Examining the effects of human additions of feed corn on the food web in an urban pond using stable isotope analysis

Background/Question/Methods

In most undisturbed lake ecosystems, primary production from aquatic autotrophs is the basis of the food web. However, inputs from terrestrial primary production can influence food webs; in this case consumers may rely more heavily on terrestrial inputs and/or producers may rely more heavily on nutrients entering the system from the land.  Webster’s Pond is a 4.2-acre urban pond in Syracuse, NY that has a wide gradient of human influence. Its southwest side is fed by a natural spring and rarely has visitors, while the northeastern side is urbanized and has dozens of visitors daily that feed corn to waterfowl. Some 40 short tons (36 metric tons) of feed corn enter this ecosystem each year.  We hypothesized that 1) Terrestrial (corn) inputs support a majority of consumer biomass production at all trophic levels, and 2) The stable isotopes of samples taken from the less human influenced side would not differ significantly from those taken from the human dominated side of the pond.  To test this, we sampled the pond for representative species at each trophic level, and performed stable isotope analysis to quantify the utilization of autochthonous (within pond) and allochthonous (derived from corn) resources in producers and consumers.

Results/Conclusions

Based upon the results of the stable isotope analysis, we reject the hypotheses that corn inputs support a majority of consumer biomass production at all trophic levels, and also that there are no significant differences in the stable isotope ratio between organisms from areas of high and low human influence.  Benthic algae appear to be the basis for a majority of the food web, although an alternative hypothesis is that carbon and nitrogen in higher trophic level consumers are derived from equal parts phytoplankton, algae, and corn. However, benthic algae and less mobile primary consumers on the human dominated side have isotope signatures more similar to those of the feed corn, than those on the less human influenced side. These results were unexpected and we are now working on a comparative study to determine if there is seasonal variation in the results and how the results compare to a nearby urban lake without corn inputs.