Satellites, Buoy Help Oceanographer Locate Red Tide in Green Waters

bowdoin buoy

Collin Roesler’s method for spotting red tide sounds downright counter intuitive—she looks for green water. Not just any green, but a certain shade of blue-green that indicates the proliferation of a phytoplankton associated with Alexandrium fundyense, the major cause of red tide.

The term “red tide” is used colloquially to describe proliferations of phytoplankton that discolor the water red. However, it is also used to describe the occurrence of toxic algal events, even in the absence of red waters. In the Gulf of Maine, the term is used to indicate the occurrence of paralytic shellfish poisoning and subsequent shut downs of shell fishing. These events are extremely difficult to predict. Scientists don’t fully understand the environmental factors that lead to a full-scale “bloom.”

“If it were just detecting red water, this would be easy,” says Roesler, scanning the ocean horizon from the back of a lobster boat. “But here in the Gulf of Maine, the water doesn’t have to turn red to be toxic. In fact, the dinoflagellate Alexandrium fundyense only needs to be present in low concentrations to produce a harmful algal bloom (HAB).”

Roesler, Bowdoin Associate Professor of Earth and Oceanographic Science, has decidedly high-tech help in her hunt for Alexandrium fundyense in the Gulf of Maine.

She and a team of scientists and student researchers are collecting data from a scientific buoy located just off Bowdoin’s Coastal Studies Center in Harpswell Sound that is operated in conjunction with St. Joseph’s College and University of Maine. It’s one of 10 buoys in the Gulf of Maine Moored Buoy Program (formerly GoMOOS).

Using cell phone and satellite technology, it beams real-time measurements of wind, waves, temperature, currents, salinity, turbidity, color, and dissolved oxygen to a central facility at the University of Maine, which updates the data on a Web site every hour. Bowdoin’s buoy is unique among the fleet because it also measures the content of nutrient the phytoplankton need to live as well as sophisticated instruments to determine small variations in ocean color.

roesler and students
Roesler, left, motors out to the Bowdoin Buoy with student researchers Tricia Thibodeau ’13, center, and Amy Anderson ’12.

Maine dodged the bullet this summer—no severe red tides occurred—but Roesler remains vigilant. HABs are occurring with greater frequency and infiltrating greater areas of oceans around the globe, she says.

“We’ve been looking at 10 years of observation in the Gulf of Maine and there are distinct changes in species composition of phytoplankton and timing of when they bloom,” she says. “And it seems that these are related to climate and weather-scale variability.”

Roesler’s colleague, Greg Teegarden from St. Joseph’s College, is collecting phytoplankton cells in nets to determine how different species vary in the phytoplankton community and what conditions favor Alexandrium fundyense population growth.

Roesler is trying to anticipate HABs by studying the pigments and optical qualities of other phytoplankton that co-occur with Alexandrium fundyense. Hers is a subtle analysis of color and light, aided by satellite remote sensing and optical sensing on the buoy.

“When there is phytoplankton in the water it turns green because the organisms absorb the blue light and the water itself absorbs red light,” she explains. “What’s left is green.”

roesler deploys sensor
Roesler deploys a sensor that will aid in satellite imagery of ocean pigment.

But there are shades of green, and more shades of green. Roesler says the distinctions between the shading of pigments that indicate the presence of the marker phytoplankton for Alexandrium is extremely hard to detect.

“The question I ask myself is, if the water is green and the phytoplankton are growing, how is the color of that green water evolving? Toward olive or blue-green? It’s like studying a whiter shade of a pale, right?”

Patricia Thibodeau ’13, who spent the summer working as one of Roesler’s undergraduate researchers, has been calculating the fluorescent properties of the phytoplankton. Fluorescence is the light they radiate from the wavelengths of sunlight they have absorbed, and is a clear indication of their pigments.

“It’s a gratifying feeling to contribute to new research,” says Thibodeau. “I’ve been going out on the research boat every week to help with the light sensor, then crunching numbers on the computer so we can plot trends on a graph.”

tricia thibodeau
Tricia Thibodeau prepares water sampling equipment.


Story posted September 20, 2010, Bowdoin College