•We estimated riverine carbon fluxes in response to river flow and watershed geology.•Soil respiration and carbonate dissolution primarily control carbon cycling in the river.•Flood events enhance ...terrestrial carbon loss to the atmosphere via river water.•River on siliciclastic catchment liberates three times more CO2 than on carbonate catchment.•Carbonate dissolution reactions limit riverine CO2 loss to the atmosphere.
Assessing the origin, transformation and transport of terrestrially derived carbon in river systems is critical to regional and global carbon cycles, particularly in carbonate terrains, which represent the largest carbon reservoir on the earth’s surface. For this reason, we evaluated sources, cycling, and fluxes of dissolved organic and inorganic carbon (DOC and DIC) and riverine CO2 degassing to the atmosphere in the Santa Fe River in north-central Florida, a sub-tropical river that flows across two distinct hydrogeological settings of a region dominated by carbonate karst. One setting occurs in the upper river catchment, where the carbonate Floridan aquifer is confined by the siliciclastic Hawthorn Group, while the other setting occurs in the lower catchment where the river flows across the unconfined Floridan aquifer. The upper catchment is characterized by DOC-rich and DIC-poor water and the DIC has more variable and lower δ13C values compared to the lower catchment. The river in the upper catchment degasses more CO2 to the atmosphere (1156gCm−2yr−1) than in the lower catchment (402gCm−2yr−1) because soil respired carbon and organic matter decomposition increase dissolved CO2 concentration, much of which is consumed during carbonate dissolution reactions in the lower catchment. The CO2 flux from the water surface to the atmosphere during a flood event is three times greater than during base flow, suggesting that excess precipitation flushes soil organic carbon to the river through interflow and enhances the loss of terrestrial carbon via river water to the atmosphere. Our values of CO2 fluxes to the atmosphere lie within the range of fluxes from the world’s rivers, but fluxes from the carbonate dominated region are at the low end, while fluxes from the siliciclastic region are at the high end. These results indicate that catchment lithologies, particularly whether carbonate or siliciclastic, as well as flow, are critical to carbon budgets in rivers and thus are linked to the global carbon cycle.
Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of ...dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+ and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+ and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3–5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm−2a−1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion.
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•In-stream metabolism controls diel cycles of DIC concentrations of karst streams.•DIC uptake by photosynthesis exceeds DIC release by respiration at the study site.•The excess uptake of 73tCkm−2a−1 is about half the oceanic biological pump.•The in-stream photosynthetic C sink could affect the global carbon budget.
CO2 fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these ...inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO2 fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca2+ concentrations) and CO2 fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO2 to and absorbs CO2 from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO2 fluxes evolve through diurnal cycles. At DM, the river evaded CO2 from early morning through noon and absorbed CO2 from afternoon through early morning. At PY in summer, the CO2 evasion decreased during the daytime and increased at night while in winter at night, CO2 uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm−2day−1), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO2 fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO2 emission.
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•Significant diel variations occur in CO2 uptake and evasion in a karst river.•A river can be both sink and source of atmospheric CO2 at different timescales.•Biological processes of sub-aquatic community control river CO2 uptake and evasion.•Karst rivers degas less CO2 to the atmosphere than non-karst rivers in the world
•Apparent age of Florida spring discharge increased with time from 1997 to 2013.•Apparent age increase unaffected by pumping or recharge from Tropical Storm Debby.•Increase in apparent age caused by ...excess precipitation from ∼1960 to 1980.•Excess precipitation results from El Niño years during warm phase AMO.•Groundwater management should consider effects of global climate on recharge.
Water quantity and quality in karst aquifers may depend on decadal-scale variations in recharge or withdrawal, which we hypothesize could be assessed through time-series measurements of apparent ages of spring water. We tested this hypothesis with analyses of various age tracers (3H/3He, SF6, CFC-11, CFC-12, CFC-113) and selected solute concentrations dissolved oxygen (DO), NO3, Mg, and SO4 from 6 springs in a single spring complex (Ichetucknee springs) in northern Florida over a 16-yr period. These springs fall into two groups that reflect shallow short (Group 1) and deep long (Group 2) flow paths. Some tracer concentrations are altered, with CFC-12 and CFC-113 concentrations yielding the most robust apparent ages. These tracers show a 10–20-yr monotonic increase in apparent age from 1997 to 2013, including the flood recession that followed Tropical Storm Debby in mid-2012. This increase in age indicates most water discharged during the study period recharged the aquifer within a few years of 1973 for Group 2 springs and 1980 for Group 1 springs. Inverse correlations between apparent age and DO and NO3 concentrations reflect reduced redox state in older water. Positive correlations between apparent age and Mg and SO4 concentrations reflect increased water-rock reactions. Concentrated recharge in the decade around 1975 resulted from nearly 2m of rain in excess of the monthly average that fell between 1960 and 2014, followed by a nearly 4m deficit to 2014. This excess rain coincided with two major El Niño events during the maximum cool phase in the Atlantic Multidecadal Oscillation. Although regional water withdrawal increased nearly 5-fold between 1980 and 2005, withdrawals represent only 2–5% of Ichetucknee River flow and are less important than decadal-long variations in precipitation. These results suggest that groundwater management should consider climate cycles as predictive tools for future water resources.
