Modeling the impact of the 1992 El Niٌo on new production in Monterey Bay, California

New primary production is estimated using a physiologically driven model of phytoplankton utilization of nitrate for the Monterey Bay, California region. Primary inputs for the model come from temperature and phytoplankton biomass (chlorophyll) from 3 moorings, allowing daily estimation of surface new production. The model successfully reproduces the seasonal pattern of nitrate utilization determined from shipboard experiments using traditional stable-isotope techniques, with a spring peak corresponding to upwelling favorable conditions, a more oceanic fall period, and low values during winter. Comparison of the climatological model output to discrete bottle measurements from 1989–1995 provided an r2 value of 0.74 for surface waters. This correlation decreased to 0.52 when bottle depths from the 50% light level were included, suggesting that the model does not perform well in subsurface waters due to the lack of an irradiance-response in the modeled N uptake. We use the model to explore the effects of the 1992 El Niño event. There was a small decrease in primary production during the 1992 even with no discernible change in vertical export of organic material (Chavez, 1996; Pilskaln et al., 1996). In contrast, this model demonstrates a substantial decrease in new production (about four fold) during 1992, while during this same period, new production was greatly reduced (more than four-fold) farther from shore and presumably downstream during the spring upwelling season. These results are consistent with the hypothesis that moderate upwelling during 1992 provided sufficient nutrients near the coast to maintain “normal” levels of primary productivity, but that productivity downstream and offshore from the upwelling centers was impacted due to the suppression of the themocline and nitracline associated with the ENSO event.