What is the composition of phytoplankton communities? How will it affect higher trophic levels and carbon cycling?

Phytoplankton are at the base of the marine food web and are responsible for about half of the biological uptake of CO2 on Earth through the process of photosynthesis. They comprise very diverse organisms with a wide range of shape, size, pigmentation, photosynthetic efficiency, nutrient uptake, or temperature tolerance, all resulting in different biogeochemical and ecological functionality in marine ecosystems. To assess phytoplankton community composition is therefore of crucial interest for improved biogeochemical characterization (e.g., improved primary production and carbon export estimates) and to better understand and potentially predict the ecosystem responses to current and future changes in environmental drivers. Important progress has recently been made in developing a capability for retrieving information on the composition of phytoplankton communities using bio-optical observations acquired in situ (field cruise, moorings) or derived from remote sensing of ocean color ([IOCCG, 2014] and references therein).

Such a capability, applied to profiling floats equipped with bio-optical and biogeochemical sensors, will provide invaluable information on the vertical distribution of relevant proxies. For instance, the shape of the vertical profile of chlorophyll a as well as its magnitude can be used to derive quantitative information on the size structure of the phytoplankton assemblage (relative contributions of pico-, nano-, and micro-phytoplankton to the total chlorophyll a concentration) ([Uitz et al., 2006]; [Sauzède et al., 2015]), which is known to be an important driver of, e.g., energy transfer to upper trophic levels or sinking of organic carbon to the deep ocean. The ratio of the chlorophyll a fluorescence to the particulate backscattering coefficient, a proxy for POC, has proved to be a useful indicator of phytoplankton composition with low values associated with a dominance of diatoms in bloom conditions [Cetinic et al., 2015]. Such conditions have strong impact on the export and potential sequestration of carbon. The chlorophyll a concentration-to-particulate backscattering ratio also provides information on the photoacclimation status of phytoplankton (e.g., [Behrenfeld et al., 2005]; [Mignot et al., 2014]), which induces horizontal and vertical variability in the chlorophyll a concentration decoupled from that of carbon biomass with consequences for primary production modeling (e.g. [Uitz et al., 2008]; [Westberry et al., 2008]; [Graff et al., 2016]).

Biogeochemical-Argo will be the first observation network to provide such depth-resolved proxies of phytoplankton community composition and photoacclimation over large spatial and temporal scales in the world’s oceans.

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