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The response of primary producers to seasonal and interannual variabilities in the hydrographic conditions observed between 1997 and 2012 is analyzed for the southern portion of the California Current System (CCS). The analysis uses the optimum rate of primary productivity (PP) normalized by units of chlorophyll (Chla) in the water column (PBopt) and Chla concentration. In situ PP estimations using the 14C method were obtained as part of the seasonal cruises conducted by the Investigaciones Mexicanas de la Corriente de California program. Supplementary data included sea surface temperature (SST) as measured by the AVHRR sensor from 1985–2009. We found the mean value of PBopt to be at 5.1 ± 3.3 mg C·(mg Chla)–1·h–1, with maximum ranges of 0.5 and 17.5 mg C·(mg Chla)–1·h–1. The relationship between PBopt and SST suggested a phytoplankton community change at around 19 ºC, which characterized the transitional nature of the southern portion of the CCS. SST data suggested, on the one hand, that on average the 19 ºC isotherm is located in the vicinity of Punta Eugenia and, on the other, that its spatial variability defined the alternating cool–warm conditions. At the seasonal scale, this isotherm showed a marked latitudinal displacement (from 24ºN to 32ºN), which was observed to be even out of this range during interannual events associated with El Niño/La Niña. Under both cool-warm hydrographic conditions, the phytoplanktonic community showed large PBopt rates (~6 mg C·(mg Chla)–1·h–1). At the interannual scale, changes were associated to changes in the abundance and composition of nano-microphytoplankton. Additionally, data suggested that high PP rates during warm periods could be attributed to an enhanced picoplankton contribution.
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