Seasonal dynamics in hydrography and biogeochemical cycling in a sub-Arctic fjord

Coastal oceans and shelf regions play a key role in marine productivity and represent an important part of the global carbon cycle, however due to strong seasonal and temporal dynamics, the processes controlling the biogeochemical cycling remain largely unresolved. This study presents the first time series measurements of hydrography, carbonate chemistry and macronutrients in a sub-Arctic fjord, Kaldfjorden (69.75 ºN, 18.68 ºE), of northern Norway during a full annual cycle. The influence of freshwater inputs and biological production were the dominant controls on fjord carbonate chemistry. Meteoric water freshened the upper layers during spring and summer, accounting for variability in surface water total alkalinity. Remineralisation of organic matter and deep mixing into carbon-rich subsurface water resupplied the water column with total inorganic carbon and macronutrients throughout the winter. Surface water saturation states (Ω) of aragonite were lowest 1.64 ± 0.04 during winter and early spring. Rapid C_T drawdown in the spring phytoplankton blooms exhausted the winter stock of nitrate to drive high Ω of 2.26-2.33 and CO₂ undersaturation (ΔfCO_2(sea-air) was -58 ± 33 μatm) in the surface layer from April. Decreases in surface water alkalinity from July to October are consistent with coccolithophore blooms in the fjord. Kaldfjorden was estimated to be an annual net sink of atmospheric CO₂. Climate change may intensify the natural variability in these important marine environments and gathering seasonal baseline data helps to unravel the contribution of fjords to oceanic CO₂ cycling.