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Further observations on a thermal front in the Sargasso Sea
Deep-Sea Research, 1975, Vol. 22, pp. 433 to 439. Pergamon Press. Printed in Great Britain.
NOTE
Further observations on a thermal front in the Sargasso Sea* J. B. COLTON, JR'r, D. E. SMITH:I: and J. W. Jossl~: (Received 22 August 1974; hz revisedJbrm 11 November 1974; accepted 17 November 1974) Abstract--Thermal fronts, which are characterized by a sharp surface or near-surface temperature gradient and which separate the Sargasso Sea into cooler, more productive northern parts, and warmer, less productive southern parts, have been observed in all months except July, August, and September. Temperature and zooplankton distributional data obtained on a bathythermograph-Continuous Plankton Recorder transect made in July, 1972, established the presence of a front at about 27°30'N and 64°55 'W. A marked discontinuity in temperature occurred between 50 and 100 m. The 20°C isotherm occurred at or above I00 m north of the front and at or below 150 m south of the front. Water shallower than 50 m was well stratified both north and south of the front. The point of change in the subsurface thermal structure corresponded to a near-surface faunal change. Of 27 species of copepods occurring in 15, 10-mile Continuous Plankton Recorder sections immediately north and south of the front, 12 occurred only north of the front and 5 occurred only south of the front. The abundance and number of species of copepods were greater north of the front. It is suggested that the near-surface faunal boundaries are related to environmental conditions in the subsurface layers inhabited by the copepods during the day. INTRODUCTION
THERMAL fronts have been observed in the Sargasso Sea by VOORH1S a n d HERSEY (1964), KATZ (1969), and BACKUS, CRADDOCK,HAEDRICHand SHORES(1969). They have been found between 24 to 37°N, and 56 to 76°W (Fig. 1) and in all months except July, August, and September. These thermal fronts, which extend in an eastwest direction, are characterized by a sharp surface or near-surface temperature gradient of at least I°C/10 km (VoORH|Sand HERSEY, 1964) and separate the Sargasso Sea into cooler northern and warmer southern parts. North of the front the upper part of the water column is stratified in summer only, while south of the front it is stratified at all seasons. In addition to differences in temperature, salinity, and density, north-south dissimilarities have been observed in net primary production (RYTHER and MENZEL, 1960), the species composition and abundance of phytoplankton (HULBERT, 1964), the abundance of planktonic Foraminifera and Radiolaria (CIFELLIand SACHS, 1966) and in the species composition and abundance of mesopelagic fishes (BAcKUS, CRADDOCK, HAEDRICHand SHOR~S, 1969). The north-south differences in the biota of the Sargasso Sea have been associated with the north-south differences in primary production (BACKUS, CRADDOCK, HAEDRICHand SHORES, 1969), which in turn are *Contribution No. 19, National Marine Fisheries Service, M A R M A P Program, Washington, D.C. 20235, U.S.A. t N M F S Northeast Fisheries Center, Narragansett Laboratory, Narragansett, Rhode Island 02882, U.S.A. :~NMFS M A R M A P Field Group, Narragansett Laboratory, Narragansett, Rhode Island 02882, U.S.A. 433
J.B. COLTON,JR., D. E. SMITHand J. W. Jossl
434
i 3,' ..OULF SrREA
f
I
I
I
35 °
I
30*
25"
80 ° ,
i
i
75 r
r
i
I
i
i
i
,
i
,
65 ° i
"-,
,~
. . . . . .
~
20 J
tv
60"
t
20 ~
55*
Fig. 1. Location of bathythermograph observations (o) and Continuous Plankton Recorder samples (insert), July, 1972, and of thermal front observations (A), 1958 1967.
attributed to the north-south variations in temperature structure; the isothermal condition in the north allowing for a renewal of nutrients in the euphotic zone (RVTHER and MENZEL, 1960). In all but one of the published temperature sections across Sargasso Sea thermal fronts, waters north of the front were isothermal to a depth of at least 50 m and the location of the front coincided with a sharp north-south increase in surface temperature. In two sections made across a front in May, 1967 (KATz, 1969, Fig. 2) there was a relatively weak vertical temperature gradient in the upper 50 m of the water column north of the front. Although in this case the front separated two vertically stratified surface layers, the horizontal gradient was such that the interface of the two water masses was still evidenced by an appreciable north-south increase in surface temperature. Because there are no marked differences in surface water temperature between the northern and southern Sargasso Sea during the summer, it was originally thought that these fronts were winter phenomena (VOORHISand HERSEY,1964). On a basis of more recent observations, BACKUS, CRADDOCK,HAEDRICH and SHORES (1969) concluded that it is probable that these fronts exist year-around at subsurface depths, but they are not readily discernible solely on a basis of surface temperature observations. Temperature and zooplankton distributional data obtained on a bathythermograph--Continuous Plankton Recorder section made in July, 1972 (Albatross IV Cruise 726) support this conclusion.
