See the
timeline graphic of activities related to our process field season.
Purpose of the Cruises: EL9904 (April '99) and EL9905 (May '99)
The objectives of the cruises were
to: (1) Examine the potential exchange of plankton in the vicinity of the
tidal flank on the southern flank of Georges Bank and in particular how
the exchange relates to retention of fish larvae in the
well-mixed cap of the Bank; (2) Use circulation model and drifters to predict
the tidal excursion for guiding the samping effort, and forcast larval
fish trajectories to simulate feeding and growth in
a near realistic prey field using a bioenergetics model; (3) Conduct site
studies to determine the vertical distribution of cod and haddock larvae
and pelagic juveniles in relation to the tidal front, their
diel variability, predator-prey relations, and biochemical content for
growth in the different water-column conditions.
Physical Oceanography
Four GPS/ARGOS drifter deployments
were made on EL9904: a surface and deep drogue on either side of the weak
tidal front. A list of times and positions of each deployment is given
in the Appendix IV Drifter Log. Temperature probes were installed on each
drifter. The shallower drifter (13m) on the mixed side of the front lost
its drogue soon after deployment and had to be recovered after only a few
tidal cycles. Its surface canister holding the electronics moved rapidly
to the east (Fig.4). For a complete description of the drifter and
the attached drogue see earlier cruise reports such as SJ9503. The shallow
units on this cluster of deployment were the Brightwater Model 104AVs and
the deeper were the newer Brightwater Model 115s. The difference is that
the newer models do not have the VHF antennae and the drogues are 1mx10m
rather than 1.6mx6m. The drag ratio of the deep to surface is designed
to be 40:1 so that there should be no difference in their water following
characteristics. The three remaining drifters were advected for at
least a few days and, in one case, for over a week. These
deployments (Fig. 5) together with Ted Durbins deployments off the
ENDEAVOR, provided a record of velocity (Fig.6) to be assimilated
into the circulation model. All processed data are posted on
the GLOBEC homepage under data|processed|1999|nmfs_drfit.
A total of seventeen drogued-drifter deployments were made on the May cruise in four different clusters (see Table 1) Clusters 1 and 2 are shown in Figure 2 and clusters 3 and 4 in Figure 3. The drifter ID, listed in column #1 below, is coded by ARGOS PTT# followed by an integer that represents the consecutive deployment. The deployment ID "3951", for example, is the first deployment of PTT#395 on this cruise. The dimension and configuration of the drifters and drogues are described in our earlier cruise reports such as SJ9503. Drifters were deployed on either side of the tidal front and at different depths for each of the cluster experiments. Cluster #1, for example, had two drifters on the mixed side with drogues at 13 and 33m, respectively, and two drogues on the stratified side at 8m and 33m, respectively. In the case of cluster #2, additional drogues were placed in the vicinity of the front. Each of the cluster experiments were conducted for 3-4 days. Drifters were then recovered and redeployed relative to the tidal front. The tidal front structure was determined by a CTD section prior to each cluster deployment.
Cross-sectional views of temperature (Fig. 10e), salinity (Fig. 10f), and sigma-t (Fig. 10g) demonstrate the degree of along-bank variability in the cross-bank structure. Notice the off-shore influence appears in transect 1, is non-existent in transects 3 and 4, and then reappears in the last transect 6. The offshore influence (mini-intrusions at depth) is also evident in some of the horizontal figures above. Significant horizontal structure of water mass properties was also visible in satellite imagery such as those on May 10,
Testing of the FCAST real-time modeling system was successful including aspects of hardware, software and links to land. Experiences and protocols developed during the EL9904 cruise were implemented in the May-June (EL9905 and EN323-4) cruises. Additional experiments during EL9905 and EN323-4 provide additional insights into the FCAST capabilities with variable meteorological forcing.
Biological Sampling
Twenty-nine bongo tows were made
with a 61-cm frame fitted with 333 and 505 mesh nets using standard MARMAP
procedures; i.e., double-oblique from surface to within 5 m of the bottom.
A SeaBird CTD (Model 19) was attached to the towing wire above the bongo
to monitor sampling depth in real time and to record temperature and salinity.
The 1-m2 MOCNESS with nine 333 mesh nets was used to sample larval fish and larger zooplankton. A total of 25 hauls were taken. Sensors on the 1-m2 MOCNESS included downwelling light, fluorometry, depth, temperature and salinity. A Video Plankton Recorder (VPR) also was attached to the MOCNESS frame to record fine-scale zooplankton during the tow. The high magnification camera was set to a field of view of 2.5 x 3.0 mm and the low magnification camera captured a 2.0 x 2.5 cm area.The tow profile for the MOCNESS was nominally 10-m strata within 5 m of the bottom; extra nets were used for special collections. The 1-m2 MOCNESS nets typically sampled for 5 minutes to filter about 250 m3 of water.
