Does terrestrial carbon affect zooplankton production? A survey of north temperate lakes

Background/Question/Methods

Inputs of terrestrial carbon into lakes are often considered a resource subsidy for aquatic consumer production.  Although there is evidence that suggests terrestrial carbon can be incorporated into the tissues of aquatic consumers, its role in subsidizing consumer production has been debated.  Given that the supply of terrestrially derived organic matter (t-OM) to aquatic systems is on the rise throughout the world, the impact that t-OM load has on zooplankton production is important in determining the overall effect on the aquatic food web as a whole. Our research addresses the role of t-OM as a resource for zooplankton, and how it impacts production.  Our primary objective was to measure zooplankton production in lakes with varying concentrations of t-OM located at the University of Notre Dame Environmental Research Center (UNDERC).  A survey of 10 lakes across a gradient of t-OM supply was conducted over a three-month period.  Zooplankton production was estimated from weekly samples based upon established regression analysis.  In addition, terrestrial resource use (allocthony) was estimated for zooplankton from each lake using carbon, nitrogen, and hydrogen stable isotopes.

Results/Conclusions

  Dissolved organic carbon across all survey lakes ranged from 5 to 26 mg L^-1, and lake color (used as a proxy for t-OM load) ranged from 1 to 20 m^-1.  Annual zooplankton production ranged from 1.2 to 17.7 g m^-2 year^-1.  Zooplankton production showed a strong negative correlation with both DOC (r² = 0.46, p = 0.03) and lake color (r² = 0.59 p < 0.01).  Despite evidence that zooplankton allochthony increases with t-OM supply, our results indicate that terrestrial carbon may not be a sufficient resource subsidy.  Although t-OM may be assimilated by zooplankton in lake systems, it appears to have reduced nutritional quality.  Past research indicates that terrestrial carbon is low in essential fatty acids and stoichiometrically poor in terms of dietary requirements, and therefore the utilization of t-OM as a food resource by aquatic consumers is likely to negatively impact their production.  Consequently, the projected increase in terrestrial carbon loading to aquatic systems may have negative impacts on zooplankton production, and aquatic consumers throughout the food web.