CO2 Input Dynamics and Air–Sea Exchange in a Large New England Estuary
Repeated surveys of the Kennebec estuary, a macrotidal river estuary in Maine, USA, between 2004 and 2008 found spatial and temporal variability both in sources of carbon dioxide (CO2) to the estuary and the air–sea flux of estuary CO2. On an annual basis, the surveyed area of the Kennebec estuary had an area-weighted average partial pressure of CO2 (pCO2) of 559 μatm. The area-weighted average CO2 flux to the atmosphere was 3.54 mol C m−2 year−1. Overall, the Kennebec estuary was an annual source of 7.2 × 107 mol CO2 to the atmosphere. Distinct seasonality in estuarine pCO2 was observed, with shifts in the seasonal pattern evident between lower and higher salinities. Fluxes of CO2 from the estuary were elevated following two summertime storms, and inputs of riverine CO2 outweighed internal estuarine CO2 inputs in nearly all months. River and estuarine inputs of CO2 represented 68 and 32 % of the total CO2 contributions to the estuary, respectively. This study examines the variability of CO2 in a large New England estuary, and highlights the comparatively high contribution of CO2 from riverine sources.
KeywordsEstuary River Carbon dioxide Dissolved inorganic carbon
The flux of water through estuaries represents a relatively small portion of the global water budget, and the surface area of estuaries is small relative to the surface area of the coastal ocean over the continental shelves. However, estuaries play a disproportionate role in the transport and transformation of carbon, both organic and inorganic, from land to sea (Borges 2005; Gattuso et al. 1998). Through coupled biological and physical processes, estuaries are generally strong sources of carbon dioxide (CO2) release to the atmosphere (Borges 2005; Cai et al. 2006; Laruelle et al. 2010; Borges and Abril 2011). The partial pressure of CO2 (pCO2) in estuaries, while usually above atmospheric levels, can range over several orders of magnitude. In an extreme example, pCO2 in the Indian Chilka lagoon ranged from 83 to 6,522 μatm (Gupta et al. 2008). Some recent work has suggested that the release of CO2 from estuaries is high enough in magnitude to offset the uptake of CO2 over the continental shelves (Cai 2011).
The typical high rates of estuarine outgassing of excess CO2 are sustained by organic and inorganic carbon from two sources: river and groundwater inputs and inputs from the surrounding estuary and coastal wetlands and marshes. Estuaries receive large loads of allochthonous carbon from rivers, and can modify these inputs as they are mixed through the estuaries and into the coastal ocean (Salisbury et al. 2009). At the same time, estuaries can also provide large inputs of autochthonous carbon through in-stream heterotrophy and lateral inputs from tidal marshes (Cai 2011). While river inputs alone could support the observed levels of estuary outgassing for many estuaries, recent work in southeastern American estuaries suggests that the majority of CO2 comes from estuarine sources such as tidal floodplains and salt marshes instead of riverine sources (Jiang et al. 2008; Cai 2011). The same authors suggest that riverine inputs of organic carbon are remineralized in the estuarine plume or coastal waters, areas outside of the estuary itself. However, work in several smaller Northeastern American river estuaries showed that riverine sources of carbon dioxide were dominant, while the estuaries were smaller sources or even sometimes sinks of CO2 (Salisbury et al. 2008; Hunt et al. 2010).
Here, we present a study of the large, river-dominated macrotidal Kennebec estuary in Maine, which was surveyed 44 times from 2004 through 2008. This time-series of data allows us to examine the atmospheric CO2 exchange of the Kennebec estuary over monthly to annual time scales, and also allows us to examine the temporal variability of internal and riverine sources of CO2.
Sampling and Analytical Methods
Surveys of the Kennebec estuary were conducted on a roughly monthly basis between September 24, 2004, and June 10, 2008, aboard the UNH research vessel R/V Gulf Challenger. A total of 44 surveys were conducted, with 22 of these on consecutive days while the ship docked overnight at Bath. Tidal state during the surveys covered the range from low tide to high, with most surveys conducted at mid-tide. Some single-day surveys turned around after sampling at Fiddler’s Reach (Fig. 1). A shipboard flow-through system was used to continuously measure physical and chemical properties of surface water. Temperature and salinity were determined by a Sea-bird SBE-45 thermosalinograph, while dissolved oxygen was measured with a Sea-bird SBE-43 sensor (Sea-bird electronics, Bellevue, WA). A temperature offset was observed between the sea surface temperature measured by the continuous-flow SBE-45 and that measured at the water surface by a SBE-37 thermosalinograph deployed as part of a profiling package. For each estuary survey, the average temperature offset between the continuous-flow and profiler sea-surface temperature was removed, to bring the continuous-flow temperature into agreement with in situ sea surface temperature. The SBE-45 thermosalinograph and SBE-43 oxygen sensor received annual manufacturer calibrations, but were not calibrated in the field against discrete measurements. The oxygen percent saturation was calculated according to Sea-bird Electronics Application Note 64 (Sea-bird Electronics 2013).
