Biogenic silica production and diatom dynamics in the Svalbard region during spring

Abstract

Abstract. Diatoms are generally the dominant contributors to the Arctic Ocean spring bloom, which is a key event in regional food webs in terms of capacity for secondary production and organic matter export. Dissolved silicic acid is an obligate nutrient for diatoms and has been declining in the European Arctic since the early 1990s. The lack of regional silicon cycling information precludes understanding the consequences of such changes for diatom productivity during the Arctic spring bloom. This study communicates the results from a cruise in the European Arctic around Svalbard, which reports the first concurrent data on biogenic silica production and export, export of diatom cells, the degree of kinetic limitation by ambient silicic acid, and diatom contribution to primary production. Regional biogenic silica production rates were significantly lower than those achievable in the Southern Ocean and silicic acid concentration limited the biogenic silica production rate in 95 % of samples. Compared to diatoms in the Atlantic subtropical gyre, regional diatoms are less adapted for silicic acid uptake at low concentration, and at some stations during the present study, silicon kinetic limitation may have been intense enough to limit diatom growth. Thus, silicic acid can play a critical role in diatom spring bloom dynamics. The diatom contribution to primary production was variable, ranging from <10 % to ∼100 % depending on the bloom stage and phytoplankton composition. While there was agreement with previous studies regarding the export rate of diatom cells, we observed significantly elevated biogenic silica export. Such a discrepancy can be resolved if a higher fraction of the diatom material exported during our study was modified by zooplankton grazers. This study provides the most direct evidence to date suggesting the important coupling of the silicon and carbon cycles during the spring bloom in the European Arctic.

Arctic (Svalbard islands) active and exported diatom stocks and cell health status

“…All other stations sampled were characterized by comparatively depleted nutrient concentrations (photic layer concentrations Si(OH) 4 = 0.99 ± 0.30 µmol Si L −1 , NO 3 +NO 2 = 1.93±0.76 µmol N L −1 , Table 1), thereby representing communities that were either in advanced blooming stages or were senescent after blooming. Stations 6 (SW Svalbard shelf) and 8 (E Svalbard shelf) supported actively blooming diatom populations, with station 8 having the highest chlorophyll a concentration (10.5 µg Chl a L −1 , as described in Krause et al, 2018), and a large fraction of living diatom cells (about 70 %, Table 1). Both locations had the highest stratification among the stations, as indicated by the low UPM values (Table 1).…”

“…Water samples were taken between the surface and the bottom (max. 500 m) for analysis of nutrients, diatom silica, productivity and other properties (Krause et al, 2018). We calculated the upper mixed layer (UPM) as the shallowest depth at which water density (σ θ ) differs from surface values by more than 0.05 kg m −3 (Mura et al, 1995).…”

“…Diatoms can support most of the Arctic primary production during the spring phytoplankton bloom (Krause et al, 2018), the key event setting the ecosystem and driving the intense carbon-uptake characteristic of the Arctic (Vaquer-Sunyer et al, 2013). However, silicic acid concentrations [Si(OH) 4 ] are characteristically low in the European sector of the Arctic, due to the inflow of Si-depleted Atlantic water (Rey, 2012).…”

“…However, silicic acid concentrations [Si(OH) 4 ] are characteristically low in the European sector of the Arctic, due to the inflow of Si-depleted Atlantic water (Rey, 2012). In the Svalbard island region, Krause et al (2018) showed diatoms to be limited by [Si(OH) 4 ] at the spring bloom and suggested that silicon limitation could collapse a diatom bloom before nitrogen, when spring conditions favor diatoms instead of favoring the haptophyte Phaeocystis. A similar observation was made during the spring bloom in Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…S. Agustí et al: Arctic active and exported diatoms southern Greenland, whereby diatom depletion of [Si(OH) 4 ] collapsed the bloom with ∼ 4 µmol L −1 remaining nitrate (Krause et al, 2019).…”