I traveled to Tela in March during the first big sunny week following an extremely hard, rainy winter. Everyone was talking about how tired of the rains they were. The potholes had grown and were only just getting filled. The dock next to the dive shop had been particularly hammered. Its mid-section had fallen into the water. Although the beautiful weather gave us the opportunity to do some fabulous exploring, all the while we think something interesting was happening in the seawater.

About four days after the sunny weather started, a storm rolled in and we were unable to dive. Antal and I were having a coffee at the Aquarium when he received a call from a friend who lives along a spit of beach on the west side of Tela Bay, called Miami. A lot of dead fish were washing up on the beach. The water was red, he said.

No filter: The sea looked red.

Antal and I grabbed some sample tubes, and jumped in his car to drive out to Miami. It was about an hour before sunset as we bounced along the dirt road’s potholes for the 40 minute drive from town. As we reached Miami, a foul smell filled the car. A lot of people from the small village were on the beach, and many of them wading through the water picking up dying fish. All along the wrack line where the waves stop on the beach were the bodies of dead fish, a lot of little sardines, but also puffer fish, angel fish, dogfish, and other reef fish. Looking out over the water and the breaking waves, the water was tinted a dull maroon.

We filled the test tubes we’d brought as well as soda bottles that the people from Miami had available. We took samples from the ocean and you could easily see the pinkish tinge to the water. One theory was that there had been some kind of spill in the lagoon, and the toxic water was passing into the ocean, killing the fish. Antal asked one of the people who live in Miami to take us into the lagoon to investigate. By this time it was night, and motoring though the mangroves that line the lagoon with the bright stars above had a spooky beauty. We took water samples from many parts of the lagoon, but the water wasn’t tinged pink or red. Everything seemed normal there.

Back at the aquarium, we froze most of the samples for analysis later. Antal had called the environmental agency to let them know about the fish kill. They would arrive in the next couple days to analyze the water for chemical contaminants.

But we were already starting to believe that since we hadn’t seen evidence of a spill, there wouldn’t be a chemical to find. The red color of the water was familiar. It reminded me of red tides I’d seen in Southern California during graduate school. The red comes not from a contaminant, but from a kind of algae, or phytoplankton, called a dinoflagellate.

Coincidentally, dinoflagellates are also the kind of algae that live symbiotically inside corals and provide the sugar from photosynthesis that makes the bulk of their nutrition. It’s so much energy that corals can build their skeletons. Dinoflagellates are the reason we have coral reefs.

When dinoflagellates live inside corals, they lose their eponymous flagellae and just hang out in a small bubble, called a symbiosome, doing the work of photosynthesis. But they can also live in seawater; and there dinoflagellates are free to swim. The have two flagella that make them mobile. One runs in a kind of ridge around the center. In the 1880s drawing below by German biologist Ernst Haeckel, it’s sort of dangling. But what that flagella does is make the algae spin, like a top, as it swims. In addition to the cool flagellae, some dinoflagellates  like the one in the drawing make plates called thecae with beautiful points and filligree.

At lest two thousand of species of dinoflagellates swim in our oceans and several dozen can live inside corals. (Remember that if you keep reading on these pages because it’ll be important down the road.) They are incredibly diverse creatures. Although they are just a single cell, they have all variety of ways of surviving in the world. Most of them can photosynthesize, but some have lost that ability and they rely on smaller symbionts like blue green algae for their energy. Some of them can eat food. Some of them do all three.

When dinoflagellates do photosynthesize they use chlorophyll like plants, and in addition they have another pigment called peridinin, which is very good at harvesting blue light, the light that reaches deepest in the ocean. So dinoflagellates can live in murky water. The peridnin also gives the dinos a faint reddish tinge. Dinos are also particularly good at using phosphate in the environment.

And you know where phosphate comes from in the ocean? Land. Especially fertilizer from agriculture.

What we think might have happened is that over the very rainy winter, a lot of fertilizer runoff from the nearby palm oil plantations ran into the bay. When the sun came out for the first time in the spring, the dinoflagellates, flush with fertilizer, swam toward the sunlight and began photosynthesizing, and dividing. And dividing. And dividing. Soon they were so numerous you could see a faint tinge of pink in the water. They formed what’s called a red tide.

Because dinos (if you’ve read this far, let’s just call them by the name that oceanographers call them) can both photosynthesize and perform cellular respiration, if there are enough around, they can lower the oxygen levels in the seawater. They can also release sugars that bacteria feed on. Bacteria will suck up oxygen too. That lack of oxygen could have been what was killing the fish. It’s also possible that the dinos released a toxin–some do–but that seems less likely. One thing I had been looking for was whether the ocean was bioluminscent. It wasn’t. But if it had been that would have been another sign that it was a bloom of dinos because some dinos do glow a beautiful blue.

When Antal and I looked at the seawater we collected under the microscope, it pretty much confirmed our thoughts. We saw lots of little cells spinning away like tops. When the scientists from the environmental agency came a few days later, they also confirmed they were dinos.

As far as anyone in Tela can remember, there’s never been a red tide in the bay. It’s easy to say this means something’s changing. But red tides have come and gone for millenia and one red tide is not a pattern.

And yet, there was one other occasion when we also witnessed a mass mortality in Tela. In 2023, a swath of reef died for unexplained reasons. It was similarly quick and had the hallmarks of low oxygen. Unfortunately, no one took water samples at that time, so we will never know what caused it.

So really the thing to do is just watch and see whether this is fleeting or becomes more regular. It’s just another reason to keep studying what’s happening in Tela.