Effect of Light and Upwelling Intensity on the Phytoplankton Community Composition in the Peruvian Upwelling System

The coast of Perú hosts the largest and most productive Eastern Boundary Upwelling System. Climate change is predicted to increase stratification, thereby increasing light availability and lowering nutrient concentrations at the surface. Moreover, the winds causing upwelling in this area are predicted to change their intensity and migrate polewards.

To better understand and predict the response of phytoplankton to changes in light and nutrient conditions, we recreated different light and nutrient scenarios in 9 off-shore mesocosms during the KOSMOS-Peru-2020 experiment in March-April 2020 off the coast of Callao (Perú). We recreated two light scenarios: high light (HL) and low light (LL); and four levels of upwelling by adding deep water (DW) in different proportions (0, 15, 30, 45 and 60 %). We monitored the phytoplankton composition every two days for 36 days. Photosynthetic pigments were measured using HPLC and the phytoplankton community composition was estimated using CHEMTAX and taxonomically determined by microscopic analyses, whereas chlorophyll-a (Chla) as a proxy for bulk phytoplankton biomass and particulate organic carbon, nitrogen and phosphorus (POC, PON and POP) provided information about biomass and stoichiometry of the total suspended matter.

The enclosed initial community was dominated by the red-tide forming raphidophyte Fibrocapsa japonica, detected for the first time off the coast of Perú during this experiment.

After an initial phase, during which F. japonica consumed the nutrients available, the DW was added and a second bloom, dominated by diatoms developed. As expected, more phytoplankton accumulated under HL and in higher DW treatments. The phytoplankton community under LL increased its Chla content per cell to maximize photosynthetic performance, whereas HL caused a significant increase in the POC:PON ratio.

Diatoms, coccolithophores and Phaeocystis were positively affected by HL, whereas the LL phytoplankton assemblage was dominated by smaller groups such as cryptophytes, prasinophytes, Synechococcus and especially the pelagophyte Octactis octonaria. F. japonica became more abundant under LL during the initial phase. Higher upwelling intensity favored diatoms as well as pelagophytes and chlorophytes under LL, whereas low nutrients conditions favored prasinophytes. Upwelling events were accompanied by high contributions of diatoms, whereas nutrient-depleted conditions were dominated by small phytoplankton groups and dinoflagellates.

From our results we conclude that although upwelling intensity did not affect stoichiometry significantly for the duration of the experiment, an intensification of stratification causing greater exposure to HL conditions might decrease the nutritional value of phytoplankton for upper trophic levels. Changes in light and nutrient availability caused by climate change will trigger a shift in the phytoplankton community composition. HL and intense upwelling areas might be dominated by diatoms and LL and low nutrient areas might be dominated by prasinophytes with distinct consequences for the trophic transfer and export efficiency of the Peruvian upwelling system.