens@iisd-ela.org)

Patrique Bulloch, IISD Experimental Lakes Area  (pbulloch@iisd-ela.org)