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Research Document - 2003/103

State of phytoplankton in the Estuary and Gulf of St. Lawrence during 2002

By Starr, M., St-Amand, L., Devine, L.


Information concerning the seasonal and interannual variations in the concentrations of chlorophyll a, nitrates, and silicates as well as the abundance of the major species of phytoplankton measured from three fixed stations (Rimouski, Anticosti Gyre and Gaspé Current) and six sections crossing the Estuary and Gulf of St. Lawrence was reviewed. The authors concentrated on conditions prevailing in 2002 but also compared those observations with previous information from the 1992-2001 period.

In 2002, the initiation of the major phytoplankton bloom at Station Rimouski in the Lower St. Lawrence Estuary occurred in late June, which is near the historical mean date. This contrasts with observations made from 1998 to 2001, when the spring phytoplankton bloom began 6-8 weeks earlier than normal (mid to early May). The average phytoplankton biomass during spring-summer 2002 at Station Rimouski was also somewhat higher compared to 1992-1994, 1998, and 2000-2001, but much lower compared to 1995, 1997, and, more especially, to 1999.

In the Anticosti Gyre and the Gaspé Current, the depletion of nutrients in the surface layer (0-50 m) during spring occurred later in 2002 compared to the 1996-2001 period, suggesting that phytoplankton growth was also initiated later in 2002 compared to recent years in the northwestern part of the Gulf of St. Lawrence. This was particularly true for the Gaspé Current. The reduction of nutrients in the surface layer during spring-summer was also somewhat more pronounced in 2002 compared to the 2000-2001 period but much less compared to 1999. Thus based on the evolution of nutrients, phytoplankton production in the northwestern Gulf of St. Lawrence could have been higher in 2002 compared to the previous two years but much lower than for 1999. This is consistent with data from Station Rimouski in the Lower St. Lawrence Estuary.

For a second consecutive year, the analysis of community composition in 2002 revealed the massive presence of the diatom Neodenticula seminae in most areas of the Gulf of St. Lawrence, with concentrations up to 1 x 106 cells per litre. This phenomenon is unusual since this species is usually found in North Pacific waters. In the Atlantic Ocean, this species has only been recorded in middle to high latitude Quaternary sediments, dating from between 0.84 and 1.2 million years ago. It is proposed that this Pacific species was introduced naturally into the Gulf (across the Arctic, down the Labrador Current, and through Strait of Belle Isle) rather than via ballast waters. The return of N. seminae to the Atlantic coast is consistent with recent observations indicating a greater influx of Pacific waters into the Atlantic and the freshening of the North Atlantic waters.

In late spring and early fall 2002, the chlorophyll levels were higher in the southern Gulf of St. Lawrence compared to the previous three years. In the eastern part of the Gulf, the chlorophyll and nitrate levels in the surface layer in late spring and fall 2002 were not notably different from those observed in 1999-2001.

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