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

construction of a large marina in Tyrell Bay in 2015–2018, likely resulted in water quality issues which, along with increased shipping and motor vessel activity, likely impacted the sea- grass community of the bay and contributed to establishment of the invasive seagrass Halo- phila stipulacea [36]. However, Tyrell Bay, the most protected bay on the island, was not included in our multi-year comparisons. The changes in seagrass beds that we observed more likely are related to broader scale phenomena that affect seagrass directly or indirectly. Seagrass cover, composition, and morphology There were significant losses in nearshore seagrass cover between 1969 and 2016 at HWE sites, but not LWE sites. These losses were associated with an increase of the inshore area devoid of seagrass (SFI) which doubled in average extent in transects at HWE sites. The power to detect differences in species composition of seagrass in the nearshore between years and between HWE and LWE sites was low because of high variability in composition between transects at both HWE and LWE sites and the relatively low number of sites in an unbalanced sampling design. The only clearly defined change was the appearance of the invasive seagrass H. stipula- cea in 2016, exclusively at LWE sites. Based on a broader survey of H. stipulacea at Carriacou [36], which included our 2016 transect and station observations as well as those at additional sites, we found that H. stipulacea occurred primarily in large, sheltered bays along the leeward,

west coast of Carriacou where it formed extensive meadows. It also occurred in smaller patches at low wave energy sites in Watering Bay on the windward, east coast and in Manchioneal Bay on the south coast. Station records provided indicators of temporal change in species composition and abun- dance of seagrass, and in morphological characteristics of T. testudinum, within broadly dis- tributed seagrass beds. T. testudinum, was the dominant species in terms of both frequency of occurrence and biomass at our stations across all three sampling years. A gradual decline in average biomass of T. testudinum, and concomitant increase in biomass of S. filiforme (although not statistically significant for either species) suggests a shift towards earlier succes- sional stages in these beds [35, 62]. Similar successional shifts in other parts of the Caribbean have been attributed to environmental deterioration through localized anthropogenic impacts [55]. At Carriacou, however, increased intensity of wave-driven erosional processes, including increases in frequency of erosional scarps (blowouts) that re-initiate seagrass successional pro- cesses [35], may be the primary driver of such shifts. There was no change in leaf width or length of T. testudinum among sampling years, although there was a significant decrease in shoot density. This suggests that any decline in mean biomass was associated with a decrease in shoot density rather than changes in plant morphology or in the overall occurrence of T. testudinum at the 17 stations. Leaf length and

width of T. testudinum varies with environmental conditions locally and regionally, decreasing with reduced light [63, 64] or salinity [65]. In a study of T. testudinum at 3 sites across a sea- grass suitability gradient in the Gulf of Mexico, mean leaf width was 4 and 5 mm in the two lower suitability sites, and 8 mm in the highest suitability site [66]. By comparison, mean leaf width at our 17 stations in ranged from 9.4 to 9.6 mm over the 3 sampling years. The constancy of total seagrass biomass regardless of composition, and large leaf width of T. testudinum, sug- gests that high quality habitat for seagrass at Carriacou prevailed in all three sampling years. Benthic macroalgae and macrofauna in seagrass habitats In contrast to progressive changes in seagrass cover and species-specific biomass across the three sampling years, taxonomic richness and species diversity of the benthic community (sea- grass and associated macroalgae and sessile macrofauna) were greatest in 1994 compared to PLOS ONE Seagrass bed biodiversity and structure and across 5 decades PLOS ONE | https://doi.org/10.1371/journal.pone.0306897 August 1, 2024 16 / 25