Further observations on a thermal front in the Sargasso Sea
435
S A M P L I N G METHODS AND ANALYSES
The majority of temperature observations in this section (Fig. l) were made en route to the first station (23°00'N, 61°00'W) occupied by the Albatross I V during the initial M A R M A P (Marine Resources Monitoring, Assessment, and Prediction) Program ichthyoplankton survey of coastal and oceanic waters from Cape Cod to the Caribbean Sea. Expendable bathythermograph casts were made at approximately 3-h intervals. To insure positional accuracy of the isotherms and maximum resolution of the widely-spaced observations, whole degree temperatures from individual traces were plotted at their depth of occurrence, rather than interpolated from readings at discrete depths. A Hardy Continuous Plankton Recorder (CPR, HARDY, 1939) was towed at a depth of l0 m and a speed of 12 knots (ca. 22 km h -~) from a point approximately 40 miles (74 km) east of Cape Hatteras to the first M A R M A P station. A 0.5-inch (l.3-cm) square nosepiece was used and the propeller pitch set for a gauze advance rate of approximately 4 in. (10'2 cm) per l0 miles (l 8.5 km) of tow during which about 3 m 3 of water are filtered. In the laboratory the gauze (24 meshes per cm) was cut into sections representing l0 miles (18.5 km) of tow. The sections were examined using 36× magnification. Abundances were reported as the number of organisms per m 3. The locations of the gauze samples analyzed tbr this study are shown in Fig. 1. TEMPERATURE DISTRIBUTION
A temperature section and plot of temperatures at specific depths are shown in Fig. 2. The vertical scale of the temperature section is expanded with respect to the horizontal scale by a factor of about 750. There are two locations within the section at which there is a rapid change in the depths of isotherms coincident with the location of thermal fronts. One, centered at approximately 35°27'N and 74°30'W, is associated with the inshore edge of the Gulf Stream, and is delimited by the 15°C isotherm at 200 m (WORTHINGTON, 1964). The other front is located at approximately 27°30'N 55o27'N 74o30'W 0 m~ ...o a r . ~
~__~.-'~-----~
4oolllli
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25
....
29
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27~30'N 64~54' W 4 6 f--45 ; '~-
35 _ : ' - ~
/.~
.
.
.
50
54
60
.... ~_ ............. ----~-
.
---
~
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~ _
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.~
-----;;-/~6oo
28°C
[28°6
26 24
124
22
22
20 18
2O 18
16 14
tSO.w/.200mm~l
/
Okm
Fig. 2.
16
14-20 JULY1972
14 200
400
600
800
I000
1200
1400
1600
I800
2000
2200
2400
12 2600
North-south bathythermograph section and comparison of temperatures at 10, 50, 100, 150, and 200m.
4 36
J . B . COLTON,JR., D. E. SMmt and J. W. Jossl
36
Fig. 3.
BT#
41 43
49
Expanded temperature prolile in the vicinity of the thermal front.
and 64°53'W, and is most readily demarcated by the 20'C isotheml, which is at or above 100 m north of the front and at or below 150 m south of the frout. An expanded profile of temperatures in the upper 250 m of the water column in the vicinity of this front is shown in Fig. 3. The vertical temperature gradient in waters above the 20°C isotherm was greater north of the front and waters between the 20 and 15"C isotherms were more sharply stratified south of the front. The maximum temperature gradient (i'~C/20 kin) occurred at 100m between bathythermograph casts 44 and 45. This gradient is appreciably less than that observed during previous crossings of fronts and results from the possibility that the section was not made normal to the front and from the incapability of the widely-spaced bathythermograph observations to resolve properly the change in isotherm depth. A marked temperature discontinuity occurred between 50 and 150 m and a thermal disturbance due to the front was present below 200-m depth. The upper 200-m integrated temperatures for the bathythermograph observations immediately north (No. 43) and immediately south (No. 44) were 20.6 and 21.7°C. The average upper 200-m integrated temperatures for the five bathythermograph observations immediately north and south of the front were 20.6 and 22.8°C. There was little or no evidence of a temperature discontinuity in waters shallower than 50 m. ZOOPLANKTON
DISTRIBUTION
Of the 18 taxonomic groups collected on the CPR transect, only the abundance and frequency of copepods were sufficient to warrant an analysis of their distribution in relation to the thermal front. Table 1 lists the copepod species composition for 15 CPR samples immediately north and immediately south of the front. Of the 27 species of copepods identified from these collections, 12 occurred only north of the front and five occurred only south of the front. In most cases these faunal differences resulted from the occurrence of a species in only one or two samples, and are possibly an artifact due to sampling techniques (small volume filtered, 10-m sampling depth). However, in the case of Calocalanus pavoninus and Clausocalanus furcatus (found only north of the front), it would appear that the distributional differences are significant. C. furcatus has previously been found both north and south of the position here reported for the front (OwR~ and FoYo, 1967; FROST and FLEMINGER, 1968). C. pavoninus has been collected well south of this latitude (OWRE and Fovo, 1967; ESTRADA, 1972). These inconsistencies are partially explained by the extensive geographic range over which thermal fronts have been observed.
