Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light attenuation. Because ...absorption by DOM depends on its composition, UV–Vis absorbance is used to constrain DOM composition, source, and amount. Ferric iron, Fe(III), also absorbs in the UV–Vis; when Fe(III) is present, DOM-attributed absorbance is overestimated. Here, we explore how differing behavior of DOM and Fe(III) at the catchment scale impacts UV–Vis absorbance and evaluate how system-specific variability impacts the effectiveness of existing Fe(III) correction factors in a temperate watershed. We sampled five sites in the Connecticut River mainstem bi-weekly for ~ 1.5 years, and seven sites in the Connecticut River watershed once during the summer 2019. We utilized size fractionation to isolate the impact of DOM and Fe(III) on absorbance and show that variable contributions of Fe(III) to absorbance at 254 nm (a
254
) and 412 nm (a
412
) by size fraction complicates correction for Fe(III). We demonstrate that the overestimation of DOM-attributed absorbance by Fe(III) is correlated to the Fe(III):dissolved organic carbon concentration ratio; thus, overestimation can be high even when Fe(III) is low. a
254
overestimation is highly variable even within a single system, but can be as high as 53%. Finally, we illustrate that UV-Vis overestimation might impart bias to seasonal, discharge, and land-use trends in DOM quality. Together, these findings argue that Fe(III) should be measured in tandem with UV–Vis absorbance for estimates of CDOM composition or amount.
River organic matter transformations impact the cycling of energy, carbon, and nutrients. The delivery of distinct dissolved organic matter (DOM) sources can alter aquatic DOM cycling and associated ...biogeochemical processes. Yet DOM source and reactivity are not well‐defined for many river systems, including in western Canada. Here, we explore DOM cycling in the mainstem of the Oldman River (stream order 6–7), a heavily regulated river network in southern Alberta (Canada). We compared seasonal river DOM content, composition, and bioavailability with nine endmember leachates from the river valley using optical properties and incubations to estimate biodegradable dissolved organic carbon (BDOC). River DOM composition was most similar to terrestrial soil leachates, followed by autochthonous DOM leachates. River DOM bioavailability was low (BDOC = 0%–16.6%, mean of 7.1%). Endmember leachate bioavailability increased from soils (BDOC = 23.9%–53.7%), to autochthonous sources (fish excretion, macrophytes, biofilm; BDOC = 49.9%–80.0%), to terrestrial vegetation (leaves, shrubs, grass; BDOC > 80%), scaling positively with protein‐like DOM content and amount of leachable dissolved organic carbon (DOC), and negatively with aromaticity. Seasonally, DOC concentrations changed little despite >15‐fold increases in discharge during spring. River DOM composition shifted modestly toward soil‐like endmembers in spring and more bioavailable autochthonous end members in autumn and winter. Low DOM bioavailability in the river mainstem and low DOC yields shown in previous work point to limited internal processing of DOM and limited bioavailable DOM delivery to downstream habitats, possibly due to upstream flow regulation. Our observations provide important insights into the functioning of western Canadian aquatic networks.
Plain Language Summary
Rivers are among the most heavily disturbed ecosystems worldwide. In agricultural regions of western Canada, human impacts on river ecosystem structure and functioning are not well defined. The study of organic matter chemistry provides important insights into aquatic ecosystem properties and biogeochemical processes. Here, we explore the seasonal trends in composition and content of dissolved organic matter in one of Canada's most heavily regulated rivers (the Oldman River, Alberta). While land use impacts on river organic matter cycling are documented elsewhere, we provide unique insights by tracking changes in river organic matter composition and microbial consumption across a complete annual cycle that includes winter. The composition and content of river organic matter changed little among seasons. Compared to potential organic matter sources from the river valley (soils, plants, aquatic organisms, and biofilm), river water had organic matter that mostly resembled soil sources, and secondarily river sources. Compared to all potential sources, river organic matter was less bioavailable to microbes and on the low end of bioavailability relative to other global aquatic systems. We hypothesize that human alterations to the flow regime upstream (including damming and irrigation) may dampen seasonal changes in river organic matter composition and cycling.
