Seasons in the Stream: River Ecosystem Phenology in a Changing Climate

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Patterns of primary productivity are primarily dictated by seasonal factors like light availability and temperature. In small rivers, primary productivity often peaks opposite that of terrestrial ecosystems, where overhanging canopies shade the river channel impeding light reaching the water surface. The degree to which river primary productivity is affected by the surrounding terrestrial environment depends on the river size. As rivers widen, river primary productivity increases, with more light reaching the stream channel. While light availability may set up the “window of metabolic opportunity” for river primary producers, factors like flow disturbance and grazing pressure from invertebrates can constrain the overall magnitude of gross primary productivity, or GPP. My dissertation seeks to evaluate what processes control the pattern and magnitude of primary productivity in rivers. I evaluate this driving question by analyzing the efficiency by which river ecosystems convert light energy into autotrophic biomass across 64 rivers (Chapter 2), through a series of in-depth experiments and careful monitoring of algae in a small forested river (Chapter 3), and in evaluating whether ice cover determines patterns of primary productivity along a river continuum (Chapter 4). Finally, I provide a framework for evaluating how climate change may be impacting the patterns of primary productivity in rivers (Chapter 5). In Chapter 2, we found that rivers are extremely efficient at converting light energy to biomass (i.e. have a high ecosystem light use efficiency, LUE). The range of LUEs reported in our study encompasses the entire range for any ecosystem LUE measured in forests, crops, lakes, and previous river studies. While river ecosystems have the potential to have high LUE on their “best days,” variability of flow constrains LUE throughout the year. In Chapter 3, we found that algae in the oligotrophic and steep headwaters of Hubbard Brook Experimental Forest were both more abundant than past studies and preferentially grew within substrates that simulate the physical structure of bryophytes. Throughout four separate nutrient diffusing substrate deployments, we found that nutrients rarely limited algal biomass. This chapter explores the previously unsuspected role of bryophytes in providing refugia for algae in this high-gradient and nutrient-poor stream. In Chapter 4, we combined three metrics of ice cover (satellite-, field camera-, and temperature-based) with a proxy for primary productivity across a river continuum. We found that satellite and field camera-based ice cover records reveal two important aspects of ice dynamics in a 3rd to 6th Strahler order river. First, river ice in narrow channels is dynamic, with more complete ice cover days and mid-winter breakups happening on the narrowest section of the reach. Second, ice cover is habitat-specific, with riffle sequences along the river never reaching complete ice cover. In comparing ice dynamics to a proxy of GPP, diel dissolved oxygen amplitude, we found that ice on rivers not only affects the overall pattern of primary productivity, but also affects our ability to measure primary productivity by affecting river gas exchange. Finally, in Chapter 5, I provide examples of relevant drivers of change for river ecosystem GPP in a warming climate. More studies that attempt to disentangle how climate change is affecting light availability, nutrients, temperature, ice cover, etc., or how climate change is shifting the “windows of metabolic opportunity” for rivers, will be imperative in the Anthropocene.






Thellman, Audrey Nicole (2023). Seasons in the Stream: River Ecosystem Phenology in a Changing Climate. Dissertation, Duke University. Retrieved from


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