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Short-term variation of ammonia in the Sargasso Sea off Bermuda
Deep-Sea Research, 1965, Vol. 12, pp. 21 to 25. PergamonPress Ltd. Printed in Great Britain.
Short-term variation of ammonia in the Sargasso Sea off Bermuda* JOHN R . BEERS a n d ALISON C. KELLY Bermuda Biological Station for Research, Inc., St. George's West, Bermuda
(Received 30 June 1964)
Abstract--Short-term variations in ammonia in the upper 500 m of the Sargasso Sea off Bermuda have been investigated at approximately 4 hr intervals over a 48-hr period. The" average "ammonia concentration in the euphotic zone ranged from 0"37-0"97/~gAN'H3-N/L and was high during midand late-morning. Low "average" levels for each day were observed near midnight. From 200500 m the "average" ranged from 0.49-0.92/~gANH3-N/L. The maximum occurred just after mid-day and the minimum was reached in the early-morning. The variations in ammonia may be correlated with diurnal behaviour of the plankton populations.
INTRODUCTION
THE diurnal variation of certain biological and chemical properties of the Sargasso Sea waters was studied by RYTHER, et aL, (1961). Levels of both nitrate and phosphate integrated from the surface to 150 m were found to be significantly higher at night than during the day. Relative to nitrate-N and nitrite-N, ammonia-N is present generally in considerably higher concentration in the euphotic zone of the Sargasso Sea (MENZEL and SPAETH,1962). Many phytoplankton organisms can use ammonia as a source of nitrogen (HARRIS, 1959; SYRETT, 1962; GUILLARD, 1963). In addition, evidence indicates a relatively rapid turnover of dissolved ammonia in the euphotic zone (e.g. HARRIS, 1959). DUGDALE and DUGOALE (1962) estimated that, in the absence of recycling mechanisms, less than 4 days would be required to deplete the ammonia present at 10 m in the Sargasso Sea. Ammonia is the chief nitrogenous excretory product of many marine invertebrates including zooplankton (PRoSSER et al., 1950; PROSSER and BROWN, 1961). Recently, KETCHUM (1962) proposed that ammonia excretion by zooplankton plays an important role in the regeneration of nitrogen in the sea. REDFIELD and KEYS (1938) were able to show tentative correlation between zooplankton distribution and the ammonia level in the Gulf of Maine. In the present study, short-term diurnal variations in the ammonia concentration in the upper 500 m of the Sargasso Sea off Bermuda have been investigated to determine if ammonia levels at given depths can be correlated with the diurnal behaviour of the plankton populations. *Contribution No. 359 from the Bermuda Biological Station under contract AT (30-1)-2646 with the Atomic Energy Commission. Also supported, in part, through National Science Foundation grant 20952 to the U.S. Program in Biology, International Indian Ocean Expedition Training Program.
21
22
JOHN R. BEERS and ALISON C. KELLY MATERIALS
AND
METHODS
A series of hydrographic stations were made from the Woods Hole Oceanographic Institution vessel, Atlantis, in November, 1962, at a location approximately 14 miles southeast of Bermuda (32 ° 10'N; 64 ° 30'W) in 1600fms of water. This site has been occupied by the Bermuda Biological Station's vessel, Panulirus, for routine hydrographic work at bi-weekly intervals since 1954. Water samples, obtained with Nansen reversing bottles, were taken approximately every 4 hr for 48 hr at selected depths in the upper 500 m. Samples for ammonia determinations were frozen in polyethylene bottles, and subsequently measured by the colorimetric method of KRUSE and MELLON (1952). An additional optical density reading at 600 m/~ was used as a " turbidity " correction (MENZEL, unpublished). The standard deviation of a series of twelve replicate standards containing 1.00 tLgA NH3-N/L was ± 0.13. RESULTS
The results of all analyses are presented in Table 1. Ammonia levels ranged from a maximum of 1.67/zgA NHa-N/L to a minimum of 0.07 #gA NHa-N/L. High levels were measured in the surface water (average 0.81/zgA NH3-N/L) and at 25 m (average 0.83/zgA NH3-N/L). Low values were found at the base of the euphoric zone (100m, average 0.32/zgANH3-N/L; 150m, average 0.38/~gA NH:
~-oo 2.5
100i~
0'8., J
X(JO -
50
3
/
m /
z,J / 0 ' 6 0
i
/e' •
1'40
.
