250
C. M. YONQE
whether it arose aa a result of that association. I n the sense that, as
Goreau and Goreau (19.59) have demonstrated, light intensity has a
profound effect on coral growth owing to the presence of zooxanthellm,
then the latter alternative appears the more probable. Droop (1963)
makes the interesting suggestion that ‘ I irritability experiments with
artificial amociations could decide whether the reaction t o an internal
oxygen gradient is inherent or not.”
XII. PRODUCTIVITY
Localized centres of high productivity in the open ocean, like oase0
in a desert, atolls are of compelling interest. Each lagoon, in the words
of Von Arx (1954), I ‘ is essentially a ‘ lake in mid-ocean ’, cupped in a
shallow saucer-like basin supported by a solitary mountain peak ”.
It also represents an area of sea bottom raised from aphotic depths into
the photic zone. Atolls lie across the trade wind belts, which fact
accounts for an asymmetry due certainly to the greater force of seas on
the windward side but possibly also to the greater quantity of zooplankton on that side. It has been claimed, as discussed later, that an atoll
represents a completely closed system effectively neither taking
material from nor releasing material into the surrounding ocean. But
atolls have grown to their present size in the past and probably continue to grow and this must have involved, and still involves, some
interception and utilization of organic and inorganic matter from the
sea. The problem also arises with fringing and barrier reefs but there,
particularly with the former, the factors are more difficult to analyse
owing to the presence of complications such aa the effect of land
drainage and of greater sedimentation.
The extent to which lagoon waters are isolated from those of the
surrounding ocean has been demonstrated by the work of Von Arx
(1954) at Bikini and Rongelap. He notes they are infiuenced by four
sources of energy, namely wind, waves, tides and the North Equatorial
Current, with the trade winds much the most important. Apart from
their effect on the seaward reefs, they cause a steady inflow of water
through channels on the reef surface into the lagoon. There is no such
major inflow on the lee where water movements are largely influenced
by the tides. The wind also drives the surface waters in the lagoon, to &
depth dependent on the force of the wind, in steady movement from
seaward to leeward side. The greater part of this water cannot escape
so is driven below to return upwind as a bottom current (Fig. 17). There
is also a secondary circulation consisting of “ two counter-rotating
compartments, which move clockwise in the southern portions and
C. M. YONQE
whether it arose aa a result of that association. I n the sense that, as
Goreau and Goreau (19.59) have demonstrated, light intensity has a
profound effect on coral growth owing to the presence of zooxanthellm,
then the latter alternative appears the more probable. Droop (1963)
makes the interesting suggestion that ‘ I irritability experiments with
artificial amociations could decide whether the reaction t o an internal
oxygen gradient is inherent or not.”
XII. PRODUCTIVITY
Localized centres of high productivity in the open ocean, like oase0
in a desert, atolls are of compelling interest. Each lagoon, in the words
of Von Arx (1954), I ‘ is essentially a ‘ lake in mid-ocean ’, cupped in a
shallow saucer-like basin supported by a solitary mountain peak ”.
It also represents an area of sea bottom raised from aphotic depths into
the photic zone. Atolls lie across the trade wind belts, which fact
accounts for an asymmetry due certainly to the greater force of seas on
the windward side but possibly also to the greater quantity of zooplankton on that side. It has been claimed, as discussed later, that an atoll
represents a completely closed system effectively neither taking
material from nor releasing material into the surrounding ocean. But
atolls have grown to their present size in the past and probably continue to grow and this must have involved, and still involves, some
interception and utilization of organic and inorganic matter from the
sea. The problem also arises with fringing and barrier reefs but there,
particularly with the former, the factors are more difficult to analyse
owing to the presence of complications such aa the effect of land
drainage and of greater sedimentation.
The extent to which lagoon waters are isolated from those of the
surrounding ocean has been demonstrated by the work of Von Arx
(1954) at Bikini and Rongelap. He notes they are infiuenced by four
sources of energy, namely wind, waves, tides and the North Equatorial
Current, with the trade winds much the most important. Apart from
their effect on the seaward reefs, they cause a steady inflow of water
through channels on the reef surface into the lagoon. There is no such
major inflow on the lee where water movements are largely influenced
by the tides. The wind also drives the surface waters in the lagoon, to &
depth dependent on the force of the wind, in steady movement from
seaward to leeward side. The greater part of this water cannot escape
so is driven below to return upwind as a bottom current (Fig. 17). There
is also a secondary circulation consisting of “ two counter-rotating
compartments, which move clockwise in the southern portions and