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 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.

中文翻译：

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2014-2015年暖水异常对北下加利福尼亚州沿海地区的养分，叶绿素a和水文状况的影响

我们报告了在北太平洋北部海洋热浪和厄尔尼诺现象发生的条件下，北下加利福尼亚州（nBC）附近近岸水域温度，养分以及总级分大小的叶绿素a（Chl- a）的季节变化。在2014–2015年期间。与年平均周期（2008- 2015年）相比，我们的研究期间的特点是温暖的水域，硝酸盐贫困，并以非常低的Chl-一浓度，与强烈的分层和减少的nBC上升流条件密切相关。到2014年底，当海洋热浪盛行时，温度异常> 2.0°C；在2015年春夏夏季上升季节中，温度异常降低；到2015年底，当厄尔尼诺现象在加利福尼亚海岸出现时，温度异常又回到> 2.5°C。 nBC。与2008-2012年一样，在上升趋势的同时，在这些异常温暖的条件下记录了总Chla的季节周期。不过，相对于年平均周期，总Chl-一个和硝酸浓度显著降低（-81％，分别为19％-55％和40％），在整个研究期间负异常。此外，季节性演变大小分级Chl-一浓度表明，较小的单元（总Chl-的<（60％）为5μm）系统具有最大分数贡献>一个。我们的研究结果表明，在2015年，2014年和厄尔尼诺海洋热浪的顺序发生了显著和持续的影响限制硝酸盐供应和降低总Chl-一个在近岸海域NBC。总之，我们的数据表明，2014-2015年温暖期，nBC附近沿海水域发生了向贫营养状态的转变。