**2. "History of terminology for phytoplankton functional types": Ecological, biogeochemical, and optical definitions**

The Ocean Color community has utilized a reductionist approach to describe phytoplankton taxonomic composition and refers to optical signatures of certain phytoplankton groups/species or PFTs. In this chapter, we will refer to Phytoplankton Functional Types (PFTs) in order to utilize current terminology. Please take into account that we may be defining groups within different levels of complexity that relate to phylogeny or bio-optically similarities etc. Taxonomic characterization has utilized different terminology over the years and we delve into the history of its classification because it may define many different levels of complexity.

Present studies on diversity are not too far removed from the original ideas invoked by G.E. Hutchinson's "Paradox of the Plankton" (Hutchinson, 1961) and J. H. Connell's "Intermediate Disturbance Hypothesis" (Connell, 1978). Ocean variability, through eddies, storms, seasonal cycles, and El Niño, have been documented to play a role in controlling diversity. How this level of stochastic variability manifests itself spatially and temporally due to climate change is unknown. Falkowski & Oliver (2007) suggest that resource competition theory is adequate to describe global distribution of marine eukaryotic phytoplankton taxa during equator-to-pole and continent-to-land thermal gradients. These results are supportive in light of describing phytoplankton beyond a bulk approach of utilizing only chlorophyll a.

A functional group is used as an ecological term to define a group of organisms that have similar biogeochemical processes e.g. nitrogen fixation, silicifiers, and calcifiers (Fauchald & Jumars 1979, Reynolds et al. 2002, Nair et al. 2008). Functional groups in phytoplankton are defined as groups of organisms related through common biogeochemical processes but are not necessarily phylogenetically related (Iglesias-Rodriguez et al. 2002). Homologously similar sets of organisms are referred to as "functional groups" or "biogeochemical guilds" following Totterdell et al. (1993). For marine phytoplankton, some attempts have been made to organize species within certain classes of phytoplankton according to their ecological and biogeochemical function or habitat distribution perspective (Iglesias-Rodríguez et al. 2002, Smayda & Reynolds, 2003,Vila & Masó, 2005). Thus, terminology has finally evolved to the usage of Phytoplankton Functional Type (PFT) or Phytoplankton Functional group but these classifications are not necessarily straightforward and include a wide range of different taxonomic classification approaches such as size, biogeochemistry, or traditional taxonomic hierarchies (see discussion by Nair et al. 2008).
