Motile macroinvertebrate assemblages in a Caribbean reef lagoon impacted by recurrent Sargassum brown tides

Recurrent inundations of holopelagic Sargassum spp. have been arriving to the Mexican Caribbean coast since 2014. The "Sargassum brown tides", murky coastal waters resulting from the decay of massive beached accumulations of this macroalga, reduce water quality, increase organic matter in sediments, and cause mortality of nearshore seagrass and fauna in reef lagoons. To examine the potential effects of Sargassum brown tides on seagrass-associated motile macroinvertebrates, we analyzed the diversity and community composition of this epifauna and the vegetation in the Puerto Morelos reef lagoon, across a gradient of increasing distance from the shore: nearshore zone, mid-lagoon zone, and backreef zone. We also examined the relationships between macroinvertebrates, marine vegetation, and organic matter content (OMC) in the sediments. On the nearshore zone, where the seagrass Syringodium filiforme prevailed, the invertebrate community was dominated by detritivorous and mesograzer gastropods (Cerithium spp.) and hermit crabs (Clibanarius tricolor), showing an association with higher OMC in sediments and cyanobacteria-diatom mats. In contrast, the invertebrate communities in the mid-lagoon and back-reef zones, dominated by Thalassia testudinum, were similar to each other and less influenced by vegetation characteristics or OMC in sediments. Although these results suggest that the impact of recurrent Sargassum brown tides may be altering the community composition of seagrass-associated invertebrates on the nearshore zone, this cannot be ascertained in the absence of previous knowledge on the composition of the macroinvertebrate communities on this zone. Future studies should investigate whether the persistence of Sargassum brown tides eventually induce long-term changes in the benthic communities.