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Research Document - 2002/067

State of phytoplankton in the Estuary and the Gulf of St. Lawrence during 2001

By Michel Starr, Liliane St-Amand & Lyse Bérard-Therriault

Abstract

We review the 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 and six sections crossing the Estuary and Gulf of St. Lawrence. We focus on the conditions prevailing in 2001 but compare those observations with previous information from the period of 1992-2000.

In 2001, the initiation of the spring phytoplankton bloom at Station Rimouski in the Lower St. Lawrence Estuary occurred in early May, that is, 6-8 weeks earlier than usual. 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 observed in the St. Lawrence basin since 1998.

For the second consecutive year, the average phytoplankton biomass at Station Rimouski during spring-summer 2001 was much lower compared to the 1995-1999 period (except for 1998) but comparable to the 1992-1994 period. In particular, the phytoplankton biomass in July 2001 was much lower compared to our previous observations. This is believed to be due to a more intense mixing period in the Lower St. Lawrence Estuary in July 2001.

In the Anticosti Gyre and the Gaspé Current, the reduction of nutrients in the surface layer during spring-summer-fall 2001 was much less pronounced compared to the 1996-1999 period (except for 1998). In the Gaspé Current, near-surface chlorophyll levels were also generally lower in 2001 compared to the previous two years. On the other hand, summertime chlorophyll levels in the Anticosti Gyre were higher in 2001 compared to those observed in 1997-2000.

Satellite observations of sea surface chlorophyll concentrations indicate that the 2001 spring bloom occurred in late April for most areas of the Gulf of St. Lawrence. This contrasts with previous observations showing a greater spatial variability in the timing of the bloom.

The analysis of community composition showed that the 2001 spring bloom over most of the Gulf was principally dominated by the diatom Neodenticula seminae. This is the first occurrence of this species in the Gulf of St. Lawrence; this species is usually found in North Pacific waters. This unusual event is consistent with recent observations indicating a greater influx of Pacific waters into the Atlantic Ocean (via the Bering Strait) and with hydrographic evidence of a major intrusion of Labrador Slope Water into the Gulf of St. Lawrence in 2001.

In late spring 2001, the chlorophyll levels were extremely low for most areas of the Gulf of St. Lawrence. The highest levels were observed in the nutrient-rich waters of the St. Lawrence Estuary and Gaspé Current system. In the eastern and southern part of the Gulf, the chlorophyll and nitrate levels in the surface layer in late spring 2001 were not notably different than those observed in 1999-2000.

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