Integrating ecosystem metabolism and consumer allochthony reveals nonlinear drivers in lake organic matter processing

Lakes process both terrestrial and aquatic organic matter, and the
relative contribution from each source is often measured via ecosystem
metabolism and terrestrial resource use in the food web (i.e., consumer
allochthony). Yet, ecosystem metabolism and consumer allochthony are
rarely considered together, despite possible interactions and potential
for them to respond to the same lake characteristics. In this study, we
compiled global datasets of lake gross primary production (GPP), ecosystem
respiration (ER), and zooplankton allochthony to compare the strength and
shape of relationships with physicochemical characteristics across a broad
set of lakes. GPP was positively related to total phosphorus (TP) in lakes
with intermediate TP concentrations (11 - 75 μg L-1) and was highest in
lakes with intermediate dissolved organic carbon (DOC) concentrations.
While ER and GPP were strongly positively correlated, decoupling occurred
at high DOC concentrations. Lastly, allochthony had a unimodal
relationship with TP and related variably to DOC. By integrating
metabolism and allochthony, we identified similar change points in GPP and
zooplankton allochthony at intermediate DOC (4.5 - 10 mg L-1) and TP (8 -
20 μg L-1) concentrations, indicating that allochthony and GPP may be
coupled and inversely related. The ratio of DOC : nutrients also helped to
identify conditions where lake organic matter processing responded more to
autochthonous or allochthonous organic matter sources. As lakes globally
face eutrophication and browning, predicting how lake organic matter
processing will respond requires an updated paradigm that incorporates
nonlinear dynamics and interactions.