In the last decades, the Arctic Ocean has been affected by climate change, leading to alterations in the sea ice cover that influence the phytoplankton spring bloom, its associated food web, and therefore carbon sequestration. During the Green Edge 2016 expedition in the central Baffin Bay, the phytoplankton spring bloom and its development around the ice edge was followed along 7 transects from open water to the ice-pack interior. Here, we studied some of the processes driving phytoplankton aggregation, using aggregate and copepod distribution profiles obtained with an underwater vision profiler deployed at several stations along the transects. Our results revealed a sequential pattern during sea ice retreat in phytoplankton production and in aggregate production and distribution. First, under sea ice, phytoplankton started to grow, but aggregates were not formed. Second, after sea ice melting, phytoplankton (diatoms and Phaeocystis spp. as the dominant groups) benefited from the light availability and stratified environment to bloom, and aggregation began coincident with nutrient depletion at the surface. Third, maxima of phytoplankton aggregates deepened in the water column and phytoplankton cells at the surface began to degrade. At most stations, silicate limitation began first, triggering aggregation of the phytoplankton cells; nitrate limitation came later. Copepods followed aggregates at the end of the phytoplankton bloom, possibly because aggregates provided higher quality food than senescing phytoplankton cells at the surface. These observations suggest that aggregation is involved in 2 export pathways constituting the biological pump: the gravitational pathway through the sinking of aggregates and fecal pellets and the migration pathway when zooplankton follow aggregates during food foraging.

Phytoplankton stoichiometry and cell size could result from both phenology and environmental change. Zooplankton graze on primary producers, and this drives both the balance of the ecosystem and the biogeochemical cycles. In this study, we performed incubations with copepods and coccolithophores including different prey sizes and particulate carbon contents by considering phytoplankton biovolume concentration instead of chlorophyll a level (Chl a) as is usually performed in such studies. The egestion of fecal pellet and ingestion rates were estimated based on a gut fluorescence method. The latter was calibrated through the relationship between prey Chl a level and the biovolume of the cell. Chl a/biovolume ratio in phytopkanton has to be considered in the copepod gut fluorescent content method. Both coccolithophore biovolume and particulate inorganic/organic carbon ratios affect the food foraging by copepods. Finally, we observed a non-linear relationship between ingestion rates and fecal pellet egestion, due to the presence of calcite inside the copepod’s gut. These results illustrate that both prey size and stoichiometry need to be considered in copepod feeding dynamics, specifically regarding the process leading to the formation of fecal pellets.

The oceanic carbonate pump corresponds to the production and sinking of particulate inorganic carbon (PIC) by calcifying planktonic organisms. In this study, global estimates of PIC standing stock, production derived from ocean colour, and the contribution of calcifying taxa were combined with PIC flux observations from short-term sediment traps deployed over the past decades, covering the global ocean. Coccolithophores are the main calcifying plankton group in the euphotic zone, exhibiting a significant seasonal blooming pattern and a pronounced latitude-dependent seasonal response. The present study highlights that PIC production in the euphotic zone, and the pelagic PIC flux vary among oceanic regions, depths, and seasons. Based on a geographic matchup between PIC flux from sediment traps and remote sensing climatological observations, a correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at a global scale, but only in specific ocean basins, such as the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC / POC ratios in the euphotic zone, temperate and subpolar areas are more efficient at exporting PIC compared to equatorial and subtropical regions, which have higher PIC production and PIC / POC ratios in the euphotic zone. Plankton phenology appears to be an important driver of PIC export efficiency (PIC Eeff) and PIC transfer efficiency (PIC Teff). This study suggests that the “packaging factor”, which includes the vehicles of the biological carbon pump (e.g., marine snow aggregates, fecal pellets) and the plankton network (e.g., zooplankton communities, microbial loop), largely determines PIC export and transfer efficiencies.

