*3.4. Inorganic Nutrients in the Water Column*

Within the uppermost 80 m, nitrite (NO2 −) and ammonium (NH4 +) concentrations ranged from 0.064 to 0.175 μmol L−<sup>1</sup> and from 0.075 to 1.04 μmol L<sup>−</sup>1, respectively, with the highest concentrations measured in May 2016 (not shown). Below 80 m, their concentrations were too low (<0.06 μmol L<sup>−</sup>1) for reliable measurements.

Nitrate (NO3 −), phosphate (PO4 <sup>3</sup>−), and silicate (SiO4 <sup>4</sup>−) concentrations all increased with depth in January 2017. In spring (May 2016 and April 2017), the concentrations were mixed beneath 80 m (Table 3, Figure 7). In January 2017 and April 2017, nutrient concentrations were higher at the southernmost (HH and NK) sites at depths >100 m than at sites farther north. In January 2017, the surface (<40 m) nutrient concentrations (<6.65, <0.51, and <3.26 μmol L−1) were higher than during spring (May 2016 and April 2017) (<5.13, <0.4, and <2.68 μmol L<sup>−</sup>1) (Table 3). Beneath 80 m, both the highest and the lowest nutrient concentrations were measured in January 2017. The highest concentrations were

measured at ~200 m depth at HH and the lowest at ~100 m depth at BV, HH, and NK. At CWC living depths, nutrient concentrations at the shallow wall (HH and SN) and deeper bank (NK and BV) reefs were on average for NO3 <sup>−</sup> ~10.34 and ~10.80 μmol L<sup>−</sup>1, for PO4 3− ~0.78 and ~0.77 μmol L<sup>−</sup>1, and for SiO4 <sup>4</sup><sup>−</sup> ~5.48 and ~5.41 μmol L<sup>−</sup>1, respectively (Table 3).

**Figure 7.** Depth profiles (y-axes) of biogeochemical water properties at depths 50–250 m of the Langenuen Fjord: (first row) January 2017, (second row) April 2017 and May 2016, for (first column) nitrate, NO3 −, (second column) phosphate, PO4 <sup>3</sup>−, (third column) silicate, SiO4 <sup>4</sup>−, (fourth column) carbon isotope, δ13C, (fifth column) oxygen isotope, δ18O. The errorbars show the range of the variable (min, max) for cruises with multiple CTD cast samples from one site, and the marker shows the median for the site (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 horizontal lines represent the definition for depths of the living CWCs (80–240 m).

At Nakken, CTD data and nutrient samples were collected from 100 to 200 m depth every three hours for 16 h between January 3–4, 2017 (Figure 8). The hydrographic range at 200 m is presented in Section 3.3. At 100 m depth, the variation patterns in Θ and SA differed from 200 m. They varied from cool (Θ < 8.7 ◦C) and saltier (SA > 34.9 g kg−1) conditions to warm and less saline and back. The overall ranges were ΔΘ = 0.14 ◦C and ΔSA = 0.10 g kg−<sup>1</sup> (Figure 8). The range of concentrations at 200 m and 100 m depth over the sampling period reached ΔNO3 <sup>−</sup> 0.88 and 0.73 μmol L−1, for ΔPO4 <sup>3</sup><sup>−</sup> 0.09 and 0.13 μmol L<sup>−</sup>1, and for ΔSiO4 <sup>4</sup><sup>−</sup> 0.84 and 0.32 μmol L<sup>−</sup>1, respectively. As for the carbonate system parameters, the change from low nutrient concentrations and low-temperature conditions to higher nutrient concentrations and warmer temperatures at 200 m depth coincided with flow velocity shift from strong southward velocity to low northward velocity (Figures 6 and 8).

**Figure 8.** Short time series (x-axes) of inorganic nutrient fluctuations on 3–4 January 2017 at Nakken bank reef at 200 m (solid line) and 100 m (dashed line) depth for (first row) temperature, Θ (o) and salinity, SA (\*), (second row) nitrate, NO3 −, (third row) phosphate, PO4 <sup>3</sup>−, and (fourth row) silicate, SiO4 <sup>4</sup>−.
