Bio-optical data and ocean colour

The colour of the ocean as “seen” from a satellite is related to the light absorbed and scattered by substances and organisms in the water. Once of the most important parameters that influences the colour of the ocean is phytoplankton, tiny plants which contain the green pigment, chlorophyll. Although microscopic, phytoplankton grow in such numbers that they can change the colour of the ocean to such a degree that it can be seen from space. Satellites measurements can then be used to estimate the number and types of plankton from how the light is scattered.

Phytoplankton are an important component of the oceanic carbon cycle. Yet, due to methodological constraints, the carbon biomass of phytoplankton is poorly characterised. To address this limitation, AMT scientists have explored the bio-optical relationship between in situ measurements of the particle backscattering coefficient at 470 nm, bbp(470), and the phytoplankton carbon concentration for cells with diameter less than 20μm (Cf). A significant relationship was found between bbp(470) and Cf for Atlantic oceanic waters with chlorophyll-a concentrations less than 0.4 mg m-3 (or bbp(470) < 0.003 m-1). This relationship could be used to estimate Cf from data collected by in situ autonomous platforms and from remote sensing of ocean colour.

The results of recent research (Martinez-Vicente et al.) demonstrated (for the first time from in situ data) a relationship between the phytoplankton carbon (derived from AMT flow cytometry) and how the light is scattered in a backward direction (optical backscattering; Figure 1 and Figure 2).

  • Figure 1. A) Map of stations occupied during the AMT19 cruise
    Figure 1. A) Map of stations occupied during the AMT19 cruise
  • B) Surface chlorophyll (TChl-a) (top) (C) . Surface particulate organic carbon concentration (POC, empty triangles, mean±SD, N=3 replicates) and phytoplankton carbon from flow cytometry at depths less than 10 m (Cf, filled triangles, median±16th and 84th percentiles).
    B) Surface chlorophyll (TChl-a) (top) (C) . Surface particulate organic carbon concentration (POC, empty triangles, mean±SD, N=3 replicates) and phytoplankton carbon from flow cytometry at depths less than 10 m (Cf, filled triangles, median±16th and 84th percentiles).
  • Vertical section of Cf (colour scale) and particle backscattering coefficient (bbp(470), contour lines). Position of euphotic depth (Zeu, black dots) and 1.5Zeu (white dots).
    Vertical section of Cf (colour scale) and particle backscattering coefficient (bbp(470), contour lines). Position of euphotic depth (Zeu, black dots) and 1.5Zeu (white dots).
  • A) Relationship between bbp(470) (see Methods for error bar definition) and Cf (median ±16th and 84th percentiles) for bbp(470)<0.003 m-1. B) Relationships between bbp(470) and Cf  for all measurements available. Black solid line: Eq. 3. Black dashed line: predicted Cm using Behrenfeld et al. [2005] model. Grey solid line: fit through the dataset including bbp(470)>0.003m-1, corresponding to log10(Cf)=2.4×log10(bbp(470))+8.1, N=237, r2=0.7 (in log-log scale, non-error weighted regression).
    A) Relationship between bbp(470) (see Methods for error bar definition) and Cf (median ±16th and 84th percentiles) for bbp(470)<0.003 m-1. B) Relationships between bbp(470) and Cf for all measurements available. Black solid line: Eq. 3. Black dashed line: predicted Cm using Behrenfeld et al. [2005] model. Grey solid line: fit through the dataset including bbp(470)>0.003m-1, corresponding to log10(Cf)=2.4×log10(bbp(470))+8.1, N=237, r2=0.7 (in log-log scale, non-error weighted regression).

Related Information

Martinez-Vicente V., Dall’Olmo G., Tarran G., Boss E. and Sathyendranath S., Optical backscattering is correlated with phytoplankton carbon across the Atlantic Ocean. (2013), Geophysical Research Letters. DOI: 10.1002/grl.50252


Author

Dr Victor Martinez-Vicente

Dr Victor Martinez-Vicente

Bio-Optical Oceanographer