"
.
/,
I J ;
,
+
i
!:)00 1800 2400 9 NOV 1962
,
i
,
.
,
060() 1200 1800 10 ~LqV 1962 I JOl.J RS
~
,
4
~
i
,
?4'qO 0600 1200 I 11 NOV 1962
Fig. 1. T h e " a v e r a g e " a m m o n i a - N level for 12 stations t a k e n at a p p r o x i m a t e l y 4 h r intervals, 9-11 N o v e m b e r 1962. Solid l i n e - - 2 5 to 100 m ; b r o k e n l i n e - - 2 0 0 to 500 m.
The " average " concentration of ammonia in the euphotic zone, i.e. 25-100 m, and from 200-500 m are shown in Fig. 1. " Average," as used throughout this paper, is defined as the integrated value f o r / z g A N H s - N / M 2 divided by the depth interval and recorded as " a v e r a g e " /zgA NH3-N/L. Since MENZEL and SPAETH (1962) have shown that surface water values are partly dependent upon rainfall, these have been omitted from the " average " for the euphoric zone.
1.19
1.04
0.57
0-83
0.74
0.56
0.37
0-40
0-24
0"85
0"59
0"63
25 m
50 m
75 m
100 m
150 m
200m
250 m
300 m
400 m
500 m
0"28
0"61
0"55
0"70
0-31
0.40
0.16
0.56
0.87
1.03
6328
1900
6327
1449
Hour
9 NOV. 1962
Atlantis Station No. 1m
9 NOV. 1962
Date
0"37
0-72
0-86
0"34
0.18
0.28
0.07
0.70
0.79
0.71
0.64
6329
2244
9 Nov. 1962
0-24
0"74
0"49
0"41
0.30
0.48
0-13
0.71
0.81
0"69
0-83
6330
0243
10 NOV. 1962
0"60
0"62
0"34
0"37
0.48
0-48
0.52
1.19
0.79
1.35
0.93
6331
0631
10 NOV. 1962
0-50
1"08
.1-18
0-76
0.64
0.70
0.28
0-50
0-76
0-58
0"85
6332
1300
10 Nov. 1962
0.53
0"47
0"63
.
0.68
0.23
0.28
0.40
0-83
0.68
0.55
6333
1443
10 Nov. 1962
.
0"38
0-53
0"80
0-59
0.44
0.36
0.53
0"51
0-70
0.52
6334
1834
10 Nov. 1962
.
0-37
0"54
0"70
.
0.55
0.21
0.23
0-30
0.34
0.71
0-54
6335
2248
10 Nov. 1962
0-45
0"53
0"60
.
0.33
0.27
0-07
0.65
0-57
0.48
0.96
6336
0240
11 Nov. 1962
.
0-24
0"74
0"96
0.59
0.04
0.23
--
0.76
0.78
0-96
6337
0649
11 NOV. 1962
Table 1. The concentration of ammonia nitrogen in Sargasso Sea water, 9-11 November, 1962. (#gA NHa-N/L).
0-71
0.43
0"57
0"97
0.71
0.89
0.46
0-76
1.67
0-68
6338
1031
11 NOV. 1962
b9
ga
o
g~
o~
K
2.
¢b
24
JoH~ R. B~ERS and ALISON C.
