Collin Roesler wants us to have a better understanding of the algae that cause red tides in Maine.
A professor in the Earth and Oceanographic Science Department at Bowdoin College, she studies phytoplankton, notably Alexandrium fundyense. This single-celled marine plant is the source of a neurotoxin that can contaminate shellfish such as mussels, clams and oysters. People who consume shellfish tainted by the toxin can experience paralytic shellfish poisoning, which can be fatal.
One of her current research projects is focused on Alexandrium in Harpswell Sound and in Lombos Hole, near the head of the sound. Her work is unlocking information that has implications for efforts to monitor and predict serious red tide events.
Some of the data she uses to understand what is going on in Harpswell Sound comes from a scientific buoy located over a deep hole to the west of the Cribstone Bridge.
The buoy’s instruments measure temperature, salinity, phytoplankton concentration and water clarity. Crucially, the instruments also measure the speed and direction of currents at depth intervals of 1 meter, from the surface to the bottom of the hole, which is about 30 meters deep (almost 100 feet). This information helps Roesler determine how much phytoplankton is in the water and where it is being transported. Harpswell Sound, it turns out, is an unusual body of water.
‘An interesting inlet’
Unlike those in most bays or estuaries, the currents in Harpswell Sound are not a simple inward flow on the rising tide and outward flow on the ebb tide. Here there is what she calls a “freshened plume of water” at the surface that tends to move up the sound.
This plume floats on the surface over the denser seawater below. “This plume of fresher water tends to move further up into the sound with each tide cycle,” she said.
Imagine dropping a hat onto the surface of Harpswell Sound on a calm day. “The hat would flow up the sound with the rising tide and back down the sound as the tide ebbed, but it wouldn’t flow back to its starting point,” Roesler said. “The net effect is that the hat, over multiple tides, would slowly, but inevitably, make its way up the sound.”
Harpswell Sound “is really an interesting inlet,” she said. “It has a reverse flow compared to most bays and estuaries, so it tends to be retentive rather than exportive.”
In other words, the flows work like a conveyor belt, carrying things near the surface on a one-way trip up the sound. Since phytoplankton need light for photosynthesis, they tend to stay in the top layer of water. As the plume makes its way up Harpswell Sound to Lombos Hole, it carries Alexandrium cells with it.
“This appears to be the likely mechanism for why Alexandrium cells accumulate in Lombos Hole,” Roesler said.
The source of this freshwater plume is the Kennebec River. How is it possible for fresh water from the Kennebec to reach Harpswell Sound, 15 miles to the southwest? Because of the Coriolis effect. In the Northern Hemisphere, this phenomenon causes moving water (like water going down your bathtub drain) to veer to the right. As it turns out, some of this clockwise-rotating fresh water exiting the Kennebec ends up right at the entrance to Harpswell Sound.
Red tides in Maine are seasonal events. Alexandrium arrives along the Maine coast in spring. And Lombos Hole is where it usually shows up first. The retentive flow of the sound may be one of the reasons why.
But there’s yet another possible explanation why Lombos is a hot spot for Alexandrium and, perhaps, a source for red tide outbreaks in other areas of the Maine coast: the life cycle of phytoplankton.
An ‘incubator’ for red tide?
When Alexandrium cells sense that the conditions they need to thrive are deteriorating, they form cysts. These encysted cells then sink to the bottom and rest in the sediment. When water temperatures rise in the spring, they emerge as regular phytoplankton cells.
The extent of a bloom largely depends on how many of these dormant cysts emerge in a given year. And that, in turn, depends on environmental conditions, according to the Maine Department of Marine Resources.
The department explains it this way on its website: “After germination, the swimming form of the cell emerges and divides. When environmental conditions are right (oxygen, light, temperature, etc.), cells continue to reproduce and become concentrated, creating blooms known as ‘harmful algal blooms.'”
Because of these cysts, Lombos Hole may have become “an incubator responsible for the transport of cells along the coast,” Roesler said. “The source is now the sediment.”
Roesler’s fieldwork includes regularly collecting water samples from Harpswell Sound and Lombos Hole. Those samples are analyzed in a lab at Bowdoin’s Schiller Coastal Studies Center on Orr’s Island, just south of Lombos Hole.
“The goal is to identify which species are incubating in Lombos, how and when are they accumulating, and where they end up,” she said.
The central question, she said, is “What is special about Lombos Hole?” Her answer: “The retentive flow.”
The research has practical implications.
Eventually Roesler may be able to show whether the environmental conditions observed in Harpswell Sound and Lombos Hole are becoming more conducive or less conducive to red tide events.
And her work may make it possible, early in each red tide season, to predict with greater precision just how severe Alexandrium blooms will be in the months to come.
Givers of life and light
Given her deep knowledge of phytoplankton, Roesler hopes these tiny plants won’t get a bad rap because of the risks posed to human health by only a few species. When it comes to phytoplankton, Alexandrium is the main threat in Maine, but there are two other species present with the potential to cause problems.
“These are just three species out of a thousand” that can make you ill, Roesler said.
We owe phytoplankton a great deal, she continued.
More than a billion years ago, the first phytoplankton evolved, and through photosynthesis, started releasing free oxygen into an atmosphere that contained almost none.
“They’re responsible for the fact that we have an oxygen-rich atmosphere,” she said.
Spread across the globe in the upper layer of the seas like the canopy of a giant rainforest, phytoplankton continue to generate more than half of all the atmospheric oxygen produced by all the plants on Earth combined.
Phytoplankton also are helping the oceans to slow climate change, she said. During photosynthesis, they absorb carbon dioxide, using the carbon as building blocks for their cellular material. When they die or are eaten, that organic material ends up sinking into deep ocean layers or even to the bottom, where the carbon remains sequestered for millennia.
Phytoplankton are at the base of the food chain. Ultimately, they support most of the life in the sea.
If that’s not enough, they also exhibit bioluminescence. If you have ever gone kayaking on a warm summer night, you probably have seen your paddle stirring up constellations of tiny lights. That, too, is a gift from phytoplankton.
So in balance, perhaps phytoplankton can be forgiven for the potential of a few species to cause illness.
“They’re small. They don’t cross people’s minds,” Roesler said. But to her, they are “just magical.”
