PLANKTON IN NITROGEN AND PHOSPHORUS CYCLES
161
ganic phosphate. The value in spring was 16.9% and that in autumn
200%. Unlike nitrogen, excess phosphorus supplied by the animals in
autumn did not accumulate in the sea.
Geen (1965), using the 14C method of measuring primary production,
estimated the average summer rates in Bras d’Or Lake and Morrison’s
Pond as 300 and 600 pg C/m3 day respectively. From these data Hargrave
and Geen (1968), assuming a C : P ratio for phytoplankton of 40 :1 by
weight, calculated that 7.5 and 15 mg of inorganic phosphate were
required daily to support photosynthesis by phytoplankton in these
two sea areas. The total excretion of inorganic phosphate by the zooplankton population, estimated in the usual way, was 14.6 mg P/m2/day
in the 20-m euphotic zone in Bras #Or Lake and 3.75 mg P/m2/day in
the 3-m cuphotic zone in Morrison’s Pond. Thus, zooplankton supplied
195% of the daily phosphorus requirement of the phytoplankton in
Bras d’Or Lake and 25% of that in Morrison’s Pond.
These various estimates of the quantitative importance of the
nitrogen excreted as ammonia, and phosphorus excreted as inorganic
phosphate by zooplankton in supplying the nitrogen and phosphorus
needed by the phytoplankton are complicated by the fact that: (a)
some of the nitrogen and much of the phosphorus excreted by the
animals may be present in an organic form and it is possible that these
substances may also be used by the plants ; (b) microzooplankton may
not be captured in the tow-nets, yet nutrient excretion by these small
animals may be of considerable importance (Conover, 1961 ; Johannes,
1964b) ; and (c) nutrients excreted by benthic organisms may be brought
into the euphotic zone by turbulence and further quantities excreted
directly into the zone by fish. Nevertheless, in most of the sea areas
studied nutrient regeneration by zooplankton appears to be of considerable, if not of primary importance in supplying the nitrogen and phosphorus requirements of the plants on which the animals feed. Moreover,
Ketchum (1962) has drawn attention t o the contribution of migrating
zooplankton to the vertical distribution of soluble nitrogen and phosphorus in the sea. Thus, some animals feed actively at the surface and
later excrete these nutrients in the depths : others feed actively in the
depths and later excrete nutrients near the surface.
We have already mentioned that the average atomic ratio N : P in
phytoplankton is about 16 :1 (Section 111), a value significantly lower
than the average of 25 :1 found for zooplankton (Section IX). Ketchum
(1962) has pointed out that zooplankton feeding on phytoplankton and
retaining excess nitrogen in accordance with their tissue composition
must show a relatively low N : P ratio in their soluble excretion products.
Few data are available, but those obtained so far are consistent with
161
ganic phosphate. The value in spring was 16.9% and that in autumn
200%. Unlike nitrogen, excess phosphorus supplied by the animals in
autumn did not accumulate in the sea.
Geen (1965), using the 14C method of measuring primary production,
estimated the average summer rates in Bras d’Or Lake and Morrison’s
Pond as 300 and 600 pg C/m3 day respectively. From these data Hargrave
and Geen (1968), assuming a C : P ratio for phytoplankton of 40 :1 by
weight, calculated that 7.5 and 15 mg of inorganic phosphate were
required daily to support photosynthesis by phytoplankton in these
two sea areas. The total excretion of inorganic phosphate by the zooplankton population, estimated in the usual way, was 14.6 mg P/m2/day
in the 20-m euphotic zone in Bras #Or Lake and 3.75 mg P/m2/day in
the 3-m cuphotic zone in Morrison’s Pond. Thus, zooplankton supplied
195% of the daily phosphorus requirement of the phytoplankton in
Bras d’Or Lake and 25% of that in Morrison’s Pond.
These various estimates of the quantitative importance of the
nitrogen excreted as ammonia, and phosphorus excreted as inorganic
phosphate by zooplankton in supplying the nitrogen and phosphorus
needed by the phytoplankton are complicated by the fact that: (a)
some of the nitrogen and much of the phosphorus excreted by the
animals may be present in an organic form and it is possible that these
substances may also be used by the plants ; (b) microzooplankton may
not be captured in the tow-nets, yet nutrient excretion by these small
animals may be of considerable importance (Conover, 1961 ; Johannes,
1964b) ; and (c) nutrients excreted by benthic organisms may be brought
into the euphotic zone by turbulence and further quantities excreted
directly into the zone by fish. Nevertheless, in most of the sea areas
studied nutrient regeneration by zooplankton appears to be of considerable, if not of primary importance in supplying the nitrogen and phosphorus requirements of the plants on which the animals feed. Moreover,
Ketchum (1962) has drawn attention t o the contribution of migrating
zooplankton to the vertical distribution of soluble nitrogen and phosphorus in the sea. Thus, some animals feed actively at the surface and
later excrete these nutrients in the depths : others feed actively in the
depths and later excrete nutrients near the surface.
We have already mentioned that the average atomic ratio N : P in
phytoplankton is about 16 :1 (Section 111), a value significantly lower
than the average of 25 :1 found for zooplankton (Section IX). Ketchum
(1962) has pointed out that zooplankton feeding on phytoplankton and
retaining excess nitrogen in accordance with their tissue composition
must show a relatively low N : P ratio in their soluble excretion products.
Few data are available, but those obtained so far are consistent with