Metabolic processes of the submerged aquatic community (photosynthesis and respiration) play important roles in regulating diel cycles of dissolved inorganic carbon (DIC) and sequestering carbon in a ...karst stream. However, little is known of whether diel DIC cycling occurs during rainfall in a karst groundwater-fed stream, even though this question is critical for the accurate estimation of what may be a major terrestrial carbon sink. Here, we measured diel variations of water chemical composition in a small karst groundwater-fed stream in southwest China during a rainfall event to assess the influences of rainfall and rising discharge on DIC diel cycling and the potential carbon sink produced by in-stream metabolism. Our results show that water chemical composition at the source spring (CK site) is relatively stable due to chemostatic behavior during rising discharge after a rainfall period. This site lacked submerged aquatic vegetation and, thus, had no diel variations in water chemistry. However, diel cycles of all hydrochemical parameters occurred at a site 1.3 km downstream (LY site). Diel variations in pH, DO, and δ
13
C
DIC
were inversely related to diel changes in SpC, DIC, Ca
2+
, and
p
CO
2
. These results indicated that diel cycling of DIC due to in-stream metabolism of submerged aquatic community was still occurring during elevated discharge from rainfall. We estimate the carbon sink through the in-stream metabolism of the submerged aquatic community to be 5.6 kg C/day during the studied rainfall event. These results imply that submerged aquatic communities in a karst stream can significantly stabilize carbon originating from the carbonate rock weathering processes in karst areas.
To understand role of biogeochemical reactions in controlling the amount and molecular form of dissolved carbon exported from carbonate terrains, spatiotemporal variations in dissolved organic carbon ...(DOC) and dissolved inorganic carbon (DIC) were observed over one year in the Santa Fe River system, a period of base flow or below. A water mixing model developed using concentrations of Na⁺, Cl⁻, and SO₄⁻²identified three major water sources: soil water, groundwater and deep aquifer water. After accounting for mixing of these water sources, additional chemical signatures resulting from biogeochemical processes in the riparian zone were identified. Net mineralization of DOC occurred throughout the Santa Fe River watershed, particularly during the lowest flow conditions and in the upper watershed. However, natural dissolved organic matter was more labile during low flow and in the lower watershed, and predominantly derived from groundwater (rather than soil water) in all samples, likely via releases during carbonate dissolution. Carbonate dissolution commonly occurred in the upper watershed during low flow conditions, while carbonate minerals precipitated during baseflow, as well as in the lower watershed during very low flow conditions. Thus, riparian zone biogeochemical processes were strongly mediated by watershed hydrology, whose spatiotemporal variations resulted in greater inorganic and organic C export production in the lower watershed than the upper watershed, and during higher flow versus lower flow periods. During this lower flow period, the Santa Fe River watershed exported ~1.0 and 10.3 ton km⁻² year⁻¹DOC and DIC, respectively, representing higher C yields than many other types of watersheds.
•We identified variable nature of GW seepage using an automated radon measurement.•GW seepage to rivers increases following rainfall events.•River geomorphology and hydrology control location and ...magnitude of GW seepage.•The automated radon technique provides near-synoptic and rapid assessment of GW seepage.•Near-synoptic assessment suggests discharge-recharge relations alter GW seepage.
Groundwater (GW) seepage can provide a major source of water, solutes, and contaminants to rivers, but identifying magnitudes, directions and locations of seepage is complicated by its diffuse and heterogeneous distributions. However, such information is necessary to develop programs and policies for protecting ecosystems and managing water resources. Here, we assess GW seepage to the Ichetucknee River, a spring-fed, low gradient, gaining stream in north-central Florida, through automated longitudinal surveys of radon (222Rn) activities at three different flow conditions. A 222Rn mass balance model, which integrates groundwater and spring water end member 222Rn activities and longitudinal 222Rn distributions in river water, shows that diffuse groundwater seepage represents about 16% of the total river baseflow, consistent with previous results obtained from ion (Ca2+, Cl−, SRP and Fe) mass balances and dye tracer methods. During high river stage, the contribution from seepage increases to 18–23% of the river flow. The spatial distribution of GW seepage is more variable in the upper 2.2-km reach of the river than the lower 2.8-km reach, regardless of river flow conditions. The upper reach has a narrower flood plain than the lower reach, which limits evapotranspiration and increases hydraulic gradients toward the river following storm events. Seepage in the lower reach is also limited by hydrologic damming by the receiving river, which inundates the floodplain during high flow conditions, and reduces the hydraulic head gradient. These results demonstrate the variable nature of seepage to a gaining river in both time and space and indicate that multiple synoptic analyses of GW seepage are required to assess seepage rates, determine time-averaged solute fluxes, and develop optimal management policies for riverine ecosystems.
CO
fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these ...inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO
fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca
concentrations) and CO
fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO
to and absorbs CO
from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO
fluxes evolve through diurnal cycles. At DM, the river evaded CO
from early morning through noon and absorbed CO
from afternoon through early morning. At PY in summer, the CO
evasion decreased during the daytime and increased at night while in winter at night, CO
uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm
day
), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO
fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO
emission.
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