Implications of freshwater ‘browning’ on zooplankton: Using Super Hume as an artificial source of dissolved organic carbon for experimentation

Background/Question/Methods

In recent decades, freshwater ecosystems in the Northern Hemisphere have increased in dissolved organic carbon (DOC) concentration, and consequently in brown color, due to increased exports of DOC from land to water.  Environmental factors driving the ‘browning’ of fresh waters are currently under intense investigation.  However, less is known about the direct and indirect effects browning will have on aquatic community structure.  One of the challenges to studying browning is identifying a feasible source of organic carbon in which to experimentally manipulate DOC concentration.  Super Hume, a liquid humic acid concentrate produced by UAS of America, has been suggested as a potential artificial source of DOC.  We used Super Hume to test the effects of increasing DOC concentration on zooplankton, which serve as important prey for planktivorous fish and grazers of phytoplankton.  Zooplankton from a local reservoir were placed in replicate 19 L tanks of Super Hume ranging from approximately 2- 15 mg/L DOC.  Equivalent amounts of algae and nutrients were also added to each tank to support zooplankton growth. 

Results/Conclusions

Over a 6-week period, zooplankton density decreased in all tanks, particularly in the highest DOC (i.e., Super Hume) treatment. Zooplankton community composition also shifted from predominantly copepod-dominated to cladoceran-dominated at all DOC concentrations.  Rotifer densities remained close to detection limits throughout the experiment in all DOC treatments. Interestingly, the guts of cladocera species, but not copepods, were often dark brown in color, suggesting their consumption of flocculent organic matter.  These experimental results using Super Hume suggest that increasing DOC concentration reduces zooplankton density and shifts zooplankton community composition, a trend supported by studies in the primary literature.  Thus, Super Hume appears to be a viable artificial source of organic carbon to experimentally examine the ecological implications of freshwater browning. However, we are further investigating the cause of the overall reduction in zooplankton densities across all treatments.