Changes in ecosystem metabolism and consumer diet in a lake with experimentally darkened water

Background/Question/Methods

A large portion of the total organic matter in many lakes is terrestrial in origin, and lake consumers can be supported by both terrestrial (allochthonous) and aquatic (autochthonous) resources. With the progression of global climate change, freshwaters may receive higher loadings of allochthonous organic matter, which would darken lake ecosystems. Darkening lakes via terrestrial DOM not only increases the availability of allochthonous carbon in the ecosystem, but could also reduce the availability of autochthonous resources by increasing the light limitation of local primary producers.  As a result, the darkening of lakes could carry consequences for the sources from which lake food webs derive their carbon, as well as their overall metabolic balance.

Ward Lake is a small, naturally productive lake in northern Wisconsin.  In 2010 we used stable isotopes to determine the origin of resources supporting its food web, and used in situ free-water measurements of dissolved oxygen to estimate ecosystem metabolism.  We previously found that Ward Lake was net autotrophic, and that its food web was primarily autochthonous. 

The objective of this study was to assess the effects of darker water on lake metabolism and food web resource use.  In 2012 we added Aquashade to Ward Lake in a whole-lake manipulation.  Aquashade is a commercially-available dye intended to control the proliferation of aquatic primary producers via increased light attenuation.  We hypothesized that the lake would shift to net heterotrophy, and that consumer allochthony would increase.

Results/Conclusions

Relative to 2010 and to a nearby reference lake, in 2012 Ward Lake had a shallower epilimnion and euphotic zone, higher pCO₂, and lower dissolved oxygen concentrations.  Estimates of metabolism indicated a shift toward net heterotrophy.  The allochthony of some fishes, a snail (Helisoma trivolvis), and the zooplankton predator Chaoborus spp. increased, while the resource use of the calanoid copepod Skistodiaptomus oregonensis was similar between years.  However, the biomass of the zooplankton community decreased dramatically between years.  Our experiment highlights that the darkening of freshwaters can have a strong influence on ecosystem metabolism, and that consumers vary in their dietary response to altered resource availability.