Food in Abundcmce
29
phytoplankton in temperate waters loses its impetus as the season
progresses.
More and more of the mineral nutrients essential to
plant growth become locked away within the living cells of the
plankton. When the animals and plants die, their cells become the
targets for marine bacteria and the dead and decaying matter sinks
slowly into deeper waters. This continuous rain of material naturally
includes a high proportion of nutrients and, as stocks become
depleted in the sunlit surface waters, the rate of production of plant
material falls
It does not stop completely; supplies of nutrients
such as those in the excreta of the zooplankton are enough to tide
the phytoplankton over the summer shortage. In the autumn the
storms chum up the waters suiciently to return nutrients to the
surface and the plankton ourishes again until the shorter days rob
them of another vital component of their energy—producing process—the sunlight. During the winter, mineral nutrients accumulate
until the warmer, longer days of spring when the cycle starts again.
The key to the seasonal changes is the separation of the warmer
surface waters from the colder water below them at a point known
as the thermocline. The creation of the thermocline stems from the
physical fact that as water (above 4°C) is heated, it expands and
becomes lighter. The warmer, lighter water oats on the colder
denser water so that no mixing takes place: the strata are stable. In
the early summer the thermocline lies within about 40 feet of the
surface and the di"erence in temperature between the two layers is
some 4°C——the difference between about 16° and 12°C. As the
summer progresses some heat is inevitably passed across from the
warm water to that immediately beneath it and so the thermocline
moves slowly deeper. However, the main point here is that the
thermocline forms a real barrier to the exchange of the mineral
nutrients which nd their way over the year into the colder water.
Thus these nutrients are lost until the surface water begins to cool,
causing the thermocline to break down, and storms chum up and
mix the waters.
In other parts of the oceans the fertility of the surface waters is
maintained by mineral—rich water welling up from below. Such up—
wellings occur along the eastern edges of the oceans, for example on
the Pacic coast of North America and the Atlantic coast of South
Africa, where prevailing winds literally push back the surface water
allowing the colder water from below to take its place in a continuous
cycle of replenishment. Naturally enough the phytoplankton and
29
phytoplankton in temperate waters loses its impetus as the season
progresses.
More and more of the mineral nutrients essential to
plant growth become locked away within the living cells of the
plankton. When the animals and plants die, their cells become the
targets for marine bacteria and the dead and decaying matter sinks
slowly into deeper waters. This continuous rain of material naturally
includes a high proportion of nutrients and, as stocks become
depleted in the sunlit surface waters, the rate of production of plant
material falls
It does not stop completely; supplies of nutrients
such as those in the excreta of the zooplankton are enough to tide
the phytoplankton over the summer shortage. In the autumn the
storms chum up the waters suiciently to return nutrients to the
surface and the plankton ourishes again until the shorter days rob
them of another vital component of their energy—producing process—the sunlight. During the winter, mineral nutrients accumulate
until the warmer, longer days of spring when the cycle starts again.
The key to the seasonal changes is the separation of the warmer
surface waters from the colder water below them at a point known
as the thermocline. The creation of the thermocline stems from the
physical fact that as water (above 4°C) is heated, it expands and
becomes lighter. The warmer, lighter water oats on the colder
denser water so that no mixing takes place: the strata are stable. In
the early summer the thermocline lies within about 40 feet of the
surface and the di"erence in temperature between the two layers is
some 4°C——the difference between about 16° and 12°C. As the
summer progresses some heat is inevitably passed across from the
warm water to that immediately beneath it and so the thermocline
moves slowly deeper. However, the main point here is that the
thermocline forms a real barrier to the exchange of the mineral
nutrients which nd their way over the year into the colder water.
Thus these nutrients are lost until the surface water begins to cool,
causing the thermocline to break down, and storms chum up and
mix the waters.
In other parts of the oceans the fertility of the surface waters is
maintained by mineral—rich water welling up from below. Such up—
wellings occur along the eastern edges of the oceans, for example on
the Pacic coast of North America and the Atlantic coast of South
Africa, where prevailing winds literally push back the surface water
allowing the colder water from below to take its place in a continuous
cycle of replenishment. Naturally enough the phytoplankton and