Research Document - 2004/123

State of phytoplankton in the Estuary and Gulf of St. Lawrence during 2003

By Starr, M., L. St-Amand, L. Devine, L. Bérard-Therriault, P.S. Galbraith

Abstract

We reviewed 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 at three fixed stations (Rimouski, Anticosti Gyre, and Gaspé Current) and six sections crossing the Estuary and Gulf of St. Lawrence. We concentrated on conditions prevailing in 2003 but also compared those observations with previous information from the 1992-2002 period.

In 2003, the initiation of the major phytoplankton bloom at Station Rimouski in the Lower St. Lawrence Estuary occurred in late May, which is one month earlier than usual. Excluding 2002, this continued a trend that began in 1998. This major shift in the timing of the phytoplankton cycle is believed to be due to the below-normal spring freshwater runoff that has been generally observed in the St. Lawrence basin since 1998. The average phytoplankton biomass during spring–summer 2003 at Station Rimouski was also higher compared to 1992-1994, 1996, 1998, and 2000-2002, but lower compared to 1995, 1997, and, more especially, to 1999. Spring–summer phytoplankton production measured at this station was also much higher in 2003 compared to the previous three years but lower than in 1999.

At the Anticosti Gyre and the Gaspé Current stations, nitrate and silicate concentrations were high in late fall–winter and low in late spring-summer due to biological consumption by phytoplankton. For both stations, the reduction of nutrients in the surface layer during spring–summer was much more pronounced in 2003 compared to the 2000-2002 period. Thus based on the evolution of nutrients, phytoplankton production in the northwestern Gulf of St. Lawrence could have been higher in 2003 compared to the previous three years. This is consistent with data from Station Rimouski in the Lower St. Lawrence Estuary.

Similarly, the surface nutrient levels in late winter 2003 were also higher in the southern and northeastern Gulf of St. Lawrence compared to the previous two years while levels were not markedly different for the late spring–summer period. This suggests again that the spring phytoplankton bloom in these regions was also more intense in 2003 compared to recent years.

For a third consecutive year, the analysis of the phytoplankton community composition in 2003 revealed the presence of the diatom Neodenticula seminae in many areas of the Gulf of St. Lawrence, with concentrations up to 197 X 10² cells per litre. This phenomenon is unusual since this species is typically 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 invasion of N. seminae on the Atlantic coast is consistent with recent observations suggesting a greater influx of Pacific waters into the Atlantic.

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