Flow to the shipboard flow-through system was also pumped to an equilibrator, similar to that described by Wanninkhof and Thoning (1993), but consisting of three Plexiglas chambers instead of a single chamber. Equilibrated air was drawn out of the third chamber, while ambient air was drawn into the first chamber and passed through the second and third chambers, equilibrating with the pumped water supply at each step. Equilibrated air was drawn at 100 mL/min through tubing containing a Nafion selectively permeable membrane (Perma Pure, Toms River NJ) with a counter-flowing stream of dry nitrogen, which dried the sample gas stream of water vapor. Due to the short run of tubing between the water source for both the continuous-flow system and the gas equilibrator, no water temperature difference was observed between that measured by the continuous-flow SBE-45 and the outflow from the equilibrator (measured with a handheld meter—YSI Yellow Springs, Ohio—manufacturer accuracy ±0.2 °C). Temperature from the continuous-flow SBE-45 was used in sea-surface temperature corrections during the calculation of pCO2. After drying, the sample was pumped to a non-dispersive infrared gas analyzer (Li-cor, LI-6262 or LI-840), which measured the molar fraction of carbon dioxide (xCO2) of the sample stream. The Li-cor was calibrated several times each survey with pure nitrogen (0 ppm CO2 molar fraction) and one span tank. Over the study period we employed a succession of span tanks containing a gas mixture with CO2 molar fraction between 819 and 851 ppm (Scott-Marin, Riverside, CA). Corrections of the data for water vapor pressure and sea surface temperature and conversion from xCO2 to the partial pressure of carbon dioxide (pCO2) were carried out according to standard methods (Dickson et al. 2007). Atmospheric pCO2 was periodically measured as well while the ship was underway. Ambient air was drawn from the ship’s bow through a length of tubing and pumped into the non-dispersive infrared gas analyzer described above. The estimated uncertainty of pCO2 measurements is ±3 μatm. All pCO2 data have been banked with the Carbon Dioxide Information Analysis Center (http://cdiac.ornl.gov/oceans/Coastal/unh_ts.html).
Discrete samples were collected from Niskin bottles in the Kennebec estuary, and transferred to sample bottles through silicone tubing to prevent bubbling. At the end-member sites, a bucket was lowered from a bridge upstream from each river’s most downstream dam, thoroughly soaked in the river water, and raised slowly to avoid promoting gas exchange. Samples for pH and total alkalinity (TAlk) were transferred without bubbling into 60-ml glass BOD bottles with greased stoppers. Beginning in 2006, dissolved inorganic carbon (DIC) was sampled from the same bottles. These were filled to leave less than 1 % headspace in the bottle, preserved with 0.1 ml of saturated mercuric chloride solution, and immediately cooled. Samples for TAlk and DIC were generally stored for several weeks before analysis, allowing for settling of particulate material, with the supernatant sample drawn for analysis. The in situ temperature of bucket samples was measured with the handheld YSI meter. DIC was measured first from each sample bottle, followed sequentially by pH and TAlk. Dissolved organic carbon (DOC) samples were collected according to JGOFS protocols (JGOFS 1996) into acid-washed high-density polyethylene bottles, and measured using a Shimadzu high temperature catalytic oxidation analyzer with chemiluminescent detection.
DIC of unfiltered water was determined using an automated analyzer built by Apollo SciTech (Bogart, GA). Immediately after opening the sample bottle, a digital syringe withdrew a small amount of sample (0.5 mL), acidified it with 10 % phosphoric acid and subsequently measured the evolved CO2 with a Li-Cor 6262 non-dispersive infrared gas analyzer (similar to the method described by Cai and Wang 1998). Certified seawater reference materials from Dr. A. Dickson were used to determine DIC concentration by preparing a calibration curve covering the range of DIC from 200 to 2000 μmol kg−1 (Dickson et al. 2003), with a resulting precision ranging from 0.05 to 0.5 % (or 0.1–10 μmol kg−1), with an average of ∼0.1 % (2 μmol kg−1).
