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Going Green: Phytoplankton fall
Gulf of Maine productivity plunges amid record rainfall, climate change
By Lisa Duchene
September 01, 2012
In Wilkinson’s Basin, an area in the Gulf of Maine east of Boston, University of Maine marine scientist Jeffrey Runge is looking for a fat-rich copepod called Calanus finmarchicus. Herring eat these tiny animals, a type of zooplankton.
Since herring are critical lobster bait — and a key forage fish for Gulf of Maine fisheries — calanus are important to the lobster market.
Runge, a biological oceanographer based at the Gulf of Maine Research Institute, hopes to see huge numbers of calanus, which spends its winters in the depths of Wilkinson’s Basin, feeding historically productive fishing areas like Jeffreys Ledge and Georges Bank.
But he may instead find a drop. This spring, another scientist reported a five-fold plunge in the amount of Gulf of Maine phytoplankton in 2007, arguing it is a result of record rainfalls and climate change. Rather than a blip, the phytoplankton drop has been sustained.
“A five-fold decrease is pretty important,” says Runge. “[For] reductions like this that are sustained, [we’ve] got to wonder if that influences the system’s carrying capacity.”
If Runge’s work shows a drop in calanus, it will be a sign that this plummet at the base of the food web is affecting other species. Phytoplankton, known as primary producers, convert the sun’s energy into food for all other life in the Gulf of Maine. Fish and shellfish abundance rely on primary production.
While collecting data in the Gulf of Maine, William Balch — a senior research scientist at Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine — noticed plumes of dissolved organic matter showing up offshore. Washed out from rivers, such plumes typically stay relatively close to shore. Balch also noticed low-salinity water far out at sea.
Those two clues hinted that something very unusual is underway in the Gulf of Maine, so Balch and his team went looking for answers. The result was a paper published March 29 in the scientific journal, Marine Ecology Progress Series.
“We’re looking at absolutely enormous changes that occurred,” says Balch, who analyzed ocean measurements taken between September 1998 and December 2010 in a West-East line from Portland, Maine, east to Yarmouth, Nova Scotia.
Four of the eight wettest years of the last century occurred between 2005 and 2010, according to that data and climatological data. “It’s an extraordinary number, when you think about it,” says Balch, “that half of the wettest years on record are since 2005.”
That influx of freshwater from rivers affected phytoplankton production in two ways, says Balch. The rivers carried larger-than-normal amounts of organic matter into the Gulf.
Think about how a teabag colors a cup of boiling water. In the Gulf, that material absorbs blue light — the same wavelength of light used by phytoplankton. “It conceivably could have been shading or depriving the plants of light to photosynthesize,” says Balch.
A surge of lower-salinity, fresh water into the salty Gulf can also disrupt the flow of nutrient-rich water from the Northeast Channel — entering just northeast of Georges Bank and the only influx of deep, outside water into the enclosed Gulf.
“If that truly stopped the inflow of water through the Northeast Channel,” says Balch, “you’d be essentially stopping the inflow of nutrients. If you take away light and nutrients from plants, they won’t be able to grow.”
The drop in productivity looks to be across the Gulf, says Balch. “When you start talking about dropping productivity that much in a short amount of time, it can’t help but have ramifications for organisms higher up in the food chain.”
Atlantic herring are currently healthy and robust, says the National Marine Fisheries Service. The 2008 year-class was estimated to be the largest on record.
Gulf of Maine lobsters are also abundant, says Rick Wahle, a research associate professor at the University of Maine’s School of Marine Sciences. “Trends in lobster abundance in the GOM are running counter to the trends in primary productivity Balch reports,” says Wahle. “By all indications we are seeing an increase in lobster abundance in the Gulf of Maine that is strongly correlated with the decline in the abundance of large predatory groundfish.”
Before Balch’s paper was published, Runge was already worried about how calanus would fare in a warming climate, as a European scientist recently predicted that the species would disappear from the Gulf of Maine in the next few decades. Also, the Gulf is the southern range for calanus and water temperatures in the Gulf are rising.
Balch’s findings are another reason for concern about calanus — and the Gulf of Maine ecosystem, says Runge.
“This is one more study that indicates we’re in a period of change in the Gulf of Maine, whether or not it’s a cyclical change or something that’s longer-term and the forcing of climate change, we don’t know yet,” says Runge. Contributing Editor Lisa Duchene lives in Bellefonte, Pa.