KELLY
The curves for the two 24-hr periods show marked similarities in general shape. Ammonia reached its high level in the euphotic zone in the mid- and late-morning hours. This was followed by a decrease with minimum values around midnight. The values from 200-500 m showed a similar pattern but the maximum level of ammonia was observed just after mid-day and it reached its minimal level in the early-morning hours. DISCUSSION
Ammonia is removed from solution in sea water through assimilation by phototrophic phytoplankton and heterotrophic or chemotrophic microorganisms, and it is added to the water by the excretion of marine organisms and decomposition of organic nitrogen compounds. Our data suggest a tentative correlation between the observed short-term variations in ammonia in the euphotic zone and previously observed diurnal behaviour of the plankton populations. Most zooplankton species in the Sargasso Sea off Bermuda which show a diurnal vertical migration move upwards in the early evening hours (MOORE, 1949). Presumably these begin to feed immediately. Since the passage of food through the gut has been found to take only approximately l hr in species of chaetognaths (DAVID, 1955) and copepods (DELALO, 1961), an increase in the ammonia level of the water might be expected to parallel, with only a short lag, the onset of their feeding activities. However, ammonia in the euphotic zone showed a decline through the evening until it reached minimum values near midnight. This may be due, in part, to assimilation by phytoplankton and other microorganisms. SYRETT (1962) reported that nitrogen deficient Chlorella vulgaris will assimilate ammonia-N in the dark until they deplete their carbohydrate reserves. GOERING,et al. (1964) give evidence of the uptake of ammonia-N in the dark by natural populations in the Sargasso Sea. High dark uptake in the early-evening hours relative to other times of the day was observed on the first day of their 48-hr study of diurnal variations in the uptake of ammonia. Factors which could contribute to the increase in ammonia concentration in the early-morning hours include an accelerated excretion rate by zooplankton, accumulation of excretory products, and a possible decline in the uptake by phytoplankton. The drop in level during the late forenoon and afternoon can be ascribed, in part, to uptake by actively photosynthesizing phytoplankton. It is similarly difficult to speculate on the factors underlying the variations in ammonia between 200 and 500m. However, the mid-morning increase until approximately noon could be due to the metabolic and excretory activity of the zooplankton, many of which fed during the night in the upper 100 m. The rapid decline of ammonia during the afternoon may possibly be due, in part, to nonbiological oxidation of ammonia to nitrite and nitrate (VACCARO, 1961).
REFERENCES
DAVIDP. (1955) Distribution of Sagitta gazellae Ritter-Zahony. Discovery Rept. 27, 235-278. D~LAIO E. P. (1961) Preliminary data on the feeding of Paracalanus parvus (Claus) in the Black Sea. (In Russian). Tr. Sevastopol'sk Biol. Sta. Akad. Nauk. SSSR., 14, 126-134. (Biol. Abstr. 1963. 43, 715).
Short-term variation of arnmonia in the Sargasso Sea off Bermuda
25
DUGDALE R. C. and DUGDALEV. A. (1962)Nitrate and ammonia uptake in the Sargasso Sea. Bermuda Biological S t a t i o n - A t o m i c Energy Commission Rept. Contr. AT(30-1)-2646. Unpublished Ms. GOERING J. J., DUGDALE R. C. and MENZEL D. W. (1964) Cyclic diurnal variations in the uptake of ammonia and nitrate by photosynthetic organisms in the Sargasso Sea. Woods Hole Oceanographic Institution Reference No. 64-8. Unpublished Ms. GUILLARD R. R. L. (1963) Organic sources of nitrogen for marine centric diatoms. In : Symposium on Marine Microbiology (Carl H. Oppenheimer, ed.), Charles C. Thomas, Springfield, Illinois, 93-104. HARRIS E. (1959) The nitrogen cycle in Long Island Sound. Bull. Bingham Oceanogr. Coll., 17, 31-65. KETCHUM B. H. (1962) Regeneration of nutrients by zooplankton. Rapp. et Proc. Verb. Cons. Int. Explor. Met., 153, 142-147. KgusE J. and MELLON M. G. (1952) Colorimetric determination of free ammonia with a pyridine-pyrazolone reagent. Sewage and Industrial Wastes. 24, 1098-I I00. MENZEL D. W. and SPAETH J. P. (1962) The occurrence of ammonia in Sargasso Sea waters and in rain water at Bermuda. Limnol. and Oceanogr., 7, 159-162. MOORE H. B. (1949) The zooplankton of the upper waters of the Bermuda area of the North Atlantic. Bull. Bingham Oceanogr. Coll., 12, 1-97. PROSSER C. L. (Ed.) (1950) Comparative Animal Physiology. W. B. Saunders Co., Philadelphia, 888 pp. PROSSER C. L. and BROWN F. A., JR. (1961) Comparative Animal Physiology. W . B . Saunders Co., Philadelphia, 688 pp. REDFIELD A. C. and KEYS A. B. (1938) The distribution of ammonia in the waters of the Gulf of Maine. Biol. Bull., 74, 83-92. RYTHER J. H., MENZEL D. W. and VACCARO R. F. (1961) Diurnal variations in some chemical and biological properties of the Sargasso Sea. Limnol. and Oceanogr., 6, 149-153. SYRETT P. T. (1962) Nitrogen assimilation. In : Physiology and biochemistry of algae (R. A. Lewis, ed.), Academic Press, New York, 161-188. VACCARO R. F. (1961) The oxidation of ammonia in sea water. J. du Conseil., 27, 1-14.