Further observations on a thermal front in the Sargasso Sea
437
Table 1. Copepod species composition north and south of the thermal front. The letters D and N following the sections designate day and night, respectively. CPR
GAUZE
SECTION
NUMBER
NORTH SPECIES 54D Calocalanus pavonlnus Corycaeus (Onychocorycaeus) latus Undltmla vulgaris Farranula g r a c i l l s Calocalanus pavo Oithona plumifera Oncaea venusta Corycaeus (Agetus) limbatus Corycaeus (Corycaeus) speelosus Oithona robusta Calanus tenulcornls Meeynocera elausl Clausoealanus furcatus Euchaeta marina Luclcuta flavlcornls Corycaeus (uroeorycaeus) lautus Pleuromama abdomlnalls Pleuromarmaa plseki Centropages vlolaceus Nannocalanus minor Oncaea conlfera Calocalanus plumulosls Oithona frigida pseudofrlglda Macrosetella gracJlls Copilla vltrea Arcartla negllgens Corycaeus (Corycaeus) clausi
55D
X X -
56D
.
.
. .
X
58D
. .
X
-
.
-
X
.
.
.
.
-
59N
X .
X
-
.
57D
.
. .
-
X
X
.
X
X
X
.
-
X
. . . . . . . . . . . . .
-
X
. . . . . . . . . . . . .
. . . . . . . . . . . . .
.
.
.
.
.
. . . . . . . . . . . . .
.
. .
.
62N
63N
64N
X
X
X .
.
.
.
.
. .
.
.
. .
.
X
.
.
.
.
. . . . . . .
.
.
.
. X X X
. . . . . .
.
. . . .
X
X
X . .
. X
.
. . .
. . X
. X X
. .
-
. -
. .
.
X
.
.
. . .
. X X
68D
.
.
. .
67N
.
.
.
. .
66N
X
.
. .
65N
X
X
. . X
. . . . . . . . . . . . . . . . . . . . .
.
61N
X
-
. . . . . . . . . . . . . . . . . . . . . . . . . .
60N
. . .
. .
. .
.
.
. -
X . X X
X . X X
. X X
X X .
X
-
82N
83N
X
-
X
-
.
.
-
. .
T
.
78N
79N
80N
X
-
-
.
.
.
SOUTH 69D Calocalanus pavonlnus Corycaeus (Onychocorycaeus) latus Undinula vulgarls Farranula gracilis Calocalanus pavo Oithona plumifeza Oncaea venusta Corycaeus (Agetus) llmbatus Corycaeus (Corycaeus) speclosus Oithona robusta Calanus tenulcornis Mecynocera clausl Clausocalanus furcatus Euchaeta marina Luclcuta flavicornls Corycaeus (Urocorycaeus) iButus Pleuromamma abdominalls Pleuroma~a plsekl Centropages vlolaceus Nannocalanus minor Oncaea conifers Calocalanus plumulosls 01thona frlglda pseudofr~glda Macrose~elIa gracills Copilia v £ t r e a Arcartia negllgens
Coryeaeus
X -
(Corycaeus)
clausi
70D
71D
X
. -
-
X
X .
X
-
X . X .
.
.
72D
. X
.
.
X
.
. -
. -
.
.
.
.
.
.
.
.
.
.
. . . .
. . . .
X
.
.
.
.
. .
74D
.
.
75N
.
76N
77N
. -
X .
X .
.
X
.
-
.
-
-
73D
.
.
.
.
.
.
. .
. X .
. .
X . .
.
.
. .
X . . . . . . . . . . . . . . . . . . . . .
.
.
X
X
X . .
.
.
.
.
X
-
. .
. .
.
. .
. .
. .
. .
X X X
.
.
X
. .
. .
-
. .
. . X
. .
.
.
.
. .
. .
.
-
.
.
. . .
X
-
r
"
-
"
-
-
X
-
X
-
X
X
X
.
