Will “Dead Zones” Spread in the St. Lawrence River?

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The historic St. Lawrence River, running from the Great Lakes to the Atlantic coast, seems unchangeable. First Nations used it for fishing and transport; fur traders and farmers made it their highway; towns and cities grew along its shores. But the populations that benefit from the St. Lawrence today may be damaging it for tomorrow.

The Maurice Lamontagne Institute (MLI) overlooks the St. Lawrence from Mont-Joli on the Gaspé Peninsula. Dr. Denis Gilbert of MLI has documented unsettling changes in the Lower St. Lawrence Estuary (LSLE). This area runs from above Tadoussac, where the Saguenay River enters the St. Lawrence, to the waters of the northwest Gulf of St. Lawrence.

Oxygen, as necessary to life in water as on land, has in parts of the LSLE fallen by half since the 1930's. When oxygen gets below 2 milligrams per litre, many species of fish and shellfish can no longer survive. Low-oxygen or “hypoxic” conditions occur naturally in some areas, such as inlets with restricted circulation. But now, these “dead zones” are on the rise in other coastal waters in many parts of the world. For example, worsening levels of hypoxia in Chesapeake Bay and in the Gulf of Mexico have prompted American authorities to launch action plans to reduce the flux of nutrients to coastal waters.

“So far, the low-oxygen areas in the Lower St. Lawrence Estuary are close to the bottom, at depths greater than 275 metres,” Denis Gilbert says. “Greenland halibut (turbot) are still very abundant in these low-oxygen waters, but they could eventually suffer and have to migrate out of the estuary if oxygen continues to drop.”

What is driving the change in the Lower St. Lawrence Estuary? Although more research needs doing, Dr. Gilbert and colleagues at MLI and universities have pieced together large parts of the picture.

Red dots in the Lower St. Lawrence Estuary indicate oxygen saturation levels that are lethal to cod, and orange dots show locations where between 5% and 50% of the cod would die after a four-day exposure. Reduced cod growth occurs below 70% oxygen saturation.

The story begins with the seabed itself. The Laurentian Channel is a deepwater gully running more than 1,200 kilometres from above Tadoussac, out past the Gaspé Peninsula, and through the Gulf of St. Lawrence to the broad Atlantic. In the surface layers of the Laurentian Channel, water flows seaward. In the deep layer, water moves slowly landward from the mouth of the Laurentian Channel, reaching far into the LSLE, a trip that takes about four years.

Outflowing waters at the upper levels of the LSLE, carrying plankton, pollutants, and whatever comes along, influence the lower levels. And those deep waters are changing.

Severe hypoxia began occurring in the mid-1980's in the LSLE, where the hypoxic area covered about 1,300 square kilometres of the seafloor in 2003. In the illustration, the reddest dots show the worst oxygen depletion, in deepwater areas where cod can no longer survive.

Denis Gilbert and colleagues have calculated that less than one third of the oxygen depletion stems from river-based factors. In particular, municipal sewage, fertilizers and manure from farmers' fields spread large quantities of nitrates and phosphates into the river. These give extra nutrients to plankton, causing them to increase. When the newly abundant plankton die and sink to the bottom, the decomposition process takes more oxygen out of the water.

Automated oxygen titration system used by MLI staff. The “whitish” samples at the left are oxygen-poor whereas the dark orange samples are oxygen-rich.

By analyzing bottom sediment samples, colleagues at l'Université du Québec à Montréal (UQAM) and McGill University have shown that the rate of organic carbon deposition started increasing in the 1600's, when French Canadian settlements were spreading along the St. Lawrence. In recent decades, organic carbon deposition and oxygen depletion have speeded up.

If river-based human activities account for only a minor part of the drop in oxygen levels, what causes the other half to two-thirds? Changing conditions in the great Atlantic Ocean, which may also be partly man-made as a consequence of global warming, are believed to have played a major role in the oxygen decline.

The ocean's surface layer exchanges gases with the atmosphere. During phytoplankton blooms, the surface ocean acts as a source of oxygen to the atmosphere. At other times, such as during intense cooling events in the fall, the surface ocean acts as an oxygen sink. But generally, the surface ocean is never very far from gas equilibrium with the atmosphere, with typical oxygen saturations in the surface ranging between 95% (undersaturated) and 110% (supersaturated).

Labrador Current Water (LCW) flowing along the southern edge of the Grand Banks of Newfoundland, a “young” water mass that had direct gas exchange with the atmosphere during the previous year, is an important component of the water mass entering the mouth of the Laurentian Channel.

But only part of the water entering the Laurentian Channel consists of Labrador Current Water (LCW). In addition, the Gulf of St. Lawrence receives large quantities of North Atlantic Central Water (NACW). Warmer, saltier, and carrying less oxygen, North Atlantic Central Water originates south of the Gulf Stream that curves by Nova Scotia and Newfoundland on its way towards Europe. Gulf Stream eddies carry large amounts of NACW into the Slope Water region between the Gulf Stream and continental shelf, and contribute to the landward flow in the deep waters of the Laurentian Channel.

In recent decades, the proportion of Labrador Current Water entering the Gulf of St. Lawrence has decreased, while North Atlantic Central Water has increased. This has contributed not only to lower oxygen levels in the deep waters of the Lower St. Lawrence Estuary, but also to an increase of their temperature, by 1.65 degrees Celsius. This warming increases bacterial respiration rates, thus further reducing oxygen concentrations.

“The ultimate causes of all this are still unknown,” Denis Gilbert says, “but it is natural to suspect a connection to global climate change.”

In pursuing the puzzle, scientists need better data on oxygen levels in the high seas, which may be changing. As part of the world-wide Argo buoy program, DFO has deployed more than 90 buoys that send in temperature and salinity readings via satellite. The department is now equipping 11 of these buoys with oxygen sensors. These will help detect oceanic changes, sometimes tiny in magnitude but huge in extent, that may affect climate world-wide.

As for the Lower St. Lawrence Estuary, Denis Gilbert says that “we can't predict changes with certainty. But forces from the river and the ocean are both stealing away oxygen. The more we can learn through research, the better prepared we'll be.”