Testing carbon limitation of a cave stream ecosystem using a whole-reach detritus amendment

Background/Question/Methods

In cave ecosystems, the absence of light and isolation from surface environments work synergistically to influence nearly all ecological processes. There is no photosynthetic primary production and chemolithoautotrophy is limited to relatively few systems, making most caves entirely reliant on allochthonous inputs of organic matter. Limited surface connectivity reduces the quantity of detrital inputs, while the quality of inputs is diminished by prior biologic processing in soil horizons and transport flow paths. Thus, carbon limitation has long been considered the primary factor influencing ecosystem and evolutionary processes in caves. However, few studies to date have provided adequate data to test this hypothesis, either because they have focused on specific ecosystem processes (e.g., decomposition) or trophic levels (e.g., microbes), or because they involved factors (e.g., heterogeneous organic pollution) that confound data interpretation. Here, we present the preliminary results from an ecosystem-level manipulation experiment designed to test the carbon-limitation hypothesis explicitly. From February 2010 to February 2011, a 100-m reach of a carbon-poor cave stream was amended with non-transgenic corn (Zea mays) litter and the response in consumer biomass was followed relative to that of an upstream reference reach.  Additionally, one year of pre-manipulation (February 2009 to January 2010) data was collected from both reaches. To assess the significance of the amended carbon for the recipient food web, the assimilation of corn-derived carbon by consumers was tracked using stable isotope analysis.  

Results/Conclusions

Prior to the corn amendment, mean standing crop organic matter was low in both study reaches (40 and 20 g ash-free dry mass [AFDM] m^-2 in the reference and manipulation reaches, respectively). The corn litter amendment increased mean standing crop organic matter in the manipulation reach to 215 g AFDM m^-2.  Initial data indicate that abundance and biomass of primary consumers were substantially higher in the manipulated reach. Eight months after the amendment (October 2011), mean macroinvertebrate biomass was more than 5 times higher in the manipulation reach (491 mg AFDM m^-2) compared to the control reach (87 mg AFDM m^-2).  Stable isotope analysis showed that corn-derived carbon contributed ~70% to macroinvertebrate biomass, indicating that the increase in macroinvertebrate biomass was driven by the assimilation of corn-derived carbon. These preliminary results provide strong support for the carbon-limitation hypothesis and illustrate that cave communities have the capability of quickly responding (e.g. increased biomass) to increases in organic matter availability.