.
' -
. .
X
.
.
X
X
X .
X
81N
.
. .
.
.
Present Not Found
Farranula gracilis and Oithona plumifera were the only relatively abundant species that occupied approximately equal rank on either side of the front. Published records for F. gracilis (GONZALES and BOWMAN, 1965) and O. plumifera (GRICE and HART, 1962) are in agreement with our findings. The day, night, and total average abundance and average number of species of copepods caught north and south of the front are given in Table 2. The total northsouth abundance and species number ratios were 2/1 and 4/3, respectively. The abundance and number of species of copepods were greater at night both north and south
438
J.B. COLTON,JR., D. E. SMITHand J. W. Jossl
Table 2. Day, night and total average abundance and average number of species of copepods collected on Continuous Plankton Recorder transect across thermal front. Day (Sunrlse-Sunset) No. Samples
~orth
No. ~m
Night (Sunset-Sunrise)
Total
No. Species
No. Samples
No. --mF
No. Species
No. Samples
2,5
9
21.4
4.8
6
8.3
South
6
1.8
2,5
9
12.9
3.3
Total
12
5.1
2.5
18
16.9
4.1
No.
No. Species
15
16.2
3.9
15
8.2
3.0
~
of the front. The average abundance was greater north of the front during both day and night. The average number of species was greater north of the front during the night and equal on either side of the front during the day. The fact that sampling in the immediate vicinity of the front occurred during daylight hours possibly obscured differences in abundance and species composition north and south of the front. DISCUSSION
Neither the density of the temperature observations nor the depth of plankton sampling was adequate to define precisely the point of physical and biological change associated with the front. However, the subsurface (50 to 150 m) temperatures and the abundance and species composition of copepods at l0 m showed a marked discontinuity at approximately the same geographical location. It is of interest that the distribution of copepods in the surface layer is associated with subsurface temperature conditions. For the most part, the species of copepods collected are epipelagic (occur in the upper 150 to 200 m of the water column) and diel migrants (move towards the surface at night and away from it in the daytime). This vertical migration is evidenced by the greater abundance and greater number of species in the night-time CPR catches. It would appear that the surface faunal boundaries indicated in this study are related to environmental conditions in the subsurface layers inhabited by these animals during the day. REFERENCES BACKUS R. H., J. E. CRADDOCK, R. L. HAEDRICH and D. L. SHORES (1969) Mesopelagic
fishes and thermal fronts in the western Sargasso Sea. Marine Biology, 3, 87-106. CIFELLIR. and K. N. SACHS,JR. (1966) Abundance relationships of planktonic Foraminifera and Radiolaria. Deep-Sea Research, 13, 751-753. ESTRADA J. C. (1972) Nueva aportaci6n al conocimiento de los cop6podos pelagios del archipielago canario. Boletin del lnstituto Espanol de Oceanografia, 155, 1-19. FROST B. and A. FLEMINGER(1968) A revision of the genus Clausocalanus (Copepoda: Calanoida) with remarks on distributional patterns in diagnostic characters. Bulletin of the Scripps Institution of Oceanography, 12, 1-235. GONZALESJ. G. and T. E. BOWMAN0965) Planktonic copepods from Bahia Fosforescente Puerto Rico, and adjacent waters. Proceedings el the United States National Museum, 117, 241-304. GRICE G. O. and A. D. HART (1962) The abundance, seasonal occurrence and distribution of the epizooplankton between New York and Bermuda. Ecological Monographs, 32, 287-309. HARDY A. C. (1939) Ecological investigations with the Continuous Plankton Recorder: object, plan and methods. Hull Bulletins of Marine Ecology, 1, 1-57.
Further observations on a thermal front in the Sargasso Sea
439
HULBERT E. M. (1964) Succession and diversity in the plankton flora of the western North Atlantic. Bulletin of Marine Science of the Gulf and Caribbean, 14, 33-44. KATZ E. J. (1969) Further study of a front in the Sargasso Sea. Tellus, 21, 259-269. OWRE H. B. and M. F o v o 0967) Copepods of the Florida Current. Fauna Caribaea, l, 1-137. RYTHER J. H. and D. W. MENZEL (1960) The seasonal and geographic range of primary production in the western Sargasso Sea. Deep-Sea Research, 6, 235-238. VOORHIS A. D. and J. B. HERSEY (1964) Ocean thermal fronts in the Sargasso Sea. Journal O/ Geophysical Research, 69, 3809-3814. WORTHINGTON L. V. (1964) Anomalous conditions in the Slope Water area in 1959. Journal t~f the Fisheries Research Board of Canada, 21,327 333.
