Processes controlling aggregate formation and distribution during the Arctic phytoplankton spring bloom in Baffin Bay

Abstract

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.

Temporal dynamics and biogeography of sympagic and planktonic autotrophic microbial eukaryotes during the under-ice Arctic bloom

“…It has also been reported in early blooms beneath snow-covered pack ice (Assmy et al 2017). In our data, a small increase in the abundance of Phaeocystis in the nano-size community was observed during the early weeks of the ice-melt stage (Figure 3), which could be due to the formation of cell aggregates (Toullec et al 2021) or of colonies in the late stages of the spring bloom. Under high-light and low-nutrient conditions, such as those found during the ice-melt stage (Oziel et al 2019), blooming Phaeocystis species tend to form polysaccharidebased mucilaginous colonies that can be millimetres in diameter and serve the functions of energy storage and defence against grazers (Nejstgaard et al 2007;Schoemann et al 2005).…”

Uptake of barium, molybdenum, and lithium and incorporation into scallop shells: Refining proxies for primary production dynamics

“…Following previous observations, Mo/Ca shell peaks in scallops ( P. maximus and C. radula ) are associated with transport of Mo‐enriched biogenic particles toward the SWI where they got ingested by the bivalves (Thébault et al 2009 a ; Barats et al 2010). More specifically, stressed phytoplankton cells (e.g., due to nutrient limitation, Toullec et al 2021, or grazing by zooplankton, Toullec et al 2019) tend to stick together, forming large particles, such as aggregates (e.g., Alldredge and Gotschalk 1988, 1989), which were proposed to sequester Mo from the dissolved phase (Dellwig et al 2007; Mori et al 2021) and induce Mo/Ca shell peaks after digestion by the scallops (Thébault et al 2009 a , 2022). Siebert et al (V. Siebert unpubl.)…”