Biogeochemical Anomalies at Two Southern California Current System Moorings During the 2014–2016 Warm Anomaly‐El Niño Sequence

Abstract

We analyzed impacts of the 2014–2015 Pacific Warm Anomaly and 2015–2016 El Niño on physical and biogeochemical variables at two southern California Current System moorings (CCE2, nearshore upwelling off Point Conception; CCE1, offshore California Current). Nitrate and Chl‐a fluorescence were <1 μM and <1 Standardized Fluorescence Unit, respectively, at CCE2 for the entire durations of the Warm Anomaly and El Niño, the two longest periods of such low values in our time series. Negative nitrate and Chl‐a anomalies at CCE2 were interrupted briefly by upwelling conditions in spring 2015. Near‐surface temperature anomalies appeared simultaneously at both moorings in spring 2014, indicating region‐wide onset of Warm Anomaly temperatures, although sustained negative nitrate and Chl‐a anomalies only occurred offshore at CCE1 during El Niño (summer 2015 to spring 2016). Warm Anomaly temperature changes were expressed more strongly in near‐surface (<40 m) than subsurface (75 m) waters at both moorings, while El Niño produced comparable temperature anomalies at near‐surface and subsurface depths. Nearshore Ωaragonite at 76 m showed notably fewer undersaturation events during both warm periods, suggesting an environment more conducive to calcifying organisms. Planktonic calcifying molluscs (pteropods and heteropods) increased markedly in springs 2014 and 2016 and remained modestly elevated in spring 2015. Moorings provide high‐frequency measurements essential for resolving the onset timing of anomalous conditions and frequency and duration of short‐term (days‐to‐weeks) perturbations (reduced nitrate and aragonite undersaturation events) that can affect marine organisms.

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

“…Consistent with our results, Lilly et al. (2019) recorded nitrate concentrations <1 μM and reported negative nitrate and Chl‐ a anomalies for the Southern California Bight throughout the 2014‐HW and El Niño in 2015. Overall, this nutrient data analysis implies that associated with ocean warming (Figures 2a, 2b, 5a, and 5b) and enhanced stratification (Figure 4c), there was a reduced vertical transport of nutrients (Figures 4d, 5e, and 5f; Gómez‐Ocampo et al., 2017; Gonzalez‐Silvera et al., 2020; Zaba & Rudnick, 2016) toward the euphotic zone between August 2014 and December 2015.…”

“…In the case of nutrient supply, there are several lines of evidence indicating a bottom‐up control on phytoplankton size during the occurrence of anomalously warm waters along the coast of nBC. First, there was a general shortage of nutrients (Figures 2e and 2f) and below average net primary production (Figure 4e) associated with ocean warming and strengthened stratification (Figures 2 and 4c), indicating that there was a limited nutrient supply toward the euphotic zone (particularly nitrate, Figure 4d; Gómez‐Ocampo et al., 2017; Gonzalez‐Silvera et al., 2020; Jacox & Edwards, 2011; Lilly et al., 2019; Zaba & Rudnick, 2016). Nitrate depletion and stratified conditions may have benefited smaller phytoplankton, since small cells have a higher surface area/volume ratio, which provides them with the ability to compete for nutrients in oligotrophic environments (Chisholm, 1992; Marañon et al., 2012, 2014; Sommer et al., 2017).…”

“…The first one is that stratification strengthening requires a larger amount of energy to raise deep isopycnals toward the surface (Cavole et al., 2016; Kosro et al., 2006). The second one is that a stratified water column acts as a barrier that reduces the vertical flux of cold subsurface water to the upper layer (Bograd & Lynn, 2003; Gómez‐Ocampo et al., 2018; Jacox & Edwards, 2011; Lilly et al., 2019; Roemmich & McGowan, 1995; Zaba & Rudnick, 2016) and, in consequence, the nutrient input toward the euphotic zone.…”

“…The second one is that a stratified water column acts as a barrier that reduces the vertical flux of cold subsurface water to the upper layer (Bograd & Lynn, 2003;Gómez-Ocampo et al, 2018;Jacox & Edwards, 2011;Lilly et al, 2019;Roemmich & McGowan, 1995; and, in consequence, the nutrient input toward the euphotic zone.…”

“…The higher the ϕ, the more stratified the water column. Here, we report ϕ 100m (h = 100 m) for outside the bay because the warm anomaly during the 2014-2015 period was a surface-intensified phenomenon across the southern CCS (Durazo et al, 2017;Lilly et al, 2019;Zaba and Rudnick, 2016), as well as to make valid comparisons with ϕ 100m values previously reported for nBC (1998-2016Gómez-Ocampo et al, 2017). Before this, ϕ 50m (h = 50 m) and ϕ 100m were calculated using 172 CTD profiles recorded outside the bay over the 2008-2015 period.…”