Hydrological and biogeochemical controls on carbon and nitrogen export from river networks
River networks are highly connected to the terrestrial landscapes they drain, and therefore play an important role in landscape-scale biogeochemical cycles. Freshwater ecosystems receive and transport materials from land to the coastal oceans, and in turn, actively process, retain, and transform terrestrial nutrient and organic carbon inputs. Characterizing the hydrological and biogeochemical processes that control aquatic carbon and nitrogen cycling is therefore critical for predicting watershed element exports as well as for understanding how streams and rivers support aquatic food webs and emit greenhouse gases.
In this dissertation, I studied the transport and transformation of carbon and nitrogen in streams and rivers. I used a combination of field experiments, high-frequency hydrochemical observations, and river-network scaling to better understand 1) how nutrients and organic carbon are delivered from terrestrial ecosystems to streams, and 2) when and where within river networks aquatic processes retain nitrogen and carbon. To assess terrestrial inputs of nitrogen and carbon to New Hampshire streams, I used high-frequency observations of hydrochemistry and analyzed the response of dissolved organic carbon (DOC) and nitrate (NO3-) to flow across land use, years, seasons, and nearly 700 episodic storm events. I found regional coherence in the response of DOC to stream flow, but the timing of solute variability differed for DOC and NO3- on seasonal and event time scales, suggesting that biotic and thermodynamic controls over nitrogen and carbon availability in catchments can affect solute delivery to streams.
The capacity for river networks to modify nutrient and organic carbon loads is also governed by biogeochemical retention in freshwater ecosystems. I found that biofilms in streams within a tropical river network in Puerto Rico have a large capacity to immobilize and nitrify dissolved ammonium (NH4+). Collectively, nitrogen uptake in streams significantly transformed the catchment nitrogen load, but small streams were relatively more important than larger streams for nitrogen retention at the river-network scale. Finally, I studied the extent to which aquatic metabolism governs net CO2 emissions from New Hampshire streams. Internal CO2 production from aquatic metabolism could explain, on average, 37 – 88% of stream CO2 emissions, indicating that additional CO2 inputs from external sources contributes to CO2 supersaturation in streams. The role of streams as transformers of terrestrial organic carbon was greatest in larger streams and rivers, but the contribution of aquatic metabolism to CO2 emissions was variable throughout the year. Collectively, these results indicate that streams and rivers can significantly modify the nitrogen and carbon loads from their catchments, but their capacity to do so may be higher in certain locations within the river network or certain times of year, and likely depends on the strength of biotic demand, river network structure, and the timing and magnitude of nutrient and organic carbon delivery from their catchments.