[1] Rivers transport globally significant amounts of carbon (C) from terrestrial ecosystems to ocean margins. Understanding and quantifying the sources and respective contributions to riverine C has emerged as an important biogeochemical problem that can be approached through natural-abundance isotope mass balance. Traditionally, the sources of riverine C have been identified either qualitatively or quantitatively through application of static mixing models. However, both source signatures and contributions can vary significantly with time. Here we apply two time-based mixing models to a study of six rivers draining the northeast U.S. In the first model, a time-averaged mixing model (TAMM), we vary only the source isotopic (d 13 C and D 14 C) signatures. In the second model, a time-varying mixing model (TVMM), we allow both isotopic signatures and contributions to vary with time. Based on results from the TVMM, drivers of variation in riverine particulate organic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC) include stream discharge, stream discharge and water temperature, and water temperature and vegetation phenology, respectively. Major sources include C 3 plant material, algal material and slow-turnover soil OC ("slow SOC"), which together account for 50%-100% (95% CI) of riverine POC; C 3 plant material and slow SOC, which together typically account for 60%-100% (95% CI) of DOC; and atmospheric exchange which alone typically accounts for 40%-60% (95% CI) of DIC. Seasonal change in relative contributions from algal material, slow SOC, and photosynthesis (in response to the identified drivers) dominates the observed variation in POC, DOC and DIC, respectively. The TVMM is a novel tool to identify component contributions under more realistic non-static conditions, and with potential application to a broad range of biogeochemical studies.Citation: Hossler, K., and J. E. Bauer (2012), Estimation of riverine carbon and organic matter source contributions using time-based isotope mixing models,

The current U.S. wetland mitigation policy of "no net loss" requires that a new wetland be created to replace any natural wetland destroyed under development pressures. This policy, however, may be resulting in a net loss of carbon-based wetland functions. We evaluated the ability of created wetlands to accumulate carbon and to mitigate loss of carbon-based functions in natural wetlands with variable hydrology. Potential limiting factors to carbon accumulation within created systems included soil aggregation, texture, and bulk density. Rates of soil development and the time required for created wetlands to accumulate the amount of carbon found in natural wetlands were estimated by an exponential model. Soils collected from five created (ages 3-8 years) and four natural freshwater marshes, located in central Ohio, USA, were analyzed for soil organic carbon (SOC), mineralizable soil carbon (Cmin), water-stable aggregates (WSA), particle-size fractions (PSD), and bulk density. Peak-standing aboveground plant biomass was also quantified. Created wetlands contained significantly less plant biomass, SOC, and Cmin than natural wetlands (c < 0.05; false discovery rate). Soil physical properties also differed significantly between created and natural wetlands, with fewer macroaggregates, more microaggregates, more silt-clay (0-5 cm only), and higher bulk density in created wetlands (a < 0.05; false discovery rate). Carbon content was positively correlated with macroaggregate content and negatively correlated with microaggregate content, silt-clay fraction, and bulk density. Fit of SOC data to the exponential model indicated that a newly created wetland would require 300 years to sequester the amount of SOC contained in a natural wetland. At this rate of carbon accumulation, a mitigation ratio of 2.7:1 (area) would be necessary for successful mitigation over a 50-year time period. However, other trajectories fit the data equally well and suggested area mitigation ratios of 2.2:1 (logistic) to 4.4:1 (linear regression) to 5.1:1 (exponential regression). Whether created wetlands are on a trajectory toward natural wetland carbon function, however, remains uncertain. Until gaps in the data are filled and a trajectory verified, the best mitigation policy will be a conservative one, with a restrictive permitting process and high mitigation ratios (5.1:1 minimum).

