Two decades of inorganic carbon dynamics along the Western Antarctic Peninsula

23 We present 20 years of seawater inorganic carbon measurements collected along the western shelf and slope of the Antarctic Peninsula. Water column observations from 25 summertime cruises and seasonal surface underway pCO 2 measurements provide unique 26 insights into the spatial, seasonal and interannual variability of this dynamic system. Discrete measurements from depths >2000 m align well with World Ocean Circulation Experiment observations across the time-series and underline the consistency of the data set. Surface total alkalinity and dissolved inorganic carbon data showed large spatial 30 gradients, with a concomitant wide range of Ω arag (< 1 up to 3.9). This spatial variability 31 was mainly driven by increasing influence of biological productivity towards the 32 southern end of the sampling grid and melt water input along the coast towards the 33 northern end. Large inorganic carbon drawdown through biological production in 34 summer caused high near-shore Ω arag despite glacial and sea-ice melt water input. In 35 support of previous studies, we observed Redfield behavior of regional C/N nutrient 36 utilization, while the C/P (80.5 ± 2.5) and N/P (11.7 ± 0.3) molar ratios were significantly 37 lower than the Redfield elemental stoichiometric values. Seasonal salinity-based predictions of Ω arag suggest that surface waters remained mostly supersaturated with 39 regard to aragonite throughout the study. However, more than 20 % of the predictions for 40 winters and springs between 1999 and 2013 resulted in Ω arag < 1.2. Such low levels of 41 Ω arag may have implications for important organisms such as pteropods. Even though we 42 did not detect any statistically significant long-term trends, the combination of ongoing 43 ocean acidification and freshwater input may soon induce more unfavorable conditions 44 than the ecosystem experiences today.


Abstract 23
We present 20 years of seawater inorganic carbon measurements collected along the 24 western shelf and slope of the Antarctic Peninsula. Water column observations from 25 summertime cruises and seasonal surface underway pCO 2 measurements provide unique 26 insights into the spatial, seasonal and interannual variability of this dynamic system. 27 Discrete measurements from depths >2000 m align well with World Ocean Circulation 28 Experiment observations across the time-series and underline the consistency of the data 29 set. Surface total alkalinity and dissolved inorganic carbon data showed large spatial 30 gradients, with a concomitant wide range of Ω arag (< 1 up to 3.9). This spatial variability 31 was mainly driven by increasing influence of biological productivity towards the 32 southern end of the sampling grid and melt water input along the coast towards the 33 northern end. Large inorganic carbon drawdown through biological production in 34 summer caused high near-shore Ω arag despite glacial and sea-ice melt water input. In 35 support of previous studies, we observed Redfield behavior of regional C/N nutrient 36 utilization, while the C/P (80.5 ± 2.5) and N/P (11.7 ± 0.3) molar ratios were significantly 37 lower than the Redfield elemental stoichiometric values. Seasonal salinity-based 38 predictions of Ω arag suggest that surface waters remained mostly supersaturated with 39 regard to aragonite throughout the study. However, more than 20 % of the predictions for 40 winters and springs between 1999 and 2013 resulted in Ω arag < 1.2. Such low levels of 41 Ω arag may have implications for important organisms such as pteropods. Even though we 42 did not detect any statistically significant long-term trends, the combination of ongoing 43 ocean acidification and freshwater input may soon induce more unfavorable conditions 44 than the ecosystem experiences today.  (Figures 2b and c). After removing five outliers, mean deep-water DIC (DIC mean 175 = 2260.6 ± 3.8 µmol kg -1 ) and TA (TA mean = 2365.4 ± 7.0 µmol kg -1 ) from PAL-LTER 176 cruises corresponded well with the data measured/calculated from WOCE cruises 177 (DIC mean = 2261.8 ± 3.0 µmol kg -1 ; TA mean = 2365.9 ± 9.3 µmol kg -1 ). 178 179

