Shifts in biodiversity and physical structure of seagrass beds across 5 decades at Carriacou, Grenadines

Importantly, our study spans a period of marked broad-scale environmental change in the Tropical Atlantic Bioregion, including increased sea temperature and frequency of heat waves [53], increased sea level and significant wave height [54, 55], and increased frequency and severity of strong storms [56], in addition to localized deterioration of the coastal seagrass hab- itat associated with eutrophication and reduced water transparency related to land based activ- ities [21, 22, 57], dieback of T. testudinum associated with drought and hypersaline conditions [58], and direct physical disturbance by recreational or commercial activities [59]. Carriacou is a relatively small island (34 km-2, population ~10,000) that remains largely free of many of the localized anthropogenic factors that have caused degradation and loss of sea- grass beds elsewhere in the Caribbean. It has retained a small-scale livelihood fishery (e.g., fish traps, spearfishing), in contrast to the more intensive fishing of reefs and lagoons recorded elsewhere in the Caribbean by 1985 [60]. There are no large freshwater inputs or restricted bays in Carriacou that might cause changes in seagrass beds associated with salinity fluctua- tions and sediment in runoff. Blooms of free-floating Sargassum seaweed, which had begun to impact coastal waters in Florida and much of the Caribbean after 2012 [61], had not been observed in Carriacou by end of our study in 2019. Dredging and infilling associated with

Fig 9. Number of scarps on transects in 1969, 1994 and 2016. Mean (± SE) scarp number for 6 HWE sites and 4 LWE sites in 1969 and 2016, and 3 HWE sites and 1 LWE site in 1994. Also shown is scarp number on replicate transects at HWE and LWE sites in each year. https://doi.org/10.1371/journal.pone.0306897.g009 PLOS ONE Seagrass bed biodiversity and structure and across 5 decades PLOS ONE | https://doi.org/10.1371/journal.pone.0306897 August 1, 2024 15 / 25