Deep-Sea Research II 49 (2002) 289–319 Assimilation of JGOFS EqPac and SeaWiFS data into a marine ecosystem model of the central equatorial Pacific Ocean Marjorie A.M. Friedrichs* Center for Coastal Physical Oceanography, Old Dominion University, Crittenton Hall, Norfolk, VA 23529, USA Accepted 11 June 2001 Abstract A five-component (phytoplankton, zooplankton, ammonium, nitrate and detritus) ecosystem model developed for the central equatorial Pacific is reformulated in a data assimilative mode, using the variationaladjointtechnique.Thismethodminimizesmodel/datamisfitsbyadjustingsixmodelparameters that were selected by assessing parameter co-dependencies and model sensitivity to parameter variations. ThroughtheassimilationofcruisedatafromtheUSJointGlobalOceanFluxStudy(JGOFS)Equatorial Pacific Process Study (EqPac), and ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS),itispossibletoreducemodel/datamisfitbyestimatingoptimalparametersgoverningprocesses such as phytoplankton and zooplankton mortality, zooplankton grazing, phytoplankton growth, and the recyclingofnutrientsfromdetritusremineralization.Thesuccessofthisapproachisevidentinthatsimilar parametersetsareobtainedevenwhenindependentdatasetsareassimilated.Forexample,theassimilation ofinsituEqPac(depth-resolved)datafromthe1991–1992ElNi * noproducesaparametersetthatisnearly identicaltothatestimatedviatheassimilationofremotelysensed(surface)SeaWiFSdatacollectedduring the1997–1998ElNi * no.Theassimilationofbiologicaldataalsoallowsobjectivedeterminationofwhether ornotagivenmodelstructureisconsistentwithaspecificsetofobservations.Forexample,theassimilation process demonstrates that data collected during and after the 1991–1992 El Ni * no are consistent with the same single-species ecosystem model, thereby suggesting that El Ni * no conditions may not necessarily be associated with shifts in species composition. In contrast, the increased abundance of diatoms associated with the passage of a tropical instability wave in October 1992 as well as a brief period of macronutrient limitation observed from November 1997 through January 1998 violate key assumptions of the model. Assimilation of data that include these dynamics results in unrealistic simulations of the lower trophic levels. The successful simulation of these particular data sets will require that the model dynamics allow for species composition changes and alternation between macro- and micronutrient limitation. In this way, assimilation of biological data into marine ecosystem models cannot necessarily overcome *Fax: +1-757-683-5550. E-mail address: marjy@ccpo.odu.edu (M.A.M. Friedrichs). 0967-0645/01/$-see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII:S0967-0645(01)00104-7