Fate of production in the Arctic seasonal ice zone. An investigation of suspended biomass, vertical export and the impact of grazers during the onset of the spring bloom north of Svalbard

In the Arctic Ocean, biological productivity is largely determined by sea ice, making the seasonal sea ice zone (SSIZ) its most productive region. The current study is a combined investigation of the suspended biomass, vertical export of organic material, and potential retention processes by zooplankton, during a crucial period of bloom development in the Eurasian SSIZ north of Svalbard, where few studies have previously been done. To evaluate the magnitude and composition of the bloom and subsequent vertical export, short-term sediment traps, at five depths between 30 and 200m, were deployed at eight sea ice stations. Daily patterns of chlorophyll a, particulate organic carbon (POC) and contribution of zooplankton fecal pellets (FP) were discovered in distinct assemblages – conditions ranging from pre- to mid-bloom development. Daily loss rates of POC increased from 0.6 to 2.7% as the bloom progressed from a pre- to mid-bloom phase, but the vertical carbon export rates in the shallower depths exceeded those in the deeper layers as the bloom developed accordingly. Phytoplankton carbon (PPC) was found to be a more important component to the vertical POC flux than FP carbon (FPC), especially as the bloom progressed. PPC and FPC contributed 5-75% and 0.5-24% to POC export respectively. The contribution of FPC flux to total POC flux was found to be in line with previous studies, revealing that the relative contribution FPC flux to vertical carbon export is variable but may diminish northward with the SSIZ. The impact of grazers was further investigated through FP production experiments of key Calanus species. The proportion of Calanus finmarchicus community-produced FPC exported to 40m decreased from 36% to 4% from early- to mid-bloom conditions, suggesting stronger zooplankton-mediated retention as the bloom intensifies. Additionally, under slower bloom development, grazers appeared to be effectively controlling and inhibiting the accumulation of biogenic biomass and subsequent vertical flux. The current study reveals that the northern ice-covered Barents Sea shelf break can provide comparable vertical export rates of organic material during the spring bloom to the productive and shallower central Barents Sea

Early spring subglacial discharge plumes fuel under-ice primary production at a Svalbard tidewater glacier

Subglacial upwelling of nutrient-rich bottom water is known to sustain elevated summer primary production in tidewater-glacier-influenced fjord systems. However, the importance of subglacial upwelling during the early spring season has not been considered yet. We hypothesized that subglacial discharge under sea ice is present in early spring and that its flux is sufficient to increase phytoplankton primary productivity. We evaluated the effects of the submarine discharge on primary production in a seasonally fast-ice covered Svalbard fjord (Billefjorden) influenced by a tidewater outlet glacier in April and May 2019. We found clear evidence for subglacial discharge and upwelling. Although the estimated bottom-water entrainment factor (1.6) and total fluxes were lower than in summer studies, we still observed substantial impact on the fjord ecosystem and primary production at this time of the year. The subglacial discharge leads to a salinity-stratified surface water layer and sea ice formation with low bulk salinity and permeability. The combination of the stratified surface layer, a 2-fold higher under-ice irradiance due to thinner snow cover, and higher N and Si concentrations at the glacier front supported phytoplankton primary production 2 orders of magnitude higher (42.6 mg C m−2 d−1) compared to a marine reference site at the fast-ice edge. Reciprocal transplant experiments showed that nutrient supply increased phytoplankton primary production by approximately 30 %. The brackish-water sea ice at the glacier front with its low bulk salinity contained a reduced brine volume, limiting the inhabitable brine channel space and nutrient exchange with the underlying seawater compared to full marine sea ice. Microbial and algal communities were substantially different in subglacial-influenced water and sea ice compared to the marine reference site, sharing taxa with the subglacial outflow water. We suggest that with climate change, the retreat of tidewater glaciers in early spring could lead to decreased under-ice phytoplankton primary production. In contrast, sea ice algae production and biomass may become increasingly important, unless sea ice disappears first, in which case spring phytoplankton primary production may increase

Carbon export in the seasonal sea ice zone north of Svalbard from winter to late summer

