Seasonal Response of Air-water Co 2 Exchange Printer-friendly Version Interactive Discussion Seasonal Response of Air-water Co 2 Exchange along the Land-ocean Aquatic Continuum of the North East American Coast Seasonal Response of Air-water Co 2 Exchange Printer-friendly Version Interactive Discussi

This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract This regional study quantifies the CO 2 exchange at the air–water interface along the land-ocean aquatic continuum (LOAC) of the North East American coast, from streams to the shelf break. Our analysis explicitly accounts for spatial and seasonal variability in the CO 2 fluxes. The yearly integrated budget reveals the gradual change in the intensity 5 of the CO 2 exchange at the air–water interface, from a strong source towards the atmosphere in streams and rivers (3.0 ± 0.5 Tg C yr −1) and estuaries (0.8 ± 0.5 Tg C yr −1) to a net sink in continental shelf waters (−1.7 ± 0.3 Tg C yr −1). Significant differences in flux intensity and their seasonal response to climate variations is observed between the North and South sections of the study area, both in rivers and coastal waters. Ice 10 cover, snow melt and estuarine surface area are identified as important control factors of the observed spatio-temporal variability in CO 2 exchange along the LOAC.

. For the purpose of this study, the area was divided in a North and a South section (Fig. 1). The boundary is set on land, to delineate the regions subject to seasonal ice freeze and snowfalls from those that are not (Armstrong and Brodzik, 2001). This delineation attributes 96 % of the estuarine surface area to the South section due, for the most part, to the contribution of Chesapeake Bay which ac-5 counts for about two thirds of the estuarine area. The delineation extends further into the coastal waters in such a way that the Scotian Shelf and the Gulf of Maine correspond to the North section and the Mid-Atlantic Bight and Georges Banks to the South section. The riverine data are calculated from pH and alkalinity measurements (previously used in Lauerwald et al., 2013) while continental shelf values are calculated from 10 the SOCAT 2.0 database which contains quality controlled direct pCO 2 measurements (http://www.socat.info/, Bakker et al., 2014).

Rivers
CO 2 evasion from rivers (F CO 2 ) was calculated monthly per 15s grid cell (resolution of the hydrological routing scheme Hydrosheds 15s, Lehner et al., 2008) from estimates 15 of the effective stream/river surface area A eff , gas exchange velocity k, and wateratmosphere CO 2 concentration gradient ∆[CO 2 ]: The calculation of A eff first requires estimation of the total stream/river surface area, 20 A. The latter was calculated from the linear stream network derived from the Hydrosheds 15s routing scheme using a minimum threshold on the catchment area of 10 km 2 and estimates of stream width derived from the annual mean discharge Q ann using the equations of Raymond et al. (2012Raymond et al. ( , 2013. A values were not calculated for each individual month, as the discharge-stream width relations only hold true for Q ann 25 . Q ann was obtained using Hydrosheds 15s to route the gridded data of average annual runoff from the UNH/GRDC composites (Fekete et al., 2002).
