Oyster Size Distributions Vary Across Time and Space in the Tampa Bay Estuary, 250–2024  <scp>ce</scp>

ABSTRACT Aim Restoration benchmarks often assume an undisturbed historic range of variability, if they include historic information at all. However, recent research highlights the need for a more nuanced understanding of shifting baselines and how to best incorporate past variabilities in modern management. We investigate trends in oyster size through time (ca. 250–2024 ce ) and space (ca. 1000 km 2 ) in the Tampa Bay Estuary to understand when and where ecologically significant changes to oyster populations occurred. By implementing a multiscalar approach, this study contributes historical ecological data relevant to contemporary restoration and management efforts and provides insights into understanding long‐term socioecological dynamics in Florida's largest open‐water estuary. Location Tampa Bay Estuary, Florida, USA. Time Period ca. 250–2024 ce . Taxon Mollusca; Crassostrea virginica . Methods We measured sizes of over 15,000 oyster shells from archaeological sites and contemporary reefs across four estuarine sub‐basins over approximately 2000 years to assess how mean and maximum shell sizes responded to shifts in climate and human harvesting and settlement patterns. Results We demonstrate a non‐linear decline in oyster sizes through time, including a steep decline in oyster size prior to European colonization and commercial harvest. Changes in oyster size varied across sub‐regions, and there was no consistent response to climatic variability. A moderate rebound in oyster size during the 19th century is evident and coincides with several social factors that alleviated harvest pressures for a relatively short period. However, most of the largest contemporary oysters remain significantly smaller than those in the deeper past. Main Conclusions Archaeological datasets provide relevant historical ecological information for understanding modern era declines in molluscs. Our data reveal that no single baseline exists for oyster size in Tampa Bay. Instead, oyster populations responded in complex ways to climatic variability, ecological stressors and human interactions. Oyster size is a valuable functional trait linked to reproduction and reef resilience, and management targets that reference past demographic states should be framed as dynamic ranges and evaluated at sub‐regional scales.