Short-term variation of ammonia in the Sargasso Sea off Bermuda* JOHN R . BEERS a n d ALISON C. KELLY Bermuda Biological Station for Research, Inc., St. George's West, Bermuda
(Received 30 June 1964)
Abstract--Short-term variations in ammonia in the upper 500 m of the Sargasso Sea off Bermuda have been investigated at approximately 4 hr intervals over a 48-hr period. The" average "ammonia concentration in the euphotic zone ranged from 0"37-0"97/~gAN'H3-N/L and was high during midand late-morning. Low "average" levels for each day were observed near midnight. From 200500 m the "average" ranged from 0.49-0.92/~gANH3-N/L. The maximum occurred just after mid-day and the minimum was reached in the early-morning. The variations in ammonia may be correlated with diurnal behaviour of the plankton populations.
INTRODUCTION
THE diurnal variation of certain biological and chemical properties of the Sargasso Sea waters was studied by RYTHER, et aL, (1961). Levels of both nitrate and phosphate integrated from the surface to 150 m were found to be significantly higher at night than during the day. Relative to nitrate-N and nitrite-N, ammonia-N is present generally in considerably higher concentration in the euphotic zone of the Sargasso Sea (MENZEL and SPAETH,1962). Many phytoplankton organisms can use ammonia as a source of nitrogen (HARRIS, 1959; SYRETT, 1962; GUILLARD, 1963). In addition, evidence indicates a relatively rapid turnover of dissolved ammonia in the euphotic zone (e.g. HARRIS, 1959). DUGDALE and DUGOALE (1962) estimated that, in the absence of recycling mechanisms, less than 4 days would be required to deplete the ammonia present at 10 m in the Sargasso Sea. Ammonia is the chief nitrogenous excretory product of many marine invertebrates including zooplankton (PRoSSER et al., 1950; PROSSER and BROWN, 1961). Recently, KETCHUM (1962) proposed that ammonia excretion by zooplankton plays an important role in the regeneration of nitrogen in the sea. REDFIELD and KEYS (1938) were able to show tentative correlation between zooplankton distribution and the ammonia level in the Gulf of Maine. In the present study, short-term diurnal variations in the ammonia concentration in the upper 500 m of the Sargasso Sea off Bermuda have been investigated to determine if ammonia levels at given depths can be correlated with the diurnal behaviour of the plankton populations. *Contribution No. 359 from the Bermuda Biological Station under contract AT (30-1)-2646 with the Atomic Energy Commission. Also supported, in part, through National Science Foundation grant 20952 to the U.S. Program in Biology, International Indian Ocean Expedition Training Program.
21
22
JOHN R. BEERS and ALISON C. KELLY MATERIALS
AND
METHODS
A series of hydrographic stations were made from the Woods Hole Oceanographic Institution vessel, Atlantis, in November, 1962, at a location approximately 14 miles southeast of Bermuda (32 ° 10'N; 64 ° 30'W) in 1600fms of water. This site has been occupied by the Bermuda Biological Station's vessel, Panulirus, for routine hydrographic work at bi-weekly intervals since 1954. Water samples, obtained with Nansen reversing bottles, were taken approximately every 4 hr for 48 hr at selected depths in the upper 500 m. Samples for ammonia determinations were frozen in polyethylene bottles, and subsequently measured by the colorimetric method of KRUSE and MELLON (1952). An additional optical density reading at 600 m/~ was used as a " turbidity " correction (MENZEL, unpublished). The standard deviation of a series of twelve replicate standards containing 1.00 tLgA NH3-N/L was ± 0.13. RESULTS
The results of all analyses are presented in Table 1. Ammonia levels ranged from a maximum of 1.67/zgA NHa-N/L to a minimum of 0.07 #gA NHa-N/L. High levels were measured in the surface water (average 0.81/zgA NH3-N/L) and at 25 m (average 0.83/zgA NH3-N/L). Low values were found at the base of the euphoric zone (100m, average 0.32/zgANH3-N/L; 150m, average 0.38/~gA NH:
~-oo 2.5
100i~
0'8., J
X(JO -
50
3
/
m /
z,J / 0 ' 6 0
i
/e' •
1'40
.
"
.
/,
I J ;
,
+
i
!:)00 1800 2400 9 NOV 1962
,
i
,
.
