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Abstract
Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Coccolithophores were generally more numerous in the surface mixed layer, whilst PIC concentrations were more homogenous with depth throughout the euphotic zone. Diatoms were notably more abundant and more diverse in the lower euphotic zone beneath the mixed layer in association with subsurface maxima in chlorophyll-a, bSi and oxygen concentrations. The four dominant coccolithophore species (Emiliania huxleyi, Gephyrocapsa muellerae, Syracosphera spp., and Rhabdosphaera clavigera) represented 78 ± 20% (range 31–100%) of the observed community across all sampled depths yet simultaneously contributed an average of only 13% to measured PIC pools. The diatom community, which was dominated by Pseudo-nitzschia spp. and by a species tentatively identified as Nanoneis longta, represented only ~1% of the bSi pool on average, with contributions increasing within the chlorophyll maximum. Despite a slow gradual deepening of the surface mixed layer in the period prior to observation, and adequate nutrient availability beneath the mixed layer, biomineral pools at this time consisted largely of detrital rather than cellular material.
Highlights
Vertical distributions of biogenic silica, particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic
For example, unclear how relationships found in spring between biomineral and biomineralizing phytoplankton distributions evolve throughout the stratified period (e.g., [20]) or, more whether biomineral pools during the late stratified period are best considered as aged detrital material retained
In particular, were more diverse and more numerous in the lower euphotic zone beneath the mixed layer, coincident with a subsurface chlorophyll-a maximum, a biogenic silica (bSi) maximum and an oxygen maximum, with these co-located features implying active biological production. These results are consistent with previous observations from the northeast Atlantic region, indicating primary production occurring at depth during the summer months [52,53] and with reports of diatom pigment markers beneath the mixed layer [19]
Summary
Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Bloom-forming taxa such as diatoms and coccolithophores precipitate significant quantities of biogenic silica (bSi) or particulate inorganic carbon (PIC; calcite) throughout the year but during bloom events [5,6,7,8,9] These biominerals can increase particle density and aid the export of organic material from the upper ocean, with PIC generally considered to be the more important biomineral for sinking fluxes, owing to its greater density [10,11]. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
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