MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol. 647: 65–78, 2020 https://doi.org/10.3354/meps13395 Published August 13 1. INTRODUCTION Seagrasses are flowering plants that can form large clonal communities, known as beds or meadows, which are capable of long-term persistence (Reusch et al. 1999); individuals (genets) may persist for 100s to 10 000s of years or more (Arnaud-Haond et al. 2012). Under stable, low-disturbance conditions, sea- grass beds can be maintained primarily through veg- etative clonal growth, providing limited opportunity for sexual recruitment by seedlings (Kaldy & Dunton 1999, Vermaat 2009). Because of the difculty in tracking seedling recruitment within dense seagrass canopies, sexual reproduction had historically been considered unimportant to seagrass meadow mainte- nance (Duarte et al. 2006), especially for long-lived, large-bodied seagrasses (Campey et al. 2002). How- ever, recent application of molecular tools has shown high levels of genetic diversity within and among seagrass populations, highlighting the role that sex- ual recruitment can play in bed maintenance and recovery (Kendrick et al. 2012, 2017, Furman et al. 2015). In fact, intermediate levels of disturbance are thought to maximize both reproductive effort and genetic diversity of tropical seagrasses (Cabaço & Santos 2012, McMahon et al. 2017). © Inter-Research 2020 · www.int-res.com *Corresponding author: durakom@uncw.edu Multiple stressors result in reduced reproductive effort by Thalassia testudinum in Florida Bay, USA Manuel M. Peñalver 1 , Michael J. Durako 2, * , Bradley T. Furman 1 , Margaret O. Hall 1 1 Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 100 8 th Avenue SE, St. Petersburg, FL 33701, USA 2 University of North Carolina Wilmington, Department of Biology and Marine Biology, Center for Marine Science, 5200 Marvin Moss Ln, Wilmington, NC 28409, USA ABSTRACT: Sexual reproduction remains an understudied aspect of seagrass ecology. We exam- ined spatiotemporal variability in the percentage of short shoots with sexual reproductive struc- tures and the proportion of sites that had flowered as an indicator of Thalassia testudinum sexual reproductive effort (RE) across Florida Bay, USA. Short shoots were collected annually during spring within 13 basins across the bay from 2006−2019. The sample period followed 2 very active hurricane seasons and included 2 subsequent major disturbance events, a large-scale die-off of seagrasses in 2015, and the passage of Hurricane Irma in 2017. On average, 4.7% of the collected short shoots had flowered between 2006 and 2019, ranging from 1.3−8.5% at the bay scale and 0−30% at the basin level. Regression analyses indicated that RE varied significantly among basins and years, with high multiyear variability in several basins. RE was negatively correlated with annual heat accumulation, and positively correlated with the number of days below 28°C. Annual heat accumulation rose steadily from 2006−2019; accordingly, bay-wide RE declined. RE was higher in western basins, which were the most affected by recent disturbance events, indicating a potentially important role for sexual reproduction in recovery from disturbance. However, signifi- cant reductions in RE following the 2015 die-off and Hurricane Irma show limits to the plasticity and resilience of T. testudinum, both in terms of reduced compensatory RE following successive disturbances and reductions in basal RE correlated with rising annual temperatures. KEY WORDS: Seagrass · Flowering · Disturbance · Die-off · Hurricanes · Resilience · Climate change · Dispersal · Sexual recruitment Resale or republication not permitted without written consent of the publisher