Georgieva, Silvia (2020) Influence of upwelling intensity on environmental variables and phytoplankton community composition in the Peruvian upwelling system during austral summer 2018-2019. (Master thesis), Ghent University, Ghent, Belgium, 38 pp.

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Abstract

The Peruvian coast is characterized by upwelling of cold and nutrient-rich subsurface waters caused by persistent equatorward winds that result in offshore Ekman transport. The supply of nutrients to the euphotic zone subsequently drives enhanced phytoplankton growth which supports highly productive ecosystem. Regional differences in wind intensity
produce a variable environment on various spatial and temporal scales that affect the biomass and composition of the phytoplankton community. Here we combine wind-derived Ekman transport, in-situ oceanographic measurements and pigment analysis to (i) characterize the upwelling intensity in six cross-shore (up to 200 km from the coast) transects sampled along the Peruvian coast (8-16°S) during summer 2018-2019, (ii) to relate the chlorophyll a (chl-a; used as a proxy for biomass) to the upwelling history in each transect and (iii) explain the patterns of phytoplankton composition. Signatures of freshly upwelled waters, including lower temperatures (17-18°C) and elevated nutrient concentrations (nitrate >25 mmolL-1) were detected close to the shore in transects with high Ekman transport recorded during the cruise. In contrast, transects with low Ekman values were characterized by the presence of stratified and oxygen-rich waters. Understanding how the environmental variables measured during sampling changed according to the Ekman transport facilitated our capacity to infer the biological dynamics that occurred in response to their environment within a week of sampling. Tracking back the upwelling history a week before the cruise took place, we show that high biomass has accumulated along transects where Ekman transport was high, supported by the nutrient input in surface waters. This was more pronounced along transects where upwelling relaxation followed, and favoured the retention of phytoplankton instead of being continuously advected offshore. High chl-a concentration remained restricted to ~25 km from the coast in transects with low upwelling intensity, while very strong and recent upwelling has caused community dilution. A clear succession in phytoplankton composition was observed, with diatoms being abundant in freshly upwelled waters, while a shift to a more mixed, dinoflagellate-dominated community was seen in more mature waters. The coastal region in the most southern part was an exception, and even though high upwelling was recorded before and during the cruise, a dinoflagellate bloom was observed, where the mixotroph Akashiwo sanguinea was >90% of the phytoplankton community. Older, offshore waters were composed of smaller phytoplankton groups, such as haptophytes (e.g. Phaeocystis globosa) and prasinophytes adapted to nutrient-deplete conditions.

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