NOTE
Further observations on a thermal front in the Sargasso Sea* J. B. COLTON, JR'r, D. E. SMITH:I: and J. W. Jossl~: (Received 22 August 1974; hz revisedJbrm 11 November 1974; accepted 17 November 1974) Abstract--Thermal fronts, which are characterized by a sharp surface or near-surface temperature gradient and which separate the Sargasso Sea into cooler, more productive northern parts, and warmer, less productive southern parts, have been observed in all months except July, August, and September. Temperature and zooplankton distributional data obtained on a bathythermograph-Continuous Plankton Recorder transect made in July, 1972, established the presence of a front at about 27°30'N and 64°55 'W. A marked discontinuity in temperature occurred between 50 and 100 m. The 20°C isotherm occurred at or above I00 m north of the front and at or below 150 m south of the front. Water shallower than 50 m was well stratified both north and south of the front. The point of change in the subsurface thermal structure corresponded to a near-surface faunal change. Of 27 species of copepods occurring in 15, 10-mile Continuous Plankton Recorder sections immediately north and south of the front, 12 occurred only north of the front and 5 occurred only south of the front. The abundance and number of species of copepods were greater north of the front. It is suggested that the near-surface faunal boundaries are related to environmental conditions in the subsurface layers inhabited by the copepods during the day. INTRODUCTION
THERMAL fronts have been observed in the Sargasso Sea by VOORH1S a n d HERSEY (1964), KATZ (1969), and BACKUS, CRADDOCK,HAEDRICHand SHORES(1969). They have been found between 24 to 37°N, and 56 to 76°W (Fig. 1) and in all months except July, August, and September. These thermal fronts, which extend in an eastwest direction, are characterized by a sharp surface or near-surface temperature gradient of at least I°C/10 km (VoORH|Sand HERSEY, 1964) and separate the Sargasso Sea into cooler northern and warmer southern parts. North of the front the upper part of the water column is stratified in summer only, while south of the front it is stratified at all seasons. In addition to differences in temperature, salinity, and density, north-south dissimilarities have been observed in net primary production (RYTHER and MENZEL, 1960), the species composition and abundance of phytoplankton (HULBERT, 1964), the abundance of planktonic Foraminifera and Radiolaria (CIFELLIand SACHS, 1966) and in the species composition and abundance of mesopelagic fishes (BAcKUS, CRADDOCK, HAEDRICHand SHOR~S, 1969). The north-south differences in the biota of the Sargasso Sea have been associated with the north-south differences in primary production (BACKUS, CRADDOCK, HAEDRICHand SHORES, 1969), which in turn are *Contribution No. 19, National Marine Fisheries Service, M A R M A P Program, Washington, D.C. 20235, U.S.A. t N M F S Northeast Fisheries Center, Narragansett Laboratory, Narragansett, Rhode Island 02882, U.S.A. :~NMFS M A R M A P Field Group, Narragansett Laboratory, Narragansett, Rhode Island 02882, U.S.A. 433
J.B. COLTON,JR., D. E. SMITHand J. W. Jossl
434
i 3,' ..OULF SrREA
f
I
I
I
35 °
I
30*
25"
80 ° ,
i
i
75 r
r
i
I
i
i
i
,
i
,
65 ° i
"-,
,~
. . . . . .
~
20 J
tv
60"
t
20 ~
55*
Fig. 1. Location of bathythermograph observations (o) and Continuous Plankton Recorder samples (insert), July, 1972, and of thermal front observations (A), 1958 1967.
attributed to the north-south variations in temperature structure; the isothermal condition in the north allowing for a renewal of nutrients in the euphotic zone (RVTHER and MENZEL, 1960). In all but one of the published temperature sections across Sargasso Sea thermal fronts, waters north of the front were isothermal to a depth of at least 50 m and the location of the front coincided with a sharp north-south increase in surface temperature. In two sections made across a front in May, 1967 (KATz, 1969, Fig. 2) there was a relatively weak vertical temperature gradient in the upper 50 m of the water column north of the front. Although in this case the front separated two vertically stratified surface layers, the horizontal gradient was such that the interface of the two water masses was still evidenced by an appreciable north-south increase in surface temperature. Because there are no marked differences in surface water temperature between the northern and southern Sargasso Sea during the summer, it was originally thought that these fronts were winter phenomena (VOORHISand HERSEY,1964). On a basis of more recent observations, BACKUS, CRADDOCK,HAEDRICH and SHORES (1969) concluded that it is probable that these fronts exist year-around at subsurface depths, but they are not readily discernible solely on a basis of surface temperature observations. Temperature and zooplankton distributional data obtained on a bathythermograph--Continuous Plankton Recorder section made in July, 1972 (Albatross IV Cruise 726) support this conclusion.
