The Trophic Role of the Endangered Caribbean Manatee Trichechus manatus in an Estuary with low Abundance of Seagrass

Abstract

Chetumal Bay is a refuge for the manatee, Trichechus manatus, a large and strictly herbivorous aquatic mammal. The ecosystem is notoriously poor in subaquatic vegetation, the main components of manatees’ diet. Due to the constant presence of manatees in the bay and their ability to consume large volumes of plant material, it is assumed that the species has a relevant trophic impact on the system. A mass–balance trophic model was designed to describe the flows of energy and matter in the bay, with the goal of assessing the role of manatees in the system. The system was aggregated in eight effective trophic levels. The biomass was intensely concentrated in the detritus, suggesting that the matter on the bottom sediment is the main regulator of the energy flow in the system. Primary producers comprised of detritus, mangrove, benthic autotrophs, and phytoplankton. The apex predators were dolphins and large piscivorous fishes. Manatees occupied the trophic level 2.0. Manatees were directly or indirectly impacted by autotrophs, mangrove, and detritus; but the competition between manatees and other groups was insignificant. In comparison to other ecosystems where manatees occur, Chetumal Bay (BCh) has the lowest relative biomass of seagrasses. Several ecological and behavioral mechanisms to compensate the lack of macrophytes biomass (or a combination of several) are suggested. Ecopath with Ecosim models are useful to describe the flow of energy and matter in the ecosystems. However, there are still critical gaps in the knowledge of BCh and its manatee population. It is difficult to assess the uncertainty associated with the estimates obtained; therefore, results should be interpreted with caution. Improving this preliminary model with robust local information on the Chetumal Bay ecology and its manatee population is recommended.