SCIENCE REVIEW 1987 - Bedford Institute of Oceanography

Research
A history of chemical oceanographic research in the
Gulf of St. Lawrence
P.M. Strain
P.M. Strain
THIS article will outline a few aspects of
the history of chemical oceanography in
the Gulf of St. Lawrence, the St. Lawrence
Estuary, and the Saguenay Fjord (Figure
l), showing how advances in several
different areas of science have led to an
improved understanding of the behaviour
of chemicals in the Gulf of St. Lawrence
system. These advances have included the
improvement of analytical chemical
methods, developments in oceanographic
sampling techniques, and the improved
design of field programs that concentrated
sampling effort on the locations most
important to the behaviour of each chemical.
At the same time, physical oceanographers
have provided estimates of water
flows into and out of the Gulf of St.
Lawrence that have been used to calculate
the amounts of chemicals moving through
the Gulf.
Organic Matter Geochemistry
Organic matter, which is composed mostly
of carbon, is the material that makes up the
soft tissues of plants and animals. It is an
important chemical component in both
fresh and salt water. As organic matter
decomposes, dissolved oxygen is consumed
and acid is produced - these chemical
reactions cause important changes in the
chemical conditions experienced by other
chemicals. In addition, chemicals such as
some toxic metals are adsorbed by organic
matter and will be deposited in bottom
sediments at the same locations. Organic
matter exists as particles suspended in the
water, dissolved in the water, and incorporated
in bottom sediments.
Fig. I Map of the Gulf of St. Lawrence, the St, Lawrence Estuary and the Saguenay Fjord
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The carbon isotope study of organic
matter provides a good example of how
advances in oceanography require advances
in chemical methods. Carbon, like most
elements, has more than one naturally
occurring non-radioactive isotope -
carbon-12 and carbon-13. There are very
small, but measurable differences in the
13 C/12 C ratio in carbon from different
sources - e.g. the amount of 13 C in land
plants is less than that in phytoplankton
growing in the St. Lawrence Estuary. A
technique for the measurement of carbon
isotope ratios in the organic matter in
sediments was developed for use at BIO. A
study of carbon isotopes in surface sediments
of the St. Lawrence Estuary and the
Gulf has shown that the organic matter that
comes into the Gulf in the St. Lawrence
River discharge at Quebec City is found in
sediments only in the St. Lawrence Estuary.
The results of this study, while
interesting, were limited. They made it
obvious that an improved understanding of
organic matter behaviour in the Gulf
would be possible only if isotope measurements
were available on additional types of
organic matter. Accordingly, the analytical
methods were improved so that the much
smaller samples available for suspended
particulate matter and the organic matter
sampled by plankton tows could also be
analyzed. These developments made it
possible to conduct an integrated field
program to examine the behaviour of
organic matter in much more detail. At the
same time, other evidence was increasingly
showing that many important chemical
changes occur in estuaries, where the most
active mixing of fresh and salt water
occurs. Therefore, field work was focussed
on the St. Lawrence Estuary.
The study of organic matter also illustrates
how an increasing understanding of
both these estuarine processes and of the
physical dynamics of the system led to
further advances in the geochemistry. The
isotope study of sedimentary carbon had