Dynamics of island mass effect – Part 2: Phytoplankton physiological responses

Abstract. Island mass effect (IME) refers to the phenomenon of elevated chlorophyll a concentrations around islands, often extending hundreds of kilometers into oligotrophic waters. In this study, we explore the physiological responses and changes in phytoplankton community composition within island mass effect zones, providing insights into the drivers and ecological impacts of this phenomenon. Here, we study IMEs associated with four South Pacific subtropical archipelagos over six-month periods. We use a combination of satellite-derived physiological indices and in situ bio-optical data collected during the Tara Pacific expedition (2016–2018) to further our mechanistic understanding of IME. We examine mechanisms such as nutrient enrichment and pigment-based proxies of ecological succession that underpin the IME. Our results demonstrate that phytoplankton populations within IME zones typically experience reduced physiological stress compared to the surrounding open ocean, likely due to an alleviation of iron limitation. Hence, recurring iron enrichment may be a significant factor of IME across the South Pacific Subtropical Ocean. In some cases, we also detected signatures of decreased phytoplankton stress due to macronutrient limitation associated with local upwellings and increased vertical mixing, highlighting the role of physical processes in supplying macronutrients to the photic zone. While iron enrichment seems to originate mostly from terrigenous/reef inputs, macronutrients can be both from terrigenous/reef origin or vertical entrainment of nutrient-rich deep water to the surface ocean. We also show that IME is often associated with changes in pigment ratios, which is suggestive of changes in phytoplankton community composition. These findings underscore the complex interplay between nutrient availability, community composition, and physiological stress in shaping IME, offering new perspectives on this phenomenon and its ecological significance.