Seagrass loss decreases abundance, diversity and composition of macrobenthic infauna in a Caribbean Bay

Abstract Seagrass meadows form extremely complex ecosystems in the coastal zone that are highly productive and sustain an abundant faunal community. This study compared the macrobenthic infaunal communities in William’s Bay, Trinidad before and after a lush seagrass meadows dominated by Thalassia testudinum (Banks ex König) was extirpated. In 2007, T. testudinum leaf aerial productivity was 1.5 ± 0.58 gm2 d− 1 and total biomass was 191 ± 55.9 gm− 2, but by 2016 the bed had collapsed with zero productivity and biomass. To assess the macrobenthic infauna, five replicate cores were taken at six stations in 2007 and in 2016. Species richness, abundance and biomass all decreased from 2007 to 2016 coinciding with the loss of the seagrass meadows. This is likely due to fewer microhabitats and protection from predators provided by seagrass cover. Relative abundance of the main species groups remained unchanged; polychaetes accounted for 48% of the species in 2007 and 2016, amphipods accounted for 17% in both years, decapods for 12% and 15% in 2007 and 2016 respectively, non-segmented worms for 11% (2007) and 6% (2016), molluscs for 2% (2007) and 13% (2016), echinoderms for less than 1% and another 1% a small miscellaneous group. Specific composition within these groups differed; in 2007, carnivorous polychaete families Lumbrineridae, Nereididae and Orbiniidae dominated, while in 2016 deposit feeding polychaete families such as Opheliidae, Paraonidae, Capitellidae and Spionidae were more abundant. Epifaunal amphipoda dominated across both periods compared to infaunal tube-dwellers. This study demonstrated a shift in the main feeding guilds of polychaete communities while vegetated from carnivorous polychaetes to deposit feeding polychaetes upon disappearance of the seagrass meadows.