Analytical phytoplankton carbon measurements spanning diverse ecosystems

This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Elsevier. The published article can be found at: http://www.journals.elsevier.com/deep-sea-research-part-i-oceanographic-research-papers/The measurement of phytoplankton carbon (C[subscript]phyto) in the field has been a long-sought but elusive goal in\ud oceanography. Proxy measurements of C[subscript]phyto have been employed in the past, but are subject to many\ud confounding influences that undermine their accuracy. Here we report the first directly measured C[subscript]phyto\ud values from the open ocean. The C[subscript]phyto samples were collected from a diversity of environments, ranging\ud from Pacific and Atlantic oligotrophic gyres to equatorial upwelling systems to temperate spring conditions.\ud When compared to earlier proxies, direct measurements of C[subscript]phyto exhibit the strongest relationship with\ud particulate backscattering coefficients (b[subscript]bp) (R²=0.69). Chlorophyll concentration and total particulate\ud organic carbon (POC) concentration accounted for ~ 20% less variability in C[subscript]phyto than b[subscript]bp. Ratios of C[subscript]phyto\ud to Chl a span an order of magnitude moving across and within distinct ecosystems. Similarly, C[subscript]phyto:POC\ud ratios were variable with the lowest values coming from productive temperate waters and the highest from\ud oligotrophic gyres. A strong relationship between C[subscript]phyto and b[subscript]bp is particularly significant because b[subscript]bp is a\ud property retrievable from satellite ocean color measurements. Our results, therefore, are highly encouraging\ud for the global monitoring of phytoplankton biomass from space. The continued application of our C[subscript]phyto\ud measurement approach will enable validation of satellite retrievals and contribute to an improved\ud understanding of environmental controls on phytoplankton biomass and physiology