Motile macrofauna associated with pelagic Sargassum in a Mexican reef lagoon.

Buildup of decaying pelagic Sargassum on the beaches and coasts of the Mexican Caribbean during the massive arrivals of 2015 and 2018 had detrimental impacts on the environment and tourist industry. To avoid ecological and economic impacts from massive beaching of Sargassum, it would be better to remove the pelagic algal masses while still at sea. However, out at sea, pelagic Sargassum rafts constitute an ecosystem with a diversified associated fauna and their removal could impact this fauna. We conducted a survey on the motile macrofauna associated to pelagic Sargassum rafts in the Puerto Morelos reef lagoon, Mexican Caribbean. Pelagic Sargassum was sampled with nets at 2 m, 50 m and 500 m from shore, at four sites during the months of September, October and November 2018. The 108 samples contained 10,296 individuals belonging to 32 taxa distributed over eight Phyla. The main phyla were Arthropoda (48%), Annelida (41%) and Mollusca (15%). Fish abundance was low (10 individuals) with only five species, of which three are typically associated with Sargassum rafts and two are common in seagrass meadows and coral reefs. Species composition and abundance of motile macrofauna varied with month and zone; the nearshore zone had the lowest abundance but there was no difference in the abundance of the fauna associated with rafts 50 or 500 m offshore. Three of the four most abundant species (together accounting for 89% of the individuals) were species typically associated with pelagic Sargassum, and the fourth was an amphipod that was only registered once near shore. Although more studies over larger time and spatial scales are required, these results suggest that the removal of pelagic Sargassum within the reef lagoon may not have a significant effect on local populations of motile macrofauna.