  We identify effective carriers of \emph{Sargassum} in the Caribbean Sea and
describe a mechanism for coastal choking. Revealed from satellite altimetry,
the carriers of \emph{Sargassum} are mesoscale eddies (vortices of 50-km radius
or larger) with coherent material (i.e., fluid) boundaries. These are
observer-independent -- \emph{unlike} eddy boundaries identified with
instantaneously closed streamlines of the altimetric sea-surface height field
-- and furthermore harbor finite-time attractors for networks of elastically
connected finite-size buoyant or "inertial" particles dragged by ocean currents
and winds, a mathematical abstraction of \emph{Sargassum} rafts. The mechanism
of coastal inundation, identified using a minimal model of surface-intensified
Caribbean Sea eddies, is thermal instability in the presence of bottom
topography.

Atmospheric and Oceanic Physics;Chaotic Dynamics