For each 15s raster cell covered by lake and reservoir areas as represented in the 11989 Introduction global lake and wetland data base of Lehner and Döll (2004), A was set to 0 km 2 . A eff was then derived from A to account for seasonal stream drying and ice cover inhibiting F CO 2 . Seasonal stream drying was assumed for each 15s cell and month when the monthly average discharge Q month is 0 m 3 s −1 . Values of Q month were calculated similarly to that of Q ann using the gridded data of average monthly runoff from the 5 UNH/GRDC composites (Fekete et al., 2002). Ice cover was assumed for each 15s cell and month when the mean air temperature (T air ), derived from the worldclim data set of Hijmans et al. (2005), is below −4.8 • C. In case of ice cover and/or stream drying, Values of k were first calculated as standardized values for CO 2 at a water temper-10 ature (T water ) of 20 • C (k 600 ), from stream channel slope CS and estimates of flowing velocity V . Using the Strahler order (Strahler, 1952) to perform the segmentation of the stream network, CS was calculated for each segment by dividing the change in its altitude by its length. Information on altitude was derived from the Hydrosheds elevation model. V was calculated from Q ann based on the equations of Raymond et al. (2012Raymond et al. ( , 15 2013). Similarly to the stream width, the V-Q relations only hold true for Q ann , and this is why only annually average values for V and k 600 could be calculated. The k value for each month was calculated from k 600 and an estimate of the in-situ air temperature T water, based on the mean monthly air temperature derived from the worldclim data set of Hijmans et al. (2005). 20 Values of ∆(CO 2 ) were derived from monitoring data with calculated pCO 2 river (12 300 water samples, from 161 locations, Lauerwald et al., 2013) and assumed pCO 2 atmosphere of 390 µatm. The pCO 2 values were converted into concentrations, [CO 2 ], using Henry's constant (Henry, 1803) for each sample at its observed temperature T water using the equation of Telmer and Veizer (1999). In order to minimize the in-25 fluence of extreme values, the results were aggregated to median values per sampling location and month for which at least three values were available. These median values per sampling location and month were then used to calculate maps of ∆[CO 2 ] at a 15s resolution using an inverse distance weighted interpolation. To account for downstream Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | decreases in pCO 2 river , which are often reported in the literature (Finlay, 2003;Teodoru et al., 2009;Butman and Raymond, 2011), the interpolation was applied separately to three different classes of streams and rivers defined by Q ann , for which sufficiently large subsets of sampling locations could be retained: (1) Q ann < 10 m 3 s −1 (n = 76), (2) 10 m 3 s −1 ≤ Q ann < 100 m 3 s −1 (n = 47), and (3) Q ann ≥ 100 m 3 s −1 (n = 38). The three 5 maps of ∆[CO 2 ] per month were then recombined according to the spatial distribution of Q ann values. The F CO 2 values were first calculated using Eq.
(1) at the high spatial resolution of 15s for each month. The results were then aggregated to a 0.25 • resolution and three-month period and reported as relative to the terrestrial surface area per raster cell including inland waters. The difference between the F CO 2 s calculated 10 using the equations of Raymond et al. (2012) and Raymond et al. (2013) was used as an estimate of the uncertainty of the mean yearly F CO 2 .

Estuaries
The yearly averaged CO 2 exchange at the air-water interface was obtained from local estimations of emission rates in seven estuaries located within the study area (Ray- 15 mond et al., 1997, 2000Raymond and Hopkinson, 2003;Hunt et al., 2010). The limited number of observation does not allow resolving the seasonality in CO 2 emissions. The yearly-average local CO 2 emission rates range from 1.1 molC m −2 yr −1 in the Parker River to 9.6 molC m −2 yr −1 in the Hudson River estuary, for a mean value of 4.2 molC m −2 yr −1 for the seven systems. This value was then multiplied by the estuar-20 ine surface areas extracted from the SRTM water body data set (NASA/NGA, 2003), to estimate the bulk outgassing for the North and South sections of COSCAT 827. Similar approaches have been used in the past to produce global estuarine CO 2 budgets (Borges et al., 2005;Laruelle et al., 2010;Cai, 2011;Chen et al., 2013;Laruelle et al., 2013). The standard deviation calculated for the emission rates of all local studies was BGD 11,2014 Seasonal response of air-water CO 2 exchange G. G. Laruelle et al.

Continental shelf waters
Monthly CO 2 exchange rates at the air-water interface were calculated in continental shelf waters using 274 291 pCO 2 measurements extracted from the SOCAT 2.0 database (Baker et al., 2014). For each measurement, an instantaneous local CO 2 exchange rate with the atmosphere was calculated using Wanninkhof's equation (Wan-5 ninkhof, 1992) which is a function of a transfer coefficient (k), dependent on the square of the wind speed above sea surface, the apparent solubility of CO 2 in water (K 0 ), which depends on surface water temperature and salinity, and the gradient of pCO 2 at the air-water interface (∆pCO 2 ).
