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

RESEARCH ARTICLE Shifts in biodiversity and physical structure of seagrass beds across 5 decades at Carriacou, Grenadines David Patriquin1, Robert E. Scheibling1, Karen Filbee-DexterID2,3* 1 Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada, 2 University of Western Australia, Perth, Australia, 3 Institute of Marine Research, His, Norway * karen.dexter@uwa.edu.au Abstract Caribbean seagrass beds are facing increasing anthropogenic stress, yet comprehensive ground-level monitoring programs that capture the structure of seagrass communities before the 1980s are rare. We measured the distribution of seagrass beds and species com- position and abundance of seagrass and associated macroalgae and macroinvertebrates in 3 years over a 47-year period (1969, 1994, 2016) at Carriacou, Granada, an area not heavily impacted by local human activity. Seagrass cover and physical parameters of fringing beds were measured in transects at high (HWE) and low wave energy (LWE) sites; frequency of occurrence of all species, and biomass and morphology of seagrasses, were measured at 100 m2 stations around the island. Losses in nearshore seagrass cover occurred at HWE but not LWE sites between 1969 and 2016 and were associated with increases in the sea- grass-free inshore zone (SFI) and erosional scarps within beds. Total biomass did not vary across years although there were progressive changes in seagrass composition: a decline in the dominant Thalassia testudinum and concomitant increase in Syringodium filiforme,

and establishment of invasive Halophila stipulacea in 2016 at LWE sites. Species richness and diversity of the seagrass community were highest in 1994, when 94% of macroalgae (excluding Caulerpa) were most abundant, and sea urchins were least abundant, compared to 1969 and 2016. Multivariate statistical analyses showed differences in community compo- sition across the 3 years that were consistent with trends in urchin abundance. Increases in SFI and scarp number in seagrass beds at HWE sites occurred mainly after 1994 and likely were related to increased wave forcing following degradation of offshore coral reefs between 1994 and 2016. Our observations suggest that landward migration of seagrass beds with rapidly rising sea level in future will not be realized in reef-protected seagrass beds at Carriacou barring reversal in the processes that have caused reef flattening. Introduction Seagrasses are major components of coastal ecosystems in the Tropical Atlantic Bioregion [1]. They colonize rubble, sand or muddy substrates on reef flats, back-reef lagoons, and in PLOS ONE PLOS ONE | https://doi.org/10.1371/journal.pone.0306897 August 1, 2024 1 / 25 a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Patriquin D, Scheibling RE, Filbee-Dexter K (2024) Shifts in biodiversity and physical structure of seagrass beds across 5 decades at Carriacou, Grenadines. PLoS ONE 19(8): e0306897. https://doi.org/10.1371/journal. pone.0306897 Editor: Judi Hewitt, The University of Auckland - City Campus: University of Auckland, NEW ZEALAND Received: July 6, 2023

Accepted: June 25, 2024 Published: August 1, 2024 Copyright: © 2024 Patriquin et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All data are available in the public iNaturalist Project https://www. inaturalist.org/projects/seagrass-bed-flora-fauna- of-carriacou-grenada-2016. Funding: In 1969 DP was supported by a McConnell Scholarship from McGill University, Montreal. RES was supported by a Discovery Grant (RGPIN-2016-04878) from the Natural Sciences and Engineering Research Council of Canada (https://www.nserc-crsng.gc.ca/). KFD was