Abstract
The Arctic Ocean is particularly affected by climate change, with changes in sea ice cover expected to impact phytoplankton primary production. During the Green Edge expedition, the development of the late spring–early summer diatom bloom was studied in relation with the sea ice retreat by multiple transects across the marginal ice zone. Biogenic silica concentrations and uptake rates were measured. In addition, diatom assemblage structures and their associated carbon biomass were determined, along with taxon-specific contributions to total biogenic silica production using the fluorescent dye PDMPO. Results indicate that a diatom bloom developed in open waters close to the ice edge, following the alleviation of light limitation, and extended 20–30 km underneath the ice pack. This actively growing diatom bloom (up to 0.19 μmol Si L–1 d–1) was associated with high biogenic silica concentrations (up to 2.15 μmol L–1), and was dominated by colonial fast-growing centric (Chaetoceros spp. and Thalassiosira spp.) and ribbon-forming pennate species (Fragilariopsis spp./Fossula arctica). The bloom remained concentrated over the shallow Greenland shelf and slope, in Atlantic-influenced waters, and weakened as it moved westwards toward ice-free Pacific-influenced waters. The development resulted in a near depletion of all nutrients eastwards of the bay, which probably induced the formation of resting spores of Melosira arctica. In contrast, under the ice pack, nutrients had not yet been consumed. Biogenic silica and uptake rates were still low (respectively <0.5 μmol L–1 and <0.05 μmol L–1 d–1), although elevated specific Si uptake rates (up to 0.23 d–1) probably reflected early stages of the bloom. These diatoms were dominated by pennate species (Pseudo-nitzschia spp., Ceratoneis closterium, and Fragilariopsis spp./Fossula arctica). This study can contribute to predictions of the future response of Arctic diatoms in the context of climate change.
Highlights
Arctic marine ecosystems are currently undergoing multiple environmental changes in relation with climate change (ACIA, 2005; IPCC, 2007; Wassmann et al, 2011)
Using the Arctic N-P relationship calculated from nutrient samples collected at 20-m depth, the fraction of Pacific waters in the water samples increased westwards (Figure 2d), in accordance with the global circulation pattern established for Baffin Bay
Phaeocystis spp. and diatoms appeared to bloom together at stations 204, 207 and 512. These results indicate that Phaeocystis spp. occurred during all stages of the bloom, from its early development under the melting sea ice to its middle and late development in open waters, and was able to maintain its population in the subsurface chlorophyll a maximum (SCM) at stations where nutrients were depleted in surface waters well after the end of the diatom bloom
Summary
Arctic marine ecosystems are currently undergoing multiple environmental changes in relation with climate change (ACIA, 2005; IPCC, 2007; Wassmann et al, 2011). The multiyear sea ice is progressively being replaced by thinner first year ice (Kwok et al, 2009; Maslanik et al, 2011), the annual phytoplankton bloom is occurring earlier (Kahru et al, 2011), and the duration of the open water season is extending (Arrigo and van Dijken, 2011). Some studies have highlighted that the total annual net primary production is already increasing in the Arctic Ocean, between 14–20% over the 1998–2010 period (Arrigo et al, 2008; Pabi et al, 2008; Arrigo and van Dijken, 2011; Bélanger et al, 2013). Ardyna et al (2014) have revealed that some Arctic regions are developing a second phytoplankton bloom during the fall, due to delayed freeze-up and increased exposure of the sea surface to wind stress
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