Detection of Coccolithophore Blooms With BioGeoChemical-Argo Floats

Abstract

Coccolithophores (calcifying phytoplankton) form extensive blooms in temperate and subpolar oceans as evidenced from ocean-color satellites. This study examines the potential to detect coccolithophore blooms with BioGeoChemical-Argo (BGC-Argo) floats, autonomous ocean profilers equipped with bio-optical and physicochemical sensors. We first matched float data to ocean-color satellite data of calcite concentration to select floats that sampled coccolithophore blooms. We identified two floats in the Southern Ocean, which measured the particulate beam attenuation coefficient (c _p) in addition to two core BGC-Argo variables, Chlorophyll-a concentration ([Chl-a]) and the particle backscattering coefficient (b _bp). We show that coccolithophore blooms can be identified from floats by distinctively high values of (1) the b _bp/c _p ratio, a proxy for the refractive index of suspended particles, and (2) the b _bp/[Chl-a] ratio, measurable by any BGC-Argo float. The latter thus paves the way to global investigations of environmental control of coccolithophore blooms and their role in carbon export.

Keywords: BGC‐Argo floats; Emiliania huxleyi; bio‐optics; bloom detection; coccolithophores; global ocean.

Figure 1

Float trajectories and coccolithophore bloom occurrences. (a) Summer climatology of satellite [PIC] (2012–2018) showing the Great Calcite Belt and (b) high‐[PIC] patches in the study area. Red rectangles outline float sampling zones. Trajectories of floats 6901583 (c) and 6902738 during (d) the Period 1 of high [Chl‐a] and (e) the Period 2 of increasing [PIC]. Background maps are satellite [PIC] averaged over the duration of float operations. Red lines correspond to isolines of satellite [Chl‐a], and concurrent [Chl‐a] maps are displayed on Figure S8 in the supporting information. (f, g) Area‐averaged time series of [PIC], [Chl‐a] and %PIC for both sampling zones, i.e., (f) is the time series for the area shown in (c) and (g) for (d)/(e). Vertical red lines indicate periods sampled by floats, and the dashed line separates Periods 1 and 2.

Figure 2

Optical measurements and satellite products during the operation of float 6901583 (a–c) and 6902738 (d–f). Time series of (a,d) [PIC], %PIC, [Chl‐a], and b _bp measured by satellites; (b,e) b _bp, c _p, and [Chl‐a] measured by floats; (c,f) b _bp/[Chl‐a] and b _bp/c _p measured by floats. Blue bands delineate the coccolithophore bloom period identified by the satellite detection method. Error bars indicate the standard deviation from the surface average.

Figure 3

Optical ratios inside (blue dots) and outside (green dots) coccolithophore blooms sampled by floats 6901583 and 6902738. The dashed lines are the b _bp/c _p and b _bp/[Chl‐a] thresholds 0.011 and 0.007 m² mg⁻¹, respectively, that best discriminate coccolithophore blooms from non‐bloom cases.

Figure 4

Detection of coccolithophore blooms with b _bp/[Chl‐a] thresholds. The map reveals locations of profiles inside coccolithophore blooms detected by satellites. The background map is the summer climatology (2012–2018) of satellite [PIC] for each hemisphere. Boxplots show distributions of b _bp/[Chl‐a] before, during, and after coccolithophore blooms in four temperate and subpolar regions identified with red polygons on the map, and in low‐latitudes (i.e., <35°) and Mediterranean Sea where no coccolithophore bloom was detected. Horizontal red dashed lines are the b _bp/[Chl‐a] thresholds reported in Table 1. Blue lines are [PIC].