Browsing by Subject "carbon cycling"

Headwater streams serve multiple important biogeochemical, hydrologic, and ecological functions, including: transporting solutes from the terrestrial landscape to downstream fluvial ecosystems; providing a surface for gas evasion to the atmosphere; integrating terrestrial, riparian and aquatic ecosystems, amalgamating surface and groundwater; accumulating and storing sediment; and transforming and retaining solutes. The numerous mechanisms mediating these physical and biological processes remain poorly understood despite their prominent influence on catchment outlet biogeochemical dynamics.

In light of this research need, this study sought to determine the influence of hydrologic, ecological, and biogeochemical processes on solute (specifically carbon and nitrogen) concentrations and fluxes in a paired set of headwater stream networks.

This research was conducted at the Tenderfoot Creek Experimental Forest in Montana. An empirical, field-based approach that combined observational monitoring using a network of high temporal resolution sensors and experimental solute additions was used to quantify carbon and nitrogen uptake, metabolism, and export across the snowmelt and baseflow recession periods.

Based on analysis of this data set, we determined that headwater streams show strong demand for carbon and nitrogen across a range of concentrations from ambient to saturating concentrations; that the variation in uptake kinetics seasonally and between sites is driven by substrate availability; that this retention capacity is linked to the magnitude of metabolic demand; and that through the metabolism of the biological community carbon and nitrogen cycles are coupled. We then demonstrate that these biological processes can have variable roles in mediating carbon and nitrogen export at the catchment scale, but during some periods of the year they can be as influential as physically driven fluxes in mediating watershed export.

This study integrates disparate ecological and hydrologic perspectives to address how energy and macronutrients move through headwater stream networks. We believe the findings presented here begin to reconcile the seemingly incompatible paradigms of streams as highly retentive biogeochemical reactors and streams as “passive pipes” that reflect and integrate the terrestrial landscape.

Marine microbes play key roles in driving patterns of important biogeochemical processes including primary production across the global ocean. Despite the importance of such interactions between the marine microbial community and ocean biogeochemistry, oceanographers have yet to attain a deep understanding of the ecological mechanisms underlying these connections. Due to the vast scale of ocean ecosystems, however, large-scale yet high-resolution surveys are necessary to uncover specific relationships between biology and elemental cycling for more detailed study.

With this need in mind, this dissertation takes advantage of recent advances in both underway techniques to measure in situ biogeochemical rates—most notably the dissolved O2/Ar method for measuring net community production (NCP)—as well as molecular sequencing methods to directly investigate relationships between marine microbial community structure, productivity, nitrogen (N2) fixation, and nutrient availability across large ocean regions. At the same time, this work also improves our understanding of the O2/Ar technique by evaluating its performance and key assumptions in a dynamic upwelling environment and by presenting recommendations to improve the accuracy of productivity estimates generated using this approach.

Presenting data and measurements from the most comprehensive survey of marine microbial community structure and patterns of productivity and N2 fixation in the western North Atlantic to date, this manuscript highlights intriguing connections between regional peaks in productivity and N2 fixation, the mixotrophic algae Chrysophyceae and Aureococcus anophagefferens, and Braarudosphaera bigelowii, a eukaryotic host organism for N2-fixing bacteria. In addition, we report a strong negative relationship between eukaryotic marine microbial diversity and productivity across the region. We further highlight the importance of considering diel cycles of productivity/respiration, other non-steady-state conditions, and vertical fluxes of O2/Ar when calculating and interpreting NCP rates obtained from surface O2/Ar measurements. Ultimately, these findings contribute to our ability to evaluate community production using surface ocean dissolved gas measurements and provide important insights into patterns of marine microbial activity and community structure into the western North Atlantic.