Total Water Column Analysis Shows the Importance of a Single Species in Subsurface Chlorophyll Maximum Thin Layers in Stratified Waters

Abstract

Marine phytoplankton form the base of marine food webs and are the driving force of the marine carbon cycle, so understanding the dynamics of their blooms is critical. While near-surface marine productivity (<10 m water depths) is extensively documented, that of the subsurface is less well characterised. Increasing evidence of the importance of subsurface chlorophyll maxima (SCM) and climatically driven increases in stratification of surface waters that promote SCM development call for improved sampling of the subsurface. To address this, we targeted the summer stratified waters of the Western English Channel, part of the NW European shelf seas, where SCM are commonly developed. In situ holography was applied to undertake the highest ever resolution, total water column, quantitative analysis of microplankton distribution, and demonstrated the importance of a SCM, co-located with the thermocline, dominated by a single species, the dinoflagellate Ceratium fusus. This species was dominant in the SCM over a wide area of the NW European shelf in the June/July 2015 study period and comprised up to 85% of the SCM biomass. Analysis of similarity and multivariate non-metric multidimensional scaling showed the phytoplankton community of the SCM to be statistically distinct from those of the surface and deep waters. Holography also revealed a fine scale layering of taxa at different levels within the SCM, likely reflecting ecological differences. Some taxa followed the peak abundance of C. fusus, while others reached maximum abundances immediately below or above the C. fusus maximum, suggesting the possible operation of exclusion mechanisms. Additionally, the detection of abundant aggregates located only within and beneath the SCM demonstrates the potential importance of this deep production for the export of carbon to the sea floor. Some predictions of phytoplankton productivity propose a shift to smaller cells in the more stratified oceans of the future resulting in declining production and export. Results presented here, however, contribute to a growing body of evidence that suggests, on the contrary, that key species among the larger celled/colonial, SCM-adapted diatoms and dinoflagellates may instead be selected in stratified conditions, driving increased production and export.

Distinct habitat and biogeochemical properties of low‐oxygen‐adapted tropical oceanic phytoplankton

“…The low density (median 1021.29 kg m −3 ), high temperature (median 28.2°C) and low saline (median 33.61 PSU) surface waters where C 1 resides, and the denser waters (median 1023.19 kg m −3 ) surrounding C 2 are in contrast to the highly dense (median 1025.90 kg m −3 ), cold (median 14.56°C), and saline (median 34.74 PSU) waters in which C 3 thrives. The strong stratification in the region, and the consequent sluggish circulation of this study site supports the formation of the vertical differences in community composition (Barnett et al, 2022).…”

Contrasting phytoplankton-zooplankton distributions observed through autonomous platforms, in-situ optical sensors and discrete sampling

“…Despite the disruption of stratification in more exposed regions, phytoplankton community composition was consistent with those usually found in stratified, thin SCM layers [8,43,44]. For instance, the dinoflagellates Tripos lineatum, T. fusus and the diatom Proboscia alata were the predominant phytoplankton species found in subsurface thin layers in seasonally-stratified waters of the Western English Channel [8,45]. In this study, these three species were found in all stations, although T. tripos and T. longipes were the dominant Tripos species.…”

“…For instance, the dinoflagellates Tripos lineatum , T . fusus and the diatom Proboscia alata were the predominant phytoplankton species found in subsurface thin layers in seasonally-stratified waters of the Western English Channel [ 8 , 45 ]. In this study, these three species were found in all stations, although T .…”

Effects of Sampling Time and Depth on Phytoplankton Metrics in Agricultural Irrigation Ponds

“…However, enumeration and identification using the Utermöhl method on an inverted microscope often cannot detect small cells like picoplankton and picocyanobacteria and these taxa are therefore overlooked in phytoplankton enumeration processes [70,78,88]. While picoplankton may not be fully represented in cell concentration data, they are likely influencing the phytoplankton pigment concentrations measured in a waterbody [89][90][91]. This indicates that pigment measurements may provide a broader picture of the phytoplankton biomass in waterbodies, whereas microscopy-based analyses are likely excluding entire groups of chlorophyll-a and phycocyanin containing organisms.…”