Amounts, isotopic character, and ages of organic and inorganic carbon exported from rivers to ocean margins: 1. Estimates of terrestrial losses and inputs to the Middle Atlantic Bight

Abstract

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.

A Contemporary Carbon Balance for the Northeast Region of the United States

“…At the watershed scale, the TEM estimates of riverine DOC export compares well with those estimates derived from field data if losses of DOC associated in-stream processing − are considered (SI Table S9). For example, if 63% of DOC is broken down by microbes to CO 2 in the Potomac River as Raymond and Bauer found for the York River, then the 110 Gg C/yr of DOC estimated by TEM to be contributed by land to the Potomac River will result in a DOC export of 41 Gg C/yr, which compares well to the 42 Gg C/yr estimated by Hossler and Bauer …”

A Contemporary Carbon Balance for the Northeast Region of the United States

“…At the watershed scale, the TEM estimates of riverine DOC export compares well with those estimates derived from field data if losses of DOC associated in-stream processing − are considered (SI Table S9). For example, if 63% of DOC is broken down by microbes to CO 2 in the Potomac River as Raymond and Bauer found for the York River, then the 110 Gg C/yr of DOC estimated by TEM to be contributed by land to the Potomac River will result in a DOC export of 41 Gg C/yr, which compares well to the 42 Gg C/yr estimated by Hossler and Bauer …”

Steady-State Land Cover but Non-Steady-State Major Ion Chemistry in Urban Streams

“…Comparisons to urban watersheds underlain by carbonate bedrock are less straightforward. On one hand, a higher degree of enrichment was observed for [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] in two studies of urban watersheds with some carbonate-containing bedrock. , On the other hand, a previous study reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. , In summary, this study and previous work ,,,,,, suggest that urban land cover results in elevated [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] (or DIC) in temperate humid regions though the degree to which concentrations are elevated depends on local geology and perhaps on watershed size.…”

“…8,13 On the other hand, a previous study 11 reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. 88,89 In summary, this study and previous work 7,8,10,11,13,15,18 − ] increased by 0.4−0.5 mg/L/year from 1986 to 2010 with concrete degradation suggested as the probable source for two urban streams in Maryland and Washington, DC. 90 Those [HCO 3 − ] increases were smaller than observed at 17% ISC (1.23 mg/L per year) and 21% ISC (1.89 mg/L per year), perhaps due to the different study durations or to the smaller watershed size in the current study.…”

Nonpoint Source Contributions Drive Elevated Major Ion and Dissolved Inorganic Carbon Concentrations in Urban Watersheds

“…Data from Smith, 60 a third study in the region, matches well with these two studies. The slopes for watersheds with extensive carbonate bedrock are higher but also consistent with each other: Baker61 (0.0769 ± 0.0180, R 2 = 0.55) and Hossler62 (0.0627 ± 0.0116, R 2 = 0.83). All of the correlation relationships are significant (p <0.01).…”

A Contemporary Carbon Balance for the Northeast Region of the United States

“…At the watershed scale, the TEM estimates of riverine DOC export compares well with those estimates derived from field data if losses of DOC associated in-stream processing − are considered (SI Table S9). For example, if 63% of DOC is broken down by microbes to CO 2 in the Potomac River as Raymond and Bauer found for the York River, then the 110 Gg C/yr of DOC estimated by TEM to be contributed by land to the Potomac River will result in a DOC export of 41 Gg C/yr, which compares well to the 42 Gg C/yr estimated by Hossler and Bauer …”

Steady-State Land Cover but Non-Steady-State Major Ion Chemistry in Urban Streams

“…Comparisons to urban watersheds underlain by carbonate bedrock are less straightforward. On one hand, a higher degree of enrichment was observed for [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] in two studies of urban watersheds with some carbonate-containing bedrock. , On the other hand, a previous study reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. , In summary, this study and previous work ,,,,,, suggest that urban land cover results in elevated [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] (or DIC) in temperate humid regions though the degree to which concentrations are elevated depends on local geology and perhaps on watershed size.…”

“…8,13 On the other hand, a previous study 11 reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. 88,89 In summary, this study and previous work 7,8,10,11,13,15,18 − ] increased by 0.4−0.5 mg/L/year from 1986 to 2010 with concrete degradation suggested as the probable source for two urban streams in Maryland and Washington, DC. 90 Those [HCO 3 − ] increases were smaller than observed at 17% ISC (1.23 mg/L per year) and 21% ISC (1.89 mg/L per year), perhaps due to the different study durations or to the smaller watershed size in the current study.…”

Nonpoint Source Contributions Drive Elevated Major Ion and Dissolved Inorganic Carbon Concentrations in Urban Watersheds

“…Data from Smith, 60 a third study in the region, matches well with these two studies. The slopes for watersheds with extensive carbonate bedrock are higher but also consistent with each other: Baker61 (0.0769 ± 0.0180, R 2 = 0.55) and Hossler62 (0.0627 ± 0.0116, R 2 = 0.83). All of the correlation relationships are significant (p <0.01).…”

A Contemporary Carbon Balance for the Northeast Region of the United States

“…At the watershed scale, the TEM estimates of riverine DOC export compares well with those estimates derived from field data if losses of DOC associated in-stream processing − are considered (SI Table S9). For example, if 63% of DOC is broken down by microbes to CO 2 in the Potomac River as Raymond and Bauer found for the York River, then the 110 Gg C/yr of DOC estimated by TEM to be contributed by land to the Potomac River will result in a DOC export of 41 Gg C/yr, which compares well to the 42 Gg C/yr estimated by Hossler and Bauer …”

Steady-State Land Cover but Non-Steady-State Major Ion Chemistry in Urban Streams

“…Comparisons to urban watersheds underlain by carbonate bedrock are less straightforward. On one hand, a higher degree of enrichment was observed for [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] in two studies of urban watersheds with some carbonate-containing bedrock. , On the other hand, a previous study reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. , In summary, this study and previous work ,,,,,, suggest that urban land cover results in elevated [Ca 2+ ], [Mg 2+ ], and [HCO 3 – ] (or DIC) in temperate humid regions though the degree to which concentrations are elevated depends on local geology and perhaps on watershed size.…”

“…8,13 On the other hand, a previous study 11 reported that [DIC] enrichment as a function of urban land cover was lower in the study watersheds than in watersheds draining carbonate bedrock in southern England and the northern United States. 88,89 In summary, this study and previous work 7,8,10,11,13,15,18 − ] increased by 0.4−0.5 mg/L/year from 1986 to 2010 with concrete degradation suggested as the probable source for two urban streams in Maryland and Washington, DC. 90 Those [HCO 3 − ] increases were smaller than observed at 17% ISC (1.23 mg/L per year) and 21% ISC (1.89 mg/L per year), perhaps due to the different study durations or to the smaller watershed size in the current study.…”

Nonpoint Source Contributions Drive Elevated Major Ion and Dissolved Inorganic Carbon Concentrations in Urban Watersheds

“…Data from Smith, 60 a third study in the region, matches well with these two studies. The slopes for watersheds with extensive carbonate bedrock are higher but also consistent with each other: Baker61 (0.0769 ± 0.0180, R 2 = 0.55) and Hossler62 (0.0627 ± 0.0116, R 2 = 0.83). All of the correlation relationships are significant (p <0.01).…”