There are millions of them – in a single drop of ocean water. They are the reason that we have everything from mussels to lobsters to stripers. They need sunlight and enough nutrients to grow and form the basis of the marine food chain. I’m talking about phytoplankton (phyto = plant, plankton = tiny living organisms). They are teeny tiny algae that turn the sun’s energy into food for everything else that can’t “eat” sunlight in the water.

These phytoplankton are critical to ocean health – unless there are too many of them, too many of certain kinds. Of the thousands of species of phytoplankton, there are a few dozen that can be toxic. Under certain conditions, they multiply like crazy and create a red tide, something we’ve recently experienced along our coast with our own local species of Alexandrium.

Plankton are one of the most basic forms of life. And yet, these dinoflagellates have the ability to move themselves around using a tiny tail called a flagellum, from which they got their name. Dinoflagellates are a type of protest; organisms made up of just a single cell, but that one cell contains photosynthetic pigments that can convert the sun’s energy into sugars for other ocean animals to consume. The pigments in these algae can be green or brown, and some even phosphoresce or make light when disturbed, a phenomenon called bioluminescence. And, only sometimes are they actually red. But, when they multiply in great numbers, the resulting murky waters that are filled with these tiny organisms create what we call a “red” tide.

While some red tides are actually red, such as those in the Gulf of Mexico, many red tides aren’t red at all. Scientists have started to refer to them instead as Harmful Algal Blooms or HABs to clarify this misnomer. Red is merely a way of putting warning into the description of the water when there are too many of toxic phytoplankton around.

So, why is a red tide a problem? When the plankton are present in such a large number and then die off, they can deplete the oxygen in the water, which means there isn’t enough for coastal fish, sea birds and other marine creatures. This can mean large-scale fish kills. Also, the saxitoxins they release can make people sick.

Things we love to eat like clams and mussels are filter feeders and consume large quantities of plankton, concentrating them in their otherwise tasty tissues. These high concentrations of saxitoxins can lead to paralytic shellfish poisoning (PSP), which causes mild responses like headaches, dizziness and nausea, or more severe responses like muscular paralysis and respiratory failure. For this reason, red tides lead to closures of shellfish harvesting areas. This has a direct impact on shellfish harvesters and supporting businesses, but also impacts peoples’ appetite for other seafood, which is still safe to eat.

We have been fortunate in the last 10 years or so not to have a significant red tide event, but back in 2005, there were record-breaking levels of saxitoxins in the water, leading to large-scale closures of shellfish harvesting areas. Under a certain deadly combination of factors, including warm enough temperatures and sufficient oxygen and light, cysts that are dormant in the winter reproduce in massive numbers. In addition, in 2005, there was an unusual amount of rain and snow melt followed by warm weather, conditions thought to trigger large blooms. This is similar to what we saw in late May in Southern and Midcoast Maine with closures after a period of lots of rain followed by a particularly warm spell.

Since this is a natural phenomenon, the question is what do we do about it. First, the state has a monitoring program to test coastal shellfish areas from early spring through the fall. It collects shellfish and measures the level of saxitoxins in their tissues. When the levels are too high, areas are closed to shellfish harvesting to keep consumers safe from the threat of PSP.

Second, scientists have developed ways to predict red tides by sampling for cysts in bottom sediment samples they take over the winter. The higher the level, the greater potential there is for major red tide events come spring.

Finally, the nutrients we put into the water through things like the fertilizers we use on our lawn, impact phytoplankton growth. They need these nutrients to grow, so if there are too many nutrients … too many phytoplankton.