Upwelling Intrusions drive Colonial Radiolarian Aggregations and Associated Zooplankton Dynamics in a Shelf Ecosystem

Colonial radiolarians are ubiquitous marine protists capable of forming dense aggregations in oceanic and coastal waters worldwide. Their taxonomic diversity and mixotrophic abilities enable them to thrive across diverse ecological niches. While recent studies have focused on radiolarians in open oceans, their role in productive shelf environments remain largely unknown. As a result of their fragile structure and dense biomass in large aggregations, conventional sampling techniques have underestimated their abundance. To address these sampling limitations, three summer cruises (2021-2023) in the South Atlantic Bight used high vertical depth resolution (~1m) shadowgraph and color imaging systems and oceanographic sensors to link fine-scale conditions to plankton aggregations. Dense aggregations of colonial radiolarians were quantified along the outer edge of the mid-shelf (45m isobath), correlating with high chlorophyll-a (12-50 µg m^-3 ) and cold bottom water upwelling intrusions. These aggregations also decreased downwelling light and influenced zooplankton community composition. Hydromedusae and siphonophores were often observed near these dense layers, potentially related to enhanced feeding opportunities. These findings suggest that radiolarian aggregations may represent a significant, previously unrecognized source of carbon flux, not only from their own population growth but interactions with other zooplankton groups. Understanding the drivers and associated organisms is crucial for revealing ecological functions and consequences of radiolarian blooms in shelf systems.

Presentation Preference: Oral

Primary Presenter: Mary Mann, University of Georgia - Skidaway Institute of Oceanography (mary.mann@uga.edu)

Authors:

Mary Mann, University of Georgia -Skidaway Institute of Oceanography  (gracemann946@icloud.com)

Patrick Duffy, University of Georgia - Skidaway Institute of Oceanography  (patrick.duffy@skio.uga.edu)

Marc Frischer, University of Georgia - Skidaway Institute of Oceanography  (marc.frischer@skio.uga.edu)

Adam Greer, University of Georgia - Skidaway Institute of Oceanography  (atgreer@uga.edu)