Beaches that appear pristine in photographs can be lined with a thick band of brown seaweed on arrival. The influxes are seasonal, driven by ocean circulation, and largely predictable in timing if not in scale.
The seaweed lives at sea, not on the shore
The floating algae involved spend their entire lives at the surface of the open ocean, held up by small gas-filled bladders rather than anchored to the seabed.
Drifting mats form an ecosystem in their own right, sheltering juvenile fish, crabs and turtles in water that is otherwise open and exposed.
Beaching is therefore an accident of circulation. The mats are carried to a coast rather than growing there, which is why the same beach can be clear for months.
Currents deliver it in seasonal pulses
Large-scale surface circulation concentrates floating material into belts, and the position of those belts shifts through the year with prevailing winds.
When the belt intersects a coastline, material comes ashore continuously for weeks, and the influx stops when the pattern moves rather than when the beach is cleared.
Arrival tends to cluster in the warmer months on affected coasts, which unhelpfully coincides with peak visitor season in several regions.
Nutrients determine how much there is
Growth at sea depends on nutrient availability, supplied by river outflow, dust deposition and upwelling of deeper water.
Years with stronger nutrient delivery produce larger mats, so the volume arriving on a given beach varies considerably from one season to the next.
This variability is why local operators can predict roughly when material will arrive but not how much, and why forecasts are issued through the season rather than annually.
Clearing it is harder than it looks
Wet seaweed is heavy, and removing it mechanically takes sand with it, so aggressive clearing accelerates beach erosion.
Once decomposing it releases gases that smell strongly, which is why the priority is removal before it sits rather than after.
Offshore barriers and collection vessels intercept material before landfall at some resorts, which works but requires equipment and staffing well beyond raking.
How it changes a beach holiday
Impact is highly local. Windward coasts receive material while leeward and sheltered bays a short distance away can remain clear through the same period.
Resorts differ enormously in their clearing effort, and morning is usually the cleanest part of the day where crews work at first light.
Regional monitoring services publish outlooks during the season, and checking one for the specific coast is more useful than checking a general destination forecast.