The parameterization used for k is that of Wanninkhof et al. (2013) and all the data necessary for the calculations are available in SOCAT 2.0 except for wind speed, which was extracted from the CCMP database (Altas et al., 2011). The resulting CO 2 exchange rates were then averaged per month for each 0.25 • cell in which data were 15 available. Monthly F CO 2 for the North and South sections were then extrapolated using the water surface area and weighted rate for each cell, multiplied by the total surface area A s of the corresponding section. To refine further the budget, a similar procedure was also applied to 5 depth segments (S1 to S5) corresponding to 0-20 m, 20-50 m, 50-80 m, 80-120 m and 120-150 m, respectively, and their respective surface areas 20 were extracted from a high resolution bathymetric files (Laruelle et al., 2013). The choice of slightly different methodologies for F CO 2 calculations in rivers and continental shelf waters stems from the better data coverage in the continental shelf, which allows capturing the spatial heterogeneity within the region without using interpolation techniques. The standard deviation calculated for all the grid cells of the integration BGD 11,2014 Seasonal response of air-water CO 2 exchange G. G. Laruelle et al.  Figure 2 shows the spatial distribution of F CO 2 along the LOAC integrated per season. Throughout the year, river waters are a strong source of CO 2 for the atmosphere. Significant differences in the intensity of the CO 2 exchange at the air-water interface can nevertheless be observed between the North and South sections, both in time and 5 space. During winter, there is nearly no CO 2 evasion from rivers in the North due to ice coverage and stream drying. Over the same period, the CO 2 emissions from the South section range from 0 to 5 g C m −2 season −1 . During spring, the pattern is reversed and northern rivers exhibit higher outgassing rates than in the South with maximum emissions rates of > 10 g C m −2 season −1 . This trend is maintained throughout summer while during fall, the entire COSCAT displays similar emission rates without clear latitudinal signal. Continental shelf waters display a very different spatial and seasonal pattern than that of rivers. During winter, the North section is predominantly a mild CO 2 sink, with rates comprised between +2 and −5 g C m −2 season −1 , which intensifies significantly in the South section (−2 to > −10 g C m −2 season −1 ). During spring, an 15 opposite trend is observed with a quasi-neutral CO 2 uptake in the South and a strong uptake in the North, especially on the Scotian shelves. The entire COSCAT becomes a net CO 2 source in summer with emission rates as high as 5 g C m −2 season −1 in the Mid-Atlantic Bight. During fall, the Gulf of Maine and Georges Banks remain CO 2 sources while the Scotian shelves and the Mid-Atlantic Bight become again regions of 20 net CO 2 uptake. The monthly integrated F CO 2 for the North and South sections provides further evidence of the contrasting seasonal dynamics for the two areas ( Fig. 3a and b). In the North section, CO 2 evasion from rivers is almost zero in January and February, rises to a maximum value of 0.26 ± 0.05 Tg C month −1 in May, and then progres- ing top-soils, which is rich in DOC and CO 2 , combined to increasing in-stream respiration rates induced by warmer water temperatures (Jones and Mulholland, 1998;Striegl et al., 2012). Compared to rivers, the continental shelf in the North section presents a close mirror behavior from winter through spring, with a mild carbon uptake rate in January and February (−0.04 ± 0.25 Tg C month −1 ) followed by a maxi-5 mum uptake rate in April (−0.50 ± 0.20 Tg C month −1 ). This CO 2 uptake in spring has been attributed to photosynthesis associated to the seasonal phytoplankton bloom (Shadwick et al., 2010). Continental shelf waters behave quasi neutral during summer (< 0.05 ± 0.09 Tg C month −1 ) and emit CO 2 at a high rate in November and December (> 0.15 ± 0.21 Tg C month −1 ). Overall, the rivers of the North section emit In the South section of the COSCAT, the warmer winter temperature leads to the 20 absence of ice cover (Armstrong and Brodzik, 2001 following that of water temperature, is consistent with the hypothesis of a CO 2 exchange in the South section regulated by variations in gas solubility, as suggested by Degrandpré et al. (2002) for the Mid-Atlantic Bight. The analysis of the intensity of the river CO 2 outgassing reveals that the smallest streams (Q < 1 m 3 s −1 , Q1 in Table 1) display the highest emission rates per unit sur-5 face area, with values ranging from 1961 g C m −2 yr −1 in the South section to 2893 g C m −2 yr −1 in the North section. These values gradually decrease with increasing river discharge to 729 g C m −2 yr −1 in the South section and 891 g C m −2 yr −1 in the North section for Q > 100 m 3 s −1 (Q4, Table 1). The emission rates for this latter class of rivers are consistent with the median emission rate of 720 g C m −2 yr −1 proposed by 10 Aufdenkampe et al. (2011) for temperate rivers with widths larger than 60-100 m. Aufdenkampe et al. (2011) also report a median emission rates of 2600 g C m −2 yr −1 for the smaller streams and rivers, which falls on the high end of the range calculated for Q1 in the present study. The surface area of the river network is relatively evenly distributed amongst the four discharges classes of rivers (Table 1). Yet, river sections 15 for which Q < 10 m 3 s −1 (Q1 + Q2) contribute to 65 % of the total CO 2 outgassing although they only represent 51 % of the surface area. This result therefore highlights that streams and small rivers are characterized by the highest surface-area specific emission rates. On the continental shelf, the shallowest depth interval is a CO 2 source while all other depth intervals are CO 2 sinks ( Table 1). The magnitude of the air-sea 20 exchange for each segment is comprised between the values calculated for estuaries (50 g C m −2 yr −1 ) and the nearby open ocean (∼ 20 g C m −2 yr −1 , according to Takahashi et al., 2009). This trend along a depth transect, suggesting a more pronounced continental influence on near-shore waters was already discussed in the regional analysis of Chavez et al. (2007) and by Jiang et al. (2013) specifically for the South Atlantic Bight. Modeling studies over a larger domain including the upper slope of the continental shelf also suggest that the coastal waters of the North East US are not a more intense CO 2 sink than the neighboring open ocean Fennel, 2010). Our analysis further suggests that the continental influence is more pronounced BGD 11,2014 Seasonal response of air-water CO 2 exchange G. G. Laruelle et al. in the North section. Here, the shallowest waters (S1) are strong net sources of CO 2 while the intensity of the CO 2 sink for the other depth intervals gradually decreases, but only to a maximum value of −4 g C m −2 yr −1 for S5. This value is about 3 times smaller than in the South section (−12 g C m −2 yr −1 ). Annually, river and estuarine waters of the entire COSCAT 827 outgas 3.0 ± 5 0.5 Tg C yr −1 and 0.8±0.5 Tg C yr −1 , respectively, while continental shelf waters take up 1.7 ± 0.3 Tg C yr −1 (Fig. 3c). The total riverine carbon load exported from rivers to estuaries for the same area has been estimated to 4.65 Tg C yr −1 , 45 % as dissolved and particulate organic carbon (2.10 Tg C yr −1 , Mayorga et al., 2010) and 55 % as dissolved inorganic carbon (2.55 Tg C yr −1 , Hartmann et al., 2009). Estimates of the total amount 10 of terrestrial carbon transferred to the riverine network are not available but the sum of the river export and the outgassing, which ignores the contribution of carbon burial and lateral exchange with wetlands, provides a lower bound estimate of 7.65 Tg C yr −1 . Under this hypothesis, ∼ 40 % of the terrestrial carbon exported to rivers is emitted to the atmosphere before reaching estuaries. In spite of higher emission rates per unit 15 surface area in the North (Table 1), the overall efficiency of the riverine carbon filter is essentially the same in the two sections (40 % and 38 % outgassing for the North and the South, respectively). On the shelf, however, the South section exhibit a significantly more intense CO 2 sink (−1.25±0.2 Tg C yr −1 ) than in the North (−0.47±0.2 Tg C yr −1 ).