Further observations on a thermal front in the Sargasso Sea
435
S A M P L I N G METHODS AND ANALYSES
The majority of temperature observations in this section (Fig. l) were made en route to the first station (23°00'N, 61°00'W) occupied by the Albatross I V during the initial M A R M A P (Marine Resources Monitoring, Assessment, and Prediction) Program ichthyoplankton survey of coastal and oceanic waters from Cape Cod to the Caribbean Sea. Expendable bathythermograph casts were made at approximately 3-h intervals. To insure positional accuracy of the isotherms and maximum resolution of the widely-spaced observations, whole degree temperatures from individual traces were plotted at their depth of occurrence, rather than interpolated from readings at discrete depths. A Hardy Continuous Plankton Recorder (CPR, HARDY, 1939) was towed at a depth of l0 m and a speed of 12 knots (ca. 22 km h -~) from a point approximately 40 miles (74 km) east of Cape Hatteras to the first M A R M A P station. A 0.5-inch (l.3-cm) square nosepiece was used and the propeller pitch set for a gauze advance rate of approximately 4 in. (10'2 cm) per l0 miles (l 8.5 km) of tow during which about 3 m 3 of water are filtered. In the laboratory the gauze (24 meshes per cm) was cut into sections representing l0 miles (18.5 km) of tow. The sections were examined using 36× magnification. Abundances were reported as the number of organisms per m 3. The locations of the gauze samples analyzed tbr this study are shown in Fig. 1. TEMPERATURE DISTRIBUTION
A temperature section and plot of temperatures at specific depths are shown in Fig. 2. The vertical scale of the temperature section is expanded with respect to the horizontal scale by a factor of about 750. There are two locations within the section at which there is a rapid change in the depths of isotherms coincident with the location of thermal fronts. One, centered at approximately 35°27'N and 74°30'W, is associated with the inshore edge of the Gulf Stream, and is delimited by the 15°C isotherm at 200 m (WORTHINGTON, 1964). The other front is located at approximately 27°30'N 55o27'N 74o30'W 0 m~ ...o a r . ~
~__~.-'~-----~
4oolllli
~"
6ooii
/-',,,
25
....
29
__
--
-~.
27~30'N 64~54' W 4 6 f--45 ; '~-
35 _ : ' - ~
/.~
.
.
.
50
54
60
.... ~_ ............. ----~-
.
---
~
I L % ~ X ; . t \ ~ / - 7 ~ ' " - - - _ - ~ - _ - - ~ - r _ ~ - S j
~ _
~
- - ~ O m
.~
-----;;-/~6oo
28°C
[28°6
26 24
124
22
22
20 18
2O 18
16 14
tSO.w/.200mm~l
/
Okm
Fig. 2.
16
14-20 JULY1972
14 200
400
600
800
I000
1200
1400
1600
I800
2000
2200
2400
12 2600
North-south bathythermograph section and comparison of temperatures at 10, 50, 100, 150, and 200m.
4 36
J . B . COLTON,JR., D. E. SMmt and J. W. Jossl
36
Fig. 3.
BT#
41 43
49
Expanded temperature prolile in the vicinity of the thermal front.
and 64°53'W, and is most readily demarcated by the 20'C isotheml, which is at or above 100 m north of the front and at or below 150 m south of the frout. An expanded profile of temperatures in the upper 250 m of the water column in the vicinity of this front is shown in Fig. 3. The vertical temperature gradient in waters above the 20°C isotherm was greater north of the front and waters between the 20 and 15"C isotherms were more sharply stratified south of the front. The maximum temperature gradient (i'~C/20 kin) occurred at 100m between bathythermograph casts 44 and 45. This gradient is appreciably less than that observed during previous crossings of fronts and results from the possibility that the section was not made normal to the front and from the incapability of the widely-spaced bathythermograph observations to resolve properly the change in isotherm depth. A marked temperature discontinuity occurred between 50 and 150 m and a thermal disturbance due to the front was present below 200-m depth. The upper 200-m integrated temperatures for the bathythermograph observations immediately north (No. 43) and immediately south (No. 44) were 20.6 and 21.7°C. The average upper 200-m integrated temperatures for the five bathythermograph observations immediately north and south of the front were 20.6 and 22.8°C. There was little or no evidence of a temperature discontinuity in waters shallower than 50 m. ZOOPLANKTON
DISTRIBUTION
Of the 18 taxonomic groups collected on the CPR transect, only the abundance and frequency of copepods were sufficient to warrant an analysis of their distribution in relation to the thermal front. Table 1 lists the copepod species composition for 15 CPR samples immediately north and immediately south of the front. Of the 27 species of copepods identified from these collections, 12 occurred only north of the front and five occurred only south of the front. In most cases these faunal differences resulted from the occurrence of a species in only one or two samples, and are possibly an artifact due to sampling techniques (small volume filtered, 10-m sampling depth). However, in the case of Calocalanus pavoninus and Clausocalanus furcatus (found only north of the front), it would appear that the distributional differences are significant. C. furcatus has previously been found both north and south of the position here reported for the front (OwR~ and FoYo, 1967; FROST and FLEMINGER, 1968). C. pavoninus has been collected well south of this latitude (OWRE and Fovo, 1967; ESTRADA, 1972). These inconsistencies are partially explained by the extensive geographic range over which thermal fronts have been observed.