Comparison with underway-surface pCO 2 data 180
We also undertook a quality check of the PAL-LTER discrete surface DIC and TA data 181 (depth < 5 m) by comparing PAL-LTER pCO 2 , which was calculated using observed 182 DIC and TA values, to LDEO pCO 2 . LDEO pCO 2 samples that were collected during the 183 PAL-LTER cruises were spatially matched with the PAL-LTER derived pCO 2 values by 184 choosing the nearest latitude and longitude pair within a 1 km distance. Four PAL-LTER 185 pCO 2 outliers that underestimate/overestimate pCO 2 relative to the underway 186 observations by more than 150 µatm were removed. Analysis of the corrected data set 187 with a Linear Regression Type II model suggests a correlation of r = 0.82 ( Figure A1, 188 Table 1). Some of the observed discrepancies may be attributed to errors in matching the 189 times of bottle samples with those of underway pCO 2 measurements. Seawater inorganic 190 carbon chemistry is highly variable along the WAP due to the influence of productivity, 191 respiration, freshwater and upwelling of CO 2 -rich subsurface water [Carrillo et al., 192 2004]. Small matching errors may therefore introduce small DIC and TA offsets, which 193 would translate into larger fractional differences in pCO 2 due to the large Revelle Factor 194 (∂ ln pCO 2 / ∂ ln DIC) common in the region [Sarmiento and Gruber, 2006]. Here, we examine the observed spatial summer patterns of DIC, TA, pHT and Ω arag along 198 the WAP and explore the underlying biological and physical drivers. We then discuss 199 regional carbon -nutrient drawdown ratios and present our seasonal Ω arag predictions that 200 give initial insights into the chemical environment in the more poorly sampled spring, fall 201 and winter months. Finally, using the LTER and LDEO data sets, we investigate temporal 202 trends over the past two decades. where samples were taken in more than 5 years ( Figure 4). The resulting pCO 2 , pHT, 219 Ω arag , TA, salinity, DIC, and nutrient fields exhibited clear onshore -offshore gradients. 220 With the exception of DIC, all variables also followed a north-south gradient. Mean 221 summertime surface pCO 2 was lowest (<200 µatm) in the southern coastal region and was 222 about 60 to 70 µatm lower than in the northern near-shore regions ( Figure 4a). The 223 highest mean summertime pCO 2 values were found in the northern slope region (300-325 224 µatm). The opposite pattern was reflected in Ω arag and pHT, with highest values (Ω arag max 225 = 2.6 and pHT max 8.3) close to the coast and south of 66.5°S (Figures 3b and c), 226 decreasing along the coast towards the north to pHT ~8.2 and Ω arag ~1.9, and reaching 227 the lowest levels in northern offshore waters (pHT min = 8.1; Ω arag min = 1.7). TA also 228 exhibited north-south and onshore -offshore gradients, with values as low as 2185 µmol 229 kg -1 in the northern near-shore regions and as high as > 2300 µmol kg -1 offshore. The low 230 TA values along the northern part of the coast coincided with the lowest salinity values of 231 31.8, suggesting dilution of TA due to freshwater input (Figures 3d and e). Higher TA 232 values offshore were also reflected in increased DIC and salinity concentrations, with 233 temperatures between 1.3 -1.5 °C. DIC also exhibited an onshore-offshore gradient with 234 values about 80 to 100 µmol kg -1 lower in the near shore region compared to offshore, but 235 there was no significant north-south gradient despite the presence of freshwater in the 236 north ( Figure 4f). Salinity normalized DIC (sDIC, normalized with UCDW salinity = 237 34.7) was lowest in the southern region, thereby indicating that biological processes 238 likely counteracted the expected north-south DIC gradient due to the pronounced 239 freshwater influence on DIC in the north (Figure 4g). 240 241