TAlk and pH of unfiltered water were simultaneously measured by the same instrument, and thus pH and TAlk measurements are both based on the same pH electrode. The pH electrode used in the TAlk titration (Orion 3-Star, Thermo Fisher Inc.) was calibrated using three low ionic strength pH buffers certified on the U.S. National Bureau of Standards (NBS) scale to ±0.01, and the initial reading before the addition of acid titrant was taken as the sample pHNBS (pH on the NBS scale, hereafter simply referred to as pH).
Non-Carbonate Alkalinity Correction
Recent work in New England rivers, including the Androscoggin and Kennebec rivers, indicates that non-carbonate species comprise a substantial portion of TAlk (Hunt et al. 2011). These non-carbonate species can include inorganic nutrients, metal complexes, and weak organic acids. Inclusion of these non-carbonate species in calculations to derive carbonate parameters can result in the overestimation of pCO2 and CO2 concentration. To account for this, we followed the methodology of Hunt et al. (2011) to derive TAlkDIC-pH from DIC and pHNBS measurements, with the resulting difference between measured TAlk and TAlkDIC-pH representing non-carbonate alkalinity (NC-Alk). All TAlk data presented in this work have been corrected for NC-Alk at levels that will be discussed later.
Air-Water CO2 Flux Estimation
Water temperature at the upper transect reach was more variable than at Fort Popham (Fig. 3). During the winter months, water temperature was lower at the uppermost transect reach than Fort Popham, while during the summer months, the uppermost transect reach was warmer than Fort Popham. For some time in the spring (usually April or May) and in the fall (usually October), the water temperature was essentially the same along the entire transect.
Summary of constituent concentrations from the Androscoggin and Kennebec rivers. The (stdev) value after each average value represents one standard deviation around the mean. Values of pCO2 were computed from pHNBS and DIC measurements, together with in situ temperature, while NC-Alk % was determined by the difference between measured TAlk and TAlkDIC-pHNBS estimated from DIC-pHNBS pairs, as described in Hunt et al. (2011). A regression between NC-Alk% and pHNBS yielded the relationship NC-Alk% = −0.31 × pHNBS + 2.45 (R2 = 0.70)
TAlk (μmol kg−1)
40 % (11 %)
DIC (μmol kg−1)
DOC (μmol kg−1)
Summary of monthly Kennebec estuary physical observations, with n indicating the number of cruises performed in each month. Discharge data were summed from the USGS gages 1049265 and 1059000 to estimate total freshwater input to the estuary. Ranges of observed data are shown for estuary water temperature, salinity, pCO2 and CO2 flux, with the area-averaged mean of each range shown in parentheses
Average 10-day antecedent discharge (m3 s−1)
Estuary surface water temperature (˚C)
Estuary surface salinity
Estuary pCO2 (μatm)
Estuary CO2 flux (mmol m−2 day−1)
Range (area average)
Range (area average)
Range (area average)
Range (area average)
Kennebec Estuary pCO2
In general, Kennebec estuary pCO2 decreased from the river to the ocean, and followed a seasonal pattern of lower pCO2 in the winter and spring and higher pCO2 in the summer and fall (Fig. 3). For the period of this study, the area-averaged pCO2 in the Kennebec estuary was 558 μatm (σ = 167 μatm), while the average observed pCO2air was 383 μatm (σ = 8 μatm). The lowest measured pCO2 was 203 μatm (at salinity 9.78) in April 2008, while the highest pCO2 was 1,771 μatm (at salinity 1.17) in June 2005. While an overall seasonal pattern of pCO2 was observed, storm events had a dramatic effect on pCO2 on shorter time scales. The two surveys with highest pCO2 (Fig. 3) were conducted on June 29, 2005, (1,725 μatm) and June 28, 2006 (1,771 μatm). River discharge on the days of these surveys was not especially high (Fig. 2), but both surveys followed very large storm events which released large amounts of river water into the estuary. A survey of three New Hampshire estuaries located approximately 150 km from the Kennebec estuary, which also experienced the same large June 2006 storm event, showed elevated pCO2 as well (Hunt et al. 2010). Storm events, especially during the warmer months when the ground is not frozen, can presumably flush out soil and groundwater enriched in CO2 (Paquay et al. 2007) as well as large amounts of labile particulate and dissolved organic matter, which could support bacterial respiration and CO2 production in the estuary (Abril et al. 2000; Abril and Borges 2004). The elevated pCO2 observed in the June 2005 and 2006 surveys was more than twice the pCO2 levels in June 2007 (highest pCO2: 780 μatm) and June 2008 (highest pCO2: 604 μatm) when river discharge was at typical low-flow summer levels (Fig. 2).
