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

Abstract

AbstractWe 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 Niño 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°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°C by the end of 2015 when El Niño 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 μm) 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 Niño 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.