518
D. J. TRANTER:
momentum, in the subsequent history of the hybrid food web. This could be part of the
reason why seasonal amplitude at the micronekton level is particularly low.
b) Recycling Mechanisms
Where processes are rapid and time lags are short, as in warm-water pelagic ecosystems
(CUSHING, 1958), there are many avenues for re-use of energy and nutrients. The present
study throws some light on 2 such mechanisms in particular - recycling of nutrients, and
recycling of detritus.
Evidence has accumulated over the last 10 years or so that zooplankton excretion
plays a major role in nutrient regeneration. HARRIS (1959) calculated that zooplankton
excretion of nitrogen compounds (particularly ammonia) accounted for 77% of the
average daily phytoplankton demand for this element during the spring bloom and over
40% of the demand throughout the year. CONOVER, MARSHALL and ORR (1959) showed
that Calanus in captivity excreted large amounts of inorganic phosphorus in soluble form,
and drew attention to earlier observations by GARDINER (1937) that zooplankton kept in
seawater for 3 hours increased the dissolved phosphate content of the water by
300-600%. The significance of these various results was emphasized by KETCHUM
(1963).
Independently, REID (1962) made the surprising discovery that zooplankton volumes
were highly correlated with inorganic phosphate, even in quite small tongues of water; he
concluded that "any reasonable allowance of time lag for growth renders this close
agreement baffling". It is now becoming clear that, whatever indirect relationships might
exist, this particular correlation is largely the result of a direct cause and effect relationship
between zooplankton and nutrients by way of zooplankton excretion and remineralization
of nutrients (CUSHING, 1969). If zooplankton excretion is an important avenue of nutrient
regeneration in cool waters, how much more likely is it to play a crucial role in the
productivity of warm waters!
The present results provide evidence of this. Perhaps for the first time in a field
situation, measures of chlorophyll and productivity are available, which demonstrate that
zooplankton is the cause, and nutrients the effect, and not the reverse. The fact that the
zooplankton-nutrients correlation was significant in the tropics and not the subtropics
is of interest. Perhaps zooplankton played a greater role in nutrient mineralization in the
tropics because here there was more continuous nutrient enrichment through the shallow
density discontinuity leading to more continuous algal production. In the subtropics
where the algal supply was less continuous (as evidenced by the fact that zooplankton
and phytoplankton were often out of phase), grazing may have resulted in less immediate
waste, hence less nutrient release in excretion. If this were the case, the consequent time
lag due to the slower remineralization of particulate matter would tend to perpetuate the
phase difference between phytoplankton and zooplankton.
Let us now look at the second avenue of recycling, i.e. the re-use of detritus.
Total particulate carbon in the study area is low by comparison with other seas
HAGMEIER, 1964 b; NEWELL, 1969; ZEITZSCHEL, 1970; FINENKO and ZAIKA, 1969). The
detrital component of this carbon averaged barely twice the living phytoplankton carbon
(Table 2); the average ratio is 5 (PARSONS, 1963) and in some upwelling areas it is as high
as 10 (RYlliER and MENZEW, 1965; ZEITZSCHEW, 1970). NEYMAN et al. (1973, this
volume) has indicated that detritus is the staple diet of the open ocean bottom fauna.
It can be shown, as follows, that part of this detritus is used by zooplankton because
D. J. TRANTER:
momentum, in the subsequent history of the hybrid food web. This could be part of the
reason why seasonal amplitude at the micronekton level is particularly low.
b) Recycling Mechanisms
Where processes are rapid and time lags are short, as in warm-water pelagic ecosystems
(CUSHING, 1958), there are many avenues for re-use of energy and nutrients. The present
study throws some light on 2 such mechanisms in particular - recycling of nutrients, and
recycling of detritus.
Evidence has accumulated over the last 10 years or so that zooplankton excretion
plays a major role in nutrient regeneration. HARRIS (1959) calculated that zooplankton
excretion of nitrogen compounds (particularly ammonia) accounted for 77% of the
average daily phytoplankton demand for this element during the spring bloom and over
40% of the demand throughout the year. CONOVER, MARSHALL and ORR (1959) showed
that Calanus in captivity excreted large amounts of inorganic phosphorus in soluble form,
and drew attention to earlier observations by GARDINER (1937) that zooplankton kept in
seawater for 3 hours increased the dissolved phosphate content of the water by
300-600%. The significance of these various results was emphasized by KETCHUM
(1963).
Independently, REID (1962) made the surprising discovery that zooplankton volumes
were highly correlated with inorganic phosphate, even in quite small tongues of water; he
concluded that "any reasonable allowance of time lag for growth renders this close
agreement baffling". It is now becoming clear that, whatever indirect relationships might
exist, this particular correlation is largely the result of a direct cause and effect relationship
between zooplankton and nutrients by way of zooplankton excretion and remineralization
of nutrients (CUSHING, 1969). If zooplankton excretion is an important avenue of nutrient
regeneration in cool waters, how much more likely is it to play a crucial role in the
productivity of warm waters!
The present results provide evidence of this. Perhaps for the first time in a field
situation, measures of chlorophyll and productivity are available, which demonstrate that
zooplankton is the cause, and nutrients the effect, and not the reverse. The fact that the
zooplankton-nutrients correlation was significant in the tropics and not the subtropics
is of interest. Perhaps zooplankton played a greater role in nutrient mineralization in the
tropics because here there was more continuous nutrient enrichment through the shallow
density discontinuity leading to more continuous algal production. In the subtropics
where the algal supply was less continuous (as evidenced by the fact that zooplankton
and phytoplankton were often out of phase), grazing may have resulted in less immediate
waste, hence less nutrient release in excretion. If this were the case, the consequent time
lag due to the slower remineralization of particulate matter would tend to perpetuate the
phase difference between phytoplankton and zooplankton.
Let us now look at the second avenue of recycling, i.e. the re-use of detritus.
Total particulate carbon in the study area is low by comparison with other seas
HAGMEIER, 1964 b; NEWELL, 1969; ZEITZSCHEL, 1970; FINENKO and ZAIKA, 1969). The
detrital component of this carbon averaged barely twice the living phytoplankton carbon
(Table 2); the average ratio is 5 (PARSONS, 1963) and in some upwelling areas it is as high
as 10 (RYlliER and MENZEW, 1965; ZEITZSCHEW, 1970). NEYMAN et al. (1973, this
volume) has indicated that detritus is the staple diet of the open ocean bottom fauna.
It can be shown, as follows, that part of this detritus is used by zooplankton because