To examine the annual pattern of CO2 in the Kennebec estuary, we constructed a yearly climatology. We combined the area-averaged pCO2 values into monthly averages, repeated the annual cycle of monthly values three times (to obtain reasonable boundary values for January and December), ran a 60-day Matlab smooth function over the resulting timeseries (‘smooth’, The MathWorks, Inc., Natick MA), then took the middle year as the final climatology (Fig. 5). The resulting line shows a clear drop in pCO2 during the late winter and early spring (February and March), a steady rise in pCO2 through the spring and summer, more or less constant pCO2 over the fall months, then a sharp decline in pCO2 through November and December. Opposing trends were seen in the saturation of dissolved oxygen (Fig. 5), which rose through the winter to a peak in April (a month later than the lowest pCO2 in March), then dropped through the late spring and summer to a low in September (a month before the peak pCO2 in October). This seasonal coupling between pCO2 and dissolved oxygen indicates that biological activity contributes noticeably to overall pCO2 in the Kennebec estuary. The cycle of pCO2 in Fig. 5 also bears a strong resemblance to the annual cycle observed at inner shelf coastal stations in the Gulf of Maine (Vandemark et al. 2011), as well as the BATS site near Bermuda (Takahashi et al. 2002) and a Spanish coastal site (Ribas-Ribas et al. 2011).
Air-Water CO2 Fluxes
Estimates of k600 in the Kennebec estuary using the Raymond and Cole (2001) parameterization resulted in area-averaged CO2 fluxes (average = 8.6 mmol m−2 day−1) that were higher than those calculated according to Ho et al. (2004, average = 6.3 mmol m−2 day−1), but lower than those from Jiang et al. (2008), average = 12.4 mmol m−2 day−1 or Borges et al. (2004b), average = 16.1. Paired t tests showed that the average fluxes from all four parameterizations were significantly different (p < 0.01). As no site-specific parameterization of gas exchange is available for the Kennebec estuary, we chose to use the Raymond and Cole (2001) parameterization, as it demonstrated reasonable results in the Hudson estuary (Ho et al. 2011), and represents a flux estimate bracketed by other published estimates. Thus, all further CO2 fluxes presented in this work were calculated using the Raymond and Cole (2001) parameterization.
Area-averaged fluxes in the Kennebec estuary were mostly positive in sign, meaning that CO2 was released from the estuary to the atmosphere. The Kennebec estuary released CO2 to the atmosphere in 38 of the 44 surveys performed. There was no significant correlation observed between tidal height and area-averaged CO2 flux (r2 = 0.0024), indicating that other factors controlled CO2 fluxes more than the tide. The overall area-averaged CO2 flux from the estuary was 8.6 mmol m−2 day−1 (σ = 8.9 mmol m−2 day−1). The flux of CO2 decreased in late winter (February) to a minimum in early spring (March), then increased rapidly to an annual peak in April, before leveling out and gradually decreasing through the summer and fall to another minimum in December. The lowest value of area-averaged CO2 flux was on February 23, 2006 (−11.5 mmol m−2 day−1), while the largest area-averaged flux was on April 19, 2005 (31.7 mmol m−2 day−1). When the daily fluxes provided by the climatology are summed and multiplied by the total surface area of the estuary (2 × 107 m2), the result is an annual flux of 7.2 × 107 mol C, or 3,160 metric tons of CO2.
The pCO2 in estuaries is controlled by inputs of CO2 from rivers, the ocean, and within the estuary itself, together with water temperature, horizontal and vertical mixing, and net community productivity. In the following sections, we will discuss the effects of changing water temperature and inputs of CO2 from river and within-estuary sources, and compare results from the Kennebec estuary to some other estuaries. It is important to note that the data presented here, particularly CO2 fluxes, represent the surveyed portion of the Kennebec estuary, not the entire estuary. Merrymeeting Bay (Fig. 1) is the upper tidal portion of the estuary, with a surface area more than twice that of the lower portion of the estuary we surveyed. So, although the surveyed portion of the estuary represented almost the entire salinity gradient (Table 1), a rigorous definition such as that of Perillo (1995), “a semi-enclosed coastal body of water that extends to the effective limit of tidal influence” would include Merrymeeting Bay and some lower portion of the Androscoggin and Kennebec rivers; thus total fluxes, as well as average pCO2 and area-specific CO2 fluxes (i.e., in millimole per square meter per day), are underestimates. Indeed, since the Merrymeeting Bay portion of the estuary would fall into the 0–5 salinity bin- the bin with highest pCO2− estimates of CO2 flux from the whole Kennebec estuary, including Merrymeeting Bay, are likely to increase by a factor of three or more.
