58
D. A. MCGILL:
Ocean. The least transparent water found (at Station 145 A: 10° 39' N, 60° 07' E) was
estimated to allow daylight perception to about 700 m in deep sea fish. "These values
indicate the ranges of depths down to which daylight would be sufficient for vision and
for the control of vertical diurnal migration." (CLARKE and KELLY, 1964, p. 150).
c) Relation to Biological Studies
The primary significance of irradiance measurements today lies in their usefulness
to the interpretation of productivity. A standard procedure that can provide adequate
data on depths of relative extinction is still not available to many of those interested in
such information. It has remained easier for many people to sample the productivity
throughout the euphotic zone at standard depth intervals which only approximate the
preferred depths of relative light extinction.
It is common practice for primary productivity workers to estimate the depth of the
euphotic zone - the depth reached by 1% of the ambient radiant energy - by multiplying
the Secchi disk depth by 3. HOLMES (1970) suggests that a factor of 3.5 is more appropriate for turbid waters where the depth reading is under 5 m, and that a factor of 2.0
would apply for depths between 5 and 12 m in his data. Thus, the relationship between
Secchi disk depth and the 1 % optical depth merits additional study.
The behavioral aspects of light penetration are currently under intensive investigation.
DUNlLEY (1962) noted that there is no simple conversion between Secchi disk data and
the sighting ranges of other objects. MURPHY (1959) felt that Secchi disk data had only
limited value in the evaluation of the vision of tuna or their ability to sense fishing gear.
"The evidence shows that trolling, which depends on a positive visual response, is most
efficient in clear water, whereas gill netting, which is dependent on non-response, is most
efficient in turbid water ... Water clarity may be an important ecological factor in the
ocean in that the efficiency of sight feeders will be reduced as turbidity increases."
(MURPHY, 1959, p. 86).
In an analysis of the distribution of primary production and chlorophyll a by RYTHER
et al. (1966), it is suggested that much of the observed light extinction, especially in areas
of patchy phytoplankton distribution, may be due to non-living organisms. MENZEL
(1967) and NEWELL and KERR (1968) agree, however, that the surface standing stock of
organic particles has no influence on the concentration of particles found at depth. In
fact, a pronounced minimum around 200 m is found just at those depths where minimum
stability might be expected to result in maximum accumulation of surface materials. It is
accordingly suggested that the amount of organic particles present at any depth is a
function of advection rather than sedimentation (NEWELL and KERR, 1968).
JERLOV (1953) has reported a high particle content in the divergence between the
South Equatorial Current of the Indian Ocean and the countercurrent and in the region
of the equatorial divergence. GORDEYEV (1964) reported the concentration of suspended
matter in surface layers from stations of R V "Vityaz" to show a range from 0.3 -1.0 g
m -3, while the remaining areas - Red Sea, Andaman Sea, Bay of Bengal, northern part of
the Arabian Sea, equatorial region and the region south of 38 0 30' S contained more
than 0.5 g m -3. Below the surface, the waters of the Indian Ocean have been shown to
contain from 1- 2 g m -3 of suspension; toward the continental slopes the concentration
reached 2-4 g m- 3 and sometimes 10 g m- 3 (LISITZIN, 1960b). Furthermore, SEROVA
D. A. MCGILL:
Ocean. The least transparent water found (at Station 145 A: 10° 39' N, 60° 07' E) was
estimated to allow daylight perception to about 700 m in deep sea fish. "These values
indicate the ranges of depths down to which daylight would be sufficient for vision and
for the control of vertical diurnal migration." (CLARKE and KELLY, 1964, p. 150).
c) Relation to Biological Studies
The primary significance of irradiance measurements today lies in their usefulness
to the interpretation of productivity. A standard procedure that can provide adequate
data on depths of relative extinction is still not available to many of those interested in
such information. It has remained easier for many people to sample the productivity
throughout the euphotic zone at standard depth intervals which only approximate the
preferred depths of relative light extinction.
It is common practice for primary productivity workers to estimate the depth of the
euphotic zone - the depth reached by 1% of the ambient radiant energy - by multiplying
the Secchi disk depth by 3. HOLMES (1970) suggests that a factor of 3.5 is more appropriate for turbid waters where the depth reading is under 5 m, and that a factor of 2.0
would apply for depths between 5 and 12 m in his data. Thus, the relationship between
Secchi disk depth and the 1 % optical depth merits additional study.
The behavioral aspects of light penetration are currently under intensive investigation.
DUNlLEY (1962) noted that there is no simple conversion between Secchi disk data and
the sighting ranges of other objects. MURPHY (1959) felt that Secchi disk data had only
limited value in the evaluation of the vision of tuna or their ability to sense fishing gear.
"The evidence shows that trolling, which depends on a positive visual response, is most
efficient in clear water, whereas gill netting, which is dependent on non-response, is most
efficient in turbid water ... Water clarity may be an important ecological factor in the
ocean in that the efficiency of sight feeders will be reduced as turbidity increases."
(MURPHY, 1959, p. 86).
In an analysis of the distribution of primary production and chlorophyll a by RYTHER
et al. (1966), it is suggested that much of the observed light extinction, especially in areas
of patchy phytoplankton distribution, may be due to non-living organisms. MENZEL
(1967) and NEWELL and KERR (1968) agree, however, that the surface standing stock of
organic particles has no influence on the concentration of particles found at depth. In
fact, a pronounced minimum around 200 m is found just at those depths where minimum
stability might be expected to result in maximum accumulation of surface materials. It is
accordingly suggested that the amount of organic particles present at any depth is a
function of advection rather than sedimentation (NEWELL and KERR, 1968).
JERLOV (1953) has reported a high particle content in the divergence between the
South Equatorial Current of the Indian Ocean and the countercurrent and in the region
of the equatorial divergence. GORDEYEV (1964) reported the concentration of suspended
matter in surface layers from stations of R V "Vityaz" to show a range from 0.3 -1.0 g
m -3, while the remaining areas - Red Sea, Andaman Sea, Bay of Bengal, northern part of
the Arabian Sea, equatorial region and the region south of 38 0 30' S contained more
than 0.5 g m -3. Below the surface, the waters of the Indian Ocean have been shown to
contain from 1- 2 g m -3 of suspension; toward the continental slopes the concentration
reached 2-4 g m- 3 and sometimes 10 g m- 3 (LISITZIN, 1960b). Furthermore, SEROVA