Key Points
Bioavailability of end member leachates increased from soils to river sources (fish, then macrophytes and biofilm), to terrestrial plants
Dissolved organic matter (DOM) composition and bioavailability were modestly responsive to elevated discharge during the spring freshet
Weakly bioavailable soil DOM dominated the downstream river pool, possibly showing the mainstem is not a hotspot for DOM transformation
Riverine dissolved iron (Fe) affects water color, nutrients, and marine carbon cycling. Fe size and coupling with dissolved organic matter (DOM), in part, modulates the biogeochemical roles of ...riverine Fe. We used size fractionation to operationally define dissolved Fe (< 0.22 μm) into soluble (< 0.02 μm) and colloidal (0.02–0.22 μm) fractions in order to characterize the downstream drivers, concentrations, and fluxes of Fe across season and hydrologic regime at the freshwater Connecticut River mainstem, which we sampled bi‐weekly for 2 yrs. Drivers of colloidal and soluble Fe concentrations were markedly different. The response of colloidal Fe concentration to changes in discharge was modulated by water temperature; colloidal Fe decreased with increasing discharge at temperatures < 10.5°C, but increased with increasing discharge at temperatures > 10.5°C. Conversely, soluble Fe concentrations were only positively correlated to discharge at high temperatures (> 20°C). Soluble Fe was strongly positively correlated to a humic‐like DOM fluorescence component, suggesting coupling with DOM subsets, potentially through complexation. While average colloidal Fe fluxes varied twofold seasonally, soluble Fe fluxes varied ninefold; therefore, soluble Fe variability was more important to the overall dissolved Fe variability than colloidal Fe, despite lower concentrations. Seasonal Fe fluxes were decoupled from discharge: dissolved and soluble Fe fluxes were greatest in the fall, whereas discharge was greatest in the spring. Fluxes of soluble Fe, which may be more bioavailable and more likely to be exported to the ocean, were lowest in the summer when downstream biological demand is high, having implications for primary productivity and iron uptake.
Nitrous oxide (N2O) evasion from streams and rivers is a significant, yet highly uncertain, flux in nitrogen cycle models. Most global estimates of lotic N2O emission assume that evasion rates are ...proportional to inorganic nitrogen inputs to a stream or river. However, many field studies do not detect relationships between lotic N2O evasion and dissolved nitrogen concentration, highlighting the need for better understanding of process‐based controls on this flux. This study reports 4‐yr time series of pN2O and N2O evasion from eight nested streams and rivers and detects an abrupt change in N2O dynamics associated with an intense rainstorm. This rainstorm, and the associated hydrologic flood event, pushed forested reaches across the watershed from consistent N2O sources to prolonged N2O sinks. We attribute this shift to disturbance of incomplete denitrification in the stream network and surrounding watershed, although alternate hypotheses are also discussed. There was continued availability of nitrate (NO3−) for in‐stream processing, eliminating the possibility that NO3−‐availability limited N2O production, and post‐storm N2O‐to‐nitrate ratios were lower than pre‐storm ratios suggesting that the large storm affected in‐situ nitrogen processing rates. The sustained period of post‐storm N2O undersaturation resulted in net negative evasion for five of the eight study sites in 2018, which mitigated emissions over the 4‐yr study. This nonlinear response in N2O dynamics illustrates the potential importance of storm events to control lotic N2O production and emissions.
Streams and rivers are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, the magnitudes of these fluxes are uncertain, in part, because dissolved greenhouse ...gases (GHGs) can exhibit high spatiotemporal variability. Concentration‐discharge (C‐Q) relationships are commonly used to describe temporal variability stemming from hydrologic controls on solute production and transport. This study assesses how the partial pressures of two GHGs—pCO2 and pCH4—vary across hydrologic conditions over 4 yr in eight nested streams and rivers, at both annual and seasonal timescales. Overall, the range of pCO2 was constrained, ranging from undersaturated to nine times oversaturated, while pCH4 was highly variable, ranging from 3 to 500 times oversaturated. We show that pCO2 exhibited chemostatic behavior (i.e., no change with Q), in part, due to carbonate buffering and seasonally specific storm responses. In contrast, we show that pCH4 generally exhibited source limitation (i.e., a negative relationship with Q), which we attribute to temperature‐mediated production. However, pCH4 exhibited chemostasis in a wetland‐draining stream, likely due to hydrologic connection to the CH4‐rich wetland. These findings have implications for CO2 and CH4 fluxes, which are controlled by concentrations and gas transfer velocities. At high Q, enhanced gas transfer velocity acts on a relatively constant CO2 stock but on a diminishing CH4 stock. In other words, CO2 fluxes increase with Q, while CH4 fluxes are modulated by the divergent Q dynamics of gas transfer velocity and concentration.
Abstract
Nitrous oxide (N
2
O) evasion from streams and rivers is a significant, yet highly uncertain, flux in nitrogen cycle models. Most global estimates of lotic N
2
O emission assume that evasion ...rates are proportional to inorganic nitrogen inputs to a stream or river. However, many field studies do not detect relationships between lotic N
2
O evasion and dissolved nitrogen concentration, highlighting the need for better understanding of process‐based controls on this flux. This study reports 4‐yr time series of
p
N
2
O and N
2
O evasion from eight nested streams and rivers and detects an abrupt change in N
2
O dynamics associated with an intense rainstorm. This rainstorm, and the associated hydrologic flood event, pushed forested reaches across the watershed from consistent N
2
O sources to prolonged N
2
O sinks. We attribute this shift to disturbance of incomplete denitrification in the stream network and surrounding watershed, although alternate hypotheses are also discussed. There was continued availability of nitrate (
) for in‐stream processing, eliminating the possibility that
‐availability limited N
2
O production, and post‐storm N
2
O‐to‐nitrate ratios were lower than pre‐storm ratios suggesting that the large storm affected in‐situ nitrogen processing rates. The sustained period of post‐storm N
2
O undersaturation resulted in net negative evasion for five of the eight study sites in 2018, which mitigated emissions over the 4‐yr study. This nonlinear response in N
2
O dynamics illustrates the potential importance of storm events to control lotic N
2
O production and emissions.
