Examing the climate drivers of phytoplankton variation along the Chilean coast

The South American east coast is a very productive region, with more than 10% of the worldwide marine fish catch, especially along the Chilean shores. The base of this production is the phytoplankton that can be easily measured by the MODIS-aqua satellite through its chlorophyll (Chl). The Chl concentration depends on specific parameters such as precipitation rate (PR), surface sea temperature (SST), or wind direction (U, V), obtained by ERA5. These parameters have a different impact along the extensive Chilean coast, having different climates. Additionally, they are forced by climate drivers such as El Niño Southern Oscillation (ENSO), the Antarctic Oscillation (AAO), the Southern Pacific Subtropical Anticyclone (SPSA), and the Pacific Decadal Oscillation (PDO) obtained by NOAA. The study area was split into ten subsections, grouped into three Zones (the Northern, Central and Southern Zone). The Chl and physical parameters were studied in each subsection from 2002 to 2018. The analysis revealed that the southerly winds driven by the presence of the SPSA and the shelf size are the main triggers for the phytoplankton bloom. Therefore, the most productive region is the middle Central Zone having a broad shelf. However, despite its extreme aridity and low southerly winds, the Northern Zone’s extreme subsection (1N) had a significant Chl signal. In contrast, although the Southern Zone had high PR and predominant westerly winds leading to stratification, its production was significant, with a slightly lower concentration further south (3S). Thus, these regions need a deeper analysis. Composites were created selecting the years when the climate drivers were either strongly positive or negative to understand their influence on the different parameters. The study period was extended towards 1979 to have enough years in the composite samples. Nonetheless, the Chl is only available from 2002 onwards. The Chl dataset from the long preMODIS period was reconstructed by Canonical Correlation Analysis (CCA), based on another parameter with a significant correlation with Chl. All the variables were tested, having better results with SST. This analysis concludes that AAO and ENSO are the predominant climate drivers that modulate the SPSA. The Chl’s impact from El Niño was significant in the Southern Zone during the summer, while the Northern Zone was not significantly affected. Although AAO altered the PR patterns significantly, the Chl activity did not show a response. Three case studies per Zone were selected to understand better the mechanisms that trigger the Chl increment. Also, a heavy dust storm and a significant red tide episode in the Northern and Southern Zone were added to determine these external factors’ effects. The results confirmed that the southerly winds are the main trigger for these cases. Although this was straightforward in the Central Zone, it was less clear in the other Zones. The Chl blooms and the wind patterns showed slight gaps in location in the Northern Zone. The Southern Zone cases occurred when the southerly winds were dominant, and the PR was low at the same time. These conditions were driven when the SPSA was moved southwards by El Niño, and the positive AAO blocked the fronts coming from the Southern Ocean. This thesis demonstrates the importance of longshore winds driven by the SPSA and the shelf width in ocean fertilisation, increasing the phytoplankton. Thus, the middle Central Zone is the most productive, where both variables are present. Due to its high PR and low southerly winds, these fluxes are more predominant in the Southern Zone leading to stratification. However, it reached an equilibrium when El Niño moved the SPSA southwards, and the positive AAO occurred. 1N and 3S’s Chl signal is linked to a more significant presence of undercurrents Equatorial Subsurface Water (ESSW) and Antarctic Intermediate Water (AAIW), respectively.