Silicon cycle in the tropical South Pacific: contribution to the global Si cycle and evidence for an active pico-sized siliceous plankton

Abstract

Abstract. This article presents data regarding the Si biogeochemical cycle during two oceanographic cruises conducted in the tropical South Pacific (BIOSOPE and OUTPACE cruises) in 2005 and 2015. It involves the first Si stock measurements in this understudied region, encompassing various oceanic systems from New Caledonia to the Chilean upwelling between 8 and 34∘ S. Some of the lowest levels of biogenic silica standing stocks ever measured were found in this area, notably in the southern Pacific gyre, where Chlorophyll a concentrations are the most depleted worldwide. Integrated biogenic silica stocks are as low as 1.08±0.95 mmol m−2 and are the lowest stocks measured in the South Pacific. Size-fractionated biogenic silica concentrations revealed a non-negligible contribution of the pico-sized fraction (<2–3 µm) to biogenic silica standing stocks, representing 26%±12% of total biogenic silica during the OUTPACE cruise and 11%±9% during the BIOSOPE cruise. These results indicate significant accumulation in this size class, which was undocumented for 2005, but has since then been related to Si uptake by Synechococcus cells. Si uptake measurements carried out during BIOSOPE confirmed biological Si uptake by this size fraction. We further present diatoms community structure associated with the stock measurements for a global overview of the Si cycle in the tropical South Pacific.

Microbial functional structure and stable isotopic variation of leptocephali across three current zones in the western South Pacific

“…The concentration of nutrients in our study showed the typical characteristics of oligotrophic ecosystems, as was observed in another recent study in the WSP (Bonnet et al, 2018 (Leblanc et al, 2018) and nitrates (Blanchot and Rodier, 1996) has been observed in that general area. DiTullio et al (2003) also found some variability in nutrient levels along 170°W in the South Pacific Ocean.…”

“…Studies on plankton in the WSP focused on the nutrient cycle and nitrogen fixation by diazotrophic microorganisms (Campbell et al, 2005;Barboza Tenório et al, 2018;Knapp et al, 2018;Leblanc et al, 2018;Van Wambeke et al, 2018), picophytoplankon distribution (Blanchot and Rodier, 1996;DiTullio et al, 2003;Grob et al, 2007;Rii et al, 2016;Zhang et al, 2008) and mesozooplankton and micronekton distribution (Hannides et al, 2013;Menkes et al 2015;Carlotti et al, 2018), but none have attempted to examine planktonic microbial functional structure and stable isotopic variation of leptocephali.…”

Automatic Detection of Optical Signatures within and around Floating Tonga-Fiji Pumice Rafts Using MODIS, VIIRS, and OLCI Satellite Sensors

“…[1] stated that the pumice clasts collected from the 2011 eruption were acidic while [10] described the 2019 ones as rhyolitic. Such silicium would be favorable to pico-and eukaryotic algae such as diatoms, which are a variable component of phytoplankton south of Fiji at the surface [27]. Indeed, silicium in this pumice-dominated environment may not be in a form readily available for phytoplankton.…”

Size-Fractionated Biogenic Silica Standing Stocks and Carbon Biomass in the Western Tropical North Pacific: Evidence for the Ecological Importance of Pico-Sized Plankton in Oligotrophic Gyres

“…However, these estimates are not enough to address spatial variation in small-sized bSi stock across the world's oceans, and more data are needed to further understand the global variability of picoplankton bSi stock. Moreover, we would like to verify the hypothesis proposed by Baines et al (2012); Krause et al (2017) and Leblanc et al (2018) that "picophytoplankton may exert an important influence on the global Si cycle." Our previous study revealed that picophytoplankton were approximately 4-5 orders of magnitude higher in abundance than large diatoms (>2 µm cells), in spite of low nutrient concentrations in the western tropical Pacific Ocean (Wei et al, 2020).…”

“…A caveat to previous studies regarding the C cycle is that the contribution of pico-sized fraction to total C biomass was estimated using traditional size-fractionated chlorophyll and particulate organic carbon (POC) analyses, i.e., detailed C biomass information in picophytoplankton could not be accurately acquired. Additionally, the significant contribution of pico-sized fraction to total bSi stocks was only estimated from the equatorial Pacific (Baines et al, 2012), the Sargasso Sea (Krause et al, 2017) and the tropical South Pacific (Leblanc et al, 2018). However, these estimates are not enough to address spatial variation in small-sized bSi stock across the world's oceans, and more data are needed to further understand the global variability of picoplankton bSi stock.…”