Further observations on a thermal front in the Sargasso Sea
437
Table 1. Copepod species composition north and south of the thermal front. The letters D and N following the sections designate day and night, respectively. CPR
GAUZE
SECTION
NUMBER
NORTH SPECIES 54D Calocalanus pavonlnus Corycaeus (Onychocorycaeus) latus Undltmla vulgaris Farranula g r a c i l l s Calocalanus pavo Oithona plumifera Oncaea venusta Corycaeus (Agetus) limbatus Corycaeus (Corycaeus) speelosus Oithona robusta Calanus tenulcornls Meeynocera elausl Clausoealanus furcatus Euchaeta marina Luclcuta flavlcornls Corycaeus (uroeorycaeus) lautus Pleuromama abdomlnalls Pleuromarmaa plseki Centropages vlolaceus Nannocalanus minor Oncaea conlfera Calocalanus plumulosls Oithona frigida pseudofrlglda Macrosetella gracJlls Copilla vltrea Arcartla negllgens Corycaeus (Corycaeus) clausi
55D
X X -
56D
.
.
. .
X
58D
. .
X
-
.
-
X
.
.
.
.
-
59N
X .
X
-
.
57D
.
. .
-
X
X
.
X
X
X
.
-
X
. . . . . . . . . . . . .
-
X
. . . . . . . . . . . . .
. . . . . . . . . . . . .
.
.
.
.
.
. . . . . . . . . . . . .
.
. .
.
62N
63N
64N
X
X
X .
.
.
.
.
. .
.
.
. .
.
X
.
.
.
.
. . . . . . .
.
.
.
. X X X
. . . . . .
.
. . . .
X
X
X . .
. X
.
. . .
. . X
. X X
. .
-
. -
. .
.
X
.
.
. . .
. X X
68D
.
.
. .
67N
.
.
.
. .
66N
X
.
. .
65N
X
X
. . X
. . . . . . . . . . . . . . . . . . . . .
.
61N
X
-
. . . . . . . . . . . . . . . . . . . . . . . . . .
60N
. . .
. .
. .
.
.
. -
X . X X
X . X X
. X X
X X .
X
-
82N
83N
X
-
X
-
.
.
-
. .
T
.
78N
79N
80N
X
-
-
.
.
.
SOUTH 69D Calocalanus pavonlnus Corycaeus (Onychocorycaeus) latus Undinula vulgarls Farranula gracilis Calocalanus pavo Oithona plumifeza Oncaea venusta Corycaeus (Agetus) llmbatus Corycaeus (Corycaeus) speclosus Oithona robusta Calanus tenulcornis Mecynocera clausl Clausocalanus furcatus Euchaeta marina Luclcuta flavicornls Corycaeus (Urocorycaeus) iButus Pleuromamma abdominalls Pleuroma~a plsekl Centropages vlolaceus Nannocalanus minor Oncaea conifers Calocalanus plumulosls 01thona frlglda pseudofr~glda Macrose~elIa gracills Copilia v £ t r e a Arcartia negllgens
Coryeaeus
X -
(Corycaeus)
clausi
70D
71D
X
. -
-
X
X .
X
-
X . X .
.
.
72D
. X
.
.
X
.
. -
. -
.
.
.
.
.
.
.
.
.
.
. . . .
. . . .
X
.
.
.
.
. .
74D
.
.
75N
.
76N
77N
. -
X .
X .
.
X
.
-
.
-
-
73D
.
.
.
.
.
.
. .
. X .
. .
X . .
.
.
. .
X . . . . . . . . . . . . . . . . . . . . .
.
.
X
X
X . .
.
.
.
.
X
-
. .
. .
.
. .
. .
. .
. .
X X X
.
.
X
. .
. .
-
. .
. . X
. .
.
.
.
. .
. .
.
-
.
.
. . .