The oceanic carbonate pump corresponds to the production and the sinking of particulate inorganic carbon (PIC) thanks to calcified planktonic organisms. In this study, global estimates of PIC standing stock, production derived from ocean colour and calcified taxa contribution were combined with PIC flux observation from short-term sediment traps deployed during the last decades covering the global ocean. Coccolithophores are the main planktonic calcified group in the euphotic zone, with a significant seasonal blooming pattern and an important latitude dependant seasonal response. The present study highlights that the PIC production in the euphotic zone and the pelagic PIC flux varied among oceanic regions, depth and season. Based on a geographic matchup between the PIC flux from sediment traps and remote sensing climatology observation, correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at global scale, but only for delimited ocean basin such as in the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC/POC ratios in the euphotic zone, temperate and subpolar areas are more efficient to export PIC compared to equatorial and subtropical areas (higher PIC production and PIC/POC ratios in the euphotic zone). The plankton phenology seems to be an important driver of PIC export efficiency (PIC E_eff) and PIC transfer efficiency (PIC T_eff). This study suggests that the 'packaging factor' corresponding to the vehicle of the biological carbon pump (marine snow aggregates, fecal pellets) and the plankton network (e.g. zooplankton community, microbial loop) determine the PIC export efficiency and the PIC transfer efficiency.

The oceanic carbonate pump corresponds to the production and the sinking of particulate inorganic carbon (PIC) thanks to calcified planktonic organisms. In this study, global estimates of PIC standing stock, production derived from ocean colour and calcified taxa contribution were combined with PIC flux observation from short-term sediment traps deployed during the last decades covering the global ocean. Coccolithophores are the main planktonic calcified group in the euphotic zone, with a significant seasonal blooming pattern and an important latitude dependant seasonal response. The present study highlights that the PIC production in the euphotic zone and the pelagic PIC flux varied among oceanic regions, depth and season. Based on a geographic matchup between the PIC flux from sediment traps and remote sensing climatology observation, correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at global scale, but only for delimited ocean basin such as in the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC/POC ratios in the euphotic zone, temperate and subpolar areas are more efficient to export PIC compared to equatorial and subtropical areas (higher PIC production and PIC/POC ratios in the euphotic zone). The plankton phenology seems to be an important driver of PIC export efficiency (PIC E_eff) and PIC transfer efficiency (PIC T_eff). This study suggests that the 'packaging factor' corresponding to the vehicle of the biological carbon pump (marine snow aggregates, fecal pellets) and the plankton network (e.g. zooplankton community, microbial loop) determine the PIC export efficiency and the PIC transfer efficiency.

The oceanic carbonate pump corresponds to the production and the sinking of particulate inorganic carbon (PIC) thanks to calcified planktonic organisms. In this study, the global PIC production from ocean colour satellite observations were combined with PIC flux observation from short-term sediment traps deployed during the last decades covering the global ocean. Coccolithophores are the main planktonic calcified group in the euphotic zone, with an important phenological blooming pattern and an important latitude dependant seasonal response. The present study highlights that the PIC production in the euphotic zone and the pelagic PIC flux varied among oceanic regions, depth and season. Based on a geographic matchup between the PIC flux from sediment traps and remote sensing climatology observation, correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at global scale, but only for delimited ocean basin such as in the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC/POC ratios in the euphotic zone, temperate and subpolar areas are more efficient to export PIC compared to equatorial and subtropical areas (higher PIC production and PIC/POC ratios in the euphotic zone). The plankton phenology seems to be an important driver of PIC export efficiency (PE_eff) and PIC transfer efficiency (T_eff). Pelagic PIC dissolution is assumed to be responsible for the decrease in PE_eff and T_eff. In a context of climate change, any modification of plankton network community as well as modification PIC T_eff is expected to have consequences on surface alkalinity balance and CO₂ exchange between ocean and atmosphere. This study suggests that the 'packaging factor' corresponding to the vehicle of the biological carbon pump (marine snow aggregates, fecal pellets) and the plankton network (e.g. zooplankton community, microbial loop) may affect the PIC export efficiency and the PIC transfer efficiency.