Changes in water temperature in a northern latitude estuary occur on a seasonal basis, resulting in higher pCO2 in the summer and lower pCO2 in the winter. The temperature of the Androscoggin and Kennebec rivers varies from a low of nearly 0 °C to a high of over 25 °C, while the ocean end-member water temperature ranges from 1.4 to 16 °C. The timing of maximum and minimum temperatures also differs between the rivers and ocean end-member. River temperature reaches its minimum in January and February, while the surface ocean temperature minimum is later, in April. Additionally, the rivers warm up faster in the spring, so that by May the rivers are warmer than the ocean end-member. Maximum river water temperature is in July or August, while the maximum ocean end-member temperature occurs in September or October. The Androscoggin and Kennebec rivers spend much of the winter covered in ice, usually from December until spring snowmelt in March or April. When the spring snowpack melts, there is a large pulse of very cold freshwater discharged to the estuary, which lowers water temperature and pCO2 and also delivers inorganic nutrients (Oczkowski et al. 2006) and presumably DOC to the estuary.
Values of ΔpCO2temp and ΔpCO2bio for each salinity bin, as well as calculated values of T − B and T/B
T − B (μatm)
For 2005, 2006, and 2007, the 3 years with the best survey coverage, the mean area-averaged CO2 fluxes were 13.9, 5.4, and 5.8 mmol m−2 day−1, respectively (σ = 9, 9 and 7 mmol m−2 day−1, respectively). The year-to-year variation in annual mean CO2 flux may be partly explained by river discharge. Annual river discharge in 2005 was the highest measured for the Kennebec river in 24 years, and the second-highest in the Androscoggin river for 82 years of record. Elevated river discharge delivers large fluxes of CO2 and DOC to estuaries, resulting in elevated CO2 fluxes from the estuary (e.g., Hunt et al. 2011). Additionally, elevated river flow in 2005 reduced the overall surface salinity in the estuary, and since lower salinity is generally associated with high pCO2 in the Kennebec estuary, the higher proportion of low-salinity waters in the estuary resulted in elevated fluxes of CO2 from the estuary as a whole.
The average annual flux of CO2 to the atmosphere from the Kennebec estuary was 3.5 mol m−2 year−1 (σ = 1.0 mol m−2 year−1) over the study period. A review of estuary CO2 fluxes (Borges and Abril 2011) provides a reference for CO2 fluxes from estuaries similar to the Kennebec, with the range of fluxes from “tidal systems and embayments” reported as 1.1–76 mol m−2 year−1. This places the CO2 flux from the Kennebec estuary on the low end of the range. The Kennebec flux is consistent with data from five other estuaries in the region, which only range from 1.1 to 4.0 mol m−2 year−1 (Raymond and Hopkinson 2003; Hunt et al. 2010). One possible explanation as to why the Kennebec and other New England estuaries seem to have lower pCO2 and CO2 fluxes than their global counterparts is a lack of CO2 input from the estuaries themselves. Some estuaries have been shown to enhance CO2 levels (Cai 2011; Gupta et al. 2009; Jiang et al. 2008; Raymond et al. 2000). In fact, some researchers have observed that the majority of CO2 in the estuary is produced internally from the lateral transport of DIC produced by microbial organic matter respiration in tidal marshes or mangroves which fringe the estuary (Cai 2011; Cai and Wang 1998, 2004), although CO2 production via respiration in the water column or sediments is also a potential contributor (Raymond et al. 2000). The Kennebec estuary does have some tidal marshes, particularly around smaller tributaries, but overall they are not a large presence along the steep, rocky shoreline. In other New England estuaries the majority of CO2 came from the rivers, and during the spring (and fall to a lesser extent) the estuarine areas removed CO2 (Salisbury et al. 2008; Hunt et al. 2010). Another possible explanation for the seeming lower CO2 fluxes from the Kennebec estuary as compared to other global estuaries is that fluxes from Merrymeeting Bay, which covers a large amount of area at the freshwater end of the estuary, were not measured and would presumably reflect high pCO2 from river inputs, raising the overall CO2 flux from the estuary as a whole.
