Abstract
Abstract. The objective of the current work is to improve our understanding of how water column diatom's abundance and assemblage composition is seasonally transferred from the photic zone to seafloor sediments. To address this, we used a dataset derived from water column, sediment trap and surface sediment samples recovered in the NW Iberian coastal upwelling system. Diatom fluxes (2.2 (±5.6) 106 valves m−2 d−1) represented the majority of the siliceous microorganisms sinking out from the photic zone during all studied years and showed seasonal variability. Contrasting results between water column and sediment trap diatom abundances were found during downwelling periods, as shown by the unexpectedly high diatom export signals when diatom-derived primary production achieved their minimum levels. They were principally related to surface sediment remobilization and intense Minho and Douro river discharge that constitute an additional source of particulate matter to the inner continental shelf. In fact, contributions of allochthonous particles to the sinking material were confirmed by the significant increase of both benthic and freshwater diatoms in the sediment trap assemblage. In contrast, we found that most of the living diatom species blooming during highly productive upwelling periods were dissolved during sinking, and only those resistant to dissolution and the Chaetoceros and Leptocylindrus spp. resting spores were susceptible to being exported and buried. Furthermore, Chaetoceros spp. dominate during spring–early summer, when persistent northerly winds lead to the upwelling of nutrient-rich waters on the shelf, while Leptocylindrus spp. appear associated with late-summer upwelling relaxation, characterized by water column stratification and nutrient depletion. These findings evidence that the contributions of these diatom genera to the sediment's total marine diatom assemblage should allow for the reconstruction of different past upwelling regimes.
Highlights
Diatoms are the most important primary producers in the ocean and play a key role in biogeochemical cycles through transferring organic carbon and biogenic silica from the surface layer to the seafloor sediments (Sancetta, 1989; Romero and Armand, 2010; Tréguer and De La Rocha, 2013)
From October to April–May, the NW Iberian margin was characterized by the prevalence of low irradiance levels and southwesterly winds as shown by the negative upwelling index (UI) values (Fig. 2a and b)
As explained in detail in Zúñiga et al (2016), hydrographically we can distinguish in a first phase the presence of the Iberian Poleward Current (IPC) (October–January), characterized by anomalously warm water (15–17 ◦C) with relatively low nutrient concentrations and chlorophyll a (Chl a) (< 4 mg m−3) (Fig. 3)
Summary
Diatoms are the most important primary producers in the ocean and play a key role in biogeochemical cycles through transferring organic carbon and biogenic silica from the surface layer to the seafloor sediments (Sancetta, 1989; Romero and Armand, 2010; Tréguer and De La Rocha, 2013). The preservation of their siliceous valves in marine sediment records has promoted their use as paleoproductivity indicators. The analysis of data provided by sediment traps has contributed significantly to improving our knowledge of this topic, because the deployment of traps still is the best approach for monitoring downward diatom fluxes as a response to oceanographic and biological processes occurring in surface waters on a long-term basis
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