X
-
r
"
-
"
-
-
X
-
X
-
X
X
X
.
.
' -
. .
X
.
.
X
X
X .
X
81N
.
. .
.
.
Present Not Found
Farranula gracilis and Oithona plumifera were the only relatively abundant species that occupied approximately equal rank on either side of the front. Published records for F. gracilis (GONZALES and BOWMAN, 1965) and O. plumifera (GRICE and HART, 1962) are in agreement with our findings. The day, night, and total average abundance and average number of species of copepods caught north and south of the front are given in Table 2. The total northsouth abundance and species number ratios were 2/1 and 4/3, respectively. The abundance and number of species of copepods were greater at night both north and south
438
J.B. COLTON,JR., D. E. SMITHand J. W. Jossl
Table 2. Day, night and total average abundance and average number of species of copepods collected on Continuous Plankton Recorder transect across thermal front. Day (Sunrlse-Sunset) No. Samples
~orth
No. ~m
Night (Sunset-Sunrise)
Total
No. Species
No. Samples
No. --mF
No. Species
No. Samples
2,5
9
21.4
4.8
6
8.3
South
6
1.8
2,5
9
12.9
3.3
Total
12
5.1
2.5
18
16.9
4.1
No.
No. Species
15
16.2
3.9
15
8.2
3.0
~
of the front. The average abundance was greater north of the front during both day and night. The average number of species was greater north of the front during the night and equal on either side of the front during the day. The fact that sampling in the immediate vicinity of the front occurred during daylight hours possibly obscured differences in abundance and species composition north and south of the front. DISCUSSION
Neither the density of the temperature observations nor the depth of plankton sampling was adequate to define precisely the point of physical and biological change associated with the front. However, the subsurface (50 to 150 m) temperatures and the abundance and species composition of copepods at l0 m showed a marked discontinuity at approximately the same geographical location. It is of interest that the distribution of copepods in the surface layer is associated with subsurface temperature conditions. For the most part, the species of copepods collected are epipelagic (occur in the upper 150 to 200 m of the water column) and diel migrants (move towards the surface at night and away from it in the daytime). This vertical migration is evidenced by the greater abundance and greater number of species in the night-time CPR catches. It would appear that the surface faunal boundaries indicated in this study are related to environmental conditions in the subsurface layers inhabited by these animals during the day. REFERENCES BACKUS R. H., J. E. CRADDOCK, R. L. HAEDRICH and D. L. SHORES (1969) Mesopelagic
fishes and thermal fronts in the western Sargasso Sea. Marine Biology, 3, 87-106. CIFELLIR. and K. N. SACHS,JR. (1966) Abundance relationships of planktonic Foraminifera and Radiolaria. Deep-Sea Research, 13, 751-753. ESTRADA J. C. (1972) Nueva aportaci6n al conocimiento de los cop6podos pelagios del archipielago canario. Boletin del lnstituto Espanol de Oceanografia, 155, 1-19. FROST B. and A. FLEMINGER(1968) A revision of the genus Clausocalanus (Copepoda: Calanoida) with remarks on distributional patterns in diagnostic characters. Bulletin of the Scripps Institution of Oceanography, 12, 1-235. GONZALESJ. G. and T. E. BOWMAN0965) Planktonic copepods from Bahia Fosforescente Puerto Rico, and adjacent waters. Proceedings el the United States National Museum, 117, 241-304. GRICE G. O. and A. D. HART (1962) The abundance, seasonal occurrence and distribution of the epizooplankton between New York and Bermuda. Ecological Monographs, 32, 287-309. HARDY A. C. (1939) Ecological investigations with the Continuous Plankton Recorder: object, plan and methods. Hull Bulletins of Marine Ecology, 1, 1-57.
Further observations on a thermal front in the Sargasso Sea
439
HULBERT E. M. (1964) Succession and diversity in the plankton flora of the western North Atlantic. Bulletin of Marine Science of the Gulf and Caribbean, 14, 33-44. KATZ E. J. (1969) Further study of a front in the Sargasso Sea. Tellus, 21, 259-269. OWRE H. B. and M. F o v o 0967) Copepods of the Florida Current. Fauna Caribaea, l, 1-137. RYTHER J. H. and D. W. MENZEL (1960) The seasonal and geographic range of primary production in the western Sargasso Sea. Deep-Sea Research, 6, 235-238. VOORHIS A. D. and J. B. HERSEY (1964) Ocean thermal fronts in the Sargasso Sea. Journal O/ Geophysical Research, 69, 3809-3814. WORTHINGTON L. V. (1964) Anomalous conditions in the Slope Water area in 1959. Journal t~f the Fisheries Research Board of Canada, 21,327 333.