Increased volumes of tea-colored water flowing down rivers into the Gulf of Maine are having an effect on the gulf that could lead to a reduction in its biological productivity, according to marine scientists.
In other words, as higher volumes of rain fall on Maine, the more the amount of fish found along its coast is expected to decline.
Working with scientists at U.S. Geological Survey, researchers at Bigelow Laboratory for Ocean Sciences in Boothbay recently published a paper that indicates the level of biological reproduction in the Gulf of Maine has decreased. The reason is an increase over the past 80 years in rainfall and the resulting dissolved organic carbon — a dark “tea” steeped from dead leaves and soil, lab officials said — that flows into the gulf, which is interfering with the ability of microscopic marine plants known as phytoplankton to grow.
Researchers at Bigelow Lab, led by Dr. William Balch, conduct transects of the Gulf of Maine each year, taking samples and logging data as they traverse the width of the gulf between Maine and Nova Scotia. They have been recording the color of the gulf and as part of a recent study compared their measurements with those collected by the lab’s namesake, Henry Bryant Bigelow, in 1912.
“The conclusion is that the gulf has yellowed over the last century, particularly in coastal Maine waters,” the lab indicated in a recent news release.
As plants, phytoplankton contain chlorophyll and grow via sun-fueled photosynthesis. The color change in the gulf is reducing the amount of sunlight that penetrates the water’s surface, however, hampering the growth rates of phytoplankton.
Despite their small size, phytoplankton are a key component of ocean life, functioning as an oxygen source and as a staple at the low end of the marine food chain.
According to Balch, there are other factors that affect oxygen levels in the Gulf of Maine. Temperature is one factor, he said, and wave action is another, with colder and stormier seas generally having higher oxygen levels than warmer and more stagnant waters.
A decrease in phytoplankton in the gulf “does not mean necessarily the oxygen in the Gulf of Maine will go down,” Balch said Friday.
The bigger impact from a decrease in phytoplankton in the gulf is a reduction in food that would result for other small organisms, such as copepods, which in turn serve as food sources for larger species. A drop in phytoplankton levels could result in a domino effect up the food chain.
“This is the bottom of the marine food web we’re talking about,” Balch said.
There are other factors that affect the population of fish species in the gulf, however. One reason often cited for the dramatic increase in lobster catches over the past 25 years is the overfishing of cod, which is one of the primary predators for younger lobster.
As for rainfall, scientific climate and hydrologic models predict higher levels this century of precipitation and resulting runoff in the Gulf of Maine watershed, according to the paper’s authors. The increased rainfall could result in dissolved organic carbon levels in the gulf going up by close to 30 percent over the next 80 years, they said, “potentially contributing to a continued decline in the productivity of this coastal marine food web.”
Balch acknowledged that this winter and spring have been fairly dry but said the wetter trend that has been documented is spread out over many decades. Of the eight wettest years Maine has had since the 1890s, he said, four of them have occurred in roughly the past decade.
“These were years of extraordinary rainfall,” Balch said.