Abstract
Streams and rivers are significant sources of carbon dioxide (CO
2
) and methane (CH
4
) to the atmosphere. However, the magnitudes of these fluxes are uncertain, in part, because dissolved ...greenhouse gases (GHGs) can exhibit high spatiotemporal variability. Concentration‐discharge (
C
‐
Q
) relationships are commonly used to describe temporal variability stemming from hydrologic controls on solute production and transport. This study assesses how the partial pressures of two GHGs—
p
CO
2
and
p
CH
4
—vary across hydrologic conditions over 4 yr in eight nested streams and rivers, at both annual and seasonal timescales. Overall, the range of
p
CO
2
was constrained, ranging from undersaturated to nine times oversaturated, while
p
CH
4
was highly variable, ranging from 3 to 500 times oversaturated. We show that
p
CO
2
exhibited chemostatic behavior (i.e., no change with
Q
), in part, due to carbonate buffering and seasonally specific storm responses. In contrast, we show that
p
CH
4
generally exhibited source limitation (i.e., a negative relationship with
Q
), which we attribute to temperature‐mediated production. However,
p
CH
4
exhibited chemostasis in a wetland‐draining stream, likely due to hydrologic connection to the CH
4
‐rich wetland. These findings have implications for CO
2
and CH
4
fluxes, which are controlled by concentrations and gas transfer velocities. At high
Q
, enhanced gas transfer velocity acts on a relatively constant CO
2
stock but on a diminishing CH
4
stock. In other words, CO
2
fluxes increase with
Q
, while CH
4
fluxes are modulated by the divergent
Q
dynamics of gas transfer velocity and concentration.
Dissolved organic matter (DOM) affects the structure and function of aquatic ecosystems. DOM determines system net heterotrophy or autotrophy, affects light availability and water color, controls the ...speciation of metals such as iron, and is one of the most reactive carbon pools. DOM quality and source, whether from terrestrial plants and soils or primary production in inland waters, modulates its impact. Here, I characterize DOM cycling in the predominantly forested, temperate Connecticut River watershed. In Chapter 1, I evaluate how commonly used optical indices for DOM source and quality are complicated by the presence of filterable ferric iron in streams and rivers. I test the effectiveness of existing ferric iron correction factors for these indices using size fractionation and demonstrate systematic biases in proxies of DOM quality due to the divergent controls on ferric iron versus DOM. In Chapter 2, I characterize the cycling of two size fractions of dissolved iron in the Connecticut River mainstem. I show that while iron reduction or iron-leaching from plant material drive colloidal iron, DOM quality and amount control soluble iron, likely through complexation. In Chapter 3, I compare dissolved organic carbon (DOC) radiocarbon in the Connecticut River mainstem to atmospheric radiocarbon records to demonstrate how the atmospheric bomb-pulse can be used to estimate the sources of riverine DOC in steady-state river. DOC age in the Connecticut River was driven by flow path depth, ancient carbonate weathering products incorporated into DOC, and the leaching of recently-fixed DOC.
Abstract
Aquatic primary productivity produces oxygen (O
2
) and consumes carbon dioxide (CO
2
) in a ratio of ~1.2. However, in aquatic ecosystems, dissolved CO
2
concentrations can be low, ...potentially limiting primary productivity. Here, results show that a large drainage basin maintains its highest levels of gross primary productivity (GPP) when dissolved CO
2
is diminished or undetectable due to photosynthetic uptake. Data show that, after CO
2
is depleted, bicarbonate, an ionized form of inorganic carbon, supports these high levels of productivity. In fact, outputs from a process‐based model suggest that bicarbonate can support up to ~58% of GPP under the most productive conditions. This is the first evidence that high levels of aquatic GPP are sustained in a riverine drainage network despite CO
2
depletion, which has implications for freshwater ecology, biogeochemistry, and isotopic analysis.
Knowledge about the origins and evolution of crop species represents an important prerequisite for efficient conservation and use of existing plant materials. This study was designed to solve the ...ongoing debate on the origins of the common bean by investigating the nucleotide diversity at five gene loci of a large sample that represents the entire geographical distribution of the wild forms of this species. Our data clearly indicate a Mesoamerican origin of the common bean. They also strongly support the occurrence of a bottleneck during the formation of the Andean gene pool that predates the domestication, which was suggested by recent studies based on multilocus molecular markers. Furthermore, a remarkable result was the genetic structure that was seen for the Mesoamerican accessions, with the identification of four different genetic groups that have different relationships with the sets of wild accessions from the Andes and northern Peru–Ecuador. This finding implies that both of the gene pools from South America originated through different migration events from the Mesoamerican populations that were characteristic of central Mexico.
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