Aragonite saturation states and pH in western Norwegian fjords: seasonal cycles and controlling factors, 2005–2009

Abstract

Abstract. The uptake of anthropogenic carbon dioxide (CO2) by the ocean leads to a process known as ocean acidification (OA), which lowers the aragonite saturation state (ΩAr) and pH, and this is poorly documented in coastal environments including fjords due to lack of appropriate observations.Here we use weekly underway data from the Voluntary Observing Ships (VOS) program covering the period 2005–2009 combined with data from research cruises to estimate ΩAr and pH values in several adjacent western Norwegian fjords, and to evaluate how seawater CO2 chemistry drives their variations in response to physical and biological factors.The OA parameters in the surface waters of the fjords are subject to strong seasonal and spatially coherent variations. These changes are governed by the seasonal changes in temperature, salinity, formation and decay of organic matter, and vertical mixing with deeper, carbon-rich coastal water. Annual mean pH and ΩAr values were 8.13 and 2.21, respectively. The former varies from minimum values ( ≈ 8.05) in late December – early January to maximum values of around 8.2 during early spring (March–April) as a consequence of the phytoplankton spring bloom, which reduces dissolved inorganic carbon (DIC). In the following months, pH decreases in response to warming. This thermodynamic decrease in pH is reinforced by the deepening of the mixed layer, which enables carbon-rich coastal water to reach the surface, and this trend continues until the low winter values of pH are reached again. ΩAr, on the other hand, reaches its seasonal maximum (> 2.5) in mid- to late summer (July–September), when the spring bloom is over and pH is decreasing. The lowest ΩAr values ( ≈ 1.3–1.6) occur during winter (January–March), when both pH and sea surface temperature (SST) are low and DIC is its highest. Consequently, seasonal ΩAr variations align with those of SST and salinity normalized DIC (nDIC).We demonstrate that underway measurements of fugacity of CO2 in seawater (fCO2) and SST from VOS lines combined with high frequency observations of the complete carbonate system at strategically placed fixed stations provide an approach to interpolate OA parameters over large areas in the fjords of western Norway.

Trends of Ocean Acidification and pCO2 in the Northern North Sea, 2003–2015

“…The seasonal changes in DIC and TA are the most important drivers for changes in Ω ar whereas the effects of variations of SST and SSS (not shown) on Ω ar are negligible. This is in agreement with Omar et al (), who analyzed data from the southwestern Norwegian coast using a different method and concluded that seasonal changes in DIC and TA are the most important drivers for changes in Ω ar , and variations of SST and SSS are less relevant. Again, we note that the TA influence is enhanced in the Skagerrak region compared to the NAW region.…”

Seasonal Changes in Carbonate Saturation State and Air‐Sea CO2 Fluxes During an Annual Cycle in a Stratified‐Temperate Fjord (Reloncaví Fjord, Chilean Patagonia)

“…Previous studies have shown that SAMI‐pH and SAMI‐CO 2 sensors are stable over a time period of several months, showing no significant drift during a deployment period (Omar et al, ), although the p CO 2 sensors require in situ validation during deployment (Cullison‐Gray et al, ; DeGrandpre et al, ). The differences between discrete and sample p CO 2 and pH, along with the comparison of in situ and calculated values, are shown and discussed below.…”

Attributing seasonal pH variability in surface ocean waters to governing factors

“…Moreover, and most importantly, they also found that at all sites T, DIC, and TA are the principal drivers of seasonal pH variability. Omar et al [2016] studied seasonal cycles of pH in western Norway fjords and found that the combined effect of T and TA changes virtually compensates for the opposing effect of DIC changes on seasonal pH variability. Figure 2 shows the quantitative contributions of changes in T, DIC, and TA on pH seasonality for two sites in the Southern Ocean (Drake Passage areas 1 and 4), the Iceland Sea station, DYFAMED, Bermuda Atlantic Time-series Study (BATS), European Station for Time series in the Ocean Canary Islands (ESTOC), and Station ALOHA.…”