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

is estimated as 1.1 m based on the slope of the shore across transects at the 6 HWE sites. In any event, it is evident that at HWE sites (with the possible exception of T9), landward migration of seagrass beds has not kept pace with an increase in accommodation space [28] over our study period. At LWE sites, SFI increased in extent at only one site (T2) between 1969 and 2016. The shore at T2 was steeply sloping and naturally fortified with large boulders, compared to the gently sloping, mangrove-lined (T3) or sandy (T10, T14) shores at the other LWE sites, where SFI was effectively stable or decreased between 1969 and 2016. Seawalls reduce sediment sup- ply from the shore and enhance wave reflection and surge levels [86], which likely accounts for the observed expansion of SFI at T2. Erosional scarps occur along the inner margins of seagrass beds where they alternate with gently sloping margins of seagrass growing towards the shore (S3 Fig) [35]. Within seagrass beds “blowouts”—characterized by an erosional scarp, a seagrass-free blowout floor and a gently sloping seagrass-colonized leeward face—form during high wave energy events and their persistence is dependent on recurrent wave- or current-induced erosion at the scarp face (S3 Fig) [35, 87, 88]. Scarps and blowouts generally are absent in more protected, low wave- energy areas [35, 87, 89]. No scarps were observed at LWE sites in 1969 and 1994, but 1 or 2 scarps occurred at 3 of the 4 sites in 2016, suggesting that mean wave energy at those sites had

increased sufficiently to maintain their erosion. At 3 HWE sites (T1, T6, T7), scarp number tri- pled between 1969 and 2016, with most of this increase occurring between 1994 and 2016. At one site (T5), the sediments were scoured down to bedrock between 1994 and 2016, removing the elevated seagrass bed and associated scarps. This is consistent with anecdotal reports by local fishers of a massive break in the barrier reef in this area in the 2000s, after which the area became more turbulent and subject to strong currents. Our findings suggest that the frequency of high wave energy events sufficient to disrupt seagrass beds, particularly those dominated by T. testudinum, increased between 1994 and 2016 compared to 1969 to 1994. This is consistent with a significant increase in Atlantic hurricane activity from 1996 to 2005 (relative to the 1950–2000 average) attributed to local sea surface warming [90]. At the remaining two HWE sites (T8, T9), both in Grand Bay, scarp number changed little over the 3 sampling years. A large increase (27 m) in SFI at T8 between 1969 and 2016 suggests that persistent wave energy at that site had increased, but that scarp-forming, high energy events affecting other HWE sites had less impact at T8, which is partially protected by a head- land to the northeast. At T9, SFI and scarp number decreased between 1994 and 2019. Bottom profiles show little change in depth between 1969 and 1994, and a small increase (30–50 cm) between 1994 and 2019 (S2 Fig), suggesting that, as at T5, there had been an increase in inten-

sity of the wave regime. The seemingly anomalous differences at T9 may be related to a plenti- ful supply of pebble- and cobble-sized materials from eroding banks onshore (https://www. inaturalist.org/observations/32264949), and accretion of a rubble layer, not seen at other HWE sites in 1969 [35] or in 2016. Impacts of reef flattening on lagoonal seagrass beds Increases in SFI and scarp number at HWE sites and their greater magnitude compared to LWE sites, together with greater changes in these parameters between 1994 and 2016 than between 1969 and 1994, are consistent with increased wave forcing associated with degrada- tion of offshore coral reefs between 1994 and 2016 [33, 34]. Observations on shallow reefs dominated by Acropora palmata at Sandy Island on the southwest coast of Carriacou show a progression from healthy A. palmata in 1969 to dead but still standing A. palmata in 1994 [91] —following the spread of white band disease in the Caribbean in the 1970s and 1980s [92]—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 18 / 25