Species composition and chemical characterization of Sargassum influx at six different locations along the Mexican Caribbean coast.

In the last decade, unprecedented arrivals of pelagic Sargassum (Sargassum influx) have been reported for the Caribbean coasts causing severe ecological and economical affectations and remarking the necessity to characterize the phenomenon. In the north coast of the Mexican Caribbean, spatial characterization of Sargassum influx was performed in terms of its abundance, species composition and chemical content using a combination of in situ sampling and remote sensing evaluations. Sargassum influx was detected within the 25 km fringe near to Quintana Roo coast in coincidence with beach-cast events during September 2018. Significant spatial variation in abundance and species composition of the stranded biomass was found for the six localities studied, which was related to the local environmental conditions. Pelagic species of Sargassum were the main component ranging from 78.1 to 99.6% of the total beach-cast (wrack) fresh biomass, whereas benthic macrophytes (seagrasses and macroalgae) were a minor component reaching up to 21% in some localities. The biochemical composition of beach-cast Sargassum resulted spatially homogeneous for most of the components analyzed; only carbon tissue content, ash metals (particularly Fe and As) and isotopic composition changed spatially. This study represents baseline information for the region. Long-term seasonal evaluations of Sargassum influx along Mexican Caribbean are required to define adequate management strategies and exploitation.

Epibiont hydroids on beachcast Sargassum in the Mexican Caribbean.

Massive accumulations of pelagic species of Sargassum have generated recent social, economic and ecological problems along Caribbean shores. In the Mexican Caribbean, these events have prompted the study of diverse biological and ecological aspects of these macroalgae. However, studies on their associated biota, including Hydrozoa, remain scarce. This research provides important species observations in an area where data is lacking. The occurrence and percent cover of hydroids on Sargassum thalli collected on the beach at Puerto Morelos, Quintana Roo, Mexico from April 2018 to March 2019 was studied. Three pelagic species and morphotypes of Sargassum from this area were analyzed: Sargassum fluitans III, S. natans I and S. natans VIII, as well as a benthic species, S. polyceratium var. ovatum. A total of 14 taxa of hydroids, belonging to the superorders "Anthoathecata" and Leptothecata, were identified. In our study, more hydroid taxa were observed on axes of the different species of Sargassum than on leaves or aerocysts. In general, the greatest species richness of hydroids was observed from February to April. Results show that live hydrozoans attached to pelagic Sargassum are transported into the area. This should be considered in future management measures that address the recurring coastal abundance of Sargassum and its associated biota in the Caribbean region.