Effects of the 2013-2016 warm anomalies on the California Current phytoplankton

Abstract

Anomalies occurring in the California Current System during 2013–2016 were examined using remote sensing data and in-situ measurements. Positive anomalies in sea surface temperature (SST, up to 4°C) and absolute dynamic topography (ADT, up to 20cm), as well as negative anomalies in satellite chlorophyll concentration (CHL, up to -3.0mgm-3) were recorded. In-situ temperature anomalies in August 2014 increased equatorward, from 2°C along the northernmost line (CalCOFI hydrographic line 76.7) to 10°C (below ~80m depth) at the southernmost hydrographic line (off Baja California). In-situ negative Chlorophyll-a (Chl-a) surface anomalies were similar in most of the lines examined (~ -1.5mgm-3), but for the CalCOFI lines, larger anomalies (~ -3.5mgm-3) were recorded. As a result of increased stratification, mirrored in the increase in mean monthly ADT and the upper water-column temperature, the 2003–2016 CHL time series showed a declining trend in phytoplankton production and biomass, which was particularly pronounced during the last four years in the central and southern zones of the California Current System. The mechanisms responsible for this trend seem to be related to a climate shift from cold to warm in the northeastern Pacific Ocean, as evidenced by large correlations between the CHL anomalies and the Pacific Decadal Oscillation, Multivariate ENSO Index, and Oceanic El Niño indices. Generalized additive models were used to explore the relationship between stratification and the decline in phytoplankton production. Two ADT ranges—characteristic of either highly dynamic or stratified environments—were defined, both of which led to a drop in primary production. The effects of these environments have been associated with the photo acclimation of phytoplankton to irradiance and nutrient availability regimes, by which primary production and biomass decrease due to the limitation of either of these factors. This study reveals some of the effects of the warm anomalies occurring from 2013 to 2016 in the California Current ecosystem, which will help to understand changes in higher trophic levels in this region.