Phytoplankton blooms in the Arctic Ocean's seasonal sea ice zone are expected to start earlier and occur further north with retreating and thinning sea ice cover. The current study is the first compilation of phytoplankton bloom development and fate in the seasonally variable sea ice zone north of Svalbard from winter to late summer, using short-term sediment trap deployments. Clear seasonal patterns were discovered, with low winter and pre-bloom phytoplankton standing stocks and export fluxes, a short and intense productive season in May and June, and low Chl a standing stocks but moderate carbon export fluxes in the autumn post-bloom conditions. We observed intense phytoplankton blooms with Chl a standing stocks of >350 mg m−2 below consolidated sea ice cover, dominated by the prymnesiophyte Phaeocystis pouchetii. The largest vertical organic carbon export fluxes to 100 m, of up to 513 mg C m−2 day−1, were recorded at stations dominated by diatoms, while those dominated by P. pouchetii recorded carbon export fluxes up to 310 mg C m−2 day−1. Fecal pellets from krill and copepods contributed a substantial fraction to carbon export in certain areas, especially where blooms of P. pouchetii dominated and Atlantic water advection was prominent. The interplay between the taxonomic composition of protist assemblages, large grazers, distance to open water, and Atlantic water advection was found to be crucial in determining the fate of the blooms and the magnitude of organic carbon exported out of the surface water column. Previously, the marginal ice zone was considered the most productive region in the area, but our study reveals intense blooms and high export events in ice-covered waters. This is the first comprehensive study on carbon export fluxes for under-ice phytoplankton blooms, a phenomenon suggested to have increased in importance under the new Arctic sea ice regime

Vertical Carbon Export in a Changing Arctic Seasonal Ice Zone: Composition and seasonality in the area north of Svalbard and in an Arctic glacial fjord

The ocean’s biological carbon pump (BCP) is a vital part of the global carbon cycle as it produces, transforms, and transfers organic carbon from the surface ocean to the deep. To model the oceans capacity to store carbon today and in the future, we need to understand these processes well. In the Eurasian Arctic, the marine ecosystems who control most of the BCP face a rapid decline in sea ice cover, ocean warming and the increased prominence of the Atlantic water (AW), as well as glacier mass loss in their fjords. The aim of this work was to investigate the current seasonal patterns and to identify the relative protist, zooplankton fecal pellet and detritus composition of vertical carbon export in the northward-retreating and AW influenced seasonal sea ice zone north of Svalbard and in an Arctic glacial fjord (Billefjorden, Svalbard), using short- and long-term sediment traps. The key findings of this thesis include, 1) clear and comparable seasonal patterns in vertical carbon export across study sites and sediment trap designs, 2) diatom blooms and zooplankton fecal pellets were important constituents of heightened vertical carbon fluxes, especially where AW advected blooms into unfavorable light conditions or brought in high abundances of zooplankton, and 3) estuarine circulation and high turnover of nutrients and production drive vertical carbon export in a fjord experiencing glacier transition. Our studies suggest than in an Arctic where seasonal ice is reduced, the annual amount of organic carbon exported to depth may not change but that it will be increasingly reworked rather than phytoplankton derived. This thesis provides the first measured annual vertical carbon export rates in the region north of Svalbard and highlights the role of AW and plankton communities in driving the BCP in these highly dynamic and rapidly changing regions of the Arctic.Koblingen av planteproduksjon i de lysrike overflatelagene i havet og havdypet der mange organismer lever påvirkes av klimaendringer. I det arktiske marine økosystemet vil utsynking av algemateriale i form av organisk karbon påvirkes av reduksjon av havis, smelting av isbreer, og varmere atlanterhavsvann som strømmer inn i Polhavet. Vi samlet inn synkende partikler i vannet ved hjelp av sedimentfeller og studerte hvordan årstidsvariasjoner og sammensetningen av dyre- og planteplankton påvirker hvor mye organisk karbon som synker ned mot havbunnen. Vi studerte havområdet nord for Svalbard, hvor havisen er sterkt redusert, og en Svalbardfjord med en isbre som har trukket seg tilbake fra havet til land. Vi fant 1) sammenlignbare årstidsvariasjoner i alle områdene; 2) havområdet nord for Svalbard hvor Atlanterhavsstrømmen tilfører ekstra alger og dyreplankton fra sør har høyere utsynking av organisk karbon; 3) at karbonutsynkingen i den isbre-påvirkede fjorden er drevet av planktonsamfunn og havstrømmer. I et framtidig Arktisk hav med mindre havis, kan det organiske materialet som synker til bunns være sammenlignbart i mengde, men kvaliteten vil være dårligere

Macrofauna and meiofauna food-web structure from Arctic fjords to deep Arctic Ocean during spring: A stable isotope approach

The knowledge on benthic trophic relations is particularly important for understanding the functioning of still pristine and less studied Arctic Ocean ecosystems. This study examines the benthic food-web structure in the European sector of the Arctic Ocean and assesses if and how it differs along depth gradients in the marginal sea-ice zone during spring. Samples of the sediment organic matter were collected in May/June 2015 and May 2016 at stations representing different sedimentary habitats (fjord, shelf, slope/basin), and stable isotopes of δ13C and δ15N were used to determine macro- and meiofauna food-web structure. Our results show that the food-web structure differed both among the three studied habitat types and between macro- and meiofauna components of benthic communities, and that these differences were related to the quality and quantity of organic matter. Meiofauna in fjords and on the shelf mainly relied on the reworked sediment organic matter while macrofauna utilized more fresh organic matter, sedimenting to the sea floor. In fjords and on the shelf, benthos displayed a high degree of omnivory and non-selective feeding while on the slope and in deep basins feeding on higher trophic levels dominated. In the latter, benthos seemed also to have utilized highly reworked organic matter. As the organic matter quantity and quality were major drivers of trophic relations in all studied areas, the benthic food webs will likely face cascading effects following the modification of pelagic food webs due to climatic changes

