Physical controls on land-water linkages: Carbon cycling and food webs in boreal watersheds
Smits, Adrianne P
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Aquatic ecosystem responses to changing climate and land-use are often extremely heterogeneous across landscapes: studies than quantify relationships between watershed features and biogeochemical and food web processes at intermediate scales (e.g., watershed-scale) would provide a means to extrapolate local findings across landscapes, potentially aiding future management efforts. I investigated how watershed features influence the movement of materials and energy between aquatic and terrestrial environments. I worked in a nearly pristine boreal river system in southwest Alaska (Wood River), where a high level of physical heterogeneity, coupled with natural landscape gradients, provided an opportunity to explore watershed geomorphic controls on ecosystem processes in streams. In Chapter 2 I used fatty acid biomarkers as novel tracers of food web pathways to assess how assimilation of heterotrophic bacteria and algae by stream insects varied across a landscape gradient in watershed features. I found that watershed features such as mean slope are correlated with the energetic base of food webs in boreal streams, with algal resources more important in steep streams and bacterial resources more important in flat watersheds. In Chapter 3 I used fatty acid and stable isotope tracers, coupled with several years of growth data, to investigate how stream thermal regimes affected the ability of juvenile coho salmon (Oncorhynchus kisutch) to benefit from marine resource subsidies (sockeye salmon eggs). I found that local environmental conditions (mean summer stream temperature) were a stronger control on fish growth than the magnitude of the resource subsidy, but also that individuals varied substantially in their metabolic responses to an energy-rich food source. In Chapter 4 I examined geomorphic controls on the magnitude and sources of stream CO2 emissions from boreal streams. I found that watershed slope interacts with precipitation events to control terrestrial carbon fluxes into and out of streams. Watershed slope influences C loading and stream CO2 fluxes by determining the amount of carbon accumulation in watersheds, and to a lesser extent by determining gas transfer velocity across the air-water interface. These patterns provide a way to extrapolate across boreal landscapes by constraining CO2 concentration and flux estimates by local geomorphic features. In this dissertation I demonstrated that interactions between geomorphology, climate, and organisms produce incredible variability in ecosystem processes (carbon cycling, food web pathways) within a single river system, but that geomorphic features of watersheds regulate that variability.
- Fisheries