Complementary Approaches to Assess Phytoplankton Groups and Size Classes on a Long Transect in the Atlantic Ocean

Abstract

Phytoplankton biomass, through its proxy, Chlorophyll a, has been assessed at synoptic temporal and spatial scales with satellite remote sensing (RS) for over two decades. Also, RS algorithms to monitor relative size classes abundance are widely used; however, differentiating functional types from RS, as well as the assessment of phytoplankton structure, in terms of carbon remains a challenge. Hence, the main motivation of this work it to discuss the links between size classes and phytoplankton groups, in order to foster the capability of assessing phytoplankton community structure and phytoplankton size fractionated carbon budgets. To accomplish our goal, we used data (on nutrients, photosynthetic pigments concentration and cell numbers per taxa) collected in surface samples along a transect on the Atlantic Ocean, during the 25th Atlantic Meridional Transect cruise (AMT25) between 50° N and 50° S, from nutrient-rich high latitudes to the oligotrophic gyres. We compared phytoplankton size classes from two methodological approaches: (i) using the concentration of diagnostic photosynthetic pigments, and assessing the abundance of the three size classes, micro-, nano-, and picoplankton, and (ii) identifying and enumerating phytoplankton taxa by microscopy or by flow cytometry, converting into carbon, and dividing the community into five size classes, according to their cell carbon content. The distribution of phytoplankton community in the different oceanographic regions is presented in terms of size classes, taxonomic groups and functional types, and discussed in relation to the environmental oceanographic conditions. The distribution of seven functional types along the transect showed the dominance of picoautotrophs in the Atlantic gyres and high biomass of diatoms and autotrophic dinoflagellates (ADinos) in higher northern and southern latitudes, where larger cells constituted the major component of the biomass. Total carbon ranged from 65 to 4 mg carbon m–3, at latitudes 45° S and 27° N, respectively. The pigment and cell carbon approaches gave good consistency for picoplankton and microplankton size classes, but nanoplankton size class was overestimated by the pigment-based approach. The limitation of enumerating methods to accurately resolve cells between 5 and 10 μm might be cause of this mismatch, and is highlighted as a knowledge gap. Finally, the three-component model of Brewin et al. was fitted to the Chlorophyll a (Chla) data and, for the first time, to the carbon data, to extract the biomass of three size classes of phytoplankton. The general pattern of the model fitted to the carbon data was in accordance with the fits to Chla data. The ratio of the parameter representing the asymptotic maximum biomass gave reasonable values for Carbon:Chla ratios, with an overall median of 112, but with higher values for picoplankton (170) than for combined pico-nanoplankton (36). The approach may be useful for inferring size-fractionated carbon from Earth Observation.

Response of coccolithophore communities to oceanographic and atmospheric processes across the North- and Equatorial Atlantic

“…Therefore, the combination of moderate MLDs, nutrient-enrichment and favorable PAR levels in the NADR (Figures 4D, E, 10E, F) induced favorable conditions for coccolithophores to reach their highest productivity during the boreal autumn in this area. Highest proportion of pigments associated with diatoms and dinoflagellates in the northernmost part of the NADR (Figures 8C, D) is in line with AMT pigment studies from previous years (Brotas et al, 2022), and earlier descriptions of NADR as the region of highest productivity by all eukaryotic phytoplankton (Tarran et al, 2006). Our data confirm existing notions of larger cell-sized microphytoplankton being dominant at higher latitude regions (e.g., Marañoń et al, 2000;Marañoń et al, 2001;Marañoń et al, 2003;Brotas et al, 2022), with which r-selected coccolithophores species are more likely to compete, reflecting their ability to thrive in light-nutrient conditions that are favourable for rapid population growth (e.g., Winter et al, 1994;Young, 1994;Baumann et al, 2000;Guerreiro et al, 2013;Poulton et al, 2017).…”

“…Highest proportion of pigments associated with diatoms and dinoflagellates in the northernmost part of the NADR (Figures 8C, D) is in line with AMT pigment studies from previous years (Brotas et al, 2022), and earlier descriptions of NADR as the region of highest productivity by all eukaryotic phytoplankton (Tarran et al, 2006). Our data confirm existing notions of larger cell-sized microphytoplankton being dominant at higher latitude regions (e.g., Marañoń et al, 2000;Marañoń et al, 2001;Marañoń et al, 2003;Brotas et al, 2022), with which r-selected coccolithophores species are more likely to compete, reflecting their ability to thrive in light-nutrient conditions that are favourable for rapid population growth (e.g., Winter et al, 1994;Young, 1994;Baumann et al, 2000;Guerreiro et al, 2013;Poulton et al, 2017). This probably contributed to the high levels of oxygen concentrations found near the surface in this region (Figure 3D), in line with existing understanding on the importance of marine phytoplankton for oxygen production.…”

“…Fucoxanthin (Fuco) was used as pigment marker for diatoms, peridinin (Perid) for autotrophic dinoflagellates, 19'hexanoyloxyfucoxanthin (HexFuco) for coccolithophores (Haptophyta Type 6), and zeaxanthin (Zea) for marine cyanobacteria sensu latu (s.l. ), i.e., including picoplankton and N 2 -fixing filamentous diazotrophs (detailed description of the pigment analysis in Brotas et al, 2022). While HexFuco is an exclusive pigment of coccolithophores, fucoxanthin is not exclusive of diatoms, nor zeaxanthin of Cyanobateria, nor is peridinin present in all Dinoflagellate species.…”

“…Nevertheless, this simplistic approach was considered the most appropriate for our studied region since the phytoplankton community has been reported to change significantly across the large hydrological gradients as those covered by the AMT, therefore less suitable for implementing other pigment analysis approaches (e.g. CHEMTAX), as discussed in Brotas et al (2022) For the phytoplankton cells count and identification, samples of 200 mL were collected at each sampling site, placed put in amber glass bottles, and fixed with acidic Lugol's iodine solution. Taxonomic analysis and abundance quantification of filamentous cyanobacteria (e.g., Trichodesmium spp.)…”

Evaluating historic and modern optical techniques for monitoring phytoplankton biomass in the Atlantic Ocean

“…Following each AMT campaign, High Performance Liquid Chromatography (HPLC) was used to determine total Chl-a (estimated from the sum of monovinyl chlorophyll-a, divinyl chlorophyll-a, and chlorophyllide-a). Details of the HPLC protocols and pigment extraction methods used on AMT23 and AMT25 are provided in Brotas et al (2022), and those used on AMT26 and AMT28 are provided in Tilstone et al (2021). At stations where neither CTD casts or underway samples were collected (four stations, one on AMT25, three on AMT26), Chl-a was estimated from underway spectrophotometric measurements of particulate absorption collected at the stations using the line-height method as described in Dall'Olmo et al (2012), a technique proven to provide very accurate estimates of Chl-a on AMT cruises (see Dall'Olmo et al, 2012;Brewin et al, 2016;Rasse et al, 2017;Tilstone et al, 2021).…”