,
060() 1200 1800 10 ~LqV 1962 I JOl.J RS
~
,
4
~
i
,
?4'qO 0600 1200 I 11 NOV 1962
Fig. 1. T h e " a v e r a g e " a m m o n i a - N level for 12 stations t a k e n at a p p r o x i m a t e l y 4 h r intervals, 9-11 N o v e m b e r 1962. Solid l i n e - - 2 5 to 100 m ; b r o k e n l i n e - - 2 0 0 to 500 m.
The " average " concentration of ammonia in the euphotic zone, i.e. 25-100 m, and from 200-500 m are shown in Fig. 1. " Average," as used throughout this paper, is defined as the integrated value f o r / z g A N H s - N / M 2 divided by the depth interval and recorded as " a v e r a g e " /zgA NH3-N/L. Since MENZEL and SPAETH (1962) have shown that surface water values are partly dependent upon rainfall, these have been omitted from the " average " for the euphoric zone.
1.19
1.04
0.57
0-83
0.74
0.56
0.37
0-40
0-24
0"85
0"59
0"63
25 m
50 m
75 m
100 m
150 m
200m
250 m
300 m
400 m
500 m
0"28
0"61
0"55
0"70
0-31
0.40
0.16
0.56
0.87
1.03
6328
1900
6327
1449
Hour
9 NOV. 1962
Atlantis Station No. 1m
9 NOV. 1962
Date
0"37
0-72
0-86
0"34
0.18
0.28
0.07
0.70
0.79
0.71
0.64
6329
2244
9 Nov. 1962
0-24
0"74
0"49
0"41
0.30
0.48
0-13
0.71
0.81
0"69
0-83
6330
0243
10 NOV. 1962
0"60
0"62
0"34
0"37
0.48
0-48
0.52
1.19
0.79
1.35
0.93
6331
0631
10 NOV. 1962
0-50
1"08
.1-18
0-76
0.64
0.70
0.28
0-50
0-76
0-58
0"85
6332
1300
10 Nov. 1962
0.53
0"47
0"63
.
0.68
0.23
0.28
0.40
0-83
0.68
0.55
6333
1443
10 Nov. 1962
.
0"38
0-53
0"80
0-59
0.44
0.36
0.53
0"51
0-70
0.52
6334
1834
10 Nov. 1962
.
0-37
0"54
0"70
.
0.55
0.21
0.23
0-30
0.34
0.71
0-54
6335
2248
10 Nov. 1962
0-45
0"53
0"60
.
0.33
0.27
0-07
0.65
0-57
0.48
0.96
6336
0240
11 Nov. 1962
.
0-24
0"74
0"96
0.59
0.04
0.23
--
0.76
0.78
0-96
6337
0649
11 NOV. 1962
Table 1. The concentration of ammonia nitrogen in Sargasso Sea water, 9-11 November, 1962. (#gA NHa-N/L).
0-71
0.43
0"57
0"97
0.71
0.89
0.46
0-76
1.67
0-68
6338
1031
11 NOV. 1962
b9
ga
o
g~
o~
K
2.
¢b
24
JoH~ R. B~ERS and ALISON C.
