Phytoplankton responses to the 2014–2016 warming anomaly in the northeast subarctic Pacific Ocean

Abstract

Significant changes in the abundance and composition of phytoplankton were observed along Line P in the northeast subarctic Pacific as a result of a rapid warming of surface waters in 2014–2015. This feature, labeled “the blob,” reached ~ 4°C above normal and restricted winter ocean–surface nutrient renewal due to increased stratification. As a result, surface nutrients were the lowest observed and nitrate depletion in summer extended farther offshore than in the last three decades. Within this nitrate‐depleted region, there was unusually low phytoplankton biomass and a dramatic increase in the dominance of cyanobacteria, including Prochlorococcus, which had not been previously observed in this region. Farther offshore, in the iron‐limited region, phytoplankton biomass and the abundance of haptophytes and chlorophytes increased during “the blob.” By 2016, surface nutrient and phytoplankton concentrations were still low, but at most of the stations, phytoplankton composition was similar to that observed before the warming occurred, except for an increase in diatoms farther offshore. These changes at the base of the food web could have ecosystem‐wide implications.

Extreme reduction in nutritional value of a key forage fish during the Pacific marine heatwave of 2014-2016

“…The apparent result was the lowest chlorophyll estimate for April through August 2016 from MODIS satellites in the 14 yr record (2003− 2016) (Hopcroft et al 2018). A similar response to the Pacific MHW with low nutrients and chlorophyll was recently documented in British Columbia (Peña et al 2018).…”

Warm Events Induce Loss of Resilience in Organic Carbon Production in the Northeast Pacific Ocean

“…In 2014 and 2015, the northern Gulf of Alaska exhibited low zooplankton abundance (McKinstry & Campbell, 2018). The warm anomalies impacted the Bering Sea from 2014 until 2016 by shifting phytoplankton blooms to early spring (Stabeno et al, 2017), similar to the early increase in phytoplankton abundance observed in the Gulf of Alaska in 2014 (Peña et al, 2019) and early NCP from this work (Figure 4c). The development of the Arc pattern along the west coast of United States between 2014 and 2015 altered the local phytoplankton community structure (Du & Peterson, 2018), with elevated SSTs stimulating harmful algae blooms in the northern California Current (McKibben et al, 2017;Zhu et al, 2017).…”

“…Our results show that early initiation of NCP in spring 2014 was promoted by a shallow and stable MLD condition induced by the first warm event (Figure 1a). The enhanced production could have been related to the high phytoplankton biomass, such as proposed by Alexander et al (2008) and observed by Peña et al (2019). Indeed, similar initial upper ocean conditions resulted in high NCP observed in summer 2009 (Figures 4 and 5).…”

“…Their study used a physicalecosystem model to simulate the impact of a PDO-driven event that happened between 1976 and 1977 in the region, with a focus on physical components driving the ecosystem's response. Indeed, in situ observations from spring 2014 show a 100% relative increase in surface Chl a in the region due to high phytoplankton abundance (Peña et al, 2019). Briefly, a more stratified winter minimizes the dilution of phytoplankton cells by vertical mixing; as a consequence, the cells remain in the surface layer at higher density and elevated rates of primary production start earlier in the year.…”

“…In this scenario, net organic carbon production should also have been enhanced in spring and declined toward summer. Indeed, in situ observations from spring 2014 show a 100% relative increase in surface Chl a in the region due to high phytoplankton abundance (Peña et al, 2019).…”