Long-Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

The Benguela Upwelling System (BUS) is among the most productive ecosystems globally, supporting numerous fisheries and ecosystem services in Southern Africa. Sea-viewing Wide Field-of-view Sensor and Moderate-resolution Imaging Spectroradiometer-Aqua chlorophyll a (Chla) concentrations between September 1997 and February 2018 were used to investigate long-term trends in phytoplankton biomass and size structure (microphytoplankton [>20m], nanophytoplankton [2-20m], and picophytoplankton [<2m]) in the Northern Benguela, Southern Benguela (SB), and Agulhas Bank (AB) shelf and open ocean regions of the BUS. Trends in upwelling and correlations with Chla and size structure were examined. Increasing Chla and microphytoplankton trends occurred in the Northern Benguela shelf and open ocean, while decreases were evident on the SB shelf in all seasons. In the SB open ocean, small increases occurred during austral winter, with a decrease in spring. On the AB shelf, increases in Chla and microphytoplankton occurred in summer with decreases during the other seasons. Patterns differed in the AB open ocean, with increases in winter and spring and decreases in summer and autumn. Although R-2 values indicated that linear trends accounted for a reasonable portion of the variance, and most trends were statistically significant, they showed only small changes on the shelf domains and little to no change in the open ocean. Strong correlations between upwelling, Chla, and the size classes were observed, but distinct seasonal differences occurred in each region. This is the first 20-year analysis of phytoplankton biomass and community structure in the BUS and provides a baseline against which future changes can be monitored. Plain Language Summary Two satellite ocean color data products were combined and used to examine trends in chlorophyll a and the proportions of microphytoplankton, nanophytoplankton, and picophytoplankton in three subregions of the Benguela Upwelling System, namely, the northern Benguela, southern Benguela, and Agulhas Bank. These trends were related to trends in upwelling-favorable winds and significant correlations were observed but with distinct seasonal differences in each region. Linear trends were mostly significant and accounted for a reasonable portion of the variance, but they showed only small changes on the shelf domains and little to no change in the open ocean regions. This study provides the first 20-year analysis of phytoplankton biomass and community structure in the Benguela Upwelling System and provides a baseline against which future changes can be monitored.