Our observations in the Kennebec estuary suggest a system whose main CO2 source is the degassing of river-borne DIC, with in-channel heterotrophy being a secondary input, and marsh inputs representing a small contributor of CO2. A study of 11 European and American estuaries calculated the median contribution of riverine DIC to estuary CO2 degassing at 10 % of total CO2 release, with heterotrophic activity accounting for the remaining 90 % of CO2 release, and riverine contributions increasing with decreasing estuary residence time (Borges et al. 2006). This seems consistent with our observations in the Kennebec estuary, as the Kennebec residence time is quite short (4 days, NOAA-CCMA) and the riverine CO2 contribution is relatively high as discussed above.
The balance of river and marsh DIC contributions to global estuaries remains unclear. Coastal marshes are generally found in northern temperate areas from about 30°N to 65°N, with mangroves becoming dominant south of 30°N (Ibanez et al. 2012), although the seagrasses which typically dominate marsh vegetation can also be found throughout the tropics (UNEP-WCMC 2005). Overall, seagrass and salt marsh together cover an order of magnitude more area than mangroves (158 × 106 and 15 × 106 ha, respectively, Duarte et al. 2008). There are large areas of the global coastline, notably the western African and South American coasts and higher-latitude coasts, which do not support documented marsh or mangrove areas (UNEP-WCMC 2005). Additionally, marsh habitat is being lost at a rapid pace- estimated to be up to 2 % per year- with over 30 % of total habitat lost since the 1940s (Duarte et al. 2008). As discussed above, CO2 release from numerous estuaries has been shown to be marsh-dominated; however, we speculate that higher-latitude temperate areas such as the Kennebec estuary may be more river-influenced, presenting a scaling challenge to integrating studies of individual estuaries into a global whole. Finally, as marsh areas continue to decline, riverine inputs of DIC could begin to dominate in more systems.
The seasonal cycle of Kennebec estuary pCO2 displayed a pattern of decreased pCO2 in late winter and early spring, coincident with colder water temperatures and the annual spring phytoplankton bloom, followed by increasing pCO2 through late spring and summer, then decreasing pCO2 through the fall and early winter. The Kennebec estuary was a net source of CO2 to the atmosphere, and monthly fluxes of CO2 from the Kennebec estuary were positive except for the month of March as the spring bloom drew down CO2. Warmer water temperatures in New England promote increased pCO2 in the summer and fall months, while cooler temperatures lead to decreased pCO2 in the winter and spring. The yearly progression of temperature-normalized pCO2 showed that water temperature was not the only factor controlling Kennebec estuary pCO2, as pCO2obs@12 °C decreased during the spring as did in situ pCO2, but unlike in situ pCO2 continued decreasing into the late spring or even summer, well past the time when in situ pCO2 was rising due to increasing water temperatures. Overall, temperature changes accounted for more of the pCO2 variability than biological activity and mixing. The balance between the influences of biology and mixing is not clear, but both appear to be important processes in the Kennebec estuary. Storm events, which generated large amounts of rainfall and subsequent river discharge to the estuary, were shown to produce elevated pCO2 in the estuary in June in two consecutive years, and these events may affect overall annual rates of CO2 efflux from estuaries. River discharge also appears to influences the CO2 flux from the estuary on an annual scale. The highest annual average flux of CO2 from the Kennebec estuary to the atmosphere was in 2005, the year with the highest annual river discharge and lowest annual average salinity. While both the estuary and river contributed to overall estuary CO2, contributions from the river were twice the magnitude of those from the estuary. The monthly estuary contribution of CO2 ranged from 14 to 56 %, with an average contribution of 33 %. There was no discernible seasonality in estuary or river contributions to overall estuary CO2. Future work should extend the findings presented here to the coastal ocean, and examine how inorganic and organic carbon releases from the Kennebec estuary spread into the coastal Gulf of Maine, as well as the persistence and fate of these fluxes.
This research was supported by NSF Grants 0961825 and 0851447, NASA Carbon—NNX08AL8OG and NOAA Joint Center for Ocean Observation Technology—NA05NOS4731206. We thank the crew of the R/V Gulf Challenger, as well as Kennebec cruise participants including Shawn Shellito, Michael Novak, Christopher Manning, Rebecca Jones, Timothy Moore, and Joshua Brown. Publication funds were provided by the National Aeronautics and Space Agency through the Coastal Carbon Synthesis (CCARS) program(NNX11AD47G).
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