GROUNDWATER FLOW PATH DEPTH INFLUENCES THE THERMAL STABILITY OF STREAMS: IMPLICATIONS FOR INSTREAM CARBON CYCLING (Invited Presentation)

The magnitude, spatial distribution, and contributing depths of groundwater inflows to surface waters affects the thermal regimes of stream reaches and networks. This influence has important consequences for temperature-sensitive ecological and biogeochemical processes, such as the biological breakdown and decomposition of organic carbon. Groundwater also routes organic carbon to streams, a process that is additionally influenced by source flow path depth. Our research tested how classifying streams based on spatial patterns of groundwater contribution and groundwater flow path depth (shallow vs. deep) provides insight into spatial patterns of stream warming, then explored how these warming patterns can influence instream carbon cycling. We characterized the contribution of groundwater to streams using paired air/stream temperature signals for 1424 publicly available datasets across the continental United States and evaluated how stream temperature is changing over time (14–30 years) among streams with varied source-depth of groundwater discharge. The results showed that streams with a shallow groundwater signature are warming at a similar proportion and rate to sites with low groundwater contributions. In contrast, streams with deep groundwater signatures tended to have long-term temperature records that were stable or cooling. These results are used to inform a stream network model based on the Coweeta Hydrologic Laboratory watershed (NC, USA), where we used empirically derived relationships between stream temperature and biologically mediated carbon responses (e.g., breakdown by shredder macroinvertebrates and respiration by microorganisms) to estimate the watershed-scale effects of warming on stream carbon cycling. We are now testing whether the influence of groundwater carbon inputs or the indirect influence of climate warming has a larger effect on stream network carbon cycling. Evaluating how stream carbon cycling processes will be altered in a changing climate requires considering how contributing groundwater source-depth influences stream temperatures over time.