A parsimonious model of longitudinal stream DOC patterns based on groundwater inputs and in-stream uptake

The supply of terrestrial dissolved organic carbon (DOC) to aquatic ecosystems affects local in-stream processes and downstream transport of DOC in the fluvial network. However, we have an incomplete understanding on how terrestrial DOC inputs alter longitudinal variations of DOC concentration along headwater stream reaches because groundwater discharge, groundwater DOC concentration and in-stream DOC uptake vary at relatively short spatial and temporal scales. In the riparian zone, the convergence of subsurface flow paths can facilitate the inflow of terrestrial DOC from large upslope contributing areas to narrow sections of the stream. We refer to these areas of flow path convergence as discrete riparian inflow points (DRIPs). In this study, we ask how longitudinal patterns of stream DOC concentrations are affected by DRIPs, as they are major inputs of terrestrial DOC and important locations for in-stream processes. We used a mixing model to simulate stream DOC concentrations along a 1.5 km headwater reach for fifteen sampling campaigns with flow conditions ranging from droughts to floods. Four sets of model scenarios were used to compare different representations of hydrology (distributed inputs of DRIPs vs diffuse groundwater inflow), and in-stream processes (passive transport vs in-stream biological uptake). Results showed that under medium (10–50 l/s) and high flow conditions (> 50 l/s), accounting for lateral groundwater inputs from DRIPs improved simulations of stream DOC concentrations along the reach. Moreover, in-stream biological uptake improved simulations across low to medium flow conditions (< 50 l/s). Only during an experimental drought, longitudinal patterns of stream DOC concentration were simulated best using diffuse groundwater inflow and passive transport scenarios. These results show that the role of hydrology and in-stream processes on modulating downstream DOC exports varies over time. Importantly, we demonstrate that accounting for preferential groundwater inputs to the stream is needed to capture longitudinal dynamics in mobilization and in-stream uptake of terrestrial DOC. The dominant role of DRIPs in these transport and reaction mechanisms suggests that consideration of DRIPs can improve stream biogeochemistry frameworks and help inform management of near-stream areas that exert a large influence on stream conditions.

Assessment of Spatial Nitrate Patterns in An Eastern Iowa Watershed Using Boat-Deployed Sensors

Water quality sensors deployed on boats, buoys, and fixed monitoring stations along rivers allow high frequency monitoring at dense spatial and temporal resolutions. Research characterizing nitrate (NO3–N) delivery along extended reaches of navigable rivers, however, is sparse. Since land use and stream biogeochemistry can vary within agricultural watersheds, identifying detailed spatial patterns of stream NO3–N can help identify source area contributions that can be used to develop strategies for water quality improvement. Identifying spatial patterns is especially critical in agricultural watersheds that span multiple landscapes and have dynamic hydrological regimes. We developed and tested a new method that quantifies NO3–N delivery to streams at a high spatial resolution by continuously measuring stream NO3–N using a boat-deployed sensor. Traveling up the Iowa and Cedar Rivers (located within agricultural Upper Mississippi River Basin) and their major tributaries with the system, we automatically measured NO3–N concentrations every 15 s during four excursions spanning the months of May to August, 2018, and characterized stream NO3–N both laterally and longitudinally in river flow. Iowa River NO3–N concentrations were highest nearest the headwaters and gradually declined as the river flowed toward the Mississippi River. Conversely, Cedar River NO3–N concentrations increased from the headwaters toward the mid-watershed areas due to elevated NO3–N delivery from tributaries of the Middle Cedar River; NO3–N concentrations declined in the lower reaches. Our results confirm that NO3–N mitigation efforts should focus on level and intensely-farmed subwatersheds. Data collected with our sensor system compliments permanently deployed sensors and provides an option to support NO3–N removal efforts.

Seasonal Fluxes of Dissolved Nutrients in Streams of Catchments Dominated by Swidden Agriculture in the Maya Forest of Belize, Central America

The biogeochemistry of nitrogen (N) and phosphorus (P) in tropical streams and rivers is strongly regulated by the pronounced seasonality of rainfall and associated changes in hydrology. Land use and land cover change (LULCC) can also be a dominant driver of changes in stream biogeochemistry yet responses are not fully understood and vary across different LULCC scenarios. We measured dissolved and total nitrogen (N) and phosphorus (P) concentrations in four tributary streams of the Temash River watershed in southern Belize, Central America. The dominant land use practice in each of the four study catchments was swidden agriculture. We documented a strong seasonal control on the export of nutrients from these study systems with daily N fluxes increasing approximately 10-fold during the onset of the rainy season. P fluxes increased almost 4-fold during the same time period. Comparisons with nutrient export coefficients from other tropical streams suggest that nutrient export in streams of the Temash River watershed is similar or slightly lower. Establishing improved understanding of the terrestrial and hydrologic controls of N and P transport across the terrestrial-aquatic boundary and developing a comprehensive nutrient budget that includes inputs and outputs associated with crop production is warranted in future work.

Linking LiDAR with streamwater biogeochemistry in coastal temperate rainforest watersheds

The goal of this study was to use watershed characteristics derived from light detection and ranging (LiDAR) data to predict stream biogeochemistry in Perhumid Coastal Temperate Rainforest (PCTR) watersheds. Over a 2-day period, we sampled 37 streams for concentrations of dissolved C, N, P, major cations, and measures of dissolved organic matter quality (specific ultraviolet absorbance, SUVA254) and bioavailability. Random forest – classification and regression tree analysis showed that aboveground biomass and structure and watershed characteristics, inclusive of mean watershed slope and elevation, watershed size, and topographic wetness, explained more than 60% of the variation in concentration for most measured constituents. These results indicate this approach may be particularly useful for predicting stream biogeochemistry in small forested watersheds where fine resolution is needed to resolve subtle differences in forest biomass, structure, and topography. Overall, we suggest that the use of LiDAR in many of the small and remote watersheds across the Southeast Alaskan PCTR as well as other forested regions could help inform land management decisions that have the potential to alter ecosystems services related to watershed biogeochemical fluxes.