ALBERT

Description: Carbon dioxide and light are two major prerequisites of photosynthesis. Rising CO2 levels in oceanic surface waters in combination with ample light supply are therefore often considered stimulatory to marine primary production. Here we show that the combination of an increase in both CO2 and light exposure negatively impacts photosynthesis and growth of marine primary producers. When exposed to CO2 concentrations projected for the end of this century, natural phytoplankton assemblages of the South China Sea responded with decreased primary production and increased light stress at light intensities representative of the upper surface layer. The phytoplankton community shifted away from diatoms, the dominant phytoplankton group during our field campaigns. To examine the underlying mechanisms of the observed responses, we grew diatoms at different CO2 concentrations and under varying levels (5-100%) of solar radiation experienced by the phytoplankton at different depths of the euphotic zone. Above 22-36% of incident surface irradiance, growth rates in the high-CO2-grown cells were inversely related to light levels and exhibited reduced thresholds at which light becomes inhibitory. Future shoaling of upper-mixed-layer depths will expose phytoplankton to increased mean light intensities. In combination with rising CO2 levels, this may cause a widespread decline in marine primary production and a community shift away from diatoms, the main algal group that supports higher trophic levels and carbon export in the ocean.

Description: The project "Lake Tsunamis: Causes, Controls, and Hazard (Characterization of subaqueous unstable slopes with geophysical and geotechnical measurements.)" was initiated to understand their governing mechanisms of genesis, propagation, frequency, and the related hazard within an interdisciplinary context. Swiss lakes served as a laboratory for this holistic approach. The project was divided into five work packages (WP), where WP1 comprised a large number of geophysical (using amongst other techniques ocean bottom seismometers (OBS)) and geotechnical measurements to characterize the structure and stability of potentially unstable subaqueous lake slopes. To evaluate the potential and applicability of ambient vibration techniques in a shallow-water offshore environment, multiple single-station and array OBS measurements were performed on subaqueous slopes in Lake Lucerne. Eight DEPAS Lobster type broadband OBS from the German Instrument Pool for Amphibian Seismology (DEPAS) and one Nammu type OBS from ETH Zürich were successfully deployed and recovered at more than 170 distinct locations in 2018-2020. In 2020-2023 the single Nammu OBS was deployed several times for supplemental measurements. Surveys with an airgun of 1-inch³ volume were used on top of the deployment locations to determine the misorientation of the horizontal components. In addition, multibeam bathymetric surveys were performed to locate the positions of the OBS on the lake floor with high accuracy. A workflow for passive seismic investigations with OBS in such shallow-water settings was developed. The seismic response and its variability at the measured sites in terms of amplification functions during earthquakes and resonance frequencies was determined. Shear-wave velocity profiles at different morphological types of slopes down to a depth of 100-150 m below the lake floor were resolved and interpreted. Combining geophysical and geotechnical measurements and interpretation, static and dynamic slope-stability analyses were performed. Thresholds for the subaqueous slope-failure triggering in terms of earthquake magnitude and epicentral distance, macroseismic intensity, and ground-motion intensity measures were derived using earthquake ground-motion modelling.