Results and discussion
A possible reason for this difference can be found in the contribution of the estuarine 20 carbon filter. In the South, where 96 % of the estuarine surface area is located, these systems contribute to an outgassing of 0.73 Tg C yr −1 while in the North, their influence is negligible. Cole and Caraco (2001) estimated that 28 % of the DOC entering the relatively short Hudson River estuary is respired in-situ before reaching the continental shelf and it is thus likely that the estuarine outgassing in the South section is 25 fueled by the respiration of the organic carbon loads from rivers. In contrast, the absence of estuaries in the North favors the direct export of terrestrial organic carbon onto continental shelf waters where it can be buried and decomposed. The respiration of terrestrial organic carbon could therefore explain why the strength of the shelf CO 2 sink is weaker in this portion of the domain. This view is further substantiated by the similar cumulated estuarine and continental shelf F CO 2 fluxes in both sections ( Fig. 3a  and b). Naturally, other environmental and physical factors, such as, for example, local coastal currents , temperature changes and phytoplankton growth could also contribute to the difference in CO 2 uptake intensity between both sections.

5
Additionally, modeling studies evidenced the potential influence of sediment denitrification on water pCO 2 through the removal of fixed nitrogen in the water column and consequent inhibition of primary production (Fennel et al., 2008;Fennel, 2010). This removal was estimated to be of similar magnitude as the lateral nitrogen loads, except for estuaries of the MAB region (Fennel, 2010). It can nonetheless be suggested that 10 the estuarine carbon filter in the South section of COSCAT 827 is an important control factor of the CO 2 sink in the Mid-Atlantic Bight, which is stronger than in any other area along the entire Atlantic coast of the US (Signorini et al., 2013).

Conclusions
Our spatially and seasonally resolved budget analysis captures the main characteris- 15 tics of the air-water CO 2 exchange along the LOAC of COSCAT 827. It evidences the contrasting dynamics of the North and South section of the study area and an overall gradual shift from a strong source in small streams oversaturated in CO 2 towards a net sink in continental shelf waters. Our study also reveals the role of ice and snow cover as an important controlling factor of the seasonal dynamics of CO 2 outgassing in 20 streams and rivers. Additionally, the incorporation of the LOAC as a whole supports an integrated analysis that highlights the contribution of estuaries as filters of the terrestrial carbon inputs and their influence on the continental shelf carbon uptake. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 177. The Surface Ocean CO 2 Atlas (SOCAT) is an international effort, supported by the International Ocean Carbon Coordination Project (IOCCP), the Surface Ocean Lower Atmosphere Study (SOLAS), and the Integrated Marine Biogeochemistry and Ecosystem Research program (IMBER), to deliver a uniformly quality-controlled surface ocean CO 2 database. The many researchers and funding agencies responsible for the collection of data and quality control are 5 thanked for their contributions to SOCAT. This work also used data extracted from the SO-CAT/MARCATS segmentation (Laruelle et al., 2013), the CCMP wind database (Atlas et al., 2011), GLOBALNEWS2 (Mayorga et al., 2010;Hartmann et al., 2009), the SRTM water body data set (NASA/NGA, 2003), Hydrosheds 15s routing scheme, the average annual runoff data extracted from the UNH/GRDC composites (Fekete et al., 2002), the global lake and wetland 10 data base of Lehner and Döll (2004) and mean air temperature derived from the worldclim data set of Hijmans et al. (2005).  Merlivat, L., Metzl, N., Murata, A., Newberger, T., Omar, A. M., Ono, T., Park, G.-H., Paterson, K., Pierrot, D., Ríos, A. F., Sabine, C. L., Saito, S., Salisbury, J., Sarma, V. V. S. S., Schlitzer, R., Sieger, R., Skjelvan, I., Steinhoff, T., Sullivan, K. F., Sun, H., Sutton, A. J., Suzuki, T., Sweeney, C., Takahashi  Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Wang, Z. A., Wanninkhof, R., Cai, W.-J., Byrne, R. H., Hu, X., Peng, T. H., and Huang, W. J.: