Pelagic ecosystem dynamics between late autumn and the post spring bloom in a sub-Arctic fjord

Abstract

Marine ecosystems, and particularly fjords, are experiencing an increasing level of human activity on a year-round basis, including the poorly studied winter period. To improve the knowledge base for environmentally sustainable management in all seasons, this study provides hydrographic and biological baseline data for the sub-Arctic fjord Kaldfjorden, Northern Norway (69.7° N, 18.7° E), between autumn 2017 and spring 2018. Field observations are integrated with results of a numerical ocean model simulation, illustrating how pelagic biomass, represented by chlorophyll a (Chl a), particulate organic carbon (POC), and zooplankton, is affected by stratification and circulation from October to May. We observed an unusually warm autumn that likely delayed the onset of cooling and may have supported the high abundances of holoplankton and meroplankton (5768 individuals m–3). With the onset of winter, the water column cooled and became vertically mixed, while suspended Chl a concentrations declined rapidly (< 0.12 mg Chl a m–3). In January and February, suspended POC concentrations and downward flux were elevated near the seafloor. The hydrodynamic model results indicate that the strongest currents at depth occurred in these months, potentially inducing resuspension events close to the seafloor. In spring (April), peak abundances of suspended biomass were observed (6.9–7.2 mg Chl a m–3 at 5–15 m; 9952 zooplankton ind. m–3 at 0–100 m), and field observations and model results suggest that zooplankton of Atlantic origin were probably advected into Kaldfjorden. During all investigated seasons, the model simulation suggests a complex circulation pattern, even in such a small fjord, which can have implications for environmental management of the fjord. We conclude that the pelagic system in Kaldfjorden changes continually from autumn to spring and that winter must be seen as a dynamic period, not a season where the fjord ecosystem is ‘at rest’.

Impact of Microsetella norvegica on carbon flux attenuation and as a secondary producer during the polar night in the subarctic Porsangerfjord

“…While also a arctic fjord similar to Porsangerfjord in depth, Hornsund is located at a higher latitude. However, our POC estimates for settling organic aggregates were similar to those measured by Walker et al (2022). They took physical samples using sediment traps in Kaldfjord, Norway, which is similar to Porsangerfjord in latitude, climate and depth, and is also a northsouth facing fjord.…”

Contrasting seasonal patterns in particle aggregation and DOM transformation in a sub-Arctic fjord

“…The spring bloom in Ramfjorden and other close fjords is initiated at the end of March/ beginning of April within a deeply mixed water column (Riebesell et al, 1995;Vonnahme et al, 2022;Walker et al, 2022); this study). The sharp increase of DOC, fluorescence, turbidity, TPM, POC, Chl-a, protist abundance, bacterial abundance and activity (HNA/LNA) as well as nano-and picophytoplankton abundances in April, along with a strong drawdown of nutrients demonstrates the onset of the spring bloom.…”

“…Arctic and sub-Arctic fjords can be periodic hotspots for biological productivity, and locations of high carbon production, turnover and export. With the beginning of spring, the return of sunlight and replenished nutrients in the upper water column fuel intense phytoplankton blooms, and with that the production of fresh POM and DOM (Paulsen et al, 2018;Walker et al, 2022;Wetz & Wheeler, 2007). Spring is influenced by autochthonous labile DOM production composed of low oxygen and high hydrogen saturation (Hansell, 2013a).…”

Ice conditions in northern Norwegian fjords: Observations and measurements from three winter seasons, 2017–2020

“…Though little work is available documenting observations of specifically Norwegian fjords in winter (Walker et al, 2021;Eilertsen and Skarðhamar, 2006), research focused on the more general field of Arctic estuaries does exist from e.g. North America and Russia.…”