Impact of phytoplankton community size on a linked global ocean optical and ecosystem model

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Journal of Marine Systems 89 (2012): 61-75, doi:10.1016/j.jmarsys.2011.08.002.

Funding for this study came from a NASA Earth and Space Science Fellowship andUniversity of Rhode Island Graduate School Oceanography Alumni Fellowship, both awarded toC. Mouw. The CCSM-3 BEC simulations were generated with support from NASA OceanBiology and Biogeochemistry Program (NNX07AL80G) and the NSF Center for MicrobialOceanography Research and Education (C-MORE, EF-0424599).

We isolated the effect phytoplankton cell size has on varying remote sensing reflectancespectra (Rrs(λ)) in the presence of optically active constituents by using optical and radiativetransfer models linked in an offline diagnostic calculation to a globalbiogeochemical/ecosystem/circulation model with explicit phytoplankton size classes. Two casestudies were carried out, each with several scenarios to isolate the effects of chlorophyllconcentration, phytoplankton cell size, and size-varying phytoplankton absorption on Rrs(λ).The goal of the study was to determine the relative contribution of phytoplankton cell size and chlorophyll to overall Rrs(λ) and to understand where a standard band ratio algorithm (OC4) mayunder/overestimate chlorophyll due to Rrs(λ) being significantly affected by phytoplankton size.Phytoplankton cell size was found to contribute secondarily to Rrs(λ) variability and to amplifyor dampen the seasonal cycle in Rrs(λ), driven by chlorophyll. Size and chlorophyll were foundto change in phase at low to mid-latitudes, but were anti-correlated or poorly correlated at highlatitudes. Phytoplankton size effects increased model calculated Rrs(443) in the subtropical ocean during local spring through early fall months in both hemispheres and decreased Rrs(443)in the Northern Hemisphere high latitude regions during local summer to fall months. This studyattempts to tease apart when/where variability about the OC4 relationship may be associatedwith cell size variability. The OC4 algorithm may underestimate [Chl] when the fraction ofmicroplankton is elevated, which occurs in the model simulations during local spring/summermonths at high latitudes in both hemispheres.