Sargassum seaweed accumulation on Caribbean and Riviera Maya beaches is tracked in detail by the University of South Florida’s Optical Oceanography Laboratory, whose Sargassum Watch System (SaWS) publishes monthly Sargassum bulletins and satellite-based inundation reports across the region, not a resort’s own marketing claims about its beach conditions.

What the most recent bulletin actually shows

According to the latest published bulletin, total Sargassum amounts declined across every monitored region through August 2026, though most areas still exceeded their historical 75th percentile levels for this point in the year. In practical terms, that means conditions were improving month over month but still running heavier than a typical August by the system’s own long-term baseline. The bulletin indicates that beaching events, meaning seaweed actually washing up on shore rather than remaining offshore, likely persisted across most regions during this period, with coastal inundation continuing through the Caribbean and Mexican Caribbean specifically. The forecast in that bulletin points to amounts continuing to decrease in the coming months, possibly into November.

Where the seaweed actually comes from

The Great Atlantic Sargassum Belt, the band of open ocean that supplies this coast’s seaweed, has produced these large-scale blooms consistently since around 2011, and researchers have spent years narrowing down why. Peer-reviewed research examining the belt has pointed to nutrient supply as a key driver behind sustained bloom conditions, with vertical mixing of ocean water masses identified as a major nutrient source, alongside contributions from river discharge and atmospheric dust deposition that scientists continue to study. A shift in ocean circulation and wind patterns around 2009 to 2010 is understood to have pushed sargassum from its traditional range into the tropical Atlantic in the first place, where conditions allowed it to thrive at a scale not previously seen. Researchers are candid that the exact mechanisms controlling year-to-year variability in the belt are still an active area of study, which is itself a reason to trust a current satellite bulletin over a general seasonal rule of thumb rather than treating any single explanation as settled science.

The seasonal pattern, per the official data

Within the Great Atlantic Sargassum Belt, quantities typically peak around June or July, with the lowest amounts during the winter months. USF’s own monitoring notes a specific early-warning signal worth knowing about: unusually high sargassum during the cold season, as was observed between December 2022 and February 2023 ahead of a subsequently heavy year, can be a signal of a heavier season to follow. That is a detail worth watching for anyone planning a trip several months out rather than booking on the current month’s conditions alone.

Why this matters for trip timing, not trip cancellation

Sargassum is a seasonal nuisance affecting beach conditions and swimming comfort, not a safety hazard, and it varies significantly by exact beach and by week. A resort a few kilometers up the coast from another can see a materially different amount on the same day, since local currents and shoreline shape affect where the seaweed actually accumulates more than regional totals do. Checking the current satellite bulletin close to departure is more useful than relying on a general seasonal rule of thumb, particularly during a month like the one covered in the most recent bulletin, where conditions are actively trending in a specific direction rather than sitting flat. A resort’s own beach photos, taken on a good day, tell you almost nothing about what a specific week is likely to bring.

How to actually use the bulletins before a trip

The system distributes monthly bulletins by the last day of each month, which means a bulletin published in the final days before departure is the most current information available and worth checking specifically rather than relying on a bulletin from the start of the same month. The Sargassum Watch System also provides higher-resolution monitoring using Sentinel-2 satellite imagery for specific coastal stretches, including targeted coverage of the Mexican Yucatan and Caribbean islands, which is a more granular view than the regional summary bulletin for anyone comparing two specific beaches rather than two broad regions.

Full detail and how to read the reports: our sargassum season guide. For overall trip timing against both sargassum and hurricane season, see when to book Cancun and the Riviera Maya and our current hurricane season update.

Source: University of South Florida, Sargassum Watch System.