Impacts of the 2014-2015 Warm-Water Anomalies on Nutrients, Chlorophyll-a and Hydrographic Conditions in the Coastal Zone of Northern Baja California

We report the seasonal variability of temperature, nutrients, and total and size-fractionated chlorophyll-a (Chl-a) in nearshore waters off northern Baja California (nBC), under conditions of the marine heatwave and El Nino that occurred in the northeastern Pacific during 2014-2015. Compared with the mean annual cycle (2008-2015), our study period was characterized by warmer waters, nitrate-impoverished, and with very low Chl-a concentration, which was closely associated with strong stratification and reduced upwelling conditions off nBC. Temperature anomalies were >2.0 degrees C by the end of 2014 when the marine heatwave prevailed, decreased during the spring-early summer upwelling season of 2015, and returned to >2.5 degrees C by the end of 2015 when El Nino appeared along the coast of nBC. As in 2008-2012 and closely coupled with upwelling, a seasonal cycle of total Chl-a was recorded under these abnormally warm conditions. However, relative to the mean annual cycle, total Chl-a and nitrate concentrations were significantly reduced (19%-55% and 40%-81%, respectively), with negative anomalies throughout the study period. Moreover, the seasonal evolution of the size-fractionated Chl-a concentration showed that smaller cells (<5 mu m) systematically contributed with the largest fraction (>60%) of the total Chl-a. Our findings indicate that the sequential occurrence of the marine heatwave in 2014 and El Nino in 2015 had a significant and sustained impact limiting the nitrate supply and reducing the total Chl-a in nearshore waters off nBC. In conclusion, our data reveal that a shift toward an oligotrophic state occurred in coastal waters off nBC during the warm period of 2014-2015.