Headwater stream network connectivity: biogeochemical consequences and carbon fate

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Virginia Tech

Headwaters may be small relative to other aquatic ecosystems, but they are neither simple nor static environments. Heterogeneous stream corridors constitute the majority of river network length and regulate cycling of carbon and oxygen as they expand and contract their connections across the landscape. Though headwater streams integrate many biogeochemical signals from the watersheds they drain and provide important ecosystem services, their diverse habitats and dynamic changes in wet length have been under- examined compared to dendritic, perennial streams. This oversight complicates efforts to identify biogeochemical patterns at larger scales. This dissertation sets out to expand our knowledge of stream biogeochemical responses to variable connections both within the channel and the wider stream corridor. First, I investigated how the presence and arrangement of different habitat patches in the stream corridor affected overall emissions of carbon dioxide (CO2) and methane (CH4) from sub-watersheds of a forested mountain stream network. To do this I measured concentration and flux of both gasses along and around 4 streams, including dry reaches and adjacent vernal pools as well as flowing water. I found that emissions were highly variable over space and time; in particular, the presence of a vernal pool enhanced total carbon emissions from the stream corridor. Next, to quantify carbon cycling and export from a non-perennial headwater stream, I monitored concentrations of CO2 and dissolved organic carbon (DOC) at the stream outlet. I found that CO2 concentration had a negative relationship with stream discharge, and that exports of both CO2 and DOC were driven by storms reconnecting isolated surface water reaches. I also found that carbon biogeochemistry of intermediate flow states were unique from driest and highest-flow conditions. Finally, to explore how isolated pools in the stream channel respond to flow decrease and cessation, I measured dissolved oxygen (DO) as well as CO2 and CH4 from persistent pools of two non- perennial streams throughout an unusually dry summer and fall. I found that hypoxia was common in all isolated pools, but swings in DO were not consistent between pools even of the same stream. In using diel changes in DO to estimate metabolism, I also found that ecosystem respiration varied by stream, but gross primary production was more driven by stream surface water connectivity. Climate change is inducing many new patterns in stream hydrology with critical implications for biogeochemical activity, from reducing durations of connectivity to causing stronger storms. Improving our understanding of how surface water and landscape connectivity both influence the movement of carbon within and through streams is essential to resolving questions about the contributions of freshwaters to the global carbon cycle.

streams, biogeochemistry, carbon, climate change, carbon dioxide, methane, greenhouse gas, freshwater ecology