NOTE Biggs et al.: Plankton and cold-core rings in the Gulf of Mexico 
373 
On shorter time scales, the biological implications 
of thermal fronts in the Gulf of Mexico are widely 
recognized by fishermen: many of them now direct 
their boats to selected fishing areas where SST im- 
agery shows sharp temperature gradients over short 
(<10 km) distances. Skipjack, blackfin tuna, sword- 
fish, and blue marlin have been reported by fisher- 
men to be locally abundant in these frontal zones 
(Roffer 2 ). Also, on seven research cruises of the 
GulfCet program 1992-94, there were frequent 
sightings of family groups of sperm whales, Physeter 
catodon, and periodic sightings of pods of killer 
whales, Orcinus orca, in association with thermal 
fronts over the continental slope of the northern Gulf 
of Mexico (Davis and Fargion 3 ). Clearly, populations 
of apex predators like these are not likely to be sus- 
tained by low or infrequent episodes of enhanced 
secondary productivity. 
One explanation for the fact that elevated stocks 
of biomass were not found in the CCR during the 
March 1993 transect is that biomass may 
“grow in” only when cyclones are “spun up” 
into surface waters. That is, if “new” ni- 
trate is but episodically injected into the 
photic zone of cyclones, there may be lag 
times of days or weeks between what we 
hypothesize should be pulses of new pro- 
duction and secondary production. Alter- 
natively, as these cyclones spin up, nitrate 
levels may be slowly domed and then de- 
crease as the ring spins down and loses its 
cold-core surface expression. 
Because Gulf of Mexico cyclones contain 
water of the same temperature-salinity 
properties as the rest of the Gulf of Mexico, 
only when they are well “spun up” will they 
have colder surface as well as colder inte- 
rior temperatures. In fact, the cyclone of 
the present study was one of the few that 
has been visible in SST as well as in al- 
timeter imagery; it may have spun up to 
have locally cool surface temperatures in 
response to cyclonically favorable wind 
curl from the passage of a strong atmo- 
spheric cold front. This strong “norther” 
passed through Texas and out across the 
Gulf of Mexico 36 hours before the cruise; 
the cloud banks that stretch NE to SW 
along the trailing edge of this norther can 
be seen in Figure 1. Rapid (hours-to-days 
scale) and intense cyclogenesis has been 
reported to occur after cold front passage 
in the northern Gulf of Mexico, especially 
when the cold fronts stall over deep water 
off the edge of the continental margin 
(Lewis and Hsu, 1992). 
.c 
Q. 
CD 
Q o 
20 
40 
60 
80 
100 
120 
140 
160 
180 
200 
- 85.0 - 80.0 - 75.0 - 70.0 - 65.0 
Acoustic backscatter intensity (db re(m x n)~') 
Figure 3 
Acoustic backscatter intensity versus depth for time-averaged ADCP 
(acoustic Doppler current profiler) records (mean of three ensembles of 
5-min duration each) that were concurrent with times of plankton net 
tows: (A) three nighttime tows in the cold-core ring (CCR); (B) night 
tow southeast of CCR (solid line) and mean of four day tows outside 
CCR (dashed line). The region 10-100 m where acoustic backscatter 
intensity (ABI) >-74 db is shaded; the inset at top left summarizes this 
integrated ABI > -74 db (IABI). 
2 Roffer, M. 1994. Ocean Fishing Forecasting Ser- 
vice, Miami, FL. Personal commun. 
3 Davis, R. W., and G. A. Fargion (eds.). 1996. Dis- 
tribution and abundance of cetaceans in the north- 
central and western Gulf of Mexico. Outer Con- 
tinental Shelf Study (OCS) Study MMS 96-0027. 
U.S. Dep. Interior, Minerals Manage. Serv., Gulf 
of Mexico OCS Region, New Orleans, LA, 357 p. 
