LB 7-65 - Warming impacts energy transfer from detritus to macroinvertebrates and microbes’

Background/Question/Methods:

Global change scenarios predict an increase in temperature along with changes in precipitation regimes leading to increased nutrient loading in streams and rivers. Due to these shifts, the rate of decomposition is expected to increase in both terrestrial and aquatic environments. Recent estimates highlight the importance of inland waters in the global carbon budget as sources of CO₂ to the atmosphere. What is yet to be elucidated is whether these climatic shifts will alter the elemental pathways of decomposition. The fate of carbon and nitrogen held in organic matter can be fundamentally different depending on the proportion taken up and respired by microbial communities or by macroinvertebrates. The use of stable isotope tracing can be used to map the flow of elements from this organic matter into the biomass of microbes and macroinvertebrates. Using these methods, we can gauge the relative size of each pathway under different experimental treatments. By manipulating temperature and tracing elements from leaf litter to key members of brown food webs we hope to shed light on how stream warming will impact carbon and nitrogen budgets in rivers.

Results/Conclusions:

We constructed an array of 48 flow-through mesocosms (2 leaf species, 3 temperatures, 8 replicates) receiving water from a constructed and aerated wetland. Water pumped from the wetland was warmed using a water heater and mixed with ambient water to create temperature treatments held at means of 4.4 (ambient), 9.0, and 15.4 ^oC over the course of the 56-day experiment. Each mesocosm contained one species of isotopically labelled leaf litter and four caddisfly larvae which fed on the decomposing litter. Our study showed that invertebrates assimilate a greater proportion of carbon and nitrogen from decomposing leaves at higher temperatures, especially on slowly decomposing Oak leaves. Microbial biomass decreases with temperature for both litter types, indicating a potential decrease in efficiency as water temperature rises. We also observed trends of higher insect biomass in treatments held at higher temperatures. These results are empirical evidence indicating shifts in carbon and nitrogen processing in aquatic systems in response to warming. We have been able to use a novel experimental design to simulate warming water temperature without the confounding environmental variables observed when using elevational or latitudinal gradients as a proxy for warming.