Carbon quality regulates the temperature dependence of aquatic ecosystem respiration

Lakes are undergoing a variety of changes that may alter their role in
the global carbon cycle. Lake temperatures are increasing at the same
time that lakes in many regions are experiencing long-term increases
in dissolved organic carbon (DOC) concentrations. The balance between
rates of ecosystem respiration (ER) and gross primary production (GPP)
is an important determinant of CO2 outgassing from lakes and thus it
is important to understand factors regulating ER rates in these
systems. Temperature is known to regulate rates of ER, but other
factors have the potential to modulate this relationship. One such
regulator may be the quality of the dissolved organic matter (DOM) in
the system. Theory suggests that ER may increase more with temperature
in lakes dominated by allochthonous, refractory material than in
systems dominated by more autochthonous DOM. To test this theory, we
conducted a mesocosm study where half of the mesocosms received water
from a naturally occurring autochthonous DOM source and the other half
from an allochthonous DOM source. We monitored water temperature and
dissolved oxygen concentrations in each mesocosm using in-situ high
frequency DO sensors. After 27 days we used this data to calculate
daily ER for each mesocosm. We then related daily ER to mean nighttime
temperature and tested whether the relationship was different in the
two DOM treatments. Treatments dominated by allochthonous DOM had a
greater temperature dependence of respiration than those dominated by
autochthonous DOM. These results suggest that as lake temperatures
continue to increase, ER will increase more in lakes dominated by
allochthonous DOM than those dominated by autochthonous DOM.