Multiple stressors result in reduced reproductive effort by Thalassia testudinum in Florida Bay, USA

Abstract

Sexual reproduction remains an understudied aspect of seagrass ecology. We examined spatiotemporal variability in the percentage of short shoots with sexual reproductive structures 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, significant 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.