Phytoplankton dynamics in three distinct embayments along the South African coastline

The South African coast is bordered by eastern and western boundary currents, from east to west the coastline hosts tropical, subtropical, warm temperate and cool temperate biogeographic regions with the two phylogenetic breaks. The coastal phytoplankton community structure in three spatially distinct embayments was investigated during austral spring. In this study we captured the daily variability in three bays contrasted by three different oceanographic upwelling processes influencing the phytoplankton community. Advection events in St Helena and Algoa Bay and the aftereffects of an upwelling event in Walker Bay were a prior processes to each sampling campaign. The hydrological properties in Algoa Bay and Walker Bay reflected warm, well oxygenated waters and minor variations in pH and salinity while St Helena Bay exhibited noticeable stratification, decreases in pH, oxygen depletion and clear day-to-day variability. Algoa Bay had significant differences in SiO44- and NH4+ between sampling dates while Walker Bay displayed more stable conditions with only NO2- showing a significant difference. Contrastingly, St Helena Bay showed the most pronounced variability, with NOxN, NO2- and PO43- all differing significantly. The dominant community structure was broadly comparable between the bays, however, site-specific assemblages and day-to-day variability was evident. Diel oxygen dynamics highlighted bay specific differences in short term metabolic activity and phytoplankton functioning. Overall, we found common genera across all three bays, including Chaetoceros, Leptocylindrus, Rhizosolenia, Protoperidinium and Tripos. Each bay displayed alternative values of diversity and community change influenced by varying hydrological and biogeochemical factors. These factors were all broadly governed by larger scale oceanographic and atmospheric forcing.