Effect of phytoplankton community size structure on remote-sensing reflectance and chlorophyll a products
- Publisher:
- ELSEVIER
- Publication Type:
- Journal Article
- Citation:
- Journal of Marine Systems, 2020, 211
- Issue Date:
- 2020-11-01
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| 1-s2.0-S0924796319303896-main.pdf | Published version | 1.9 MB | |||
| 1-s2.0-S0924796320300968-main.pdf | Published version | 2.55 MB |
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© 2020 Elsevier B.V. Remotely-sensed ocean colour is the main tool for estimating chlorophyll a (Chl-a) concentration and primary productivity on the global scale. In order to investigate the source of errors in remotely-sensed Chl-a concentration we obtained in situ bio-optical properties, in situ reflectances, satellite-derived reflectances and the Chl-a concentration satellite products of the Ocean and Land Colour Instrument (OLCI) Instrument on board Sentinel-3 A in waters off eastern Australia. The mesoscale eddies of these oligotrophic waters provide contrasting phytoplankton communities that allowed us to focus on the effect of phytoplankton size as a source of errors. In these waters, cold-core cyclonic eddies (CE) are dominated by large phytoplankton cells, while small cells dominate warm-core anticyclonic eddies (ACE). The chlorophyll-specific absorption and backscattering from contrasting sites show significant difference due to the differing package effect of phytoplankton size distributions. After normalising the absorption and backscattering spectra to Chl-a associated with just small phytoplankton, the spectra of optical properties become much more similar, showing that small-sized phytoplankton dominate IOPs even when large cells contain the greater fraction of Chl-a concentration of the phytoplankton community. Measured in situ reflectances agreed with reflectances calculated using a simple optical model based on measured IOPs. Furthermore, the in situ measured reflectances agreed well with the OLCI reflectance (mean normalised bias (MNB) of 7% for wavelengths <600 nm). However, a systematic underestimation of Chl-a concentrations by the OLCI algorithms was found in the region of cyclonic eddies characterised by increased Chl-a concentration and dominance of large-sized phytoplankton. A similar underprediction was found in Chl-a concentration calculated with the band-ratio OC4Me algorithm using in situ and IOP-calculated reflectance. Excluding Chl-a associated with large-sized phytoplankton, reduced the bias in the OC4Me from −31% to −0.1%. The large cells absorbed and scattered less per unit of chlorophyll resulting in a smaller impact on the reflectance, and therefore are less detectable by remotely-sensed band-ratio-based Chl-a algorithms than small cells.
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