KELLY
The curves for the two 24-hr periods show marked similarities in general shape. Ammonia reached its high level in the euphotic zone in the mid- and late-morning hours. This was followed by a decrease with minimum values around midnight. The values from 200-500 m showed a similar pattern but the maximum level of ammonia was observed just after mid-day and it reached its minimal level in the early-morning hours. DISCUSSION
Ammonia is removed from solution in sea water through assimilation by phototrophic phytoplankton and heterotrophic or chemotrophic microorganisms, and it is added to the water by the excretion of marine organisms and decomposition of organic nitrogen compounds. Our data suggest a tentative correlation between the observed short-term variations in ammonia in the euphotic zone and previously observed diurnal behaviour of the plankton populations. Most zooplankton species in the Sargasso Sea off Bermuda which show a diurnal vertical migration move upwards in the early evening hours (MOORE, 1949). Presumably these begin to feed immediately. Since the passage of food through the gut has been found to take only approximately l hr in species of chaetognaths (DAVID, 1955) and copepods (DELALO, 1961), an increase in the ammonia level of the water might be expected to parallel, with only a short lag, the onset of their feeding activities. However, ammonia in the euphotic zone showed a decline through the evening until it reached minimum values near midnight. This may be due, in part, to assimilation by phytoplankton and other microorganisms. SYRETT (1962) reported that nitrogen deficient Chlorella vulgaris will assimilate ammonia-N in the dark until they deplete their carbohydrate reserves. GOERING,et al. (1964) give evidence of the uptake of ammonia-N in the dark by natural populations in the Sargasso Sea. High dark uptake in the early-evening hours relative to other times of the day was observed on the first day of their 48-hr study of diurnal variations in the uptake of ammonia. Factors which could contribute to the increase in ammonia concentration in the early-morning hours include an accelerated excretion rate by zooplankton, accumulation of excretory products, and a possible decline in the uptake by phytoplankton. The drop in level during the late forenoon and afternoon can be ascribed, in part, to uptake by actively photosynthesizing phytoplankton. It is similarly difficult to speculate on the factors underlying the variations in ammonia between 200 and 500m. However, the mid-morning increase until approximately noon could be due to the metabolic and excretory activity of the zooplankton, many of which fed during the night in the upper 100 m. The rapid decline of ammonia during the afternoon may possibly be due, in part, to nonbiological oxidation of ammonia to nitrite and nitrate (VACCARO, 1961).
REFERENCES
DAVIDP. (1955) Distribution of Sagitta gazellae Ritter-Zahony. Discovery Rept. 27, 235-278. D~LAIO E. P. (1961) Preliminary data on the feeding of Paracalanus parvus (Claus) in the Black Sea. (In Russian). Tr. Sevastopol'sk Biol. Sta. Akad. Nauk. SSSR., 14, 126-134. (Biol. Abstr. 1963. 43, 715).
Short-term variation of arnmonia in the Sargasso Sea off Bermuda
25
DUGDALE R. C. and DUGDALEV. A. (1962)Nitrate and ammonia uptake in the Sargasso Sea. Bermuda Biological S t a t i o n - A t o m i c Energy Commission Rept. Contr. AT(30-1)-2646. Unpublished Ms. GOERING J. J., DUGDALE R. C. and MENZEL D. W. (1964) Cyclic diurnal variations in the uptake of ammonia and nitrate by photosynthetic organisms in the Sargasso Sea. Woods Hole Oceanographic Institution Reference No. 64-8. Unpublished Ms. GUILLARD R. R. L. (1963) Organic sources of nitrogen for marine centric diatoms. In : Symposium on Marine Microbiology (Carl H. Oppenheimer, ed.), Charles C. Thomas, Springfield, Illinois, 93-104. HARRIS E. (1959) The nitrogen cycle in Long Island Sound. Bull. Bingham Oceanogr. Coll., 17, 31-65. KETCHUM B. H. (1962) Regeneration of nutrients by zooplankton. Rapp. et Proc. Verb. Cons. Int. Explor. Met., 153, 142-147. KgusE J. and MELLON M. G. (1952) Colorimetric determination of free ammonia with a pyridine-pyrazolone reagent. Sewage and Industrial Wastes. 24, 1098-I I00. MENZEL D. W. and SPAETH J. P. (1962) The occurrence of ammonia in Sargasso Sea waters and in rain water at Bermuda. Limnol. and Oceanogr., 7, 159-162. MOORE H. B. (1949) The zooplankton of the upper waters of the Bermuda area of the North Atlantic. Bull. Bingham Oceanogr. Coll., 12, 1-97. PROSSER C. L. (Ed.) (1950) Comparative Animal Physiology. W. B. Saunders Co., Philadelphia, 888 pp. PROSSER C. L. and BROWN F. A., JR. (1961) Comparative Animal Physiology. W . B . Saunders Co., Philadelphia, 688 pp. REDFIELD A. C. and KEYS A. B. (1938) The distribution of ammonia in the waters of the Gulf of Maine. Biol. Bull., 74, 83-92. RYTHER J. H., MENZEL D. W. and VACCARO R. F. (1961) Diurnal variations in some chemical and biological properties of the Sargasso Sea. Limnol. and Oceanogr., 6, 149-153. SYRETT P. T. (1962) Nitrogen assimilation. In : Physiology and biochemistry of algae (R. A. Lewis, ed.), Academic Press, New York, 161-188. VACCARO R. F. (1961) The oxidation of ammonia in sea water. J. du Conseil., 27, 1-14.