Seasonal dynamics of carbonate chemistry, nutrients and CO2 uptake in a sub-Arctic fjord

Abstract

Environmental change can have a significant impact on biogeochemical cycles at high latitudes and be particularly important in ecologically valuable fjord ecosystems. Seasonality in biogeochemical cycling in a sub-Arctic fjord of northern Norway (Kaldfjorden) was investigated from October 2016 to September 2018. Monthly changes in total inorganic carbon (CT), alkalinity (AT), major nutrients and calcium carbonate saturation (Ω) were driven by freshwater discharge, biological production and mixing with subsurface carbon-rich coastal water. Stable oxygen isotope ratios indicated that meteoric water (snow melt, river runoff, precipitation) had stratified and freshened surface waters, contributing to 81% of the monthly CT deficit in the surface layer. The timing and magnitude of freshwater inputs played an important role in Ω variability, reducing AT and CT by dilution. This dilution effect was strongly counteracted by the opposing effect of primary production that dominated surface water Ω seasonality. The spring phytoplankton bloom rapidly depleted nitrate and CT to drive highest Ω (~2.3) in surface waters. Calcification reduced AT and CT, which accounted for 21% of the monthly decrease in Ω during a coccolithophore bloom. Freshwater runoff contributed CT, AT and silicates of terrestrial origin to the fjord. Lowest surface water Ω (~1.6) resulted from organic matter remineralisation and mixing into subsurface water during winter and spring. Surface waters were undersaturated with respect to atmospheric CO2, resulting in modest uptake of –0.32 ± 0.03 mol C m–2 yr–1. Net community production estimated from carbon drawdown was 14 ± 2 g C m–2 yr–1 during the productive season. Kaldfjorden currently functions as an atmospheric CO2 sink of 3.9 ± 0.3 g C m–2 yr–1. Time-series data are vital to better understand the processes and natural variability affecting biogeochemical cycling in dynamic coastal regions and thus better predict the impact of future changes on important fjord ecosystems.

Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

“…High-latitude fjords and continental shelf regions are sinks for atmospheric carbon dioxide (CO 2 ) due to prominent undersaturation in surface water partial pressure (pCO 2 ) with respect to atmosphere, however, there exists large spatial-temporal variability as a result of heterogeneity in biogeochemical cycles and seasonal abiotic and biological processes (Takahashi et al, 2002;Bates, 2006;Chen et al, 2013;Yasunaka et al, 2016;Jones et al, 2020). The primary cause of undersaturation is complex but may be attributed to several combined processes, including:…”

“…However, the unique oceanographic characteristics of the semi-enclosed fjord systems add to the complexity of carbon cycling and relatively little is known about their role in global atmospheric uptake or release of carbon. For example, the influence of substantial freshwater inflow and strong spatialtemporal variability in phytoplankton blooms are known to strongly influence surface water pCO 2 and corresponding CO 2 flux (Rysgaard et al, 2012;Meire et al, 2015;Ericson et al, 2018Ericson et al, , 2019Jones et al, 2020). Yet, there is still poor seasonal and regional coverage of how these biophysical factors interact with fjord specific hydrography to influence air-sea CO 2 exchange.…”

“…The summertime halocline caused by glacial meltwater or river discharge into fjords can prevent CO 2 released by remineralization of organic material in subsurface layer to entrain surface water during summer that also helps to maintain the low summertime pCO 2 level (Rysgaard et al, 2012). Often, the surface water pCO 2 increases from autumn to winter maximum near atmospheric equilibrium due to erosion of stratification, i.e., entrain of subsurface water, and increasing salinity, low biological production and sea-air CO 2 exchange in seasonal ice-free fjords (Ericson et al, 2018;Jones et al, 2020). The river runoff and glacial meltwater can have different impacts on the fjord's surface water pCO 2 , as the glacial origin meltwater is usually combined with snow melt and it is low in dissolved inorganic carbon, total alkalinity (TA) and organic matter, but not necessarily undersaturated with respect to atmospheric CO 2 (Meire et al, 2015).…”

“…Short periods for PP are defining ecological features to highlatitude fjords and responsible for significant, seasonal drops in surface water pCO 2 , which usually occurs in April-May prior to major freshwater input (Meire et al, 2015;Ericson et al, 2018;Jones et al, 2020). Phytoplankton production in northern Norwegian fjords is limited between the end of March and September/October when light is available for photosynthesis (Eilertsen and Degerlund, 2010).…”

“…The weak stratification allows occasionally the introduction of nutrients to surface with mixing caused by increased wind events. The summertime riverine input in the area contributes only to a small extent to available nutrients (Wassmann et al, 1996;Jones et al, 2020). Sometimes the mixed layer is so deep that it hinders the growth as the cells sink below euphotic zone (Eilertsen and Taasen, 1984).…”