THE PRODUCTION O F MARINE PLANKTON
135
Similar work by Pintner and Provasoli (1963) has shown that marine
chrysomonads may also be either stenohaliiie or euryhaline forms. For
example, Coccolithus huxleyi has a remarkaldy widespread distribution
in the seas and can tolerate salinities certainly down to IS%,. Although
salinity therefore may have some effect on productivity rates of individual species, generally in the sea even near the surface, the variations
in salinity are so very slight that this factor cannot be important in
oceanic productivity. Even in inshore waters the variation in salinity is
much more likely to operate in the successicm of phytoplankton species
than as a factor in overall production. Since salinity and temperature,
however, influence the density of sea water, salinity can affect the
flotation of phytoplanktonic species. Thue Braarud (1962) suggests
that oceanic phytoplankton usually consisto of either species with large
cells with good flotation, or of minute forms with a higher reproductive
rate. A number of coastal species are unable to avoid sinking when
temperature and salinity changes reduce the specific gravity of the
water. They therefore may disappear for some time, and only spore
formation will carry the species over to a later flowering when changes
in the density of sea water permit active reproduction again. Significant
indirect effects of lowered salinity on production in relation to stratification are mentioned later.
4. Nutrients - Phsphate and Nitrate
For many years the concentration of two major plant nutrients,
nitrate and phosphate, has been recognized as one of the major factors
limiting primary production in the oceans. The cycle of phytoplankton
growth in temperate latitudes with the marked spring and autumn
peaks and the depression of production during the summer has been
linked with the changes in nutrient levels. Investigations such as those
of Atkins, Harvey, and Cooper at Plymouth, and of Marshall and Orr in
the Clyde sea area, as well as those of Bigelcw, Lillick, and Sears (1940)
for the Gulf of Maine area, suggest that the lack of nitrate or phosphate,
whichever is in shortest supply, during summer when a thermocline is
strongly established, acts as a marked brake on phytoplankton production. The work of Riley and his colleagues in Long Island Sound (e.g.
Riley and Conover, 1956), where greater quantities of nitrate and
phosphate are present over winter, indicateri that a limitation is placed
on phytoplankton production during the summer by the depletion of
nitrogen. In certain areas of the Gulf of Maine, over the Faroe-Shetland
Ridge and in Friday Harbour, where marked turbulence occurs and
there is a lack of stratification so that nutrients are not normally depleted over the summer, production may be increased. The clearest
demonstration of the relationship between EL constant supply of nitrate
135
Similar work by Pintner and Provasoli (1963) has shown that marine
chrysomonads may also be either stenohaliiie or euryhaline forms. For
example, Coccolithus huxleyi has a remarkaldy widespread distribution
in the seas and can tolerate salinities certainly down to IS%,. Although
salinity therefore may have some effect on productivity rates of individual species, generally in the sea even near the surface, the variations
in salinity are so very slight that this factor cannot be important in
oceanic productivity. Even in inshore waters the variation in salinity is
much more likely to operate in the successicm of phytoplankton species
than as a factor in overall production. Since salinity and temperature,
however, influence the density of sea water, salinity can affect the
flotation of phytoplanktonic species. Thue Braarud (1962) suggests
that oceanic phytoplankton usually consisto of either species with large
cells with good flotation, or of minute forms with a higher reproductive
rate. A number of coastal species are unable to avoid sinking when
temperature and salinity changes reduce the specific gravity of the
water. They therefore may disappear for some time, and only spore
formation will carry the species over to a later flowering when changes
in the density of sea water permit active reproduction again. Significant
indirect effects of lowered salinity on production in relation to stratification are mentioned later.
4. Nutrients - Phsphate and Nitrate
For many years the concentration of two major plant nutrients,
nitrate and phosphate, has been recognized as one of the major factors
limiting primary production in the oceans. The cycle of phytoplankton
growth in temperate latitudes with the marked spring and autumn
peaks and the depression of production during the summer has been
linked with the changes in nutrient levels. Investigations such as those
of Atkins, Harvey, and Cooper at Plymouth, and of Marshall and Orr in
the Clyde sea area, as well as those of Bigelcw, Lillick, and Sears (1940)
for the Gulf of Maine area, suggest that the lack of nitrate or phosphate,
whichever is in shortest supply, during summer when a thermocline is
strongly established, acts as a marked brake on phytoplankton production. The work of Riley and his colleagues in Long Island Sound (e.g.
Riley and Conover, 1956), where greater quantities of nitrate and
phosphate are present over winter, indicateri that a limitation is placed
on phytoplankton production during the summer by the depletion of
nitrogen. In certain areas of the Gulf of Maine, over the Faroe-Shetland
Ridge and in Friday Harbour, where marked turbulence occurs and
there is a lack of stratification so that nutrients are not normally depleted over the summer, production may be increased. The clearest
demonstration of the relationship between EL constant supply of nitrate