Zooplankton response to an increase in lake dissolved organic carbon: Results from a whole-lake manipulation experiment
The observed trend of increasing dissolved organic carbon (DOC) concentration in lakes across the northern hemisphere has amplified the importance of understanding how consumer productivity varies with DOC concentrations. Recent evidence from lake surveys suggests that increased DOC may reduce zooplankton productivity in lakes due to impacts of terrestrial organic matter on resource quality and/or a reduction in suitable habitat. Although these spatial lake surveys have provided useful information on how lake food webs may respond to increases in DOC, they may not represent an ideal replacement for determining how specific lake consumers may respond to such changes through time. We used a whole-lake manipulation experiment to observe how zooplankton biomass and production change in response to an increase in DOC in one basin of a separated lake. Zooplankton samples were collected weekly over four years (May-August) via vertical tow, and were counted and measured using imaging software. Biomass estimates were made using length-weight regressions from the literature.
Dissolved organic carbon increased from approximately 7 to 11 mg L-1 in the treatment basin post-manipulation while DOC remained similar throughout the experiment in the reference basin. Zooplankton biomass was greater in the reference basin relative to the treatment pre-manipulation, but greater in the treatment basin post-manipulation (p = 0.003). Additionally, there were also significant differences in gross primary production (GPP), mixed layer depth (Zmix), and resource quality (seston carbon-to-phosphorus; C:P) between basins post-manipulation. Multiple model comparisons suggest resource quality may have been a significant factor contributing to greater relative zooplankton biomass. Lower C:P corresponding to increases in DOC may have been due to changes in the light-to-nutrient ratio of the treatment basin, with lower light from higher DOC and increased total phosphorus concentrations. These results suggest temporal changes in lake characteristics due to increased DOC may produce a response of consumer productivity that differs from previously observed patterns, and the impact of projected global increases in lake DOC concentrations on zooplankton production may be non-linear and context-dependent.