Skip to main content

Ooooh that smell. Can’t you smell that smell? And no, Lynard Skynard, we’re not talking about the smell of death.

It’s summer in the keys. And if that doesn’t mean oppressive heat that lets you fry an egg on the road, it means sea grass on the beaches and in the water. It’s not just a summer issue—sargassum is a year round thing on our beaches—but it’s been quite a busy summer so far for these floating mats of macroalgae. So what exactly are we looking at here?

“Sargassum is a genus of brown (class Phaeophyceae) macroalgae (seaweed) in the order Fucales. Numerous species are distributed throughout the temperate and tropical oceans of the world, where they generally inhabit shallow water and coral reefs, and the genus is widely known for its planktonic (free-floating) species.”
A line of sargassum floating on the ocean.

A “weed line”.

Geographically, Marathon (and the keys) are in a tropical climate region. Specifically, a “tropical savanna climate“. This is a climate that most North Americans are entirely unfamiliar with and why South Florida and the keys are an ideal vacation destination. Recently in our Facebook group there have been questions about the quantity of, or expectations when it comes to, sargassum (colloquially: seaweed). Many folks don’t understand that this is a naturally occurring thing that is most often relegated to the Sargasso Sea, an area that is not technically a sea at all (it has no land boundaries) but rather the pelagic area of the North Atlantic Ocean that is bounded by four ocean currents. These current boundaries create an ocean gyre and occasionally a mat of sargassum will break free of this gyre and follow the currents to shore.

This year is an exceptionally busy year for sargassum. As temperatures rise and ocean water is warming, it creates an ideal breeding ground for these algal blooms. Think of it like having the perfect weather for your garden to grow. As more sargassum blooms in the Sargasso Sea, more chunks break off and float where the currents take them.

Now that we know what it is, what can we do about it? Well, nothing really. The amount that washes up on our beaches is related to the wind and the currents. You’ll notice that a north wind will mean clearer beaches on the ocean side and vice versa. There are tractors that come and rake the beaches to make them more usable, but what we really want our visitors to understand is that this is nature. This is how the keys are. The Florida Keys have never really been a place for pristine, mile-long beaches. It’s a series of tiny islands (an archipelago) formed from the remains of ancient coral reefs upon which tens of thousands of people live and work. White, sandy beaches have never really been a part of the ecology here and the beaches you see are mostly the result of manmade efforts.

If you make it out past the sargassum along the shore the water is just as nice as you’d expect. Snorkeling at the reef is still beautiful and awe-inspiring. Yes, it can smell, but that’s part of the cycle. Aside from messing up our shorelines, sargassum plays a vital role in the marine ecology providing a home for small creatures, protection for baby sea turtles, nutrients for organisms, and out in the deep water (where it’s often called the “weed line”) it presents fishermen with fantastic opportunities to catch the larger fish that are preying on the smaller ones that ride the mats along the current.

So when you step out and catch a whiff of the sargassum, just know that you are experiencing the true keys, just as nature intended.