Artificial lighting at night (ALAN) is a global phenomenon that can be detrimental to organisms at individual and population levels, yet potential consequences for communities and ecosystem functions are less resolved. Riparian systems may be particularly vulnerable to ALAN. We investigated the impacts of ALAN on invertebrate community composition and food web characteristics for linked aquatic‐terrestrial ecosystems. We focused on food chain length (FCL), a central property of ecological communities that can influence their structure, function, and stability; and the contribution of aquatically derived energy (i.e., nutritional subsidies originating from stream periphyton). We collected terrestrial arthropods and emergent aquatic insects from a suite of stream and wetland sites in Columbus, Ohio, USA. Stable isotopes of carbon (13C) and nitrogen (15N) were used to infer FCL and contribution of aquatically derived energy. We found that moderate‐to‐high levels of ALAN altered invertebrate community composition, favoring primarily predators and detritivores. Impacts of ALAN, however, were very taxon specific as illustrated, for example, by the negative impact of ALAN on the abundance of orb‐web spiders belonging to the families Tetragnathidae and Araneidae: key invertebrate riparian predators. Most notably, we observed decreases in both invertebrate FCL and reliance on aquatically derived energy under ALAN (although aquatic energetic contributions appeared to increase again at higher levels of ALAN), in addition to shifts in the timing of reciprocal nutritional subsidies. Our study demonstrates that ALAN can alter the flows of energy between aquatic and terrestrial systems, thereby representing an environmental perturbation that can cross ecosystem boundaries. Given projections for global increases in ALAN, both in terms of coverage and intensity, these results have broad implications for stream ecosystem structure and function.

Rivers transport carbon (C) from terrestrial ecosystems to the coastal ocean, providing significant heterotrophic support within both rivers and receiving coastal waters. The amounts and ages of these terrestrial‐river‐coastal ocean C fluxes, however, are still poorly constrained. To address this uncertainty, a study of eight rivers discharging to the Middle Atlantic Bight (MAB) was undertaken. The rivers were sampled periodically over 2 years for concentrations and δ13C and Δ14C signatures of particulate organic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC). For the watersheds draining to the MAB, it was estimated that ∼3800 Gg of terrestrial organic C (OC) and 700 Gg of terrestrial inorganic C was removed annually by fluvial transport. Of the terrestrial OC loss, ∼64% was contemporary C representing approximately 1% of the annual terrestrial net primary productivity. Net fluvial C inputs to the MAB shelf were estimated to be ∼70 Gg·yr − 1 of POC, 280 Gg·yr − 1 of DOC, and 800 Gg·yr − 1 of DIC. Terrestrial C, as opposed to in situ produced river C, comprised the majority of the riverine POC and DOC flux and around half of the total C flux. A smaller but significant fraction (<25%) of the river C flux was further composed of aged materials deriving from fossil C and aged soil OC. The timing of fluvial OC inputs to the MAB, which appear to be temporally offset from peak MAB primary production, could help support the net heterotrophy that has been observed there during periods of low productivity.

[1] Riverine exports of carbon (C) and organic matter (OM) are regulated by a variety of natural and anthropogenic factors. Understanding the relationships between these various factors and C and OM exports can help to constrain global C budgets and allow assessment of current and future anthropogenic impacts on both riverine and global C cycles. We quantified the effects of multiple natural and anthropogenic controls on riverine export fluxes and compositions of particulate organic C, dissolved organic C, and dissolved inorganic C for a regional group of eight rivers in the northeastern U.S. Potential controls related to hydrogeomorphology and regional climate, soil order, soil texture, bedrock lithology, land use, and anthropogenic factors were analyzed individually, collectively, and at scales of both local and regional influence. Factors related to hydrogeomorphology and climate, followed in importance by land use and anthropogenic factors, exhibited the strongest impacts on riverine C exports and compositions, particularly at smaller localized scales. The effects of hydrogeomorphology and climate were primarily related to volumetric flow, which resulted in greater exports of terrestrial and total C. Principal anthropogenic factors included impacts of wastewater treatment plants (WWTPs) and river impoundments. The presence of WWTPs as well as anthropogenic use of carbonate-based materials (e.g., limestone) may have substantially increased riverine C exports, particularly fossil C exports, in the study region. The presence of nuclear power plants in the associated watersheds is also discussed because of the potential for anthropogenic 14 C inputs and subsequent biasing of aquatic C studies utilizing natural abundance 14 C.Citation: Hossler, K., and J. E. Bauer (2013), Amounts, isotopic character, and ages of organic and inorganic carbon exported from rivers to ocean margins: 2. Assessment of natural and anthropogenic controls, Global Biogeochem. Cycles, 27,[347][348][349][350][351][352][353][354][355][356][357][358][359][360][361][362]