Subsidy or subtraction? Whole-lake experiments, surveys, and models to test the effects of terrestrial DOC on aquatic food webs

Background/Question/Methods

Lakes receive substantial inputs of terrestrial organic matter (t-OM) from their watersheds, including inputs of dissolved (DOC) and particulate (POC) organic carbon. These inputs are ingested and assimilated by aquatic consumers, and are generally assumed to represent a classic ecological subsidy. Yet recent theoretical work and limited empirical evidence suggests that t-OM inputs may in fact subtract from, rather than subsidize, consumer production. We used simulation models and a multi-lake survey of consumer production to test these alternative hypotheses.

Results/Conclusions

Zoobenthos production was negatively related to t-OM loads in all simulation model scenarios that we considered except those with extreme and unrealistic parameterizations, indicating that terrestrial inputs are not a subsidy for zoobenthos. Zooplankton production response to t-OM loads was more dependent on model assumptions; it was positively related to t-OM inputs only if the ratio of DOC load to concentration and the zooplankton growth efficiency on terrestrially-derived materials were both high. These conditions appear not to be met in real systems, because an empirical study of 10 lakes spanning a wide DOC gradient demonstrated that zooplankton production was in fact negatively related to presumed t-OM loads. An ongoing whole-lake manipulation of t-OM loads will provide an experimental test of these modeling and survey results.