Oral Session 6

The role of extreme temperatures on marine phytoplankton communities and higher trophic levels using multiple observational systems

Always more often, the ocean is experiencing periods of anomalously warm temperature conditions (called marine heatwaves, MHWs) that propagate from the sea surface to depth, and may affect marine life and ecosystem functioning. While the role of MHWs has been widely assessed in coastal and benthic environments, the MHW impact on offshore and pelagic ecosystems is overlooked. The main reason is that we need multiple observational systems that cover required temporal and spatial scales. Here, we synergistically leverage bio-optical and biogeochemical information derived from autonomous BioGeoChemical-Argo profilers, ocean color satellites and ecosystem population models to evaluate the role of MHWs on the offshore productive Northwestern Mediterranean Sea between 2012 and 2022. We analyze phytoplankton size structure and productivity during the annual spring bloom, and the cascading effects on carbon fluxes and higher trophic levels. Through optically-derived carbon proxies, we show that MHW events occurring during winter drastically inhibit spring phytoplankton carbon biomass up to 70%, while phytoplankton shifts to smaller cells and water transparency increases. Net primary production and chlorophyll-a also decrease up to 80% and 70%. Such reduction is due to the enhanced stratification of the water column under MHWs, that hinders the renewal of nutrients from deep-ocean reservoirs and leads to 80% decline of surface nitrate concentrations. Finally, we show that the inhibition of the annual spring phytoplankton bloom negatively impacts pools and fluxes of particulate organic carbon down to 1000 m depth and generates chain effects starting from zooplankton.

Emanuele Organelli, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy, https://orcid.org/0000-0001-8191-8179

Mengyu Li, East China Normal University, China

Federico Serva, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

Marco Bellacicco, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

Angela Landolfi, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

Andrea Pisano, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

Salvatore Marullo, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

Fang Shen, East China Normal University, China

Alexandre Mignot, Mercator Ocean International, France

Simon van Gennip, Mercator Ocean International, France

Rosalia Santoleri, National Research Council Institute of Marine Sciences (CNR ISMAR), Italy

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