Physical and biological drivers of the inorganic carbon system 242
In this section we examine the physical and biological mechanisms that control the 243 observed variability in DIC and TA. DIC can decrease (increase) through dilution with 244 freshwater (evaporation), organic matter production (remineralization), CO 2 outgassing to 245 the atmosphere (CO 2 uptake) and/or precipitation of CaCO 3 (dissolution). While positive 246 net community production decreases DIC, the biological effect of organic matter 247 production on TA depends on the source of nitrogen, where nitrate consumption 248 increases TA and ammonium consumption decreases TA [Goldman and Brewer, 1980].  kg -1 is visible in the winter water (grey diamonds), which increased to more than 200 266 µmol kg -1 in the mixed layer, leading to Ω arag as low as 1.5 and as high as 3.9. 267 The DIC drawdown relative to the salinity mixing-dilution line is most likely due 268 to biological production of organic matter. Figure 6 shows sDIC as a function of salinity-269 -3 , which leads to this slightly shallower slope of 6.2. 280 The intense, biologically driven DIC drawdown and resulting pCO 2 281 undersaturation in the mixed layer may have led to some CO 2 uptake from the 282 atmosphere that tends to reduce the apparent DIC deficit; thus the estimated biological 283 drawdown from observed DIC values in Figure 6 may be underestimated and needs to be 284 corrected for air-sea CO 2 gas exchange from the period of biological drawdown to the 285 sampling time. To account for DIC concentration changes due to gas exchange with the 286 atmosphere, we assumed a constant atmospheric concentration of 390 µatm between 287 1993 and 2012, and a gas transfer rate (k) of 5 (±1) milli-mol CO 2 m -2 µatm -1 month -1 , 288 which is the estimated mean rate for the Southern Ocean area south of 62 °S [Takahashi 289 et al. 2009]. The change in DIC (µmol kg -1 month -1 ) due to gas transfer into the mixed 290 layer (ML) of d meters depth is: 291 ΔpCO 2 (pCO 2 atm -pCO 2 ML ) was between -143 µatm and 312 µatm, as pCO 2 ML ranged 293 from 533 µatm to 78 µatm, indicating that there was potential for both oceanic CO 2 294 uptake and outgassing. Assuming that d = 50 m [Ducklow et al., 2013], we estimate that 295 the monthly ΔDIC due to air-to-sea CO 2 gas exchange was in the range of -14 to 31 µmol 296 kg -1 month -1 . Since the first large phytoplankton blooms generally occur after the sea-ice 297 retreats in November (Δt ~3 months), we assume that by the time of sampling at the end 298 of January, ΔDIC would fall in the range -43 to 94 µmol kg -1 . The DIC corrected for gas 299 exchange is illustrated as grey dots in Figure 6. While applying the gas exchange

Nutrient vs. carbon drawdown 306
Ocean carbon, nitrogen and phosphorus cycles are governed by organic matter production 307 and subsequent remineralization and are strongly correlated on a global average with the 308 proportions C/N/P = 106:16:1 [Redfield, 1958]. Our findings suggest that the carbon-309 nutrient cycles along the WAP depart from the standard Redfield values (Figure 7). In a 310 few samples, the standing stock of PO 4 3became depleted before NO 3 -, and overall the 311 regression indicates a low N:P ratio of 9.8 ± 0.4 in the mixed layer (Figure 7a, black) and 312 N:P = 11.7 ± 0.3 for all data (dark grey) relative to the standard Redfield value of 16 313 molN/mol P. The mole/mole C:P ratio was also considerably smaller than the Redfield 314 ratio ( Figure 7b). C:P yielded 43.1 ± 2.3 in the mixed layer and 55.0 ± 1.7 for all data. 315 However, after applying the gas exchange correction on DIC (see section 3.2), the C:P 316 ratio shifted closer to the Redfield Ratio and resulted in a value of 80.5 ± 2.5 (light grey 317 dots and lines). Correcting the DIC for gas exchange shifted the molar ratio from 4.5 ± 318 0.2 (mixed layer depth) and 4.7 ± 0.1 (all data) to 6.7 ± 0.2 and resulted in a Redfield-like 319 C:N ratio. 320 321