Carbon export in the seasonal sea ice zone north of Svalbard from winter to late summer

Phytoplankton blooms in the Arctic Ocean's seasonal sea ice zone are expected to start earlier and occur further north with retreating and thinning sea ice cover. The current study is the first compilation of phytoplankton bloom development and fate in the seasonally variable sea ice zone north of Svalbard from winter to late summer, using short-term sediment trap deployments. Clear seasonal patterns were discovered, with low winter and pre-bloom phytoplankton standing stocks and export fluxes, a short and intense productive season in May and June, and low Chl a standing stocks but moderate carbon export fluxes in the autumn post-bloom conditions. We observed intense phytoplankton blooms with Chl a standing stocks of >350 mg m−2 below consolidated sea ice cover, dominated by the prymnesiophyte Phaeocystis pouchetii. The largest vertical organic carbon export fluxes to 100 m, of up to 513 mg C m−2 day−1, were recorded at stations dominated by diatoms, while those dominated by P. pouchetii recorded carbon export fluxes up to 310 mg C m−2 day−1. Fecal pellets from krill and copepods contributed a substantial fraction to carbon export in certain areas, especially where blooms of P. pouchetii dominated and Atlantic water advection was prominent. The interplay between the taxonomic composition of protist assemblages, large grazers, distance to open water, and Atlantic water advection was found to be crucial in determining the fate of the blooms and the magnitude of organic carbon exported out of the surface water column. Previously, the marginal ice zone was considered the most productive region in the area, but our study reveals intense blooms and high export events in ice-covered waters. This is the first comprehensive study on carbon export fluxes for under-ice phytoplankton blooms, a phenomenon suggested to have increased in importance under the new Arctic sea ice regime

The Influence of Sea Ice Cover and Atlantic Water Advection on Annual Particle Export North of Svalbard

The Arctic Ocean north of Svalbard has recently experienced large sea ice losses and the increasing prominence of Atlantic water (AW) advection. To investigate the impact of these ongoing changes on annual particle export, two moorings with sequential sediment traps were deployed in ice-free and seasonally ice-covered waters on the shelf north (NSv) and east (ESv) of Svalbard, collecting sinking particles nearly continuously from October 2017 to October 2018. Vertical export of particulate organic carbon (POC), total particulate matter (TPM), planktonic protists, chlorophyll a, and zooplankton fecal pellets were measured, and swimmers were quantified and identified. Combined with sensor data from the moorings, these time-series measurements provided a first assessment of the factors influencing particle export in this region of the Arctic Ocean. Higher annual TPM and POC fluxes at the ice-free NSv site were primarily driven by the advection of AW, higher grazing by large copepods, and a wind-induced mixing event during winter. Higher diatom fluxes were observed during spring in the presence of sea ice at the ESv site. Along with sea ice cover, regional differences in AW advection and the seasonal presence of grazers played a prominent role in the biological carbon pump along the continental shelf off Svalbard

Arctic observations identify phytoplankton community composition as driver of carbon flux attenuation

The attenuation coefficient b is one of the most common ways to describe how strong the carbon flux is attenuated throughout the water column. Therefore, b is an essential input variable in many carbon flux and climate models. Marsay et al. (2015, https://doi.org/10.1073/pnas.1415311112) proposed that the median surface water temperature (0–500 m) may be a predictor of b, but our observations from Arctic waters challenge this hypothesis. We found a highly variable attenuation coefficient (b = 0.43–1.84) in cold Arctic waters (<4.1 °C). Accordingly, we suggest that water temperature is not a globally valid predictor of the attenuation coefficient. We advocate instead that the phytoplankton composition and especially the relative abundance of diatoms can be used to parametrize the carbon flux attenuation in local and global carbon flux models

Arctic Observations Identify Phytoplankton Community Composition as Driver of Carbon Flux Attenuation

The attenuation coefficient b is one of the most common ways to describe how strong the carbon flux is attenuated throughout the water column. Therefore, b is an essential input variable in many carbon flux and climate models. Marsay et al. (2015, https://doi.org/10.1073/pnas.1415311112) proposed that the median surface water temperature (0–500 m) may be a predictor of b, but our observations from Arctic waters challenge this hypothesis. We found a highly variable attenuation coefficient (b = 0.43–1.84) in cold Arctic waters (<4.1 °C). Accordingly, we suggest that water temperature is not a globally valid predictor of the attenuation coefficient. We advocate instead that the phytoplankton composition and especially the relative abundance of diatoms can be used to parametrize the carbon flux attenuation in local and global carbon flux models