Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios

Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. To assess dominant sources contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3-) isotope ratios (15N / 14N, 18O / 16O, and 17O / 16O) at the mouth of the river and from the larger of two wastewater treatment facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site more important at lower river flows during warmer months, comprising ∼ 23 % and ∼ 18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver in summer and from shallower groundwater and surface flow during colder months – wherein NO3- associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater. Corresponding NO3- 18O / 16O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NO3-, detected from its unique mass-independent NO3- 17O / 16O vs. 18O / 16O fractionation, accounted for < 3 % of riverine NO3-, even at elevated discharge. Along-river, NO3- 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of 18O / 16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO3- from the catchment and from in-river nitrification and its coincident removal by biological consumption. These findings stress the importance of considering seasonality of riverine N sources and loading to mitigate eutrophication in receiving estuaries. Our study further advances a conceptual framework that reconciles with the current theory of riverine nutrient cycling, from which to robustly interpret NO3- isotope ratios to constrain cycling and source partitioning in river systems.

Residence Time in Hyporheic Bioactive Layers Explains Nutrient Uptake in Streams

&lt;p&gt;Human activities negatively impact water quality by supplying excessive nutrients to streams. To investigate the capacity of streams to take up nutrients from the water column, we usually add nutrients to stream reaches, calculate the fraction of added nutrients that is taken up, and identify the environmental conditions controlling nutrient uptake. A common idea is that nutrient uptake increases with increasing water residence time because of increased contact time between solutes and organisms. Yet, water residence time only partially explains the temporal and spatial variability of nutrient uptake, and the reasons behind this variability are still not well understood. In this talk I&amp;#8217;ll present a study which shows that good characterization of spatial heterogeneity of surface-subsurface flow paths and bioactive hot spots within streams is essential to understanding the mechanisms of in-stream nutrient uptake. The basis of this study arises from the use and interpretation of nutrient uptake results from the Tracer Additions for Spiraling Curve Characterization (TASCC) method. This model has been rapidly adopted to interpret in-stream nutrient spiraling metrics (e.g, nutrient uptake) over a range of concentrations from breakthrough curves (BTCs) obtained during pulse solute injection experiments. TASCC analyses often identify hysteresis in the relationship between spiraling metrics and concentration as nutrient concentration in BTCs rises and falls. The mechanisms behind these hysteresis patterns have yet to be determined. We hypothesized that difference in the time a solute is exposed to bioactive environments (i.e., biophysical opportunity) between the rising and falling limbs of BTCs causes hysteresis in TASCCs. We tested this hypothesis using nitrate empirical data from a solute addition combined with a process-based particle-tracking model representing travel times and transformations along each flow path in the water column and hyporheic zone, from which the bioactive zone comprised only a thin superficial layer. In-stream nitrate uptake was controlled by hyporheic exchange and the cumulative time nitrate spend in the bioactive layer. This bioactive residence time generally increased from the rising to the falling limb of the BTC, systematically generating hysteresis in the TASCC curves. Hysteresis decreased when nutrient uptake primarily occurred in the water column compared to the hyporheic zone, and with increasing the distance between the injection and sampling points. Hysteresis increased with the depth of the hyporheic bioactive layer. Our results indicate that the organisms responsible for nutrient uptake are confined within a thin layer in the stream sediments and that the bioactive residence time at the surface-subsurface water interface is important for nutrient uptake. I will end the talk illustrating how these findings can have important implications for in-stream nutrient uptake within the context of restoration practices addressed to modify the hydro-morphological characteristics of stream channels.&lt;/p&gt;

Sources and cycling of nitrogen in a New England river discerned from nitrate isotope ratios

Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. In order to assess dominant sources of N contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3−) isotope ratios (15N / 14N, 18O / 16O and 17O / 16O) at the mouth of the river and from the larger of two Waste Water Treatment Facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge, and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site upriver more important at lower river flows during warmer months, comprising ~23 % and ~18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver during low base flow in summer, and from shallower groundwater and surface flow at higher river flows during colder months. Loading of dissolved organic nitrogen appeared to increase with river discharge, ostensibly delivered by surface water. The NO3− associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater, consistent with the expectation fractionation due to partial denitrification. Along-river, NO3− 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver, with the agricultural and urbanized portions of the watershed contributing disproportionately to total N loading. Corresponding NO3− 18O / 16O ratios were lower during the warm season, a dynamic that we ascribe to increased biological cycling in-river. The 18O / 16O isotope ratios along-river were consistent with the notion of nutrient spiraling, reflecting NO3− input from the watershed and in-river nitrification and its coincident removal by biological consumption. Uncycled atmospheric NO3−, detected from its unique mass-independent NO3− 17O / 16O vs. 18O / 16O fractionation, accounted for

Pulses of Biogenic Nitrogen Cycling Lead to Atmospheric-Based Nutrient Spiraling in Southern California

&lt;p&gt;Nitrogen deposition into arid ecosystems is increasingly shaping biogeochemical dynamics worldwide. We propose a framework to investigate the role of pulsed soil N emission as a mechanism that relays the influences of atmospheric anthropogenic N deposition to areas that otherwise would be minimally affected. We use nutrient spiraling theory, developed in lotic ecosystems, to quantify how regeneration of N by soils influences N deposition downwind of an urban plume. Our hierarchical framework of landscape functioning thereby connects ecosystem processes occurring from microbial (&lt;1cm and hours) to regional (&gt;100km and inter-annual) scales through reciprocal interactions among soils and microbes, pollution sources, and the atmosphere. We use southern California, USA as a case study for evaluation where we test the terrestrial nitrogen spiraling framework using a combination of field experiments, isotopic measurements, theoretical models, and atmospheric transport and chemistry model outputs. Initial results from field wetting experiments, isotope measurements and contrasting modeling approaches all support a spiraling framework and the increasing importance of soil regeneration of nitrogen to deposition farther from the urban source. Soil microbiome communities associated with nitrogen cycling vary both spatially across the deposition gradient and temporally in response to wetting events. From these results we derive terrestrial spiraling metrics that can identify consequences of both soil and anthropogenic inputs to regional nitrogen cycling. New landscape frameworks for evaluating the role of transport and transformations on N cycling can help understand and predict spatial variation in ecosystems connected across multiple scales.&lt;/p&gt;

Characteristics, Main Impacts, and Stewardship of Natural and Artificial Freshwater Environments: Consequences for Biodiversity Conservation

In this overview (introductory article to a special issue including 14 papers), we consider all main types of natural and artificial inland freshwater habitas (fwh). For each type, we identify the main biodiversity patterns and ecological features, human impacts on the system and environmental issues, and discuss ways to use this information to improve stewardship. Examples of selected key biodiversity/ecological features (habitat type): narrow endemics, sensitive (groundwater and GDEs); crenobionts, LIHRes (springs); unidirectional flow, nutrient spiraling (streams); naturally turbid, floodplains, large-bodied species (large rivers); depth-variation in benthic communities (lakes); endemism and diversity (ancient lakes); threatened, sensitive species (oxbow lakes, SWE); diverse, reduced littoral (reservoirs); cold-adapted species (Boreal and Arctic fwh); endemism, depauperate (Antarctic fwh); flood pulse, intermittent wetlands, biggest river basins (tropical fwh); variable hydrologic regime—periods of drying, flash floods (arid-climate fwh). Selected impacts: eutrophication and other pollution, hydrologic modifications, overexploitation, habitat destruction, invasive species, salinization. Climate change is a threat multiplier, and it is important to quantify resistance, resilience, and recovery to assess the strategic role of the different types of freshwater ecosystems and their value for biodiversity conservation. Effective conservation solutions are dependent on an understanding of connectivity between different freshwater ecosystems (including related terrestrial, coastal and marine systems).