An important hydrographic characteristic of the Lower Estuary–Northern Gulf of St. Lawrence system is the vertical three-layered thermal structure of the water column. Following the winter months when cold weather decreases the water surface temperatures, the air warming in the spring and summer creates a shallow warm surface layer and a strong temperature gradient (thermocline) that isolates a cold intermediate layer (CIL) below the surface.

In general, minimum water temperatures in the CIL do not raise above 1°C. Below the CIL, the intrusion of water from the continental Shelf into the deep channels of the Gulf creates a deep layer of warmer and saltier water.

The water column thermal structure and the strong thermocline have significant impacts on the distribution of the organisms and on the functioning of the ecosystem. This is why, for example, biologists need to know the distribution of the organisms within the different temperature fields in order to correctly estimate growth and development rates. Moreover, the position held by planktonic species in the water column defines in part their trophic position in the ecosystem.

Project Team
P. Ouellet, scientist in charge
J.P. Allard, design and realisation of the experimental setup

Funding
This study was made possible through funds provided by the Department of Fisheries and Oceans under the Science Strategic Fund program on Growth and Recruitment of Northern Shrimp and by the shrimp fisherman associations of Québec and New-Brunswick under co-management agreement with DFO.

References
Cronin, T. W. and R. N. Jinks. 2001. Ontogeny of vision in marine crustaceans. American Zoologist 41: 1098-1107.

Johnson, M. L., Gaten, E. and P. M. J. Shelton. 2002. Spectral sensitivities of five marine decapod crustaceans and a review of spectral sensitivity variation in relation to habitat. Journal of the marine biological Association of the United Kingdom 82: 835-842.

Ouellet, P. and J.-P. Allard. Vertical distribution and behaviour of shrimp Pandalus borealis larval stages in thermally stratified water columns: Laboratory experiments and field observations. Fisheries Oceanography (Submitted Feb. 2005).