**6. Modeling trophic structure through correspondence between taxa, size and function**

The relationship between phytoplankton size and biogeochemical function has been well established. Diatoms (silicifiers) tend to be large, and their negative buoyancy leads to their significant contribution to export production (Sarmiento & Gruber 2004). Picophytoplankton, on the other hand, are more important in microbial food webs were rapid recycling of organic matter in the surface ocean leads to a reduction in biogenic export to the deep ocean. Recent molecular studies on the picocyanobacterium *Prochlorococcus*, which dominates the chlorophyll-a biomass in the oligotrophic gyres, have revealed that many strains are unable to utilize nitrate as a nutrient source, and consequently can be considered obligate participants in regenerative production.

The use of pigment markers to provide insight on the trophic structure of marine ecosystems was examined by Claustre (1994). He proposes that the contribution of microphytoplankton (in particular diatoms) to the integrated concentration of diagnostic pigments, which can be considered to be a indicator of the *f*-ratio and hence new production. In addition, information on the contribution of various phytoplankton taxa to chlorophyll-a standing stock has led to the generation of taxon-specific maps of primary production using global relationships on the relationship between surface chlorophyll-a concentration and the ratio of integrated diagnostic pigments to integrated chlorophyll-a concentration (Uitz et al. 2006). Although the simple pigment indices invoked by Claustre (1994) and Uitz et al. (2006) cannot resolve instances where other taxa contribute to the diagnostic pigment markers used in their analysis, it provides a useful first attempt at obtaining information on the global distribution of phytoplankton size classes (Uitz et al. 2006), the environmental factors that control their biogeography (Bouman et al. 2003) and their relative contribution to marine productivity (Bouman et al. 2005, Uitz et al. 2006).
