Changes in abundance, composition and distributin of macrozooplancton in the St. Lawrence Estuary

Michel Harvey and his team have been closely monitoring the abundance, composition and distribution of macrozooplankton in the Lower St. Lawrence Estuary since 1994. A more extensive Fisheries and Oceans Canada programme to monitor the Atlantic zone—implemented in 1998—is aimed at collecting and analyzing biological, chemical and physical information over a greater area that includes the Gulf and Estuary of the St. Lawrence, the Maritimes and Newfoundland. Though this programme was only implemented as recently as 1998, it relies on data that has been compiled since the middle of the XX Century.

According to samples taken and estimates put forward by Michel Harvey’s team, the quantity of macrozooplankton in the Lower St. Lawrence Estuary has decreased from 32 t/km² in 1994 to 10 t/km² in 2002: a 70 percent drop in 10 years. Krill, which is essentially composed of the species Meganyctiphanes novergica and Thysanoessa raschii, accounted for 80 percent of macrozooplankton in 1994. However, it represented only 40 percent of macrozooplankton in 2003. Researchers are looking for the causes of this decline and changes in the composition of macrozooplankton in the Lower Estuary. Could changes in circulation of water masses and in the properties of the cold intermediate layer of the Estuary and Gulf—thicker and colder than it was in the mid-1980s—be responsible? Whatever the case, the decline in the proportion of krill within the macrozooplankton group of animals has also been measured in the southern Gulf of St. Lawrence since 1987, which indicates that it is a widespread occurrence.

Another major change is the arrival of a new species among St. Lawrence macrozooplankton since the early 1990s. Its advent has not gone unnoticed: from 1994 to 2003 it represented 2 percent and 45 percent of the macrozooplankton biomass respectively. This species is a cold-water Arctic amphipod known as Themisto libellula, or simply . Before the mid-1980s, was virtually absent from the waters of the Lower Estuary and Gulf of St. Lawrence. Since the early 1990s, the abundance of this species in the Estuary and Gulf has been proportional to the amount of cold water penetrating into the Gulf of St. Lawrence from the Labrador Plateau during winter; an increase in cold water from Labrador in the Gulf leads to an increase of the following summer. This does not mean that cold water from Labrador did not enter the St. Lawrence before the mid-1980s, several studies have demonstrated just the opposite. On the other hand, it is possible that these waters did not move as far upstream into the Lower St. Lawrence Estuary as they do now. It is also possible that there has been a change in the way water from the Labrador Sea circulates and that this has had the effect of increasing the presence of T. libellula in these waters. The mystery remains.

Will the reduction of krill measured in the Estuary have a long-term impact on the abundance and distribution of whales in the Lower St. Lawrence Estuary? Could T. libellula be an alternative prey species for baleen whales? Researchers are addressing these questions. They hope to come up with answers in the coming years.