Seasonal variability 322
To get insights into the carbon dynamics during winter, spring, and fall, when direct 323 measurements of DIC, TA and nutrients are either scarce or not available, we developed a 324 regional TA algorithm (based on PAL-LTER summertime data). In combination with 325 seasonal LDEO pCO 2 , salinity and temperature data, we calculated Ω arag for the missing 326 seasons. Due to the weak correlation between PAL-LTER temperature and TA (r = 0.50), 327 we based the TA algorithm on salinity only ( Figure A2, r = 0.88). Applying the Akaike 328 information criterion [Burnham and Anderson, 2002], we determined that TA along the 329 WAP will be best represented by a first order linear model. We then randomly divided 330 the PAL-LTER surface measurements (depth <5 m) into 10 data subsets using the 10-fold 331 cross validation method [Stone, 1974;Breiman, 1996]. Using 9 of the ten data sets we 332 derived a regression model, predicted the TA with the model, and calculated the model 333 coefficients and root mean square errors (RMSE). We repeated these steps so every data 334 subset was left out once. The coefficients for the final model were calculated from the 335 mean of the ten regression coefficients. We found the best fit in the following equation:

Temporal trends 365
Trend analysis of the PAL-LTER data showed no statistically significant annual trends 366 (at the 95% confidence level) in the measured carbon parameters, temperature or salinity 367 in surface waters in summer between 1993 and 2012 ( Table 2). As a comparison, we 368 conducted a trend analysis for the LDEO surface underway pCO 2 data set (1999 -2013) 369 in the same region. LDEO observations show an increasing, but not statistically 370 significant trend in surface pCO 2 , supporting our results above (Table 3). The largest 371 increasing trend was found in fall, (1.9 ± 0.95 µatm yr -1 ), but this trend was also slightly 372 outside the confidence interval and therefore statistically not significant. The 20 year-long PAL-LTER seawater inorganic carbon time-series showed a distinct 376 upper-ocean spatial pattern of onshore-offshore and north -south gradients and suggests 377 that the summertime carbon dynamics are primarily controlled by biological productivity 378 and freshwater input in near-shore areas. 379 Surface Ω arag was distributed across a wide range (<1 to values > 3) in freshwater- Subantarctic South Pacific [Hales and Takahashi, 2012]. Consistent with the low N/P 438 ratio, the observed C:P ratio (80.5 ± 2.5 ,corrected for gas exchange) was also lower than 439 the classic Redfield ratio. This indicates that the regional phosphate cycle shows non-440 Redfield behavior, which is in agreement with the observed C:P ratio of 91.4 ± 7.9 in the 441  The large uncertainties in our estimated temporal trends are caused inherently by 482 the large spatial and temporal variability of our data. Nevertheless, our mean rates of 1.45 483 ± 2.97 for summer and 0.43± 0.77 µatm yr -1 for winter suggest that the surface water 484 pCO 2 has been increasing at a slower rate than the atmospheric pCO 2 rate of about 1.9 485 µatm yr -1 , and that the air-to-sea CO 2 driving potential has been increasing. Our results This study gives new insights into the spatial and temporal variability of the WAP 508 inorganic carbon system and its main physical and biological drivers. In particular, we 509 found that large inorganic carbon drawdown through biological production in summer 510 caused high near-shore Ω arag , despite glacial and sea-ice melt water input. Furthermore, 511 the data do not show a significant long-term trend in any of the inorganic carbon 512 chemistry variables measured. Continuation and expansion of the inorganic carbon 513 chemistry timeseries across other seasons is necessary to distinguish between natural 514 variability and secular trends and to better understand synergistic effects of ocean 515 acidification and climate change. Due to the region's physical complexity of circulation 516 and forcing, and strong dynamic response to climate variability, we recommend 517 development of a highly resolved biogeochemical model to complement our 518 observational work. Implementation of modeling studies will improve our mechanistic 519 understanding of how interannual variability and anthropogenic climate change impact 520 the inorganic carbon chemistry along the WAP, which is imperative to predict the 521 potential impact on the unique WAP ecosystem. 522

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Author Contributions 524 Designed research: HD and TT. Field sampling and analytical measurements: TT, HD 525 and ME. Data analysis and interpretation: CH with help from all co-authors. Wrote the 526 paper: CH with help from SD, TT, and HD. 527 528 Acknowledgements 529 We thank past and present members of the Palmer LTER program as well as the captains 530 and crew of the U.S. Antarctic research vessels. We are especially grateful to Richard 531 Iannuzzi and James Conners for their support with data management, and to Tim 532 Newberger for underway pCO 2 measurements. We gladly acknowledge support from the 533 National