T H E PRODUCTION O F MARINE PLANKTON
147
third consisting mainly of more slowly dividing motile algae characterized by species with metabolites of high toxicity. He suggests that
these phytoplankton species havc n relatively high immunity to these
toxic substances.
It has long been recognized that in a given region, inshore areas
usually show greater production and standing crop than more oceanic
areas. The difference is normally attributed to the more turbulent
mixing of coastal regions assisting in the replenishment of essential
nutrients in the euphotic zone (vide infra). But examples were known
when even though nitrate and phosphate were relatively plentiful,
inshore areas displayed increased production. Bran (1931) suggested
that coastal regions might benefit from other micronutrients brought
down by land drainage. In the Antarctic where phosphate and nitrate
is abundant, Hart (1942) noticed that neritic areas, which could stretch
out to a considerable distance from land, showed greater production.
He suggested that the important factor might be micronutrients, such
as trace metals, which were carried out from che land. It may well be
that an organic growth-promoting substance, rather than a trace
element, is in part responsible for this beneficial neritic influence. To
some extent a similar condition is found at the boundaries of ocean
currents. For many years it has been widely recognized that standing
crop and productivity tends to be higher at current boundaries. Undoubtedly part of this increased production is due to extra nutrients
which are made available in the euphotic zone in such areas. But even
in situations where there is no nutrient lack or appreciable difference in
concentration, a marked improvement in production is often found in
areas of water mixture. The so-called “island effect” is possibly explained on the same hypothesis. This effect (e.1;. Doty and Oguri, 1956;
Jones, 1962) is a tendency to an increase in primary production and also
to a genera1,rise in standing crop of plankton in the regions round
oceanic islands. Jones found, for example, an increase in plankton crop
to a distance of nearly 200 miles from the Marquesas Islands. Although
increased nutrients such as nitrate and phosphate can certainly occur
around oceanic islands, it seems impossible from Jones’ results to
explain the increased plankton as due to nutrient level at such a distance
from the island. The presence of an increased concentration of a trace
element such as iron, or equally likely, of a mi cronutrient organic substance draining from the regions of the islands, seems a more probable
explanation.
c. PRODUCTION - TEMPERATURE AND STRATIFICATION
In tropical regions the small variation in light intensity over the year
means that the euphotic zone is more or less constant in depth. On the
147
third consisting mainly of more slowly dividing motile algae characterized by species with metabolites of high toxicity. He suggests that
these phytoplankton species havc n relatively high immunity to these
toxic substances.
It has long been recognized that in a given region, inshore areas
usually show greater production and standing crop than more oceanic
areas. The difference is normally attributed to the more turbulent
mixing of coastal regions assisting in the replenishment of essential
nutrients in the euphotic zone (vide infra). But examples were known
when even though nitrate and phosphate were relatively plentiful,
inshore areas displayed increased production. Bran (1931) suggested
that coastal regions might benefit from other micronutrients brought
down by land drainage. In the Antarctic where phosphate and nitrate
is abundant, Hart (1942) noticed that neritic areas, which could stretch
out to a considerable distance from land, showed greater production.
He suggested that the important factor might be micronutrients, such
as trace metals, which were carried out from che land. It may well be
that an organic growth-promoting substance, rather than a trace
element, is in part responsible for this beneficial neritic influence. To
some extent a similar condition is found at the boundaries of ocean
currents. For many years it has been widely recognized that standing
crop and productivity tends to be higher at current boundaries. Undoubtedly part of this increased production is due to extra nutrients
which are made available in the euphotic zone in such areas. But even
in situations where there is no nutrient lack or appreciable difference in
concentration, a marked improvement in production is often found in
areas of water mixture. The so-called “island effect” is possibly explained on the same hypothesis. This effect (e.1;. Doty and Oguri, 1956;
Jones, 1962) is a tendency to an increase in primary production and also
to a genera1,rise in standing crop of plankton in the regions round
oceanic islands. Jones found, for example, an increase in plankton crop
to a distance of nearly 200 miles from the Marquesas Islands. Although
increased nutrients such as nitrate and phosphate can certainly occur
around oceanic islands, it seems impossible from Jones’ results to
explain the increased plankton as due to nutrient level at such a distance
from the island. The presence of an increased concentration of a trace
element such as iron, or equally likely, of a mi cronutrient organic substance draining from the regions of the islands, seems a more probable
explanation.
c. PRODUCTION - TEMPERATURE AND STRATIFICATION
In tropical regions the small variation in light intensity over the year
means that the euphotic zone is more or less constant in depth. On the