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

sheltered bays and estuaries. Seagrass beds provide significant ecosystem services locally as nurseries and feeding areas for fish on coral reefs and mangroves [2–4], and as habitat for tra- ditionally fished sea urchins [5], green turtle (now protected) [6], conch [7] and lobster [8]. By dampening wave action and stabilizing sediments, seagrasses enhance coastal protection and water quality [9–11] and calcareous epiphytes and seaweeds in seagrass beds are significant contributors to sediments [12]. Interactions between seagrass habitats and nearby coral reefs and mangroves increase overall health of various habitat types [10, 13]. Seagrasses also coun- teract ocean acidification locally by raising pH leading to significantly increased calcification by corals downstream of seagrass beds [14] and by calcareous algae within beds [15]. Sea- grasses, like mangroves and saltmarshes, are significant contributors to blue carbon [16] due to their high productivity and belowground carbon storage [17]. However, there remains con- siderable uncertainty about the total area of seagrass beds globally [18], and carbon storage varies widely according to site conditions and seagrass species [19]. Recent estimates [20] indicate that seagrass cover globally has declined by 19% since the 1880s, with declines occurring in all seven of seagrass bioregions defined by [1]. Absolute losses were highest in the Tropical Atlantic Bioregion, although there has been a trend of increase in seagrass cover in this region since the late 1990s. Principal drivers of sustained

losses globally, and in the Tropical Atlantic Bioregion specifically, are coastal development and poor water quality [20–22]. Increases in seagrass cover have been observed in some areas with- out any management [23] and others after deliberate actions such as reduction of nutrient input [24]. In general, seagrass beds in the Caribbean have proved remarkably resilient to extreme storms, either being minimally impacted (especially those dominated by the climax species Thalassia testudinum) or recovering over time [11, 21]. However there have been shifts towards earlier successional seagrass species, and disruptions by intense storm activity appear to facilitate expansion of Halophila stipulacea [25], an invasive seagrass first noted in the Trop- ical Atlantic at Grenada in 2002 [26]. Such changes can affect ecosystem services such as coastal protection [27]. Sea level rise can have negative or positive effects on the area of seagrass and other coastal wetlands depending on their capacity to build vertically, sediment supply, and the lateral and vertical “accommodation space” for landward migration of wetlands, and extent to which this is realized through local coastal management [19, 28, 29]. Global mean sea level rise for the Carib- bean, estimated as 2.8 mm yr−1 for 1993–2018, is predicted to rise dramatically after 2050, increasing to as much as 30 mm yr−1 by 2100 [30]. Such increases could lead to disproportion- ally large increases in wave energy and negative impacts on seagrass beds in back-reef environ-

ments if vertical reef accretion does not keep pace [31, 32]. Reef “flattening”, the loss of hard coral and reef structural complexity, has been widespread in the Caribbean since the early 1980s [33]. Based on modelling studies [34], showed that a significant increase in mean wave height could occur at present sea level given sustained degradation of reef structural complexity. Observations conducted on seagrass beds at Carriacou, Grenada in the eastern Caribbean in 1969, 1994 and 2016 provide an opportunity to explore changes in seagrass distribution and community composition on a multidecadal scale in an area not heavily impacted by local human activity aside from subsistence fishing. These seagrass beds occur in windward lagoons behind barrier reefs as well as in leeward areas of the island [35, 36]. The barrier reefs have undergone the types of degradation [37] observed more generally for the Caribbean [38, 39]. The initial set of observations in 1969 was conducted as part of a broad characterization of T. testudinum and “turtle grass beds” at Barbados and Carriacou in the eastern Caribbean [35, 40]. Subsets of those observations were repeated at Carriacou, the less developed island, in 1994 and 2016. Scheibling et al. [36] reported on the distribution and abundance of the inva- sive seagrass H. stipulacea at these and other sites in Carriacou in 2016. PLOS ONE Seagrass bed biodiversity and structure and across 5 decades PLOS ONE | https://doi.org/10.1371/journal.pone.0306897 August 1, 2024 2 / 25 supported by the Australia Research Council

(DE190100692, DP220100650) and the Norwegian Blue Forest Network (nbfn.no). The funders had no role in study design, data collection and analysis, decision to publish, or reparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.