*3.3. Carbonate Chemistry in the Water Column*

Concentrations of dissolved inorganic carbon (CT) and total alkalinity (AT) increased with depth at all sites and times. Below 50 m (Figure 5), the variability between the sites was largest in winter (February 2016 and January 2017) and smallest in autumn (August 2016 and October 2017). Variability was higher at 200 m than at intermediate depths of 100–150 m. Relatively low surface values (AT < 2100 μmol kg−<sup>1</sup> and CT < 2000 μmol kg−1) occurred in autumn (August 2017 and October 2016) (Table 3). Both the lowest (CT = 2135 μmol kg<sup>−</sup>1) and the highest (CT = 2192 μmol kg<sup>−</sup>1) CT values at CWC living depths were measured at the NK bank reef at 200 m depth during the 16 h sampling period in January 2017. The highest AT value (2343 μmol kg−1) was also measured during the same period and place. The lowest AT of 2300 μmol kg−<sup>1</sup> was measured at HH wall reef at 200 m depth in August 2017. In general, the northernmost sites (BV and SN) had higher AT in autumn and higher CT in winter and autumn at 200 m depth than sites farther north, but short-term variability recorded over tidal cycle at a single location (HH or NK) was larger than this between sites difference.

pHT ranged from 7.94 to 8.08 throughout the water column over the study period (Figure 5, Table 3). Low values (<7.98) were measured at depths >140 m at BV bank reef and at depths >100 m at NK bank reef in winter (January 2017 and February 2016), at ~80 m depth at wall reefs HH and SN in May 2016, at ~200 m depth at HH and NK in April 2017 and at depths >100 m at all sites except BV in autumn with a minimum at 150 m depth. High values of >8.03 were measured at the surface (<40 m) at all sites and times (Table 3). Below 80 m, high values (>8.03) were measured at HH at ~100 m and at NK at ~ 200 m depth in winter (January 2017 and February 2016) and in April 2017. The pCO2 ranged from 293 to 452 μatm above 80 m. Beneath 80 m, it ranged from 356 μatm at ~200 m at NK in January 2017 to 506 μatm at ~200 m at HH in October 2016 (Figure 5, Table 3). Above 80 m, ΩAr ranged from 1.41 at NK in January 2017 to 2.28 at NK in August 2017, and below 80 m, it ranged from 1.50 at ~200 m at HH in October 2016 to 2.00 at ~200 m at NK in January 2017. ΩAr and ΩCa had maxima at the surface in autumn

(August 2017 and October 2016) (ΩAr > 2.1 and ΩCa > 3.3). The low (ΩAr < 1.59) aragonite saturation levels were measured at 150–200 m depths at wall reefs (HH, SN, HN) in autumn (August 2017 and October 2016), at ~200 m at HH in April 2017, and at depths >140 m at BV in January 2017 (Figure 5, Table 3).

**Figure 5.** Depth profiles (y-axes) of carbonate chemistry at depths 50–250 m of the Langenuen Fjord: (first row) winter (February 2016 and January 2017), (second row) spring (April 2017 and May 2016), and (third row) autumn (August 2017 and October 2016), for (first column) total alkalinity, AT, (second column) total inorganic carbon, CT, with 2070 μmol kg−<sup>1</sup> marked, (third column) pHT, (fourth column) partial CO2 pressure, pCO2, and (fifth column) aragonite saturation, ΩAr. The errorbars show the range of the variable (min, max) for cruises with multiple CTD cast samples at one station, and the marker shows the median for the station (HH—blue, NK—gray, HN—green, SN—orange, BV—purple) over the cruises. The 2016 measured values are shown with asterisks (\*) and 2017 with circles (o). The dashed line represents the definition for depths of the living CWCs (80–240 m).

Carbonate chemistry sampling was performed over several hours at two stations at two different times: At wall reef HH (corals at depths 80–220) in October 2016 and at bank reef NK (corals at depths 190–220) in January 2017. During a 26 h sampling period at 200 m depth at the HH wall reef in October 2016, Θ and SA were relatively stable with ranges of ΔΘ = 0.14 ◦C and ΔSA = 0.11 g kg−<sup>1</sup> (Figure 6). Over the sampling period, the carbonate system parameters changed by: ΔAT = 10 μmol kg−1, ΔCT = 10 μmol kg−1, ΔpHT = 0.032, ΔpCO2 = 45 μatm, ΔΩAr = 0.12, and ΔΩCa = 0.19. This is comparable to the seasonal variability between winter and summer from single measurements at HH at this depth ΔAT = 11 μmol kg<sup>−</sup>1, ΔCT = 34 μmol kg<sup>−</sup>1, ΔpHT = 0.112, ΔpCO2 = 123.6 μatm and ΔΩAr = 0.4 (Figure 6). The flow direction at nearby Nakken reef was southward with speeds between 4 and 43 cm s<sup>−</sup>1.

**Figure 6.** Short time series (x-axes) of carbonate system fluctuations on 20–21 October 2016 at Huglhammaren wall reef (left panels) and on 3–4 January 2017 at Nakken bank reef (right panels) at 200 m depth for (first row) temperature, Θ (o) and salinity, SA (\*), (second row) total alkalinity, AT (o) and total inorganic carbon, CT (\*), (third row) pHT (o) and partial CO2 pressure, pCO2 (\*), (fourth row) aragonite saturation, ΩAr (o) and calcite saturation, ΩCa (\*), and (fifth row) 1 h running mean of the horizontal flow at 200 m depth from SLM lander at NK bank reef, U with southward (solid line), and northward (dashed line) flow.

Short-term temporal variability of carbonate parameters at Nakken bank reef in January 2017 was larger than the short-term variability at Huglhammaren in October 2016. During this sampling period of 16 h, Θ and SA at 200 m varied from warm (Θ >8 ◦C) and less saline (SA < 35.23 g kg−1) conditions to cool and saltier conditions and back. The overall ranges were ΔΘ = 0.32 ◦C and ΔSA = 0.05 g kg−<sup>1</sup> (Figure 6). The carbonate chemistry followed the changes in flow direction with high CT and low AT values observed with northward flow (=warm and fresh phase). The highest AT value (2343 μmol kg<sup>−</sup>1) was also measured during northward flow conditions. Over the sampling period, the carbonate system parameters showed large ranges of: ΔAT = 26 μmol kg−1, ΔCT = 57 μmol kg−1, ΔpHT = 0.11, ΔpCO2 = 118 μatm, ΔΩAr = 0.41, and ΔΩCa = 0.64. This is larger than the seasonal variability between spring and summer from single measurements at NK at this depth ΔAT = 10 μmol kg−1, ΔCT = 41 μmol kg−1, ΔpHT = 0.04, ΔpCO2 = 47.6 μatm, and ΔΩAr = 0.14 (Figure 6).
