Jessie Turner - SMSP Fall Seminar Series 2024

Abstract: Shifts in phenology, the timing of annually occurring biological events, are being altered in the global ocean with climate change. The timing and magnitude of phytoplankton blooms in the marginal ice zone are critical drivers of carbon uptake and food quality for higher trophic levels. We examined phenological changes in the phytoplankton accumulation season in a polar region using long-term patterns in satellite-derived chlorophyll-a concentration. A merged multi-sensor cloud-filled remote sensing dataset was analyzed to determine shifts in the phenology of the phytoplankton growing season from 1997-2021 for the West Antarctic Peninsula, a rapidly warming ocean region. In spring, the phytoplankton accumulation season is beginning later in the season over time, initiating up to three weeks later in recent years compared to the late 1990s. In fall, the marginal ice zone is experiencing elevated surface chlorophyll concentrations during recent years compared to the past. Correspondingly, the seasonality of the air-sea CO2 flux has also shifted, showing evidence of a longer-lasting CO2 sink extending later into the fall. Possible mechanisms for spring shifts include increased wind speed and decreased ice-associated early season water column stability. Possible mechanisms for fall shifts include later sea ice advance and nutrient availability during the extended ice-free season. Mechanisms were tested with coupled physical-biogeochemical modeling experiments, using an ice-ocean-ecosystem model and an advanced data-assimilative ocean biogeochemistry model.  Experiments included scenarios with varying mixed layer depths and wind conditions, including higher wind speeds during episodic windy storm events. Modeling experiments suggest that increased wind speed is one likely mechanism for the observed changes in the spring season. Observed shifts in phytoplankton phenology will have important implications for carbon cycling and food web dynamics.