Mechanisms of decomposition of terrestrial organic matter in natural aquatic ecosystems

Background/Question/Methods

Terrigenous carbon inputs often account for greater than 50% of the dissolved organic carbon (DOC) in natural aquatic ecosystems. In fact, recent estimates suggest that the DOC pool in some systems may be over 90% terrestrial in origin. As such, understanding the linkages between terrestrial and aquatic ecosystems is imperative for modeling an integrated global carbon cycle. However, the factors that influence how aquatic microbes modify terrigenous carbon subsidies remain largely unknown. Some have proposed the priming effect, the dependence of recalcitrant carbon decomposition on the availability of labile carbon, to potentially explain how aquatic microbial communities degrade terrigenous carbon. Here, we added glucose, nitrogen and phosphorus, and freeze-dried algae to whole water samples from Lake Ozawindib and Long Lake in Itasca, MN. Long-term dark bottle incubations of these samples explicitly test for the priming effect in these lakes.

Results/Conclusions

After three-month incubations, we have found no evidence for the priming effect in these lakes. However, the incubations are still in the active decay phase, demonstrated by equivalent correlation coefficients for linear and exponential decay models. Consequently, these data cannot describe changes in the recalcitrant portion of organic matter. However, we were able to clearly demonstrate differences in lability between our two lake systems. In our phytoplankton dominated lake around 30% of the dissolved carbon was consumed over three months whereas the terrestrial dominated lake only consumed around 10% of the carbon in the same time period. We conclude that incubations longer than three months are necessary to reasonably describe the recalcitrant pool of organic matter and development of techniques that can address the question of the priming effect in a shorter timeframe is imperative to gather data more quickly.