Phytoplankton adaptation and absorption properties in an Agulhas Current ecosystem

The Agulhas Current is a major Western Boundary Current that flows along the southeast slope of southern Africa and has an influence on the biogeochemistry of adjacent ecosystems through localised upwelling of cool, nutrient-rich water up the slope and onto the shelf. Variability in pigments and absorption were therefore examined to determine the adaptation of phytoplankton communities to changing nutrient and irradiance conditions during mid-summer. Chlorophyll a concentrations varied from 0.1 mg m−3 offshore to 7.3 mg m−3 inshore in response to the nutrient enrichment. CHEMTAX analysis revealed that diatoms were the dominant group at most of the inshore stations, mixed populations of diatoms, haptophytes and prasinophytes characterised the shelf edge populations, while haptophytes and Prochlorococcus dominated the offshore stations over the slope. Diatom populations were associated with cool, lower salinity water and prokaryotes with warm, higher salinity water, while flagellates occurred in both water types. Nutrient relationships deviated from Redfield ratios and there appeared to be a greater uptake of silicate at elevated nutrient levels where diatoms were dominant, but nitrate was preferably utilized at low concentrations when flagellates and prokaryotes comprised the communities. Populations acclimated to changing irradiance by adjustments in their absorption and proportion of accessory pigments. Prokaryote dominated communities had high chlorophyll-specific absorption coefficients and a high proportion of chlorophyll a and photoprotective carotenoids. Diatoms had low chlorophyll-specific absorption and elevated proportions of photosynthetic carotenoids and chlorophyll a. Flagellate dominated communities had intermediate chlorophyll-specific absorption and had elevated absorption by chlorophyll a, chlorophyll b and photosynthetic carotenoids. Ratios of UV absorption at 330 nm to chlorophyll a absorption at 676 nm displayed an increasing trend with increased irradiance near the surface, particularly for prokaryotes. Overall, nutrients and irradiance were the main drivers influencing phytoplankton adaptation and acclimation. •Upwelling of cool nutrient-rich water occurred up the slope onto the shelf.•Diatoms-flagellates on the shelf; prokaryotes-flagellates in offshore Agulhas water.•Prokaryotes had high specific absorption and high proportion of PPCs and TChla.•Diatoms had low specific absorption and elevated proportion of PSCs and TChla.•Intermedia