The structure of diatom communities constrains biogeochemical properties in surface waters of the Southern Ocean (Kerguelen Plateau)

Abstract

In the context of climate change, understanding the ecological processes controlling the functioning and the efficiency of the biological pump is of primary importance. Plankton community structure and species-specific properties are often invoked as likely to affect biogeochemistry and the export of organic and biogenic material to the ocean interior. Although a major player in this respect, diatoms are still viewed as a single functional type whose diversity is generally overlooked. Here we examine that question, building on the results achieved during the MOBYDICK expedition, which occurred in the vicinity of the Kerguelen Islands (Southern Ocean) in late summer, a time window corresponding to the demise of the annually recurrent phytoplankton blooms already known to be controlled by iron availability. The Si/C/N stoichiometry of the particulate matter was studied in conjunction with the different diatom community structures, their physiological states, as well as their species-specific carbon contents and silicification degrees. Our results show that diatoms outside the iron-fertilized plateau were more heavily silicified, due to the combined effects of both taxonomic composition of the resident community and a direct physiological response to iron stress, resulting in higher Si:C elemental ratios in diatoms as well as in the bulk particulate matter. Despite low silicic acid concentrations, large chains of weakly silicified Corethron inerme were able to grow in the upper mixed layer above the plateau, while in adjacent high nutrient low chlorophyll (HNLC) waters, communities were dominated by Fragilariopsis spp., Cylindrotheca closterium and the centric genera Actinocyclus/Thalassiosira spp. Depth was also an important factor shaping diatom communities, with the presence of a deep and inactive assemblage located within the pycnocline gradient, both on-and offplateau, which likely resulted from the differential sinking and accumulation of species previously grown at the surface. In HNLC waters, below the mixed layer, detrital frustules of the heavily silicified species Fragilariopsis kerguelensis carried mostly Si, while above the plateau, Eucampia antarctica and Chaetoceros spp. (resting spores and vegetative stages) were efficient vectors of both Si and C to the deeper layers. Our study shows that the stoichiometry of the biological pump cannot be considered solely as a simple response to a single limiting factor (here iron) highlighting the importance of a species-centered approach in order to finely resolve biogeochemical fluxes and improve our understanding of the biological pump.

Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean

“…Off-plateau, large diatoms are likely primarily limited by iron. The high proportion of inactive large diatoms observed with SIMS was in line with microscopic observations of surface samples during MOBYDICK showing 33 ± 7 % of empty/broken frustules [ 61 ]. It is worthy to note that highly heterogeneous division rates have been observed in culture studies within large diatoms (Table S4 ) in Fe-limited cultures (e.g., daily division rates from 0.03 to 0.43 d −1 ) and also within specific genera in Fe-replete conditions (e.g., daily division rates from 0.16 to 0.64 d −1 for Fragilariopsis sp.).…”

“…Diatoms were the dominant phytoplankton class of the large size fraction (>20 µm in size; 55–97% of the reads). Microscopic observations confirmed that the size range of several diatom genera overlapped the two size fractions [ 61 ]. Off-plateau, diatom communities were composed of pennate ( Fragilariopsis , Pseudo-nitzschia ) and centric diatoms ( Thalassiosira, Chaetoceros, Proboscia , and Rhizosolenia ).…”

Deep particle stocks following the summer bloom around the Kerguelen islands: Insights into diatoms physiological state, community structure and mortality modes

“…The number of identifiable individual diatom frustules collected with the Bottle net in deep waters around Kerguelen islands by the end of summer was not negligible (10 7 -10 8 cells m -2 ) and the deep diatom stock (below 125 m) was one order of magnitude higher than the stock present in the upper (0-125 m) layer (Lafond et al, 2020). It was also 3 orders of magnitude higher than the concentration of single phytoplankton cells measured on average by Agusti et al (2015) in the 2,000-4,000 m layer in the subtropical Ocean.…”

“…Fig. 2 in Lafond et al, 2020), were the next contributors to winter/resting stage but were much less abundant, even though present at all sites, with a dominance at M4.…”

“…The study region is considered as a natural fertilization laboratory (Blain et al, 2008), and several previous studies focused on the large phytoplankton bloom resulting from the island's Fe-enrichment of the plateau area by comparison to the the neighboring HNLC (High Nutrient Low Chlorophyll) region (Cornet-Barthaux et al, 2007;Mosseri et al, 2008;Armand et al, 2008a;Quéguiner, 2013;Lasbleiz et al, 2014;Lasbleiz et al, 2016). Here we compare results from phytoplankton nets (Phytonets) and Bottle net casts, while a companion paper describes in similar details the surface diatom communities from CTD Niskin collection (Lafond et al, 2020). We observed a dominant contribution of single cell diatoms (93 ± 6 %) to the deep particle stocks but a very small contribution of intact diatom cells (~0.3 %) to C content in the intermediate layer (125-500 m), together with a very small fecal pellet contribution, that was dominated by the minipellet size-class.…”