Ocean Colour and the monitoring of Phytoplankton off the Portuguese coast
Carolina Sa

Phytoplankton are responsible for ~90% of primary production in the oceans, and respond for about half of the global carbon sequestration. They are a very diverse group, including pico- (0.2-2 µm) up to micro- (>20 µm) sized cells, with its variations having implications in the marine food webs and several biogeochemical cycles. Phytoplankton can be generally observable from space as its ubiquitous photosynthetic pigment, Chlorophyll-a (Chl-a), impacts the visible spectrum, allowing the estimate of phytoplankton biomass using ocean colour (OC) sensors. Sensors such as MODIS or VIIRS managed by NASA, or OLCI sensors on board of European Space Agency (ESA) Sentinel satellites, have been designed for such purpose. There are however other optically active components in the water, such as sediments and/or coloured dissolved organic matter (CDOM) that may impact Chl-a retrieval from space, particularly in coastal waters. Furthermore, OC remote sensing is limited to the first meters of the water column (optical zone) and has its spatial and temporal coverage restricted by clouds. The assessment of satellite products and algorithm validation is therefore an essential requirement to fully profit from these sensors capabilities. Validation activities for the Portuguese coast will be presented. Satellite remote sensing reflectance (Rrs) and Chl-a data have been compared with in situ measurements and results reveal larger Rrs uncertainties in the blue bands, probably due to inaccuracies in atmospheric aerosol estimation. Satellite-derived Chl-a values tend to overestimate field measurements, emphasizing the need for regional algorithms [Sá et al. 2015, Cristina et al. 2016]. Variations in the optical-relevant components are associated with variations in oceanographic conditions and a strong seasonal component has been observed for the Iberian coast [Sá, 2013, Mélin and Vantrepotte, 2015]. Although validated Chl-a products can be very useful in providing a synoptic view of phytoplankton biomass surface distribution, for ecological modelling and ecosystem studies, Chl-a products lack information on the community structure of the phytoplankton. This is an active area of research and several approaches have already been put forward to distinguish either phytoplankton size classes, functional types or specific taxa (Bracher et al. 2017 and references there in). Efforts have been made to test models for deriving phytoplankton size classes in the Portuguese area (Brotas et al., 2013; Brito et al., 2015) and main results will be presented.

References:

Bracher, A., Bouman, H.A., Brewin, R.J.W., Bricaud, A., Brotas, V., Ciotti, A.M., Clementson, L., Devred, E., Di Cicco, A., Dutkiewicz, S., Hardman-Mountford, N.J., Hickman, A.E., Hieronymi, M., Hirata, T., Losa, S.N., Mouw, C.B., Organelli, E., Raitsos, D.E., Uitz, J., Vogt, M. and Wolanin, A. (2017) Obtaining Phytoplankton Diversity from Ocean Color: A Scientific Roadmap for Future Development. Frontiers in Marine Sciences, 4-55.

Brito, A.C., Sá, C., Brotas, V., Brewin, R.J.W., Silva, T., Vitorino, J., et al. (2015). Effect of phytoplankton size classes on bio-optical properties of phytoplankton in the Western Iberian coast: Application of models. Remote Sensing of Environment, 156, 537–550.

Brotas, V., Brewin, R., Sá, C., Brito, A., Silva, A., Mendes, R., et al. (2013). Deriving phytoplankton size classes from satellite data: Validation along a trophic gradient in the Eastern Atlantic Ocean. Remote Sensing of Environment, 134, 66–77.

Cristina, S., D’Alimonte, D., Goela, P.C., Kajiyama, T., Icely, J., Moore, Fragoso, B., and Newton, A. (2016). Standard and Regional Bio-Optical Algorithms for Chlorophyll a Estimates in the Atlantic off the Southwestern Iberian Peninsula. IEEE Geoscience Remote Sensing Letters, 13(6): 757-761.

Mélin, F., & Vantrepotte, V. (2015). How optically diverse is the coastal ocean? Remote Sensing of Environment, 160: 235–251.

Sá, C. (2013). Ocean Colour off the Portuguese coast: chlorophyll a products validation and applicability, PhD Marine Sciences Degree thesis, Faculty of Sciences – University of Lisbon, 228 pp.

Sá, C., D’Alimonte, D., Brito, A., Kajiyama, T., Mendes, C.R., Vitorino, J., Oliveira, P.B., da Silva, J.C.B., Brotas, V. (2015). Validation of Standard and Alternative Satellite Ocean-Colour Chlorophyll Products off Western Iberia. Remote Sensing of Environment, 168: 403-419