Optical observation of phytoplankton photophysiology and dynamics using a novel mesocosm facility

durch Jenna Balaguer (GEOMAR)

Mittwoch 17.12.2025, 13:15 → 14:30 Europe/Berlin

8A-002 - Hörsaal Ostufer / Lecture Hall East (GEOMAR - Standort Ostufer / GEOMAR - East Shore)

Abstract:

Understanding the environmental drivers controlling primary productivity and how they vary across time and space is essential for assessing ecosystem function and predicting future change. Current observational approaches range from shipboard measurements and autonomous platforms to satellite remote sensing. Whilst remote sensing has revolutionised the observational capacity of phytoplankton abundance and productivity, physiological insights remained restricted; however, sunlight-stimulated (or passive) chlorophyll a fluorescence has shown potential as a satellite-based proxy for phytoplankton physiology. Passive fluorescence is regulated by chlorophyll a concentration, absorbed light, and fluorescence yield, being influenced by the physiological state of phytoplankton (modulated by temperature, light exposure, and nutrient availability). Although recent field studies have advanced our understanding of how, for instance, nutrient limitations affect passive chlorophyll fluorescence, observations alone often cannot disentangle the co-varying influences of these abiotic stressors; hence, key questions remain unresolved. In addition, the recent launch of the hyperspectral PACE satellite offers new opportunities for finer-resolution chlorophyll fluorescence observations, though more understanding is needed to fully harness its potential. Experimental approaches are therefore needed to identify the roles of these variables on passive fluorescence. Building on previous approaches, we developed a novel indoor mesocosm facility designed to measure passive fluorescence while independently manipulating temperature, light, and nutrient availability. The facility was specifically designed to tackle key limitations of previous studies: (i) replicating realistic ocean conditions (i.e., diurnal light cycles and high irradiance levels >2000 μmol photons m⁻² s⁻¹); (ii) detecting weak fluorescence signals under realistic low chlorophyll concentrations (<2 µg L⁻¹); and (iii) enabling simulations of major oceanic nutrient limitations (nitrogen and iron) under trace-metal-clean conditions. Initial tests using the ubiquitous diatom Chaetoceros calcitrans demonstrate the potential of this facility to advance our understanding of phytoplankton physiology and ecology at global scales.