Response of primary producers to the hydrographic variability in the southern region of the California current system

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 (P_opt^B) and Chla concentration. In situ PP estimations using the¹⁴C 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 P_opt^B 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 and P_opt^B SST suggested a phytoplankton community change at around 19 °C, which characterized the transitional nature of the southern portion of the CCS. SST data suggested, on the one hand, that on average the 19 °C 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 24°N to 32°N), which was observed to be even out of this range during interannual events associated wiThel Niño/La Niña. Under both cool-warm hydrographic conditions, the phytoplanktonic community showed large P_opt^B rates (~6 mg C·(mg Chla)^-1·h^-1). At the interannual scale P_opt^B, 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.