Fall phytoplankton depend on light, nutrients

First published in Landings, October, 2015.

It’s fall. Night and day are approximately the same number of hours. The air is cool, and brisk northeasterly storms swell across the Gulf of Maine. The Gulf, however, still remains warm since water retains the summer heat much longer than the land. And out there, once again, the engine of the Gulf’s food web is kicking into gear.

Close-up images of Gulf of Maine phytoplankton reveal their beauty. Photos by D. Townsend.
Close-up images of Gulf of Maine phytoplankton reveal their beauty. Photos by D. Townsend.

Most recognize the tell-tale green hue of the Gulf of Maine in the spring when microscopic plants produce an abundance of chlorophyll known as the phytoplankton bloom. The spring bloom is fueled by the warming surface of the Gulf combining with the oxygen-rich water churned up from the depths from the winter’s storms and nutrients brought in by meltwater from the region’s many rivers. But what happens in the fall? The phytoplankton once again bloom, but in patterns that are patchy and unpredictable, quite dissimilar to the spring bloom. “No one has really studied the fall bloom,” explained David Townsend, professor of oceanography at the University of Maine School of Marine Sciences. “In theory it does occur.”

During the summer, the Gulf of Maine is like a Jello parfait. At the surface is a deep layer of warm, nutrient-depleted water. As one moves deeper, water temperatures drop very gradually. In the middle is a layer of slightly mixed water, called the thermocline. Below the thermocline layer, water temperature will drop abruptly, leaving a layer of very cold, nutrient- and oxygen-filled water at the bottom of the Gulf. The three layers coexist together but do not mingle, like layers of different-colored Jello in a parfait.

Come fall, when the hours of sunlight decrease, that surface layer of warm water begins to diminish. Autumn storms will churn it up, allowing the cold deeper water to rise up to replace it through a process called convection. The thermocline layer expands in size as nutrients from the bottom layer mix with the warmer surface water layer. “The thermocline (slope) is gentle,” Townsend said. “Phytoplankton are getting less light but more nutrients [than during the summer].”

Townsend, who has tracked phytoplankton in the Gulf for decades, finds the fall phytoplankton bloom somewhat mysterious. “The satellite images [which show concentrations of chlorophyll produced by the phytoplankton] aggregate the data. You could have a high concentration in one pixel [of the image] and zero in the next,” he noted. It’s the variation among areas of the Gulf that he believes tells the story of the fall bloom. “In all years there must be the same productivity but it is expressed in a different way. Instead of a bloom it’s more of a gradual increase,” he said.

One of the factors that influences the fall phytoplankton bloom is the temperature of the water at depth. In some years bottom waters in the Gulf are particularly cold; in other years much warmer and saltier. Townsend explained that the difference in temperature is related to the amount of melting occurring in the Arctic Ocean.

davetownsend photoThe Arctic meltwater flows into the Labrador Sea. That cold fairly fresh water fuels the Labrador Current, an arm of which winds its way into the Gulf of Maine at the surface. Scientists refer to that water as Scotian Shelf Water. Warmer, nutrient-rich water slips in over Georges Bank and the continental shelf; that deep water is called Slope Water. “If lots of freshwater is coming in, then the deep slope water can’t come in,” Townsend said. As a result, phytoplankton may find themselves without the nutrients they need to explode into a bloom, both in the spring and in the winter months.

The Gulf of Maine is a complex system, whose rich productivity is a result of interactions among the sun, the currents, and the wind. As the Earth’s climate warms, that complex interplay has begun to change as well.

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