This research was published in the INSPIRED: A Publication of the New Hampshire Agricultural Experiment Station (Spring 2024)
Researchers: H. M. Fazekas, J. Brun and A. S. Wymore
Freshwater ecosystems are integral components of the ecosystem and crucial for supporting biodiversity and providing ecosystem services. Streams and rivers are the conduits of energy and nutrients from landscapes to aquatic systems. These water bodies often reflect a complex interplay between their catchment characteristics—the landscape features that collect precipitation and direct it as runoff to streams, rivers and other water bodies—and the health and productivity of aquatic ecosystems, as well as broader environmental factors. However, the roles of climate, soil and watershed characteristics on the drivers of carbon and nitrogen availability in streams and rivers is not well identified, limiting a fuller understanding of how multilayered interactions play a role in freshwater biogeochemistry and water quality.
Background and Key Concepts
For this study, three scales were examined:
- Continental Scale: The broadest geographical scope of the study, looking at streams and rivers across the entire continental United States. At this scale, the study observes general patterns and drivers of stream chemistry that are applicable over vast regions.
- Ecoregion Scale: A more focused geographical area within the continental scale that shares similar ecological and climatic characteristics. This scale allows for the examination of stream chemistry within regions that have similar ecological features, climate and landscape forms.
- Biogeoclimatic Region Scale: An even more specific areas characterized by unique biological, geological and climatic conditions. This finer scale provides insights into how stream chemistry is influenced by localized conditions and interactions.
This study also examined direct and indirect drivers of nutrients and energy:
- Direct Drivers: These are factors that have an immediate and measurable impact on stream chemistry. For example, the soil characteristics such as porosity or hydraulic conductivity (the ease in which fluid moves through a soil matrix) can directly affect the amount of total organic carbon (TOC) in streams by influencing the movement and filtration of water and carbon compounds through the soil into the waterways.
- Indirect Drivers: These factors influence stream chemistry through one or more intermediary steps or processes. For example, the study found that climate influences nitrate (NO3-) concentrations indirectly through watershed characteristics. That is, climate factors such as precipitation patterns and temperature may not alter nitrate levels in the water directly, but instead, they impact other aspects such as vegetation growth or soil processes, which in turn affect how nitrates are transported into streams.
Key Findings
- Climate, watershed and soil characteristics are key mediators of carbon (C) and nitrogen (N) availability in streams, but their influence varies by scale—the different levels of geographical or spatial size at which observations and measurements of stream chemistry are taken and analyzed.
- At broader scales, C and N variability is poorly explained, suggesting complex, scale-dependent processes.
- Unique pathways and interactions between climate, soil and watershed characteristics control stream C and N across spatial scales, with hydrometeorological processes being a consistent driving force.
About the Co-author
Adam Wymore, Associate Professor of Natural Resources and the Environment
Contact information: Adam.Wymore@unh.edu, FindScholars profile
Fig. 1. Median total organic carbon (top) and NO3 − (bottom) concentrations (in mg/L) for streams used in partial least squares structural equation modeling (PLS-SEM) analyses.
Methodology
The research integrated stream chemistry data on TOC and NO3- from the United States Geological Survey with data from the Catchment Attributes and Meteorology for Large-sample Studies database. The initial dataset covered 671 basins across the contiguous United States that were selected for their minimal human impact, ensuring that observed chemical patterns were less likely to be anthropogenically altered. This larger dataset was used to compile and analyze more than 37,000 chemistry measurements taken from 459 streams and rivers, alongside landscape and climatic variables.
Results and Impacts
At the continental scale, the study uncovered that while climate, watershed and soil characteristics collectively explain a fraction of the variance in TOC and NO3- concentrations, with 25% for TOC and a mere 6% for NO3-, this coverage substantially increases at smaller spatial scales. Notably, the explained variance for TOC and NO3- rises to 61% and 40%, respectively, when observed at narrower scales. This scale-dependent nature of environmental influence supports the hypothesis that broader scale evaluations capture a more diverse set of biogeochemical processes which are less discernible than those at localized scales.
Carbon Dynamics Across Scales
The direct and indirect drivers of stream chemistry unveiled scale-sensitive interactions. At the continental scale, carbon dynamics are modestly explained by environmental drivers; however, the intricacies of these dynamics become more pronounced at reduced spatial scales. Soil properties, such as hydraulic conductivity and porosity, were found to be significant direct predictors of TOC concentrations, highlighting the critical role of terrestrial–aquatic interfaces in biogeochemical cycling. It is at the ecoregion and biogeoclimatic scales that the complexity of climate–soil interactions becomes evident.
Notably, the impact of hydrometeorological forces like seasonality of precipitation play a contrasting role across scales, directly influencing carbon concentrations at broader scales and indirectly at finer scales via soil moisture interactions.
Nitrogen Dynamics Through the Lens of Scale
The study also sheds light on nitrogen dynamics, demonstrating strong associations with watershed attributes like forest cover and slope. These findings underscore the importance of physical catchment features in controlling nitrogen transport, a critical component for informing watershed management strategies. Climate’s indirect effect on nitrogen, primarily through watershed characteristics, suggests that management efforts may need to focus on both the physical catchment structure and the broader climatic forces that modulate these characteristics.
Strategic Implications for Management
Insights from this study have significant implications for strategic environmental management. Policies and conservation efforts need to be fine-tuned to the spatial scale at which they are implemented to address the nuanced variations revealed in carbon and nitrogen dynamics. Soil conservation emerges as a priority in carbon-rich areas, while nitrogen transport concerns may call for an emphasis on watershed management.
This material is based on work supported by the NH Agricultural Experiment Station through joint funding from the USDA National Institute of Food and Agriculture (under Hatch award number 1022291) and the state of New Hampshire. Additional support came from the National Science Foundation's Experimental Program to Stimulate Competitive Research (EPSCoR) program award EPS-1929148.
