NOTE Biggs et a I.: Plankton and cold-core rings in the Gulf of Mexico 
371 
The plankton tows were made with an open net of 
333-pm mesh and 1 m in diameter, at every 3rd XBT 
site beginning at 27°00'N. The net was outfitted with 
an impeller-type flowmeter (General Oceanics) that 
allowed the volume of water fished to be determined 
upon recovery of the net. In 15-20 min oblique tows 
to 100 m depth, volumes of water fished ranged from 
450 to 800 m 3 . Collections were preserved in a 4% 
formaldehyde solution buffered with borax, and then 
bulk sample displacement volume was measured 
according to the method of Ahlstrom and Thrailkill 
(1960). Four of the 8 tows were made during day- 
light hours and the other 4 were made at night. Tow 
1 was made in daylight outside and to the northwest 
of the CCR, whereas tows 2-4 were at night, within 
the CCR. Tows 5-7 were daylight tows outside and 
to the southeast of the CCR, and tow 8 the following 
night was also outside and to the southeast of the 
CCR. Because only nighttime tows were able to be 
made in the CCR, we chose to enumerate the taxo- 
nomic composition of all eight samples for three 
groups of macrozooplankton that are well known to 
exhibit diel vertical migration (Gasca et al., 1995). 
Each sample was split 1:4 with a Folsom plankton 
splitter and then euphausiids, thecosome pteropods, 
and siphonophores were enumerated to species at 
the Centro de Investigaciones Quintana Roo. 
| i i i i 
LAT: 27 40' 26 40' 25 40' 24 40' 23 40' 
Figure 2 
Along-track plot of the doming of 15°C and 8°C isotherms, show- 
ing the cyclone as a 150-km wide region where 8°C <500 m. 
Results 
Zooplankton biovolume averaged 2.4-fold higher 
in nighttime tows than in daytime tows (90 ver- 
sus 38 mL/1,000 m 3 ; see Table 2). This eleva- 
tion of stock at night reflects greatly increased 
numbers of euphausiids at night, for most of 
the euphausiid species present in the western 
Gulf of Mexico perform vertical migratory pat- 
terns during a day-night cycle. During the night 
hours, these euphausiid species can be collected 
in the upper 200 m (Mauchline, 1980). How- 
ever, the numbers and kinds of euphausiids 
present at night inside versus outside the CCR 
were quite different: 56% of the number of eu- 
phausiids inside the CCR were species of the 
genus Euphausia, whereas 63% of the individu- 
als in the night tow outside the CCR belonged 
to two species of the smaller-size genus 
Stylocheiron. Moreover, euphausiid species of 
the genus Euphausia at night were, on aver- 
age, 1.8-fold more abundant within the CCR 
than outside (321 individuals/1,000 m 3 inside, 
Table 1 
Surface temperature, mixed layer (ML) depth, temperature at 100 m, and the calculated dynamic height (relative to 800 db) for 
stations where plankton tows were made. 
Plankton 
Tow 
Local 
Time 
XBT 
station 
Temp (°C) 
at surface 
Depth (m) to reach 
surface temp, minus 1°C 
(= ML depth) 
Temp (°C) 
at 
100 m 
Dynamic 
height 
(cm) 
1 
15:52-16:09 
7 
23.24 
54 
19.7 
102 
2 
19:23-19:38 
10 
22.34 
72 
19.2 
97 
3 
23:11-23:27 
13 
21.50 
75 
17.8 
88 
4 
03:08-03:23 
16 
21.75 
71 
19.0 
94 
5 
07:08-07:19 
19 
22.50 
99 
21.4 
106 
6 
12:11-12:26 
22 
23.62 
98 
22.4 
115 
7 
16:08-16:25 
25 
23.39 
112 
22.5 
113 
8 
21:17-21:34 
28 
23.36 
95 
21.9 
111 
