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

Mean shoot density of T. testudinum declined from 572 to 348 shoots m-2 between 1969 and 2016 (Fig 4A), a significant decrease to 61% of the 1969 value (1-way ANOVA, F2,48 = 3.7, P = 0.033; Tukey’s HSD test: P = 0.031). Mean maximum leaf length (range: 21–25 cm, (Fig 4B) did not vary significantly among years (F2,48 = 1.7, P = 0.201) and mean width (9.4– 9.5 mm, Fig 4C) was effectively constant (F2,48 = 0.132, P = 0.877). Mean number of leaves per shoot also varied little between 1994 (2.8) and 2016 (2.7); leaf number was not measured in 1969. Therefore, the possible decline in biomass of T. testudinum during our study likely was related to a decrease in shoot density, rather than changes in leaf number or dimensions. Macroalgae and macrofauna at stations The frequency of abundance of the more common sessile species and species groups, defined as those occurring at a mean frequency of � 0.75 per 12 quadrats (6.25%) in at least one of the 17 stations in at least one year, are shown in Fig 5 (a complete list of species including the less common forms is given in S2 Table). Among macroalgae, 19 of the 25 species/groups were more frequent in 1994 (grand mean frequency, 16.5%) than in 1969 (5.0%) or 2016 (2.9%). Only the red alga Amphiroa tribulus, the green alga Halimeda simulans, and 4 of 6 Caulerpa spp. did not conform to this pattern. The most common Caulerpa species was C. prolifera, which occurred at 8, 8 and 5 stations in 1969, 1994 and 2016 respectively. Caulerpa scapellifor- mis, an invasive Indo-Pacific species, was absent at our stations in 1969, occurred at 1 station

in 1994, and at 6 stations (5 with C. prolifera) in 2016. The 2 more common corals, Manacina areolata and the Porites furcata group (P. furcata / P. divaricata), also were most frequent in 1994. There was no consistent trend with time for sponges, which as a group were common in all years. Of the 3 mobile macrofaunal species or species groups observed, only sea urchins (Tripneustes ventricosus group) were more common. Unlike the corals and most macroalgae, sea urchins were less frequent in 1994 (mean fre- quency, 0.01%) compared to 1969 (10.3%) or 2016 (11.8%). The other 2 mobile species were recorded only at low frequency in quadrats and in only 1 of the 3 sampling years (Oreaster reti- culatus: 1.4% in 2016, Alger gigas: 0.98% in 1969). Seagrass community comparisons Taxonomic richness and diversity (Shannon Index) of seagrass and associated macroalgae and sessile macrofauna differed significantly across the three sample years (F2,48 = 8.4, P <0.001 Fig 4. Shoot density and leaf dimensions of Thalassia testudinum at stations (n = 17) in 1969, 1994 and 2016. Mean (+SE) of a) shoot density (shoots m- 2), b) maximum leaf length (cm), and c) leaf width (cm). https://doi.org/10.1371/journal.pone.0306897.g004 PLOS ONE Seagrass bed biodiversity and structure and across 5 decades PLOS ONE | https://doi.org/10.1371/journal.pone.0306897 August 1, 2024 10 / 25