The MOCNESS sampling strategy was to make four tows every 24 hours at 0600h, 1200h, 1800h, and 2400h.The plankton from the first down profile would be preserved in formalin for gut content analysis. Two nets were samples from 0-20m and 20m-the bottom to be used for biochemical specimens and special samples.
The Video Plankton Recorder, an underwater imaging video microscope, was mounted above the net opening on the 1-m2 MOCNESS.
CTD/pump casts followed the MOC1 tows so that we can examine the prey fields for larval fish and obtain complementary zooplankton distributions with smaller mesh size (40 m vs. the 150 m of the MOC) and finer vertical resolution (5 m in the upper 40 m, 10 m intervals below that).
Information on depth distributions will be used in conjunction with models and other GLOBEC findings relative to larval feeding and growth and cross-frontal mixing of various species and stages. We sampled 14 profiles with a total of 123 samples using a gas-driven diaphragm pump on deck. Generally, 3 profiles were obtained each day of sampling: once each in the forenoon, mid afternoon, and after sunset.
Nutrient samples were taken for Dave Townsend at the second occupation of the frontal site, cruise station 33, when we remained at a fixed location and let the tide pass beneath us. Five profiles were taken which included the top and bottom of the tidal excursion. Additional profles were taken at cruise station 30 (mixed site, shallow end of Schlitz mooring line) and at a flank station (cruise station 36, in 86 m water). Seven profiles, 44 samples frozen.
A total of 2,597 cod and haddock
larvae were collected for biochemical analysis from the bongo-net survey
hauls and extra net profiles of the 1-m2 MOCNESS hauls. Species
distribution was almost even: 51% cod, 49% haddock. The larvae will be
analyzed for their RNA, DNA, and protein content and the data used to determine
the growth rate and nutritional condition of the individual fish. A comparison
will be made of fish taken from the different sites and at discrete depths.
A subsample of 107 larvae will be shipped to Dr. Mike St. John at the Danish
Institute for Fisheries Research for lipid analysis.
The 1-m2 MOCNESS with nine 333 mesh nets was used to sample larval fish and larger zooplankton. A total of 41 hauls were taken. Sensors on the 1-m2 MOCNESS included downwelling light, fluorometry, depth, temperature, and salinity. A Video Plankton Recorder (VPR) was attached to the 1-m2 frame to record fine-scale zooplankton distribution during the tow. The 10-m2 MOCNESS with five 3-mm mesh nets sampled juvenile ichthyoplankon and larger zooplankton predators. A total of 17 hauls weretaken: four from the "mixed" or shoal side of the tidal front, seven from the stratified or off-bank, southern side of the tidal front, and four hauls within the tidal front. The tow profile for the 10-m2 MOCNESS was nominally 10-m or 20-m strata within 5-m of the bottom.
Samples for biochemical and age analysis were taken from fifty one 505 and 333 , 61-cm bongo nets, and forty one 333 mesh 1-m2 MOCNESS hauls. A total of 63 individuals predators from 5 invertebrate and four fish taxa were preserved in 95% ethanol for later analysis for the presenceof Calanus, Pseudocalanus, cod and haddock DNA in the stomach contents.
The Video Plankton Recorder, an underwater imaging video microscope, was mounted above the net opening on the 1-m2 MOCNESS.
A total of 3,189 cod and haddock larvae were collected for biochemical
analysis from the bongo-net survey hauls and extra net profiles of the
1-m2 MOCNESS hauls. Species distribution was 34% cod, 66% haddock.
The larvae will be analyzed for their RNA, DNA, and protein content and
the data used to determine the growth rate and nutritional condition of
the individual fish. A comparison will be made of fish taken from the different
sites and at discrete depths. A subsample of 232 larvae will be shipped
to Dr. Mike St. John at the Danish Institute for Fisheries Research for
lipid analysis.
Studies on the Predation of omnivorous copepods on early developmental
stages of Calanus finmarchicus and Pseudocalanus spp. were conducted with
the following objectives:
(1) To catch and maintain cultures of several species of omnivorous
copepods that are abundant and potentially important as predators of early
life stages of Calanus finmarchicus and Pseudocalanus spp.
(2) To catch adult female C. finmarchicus and Pseudocalanus spp. for
cultures producing eggs and nauplii to be used in predation experiments.
(3) To run predation experiments at ambient sea water temperature using
deck incubations in a plankton wheel.
Studies on GELATINOUS PREDATORS were conducted as well with the following objectives:
Thirty bongo samples were examine, both 333µ and 505µ nets representing almost all the stations along the transects. Numbers of Pleurobrachia collected in these samples are shown . These are raw numbers and have not been converted to numbers per volume towed. Seventeen complete MOC-1 hauls and 4 partial hauls at station 106 were examined and numbers of Pleurobrachia counted; 13 complete and 3 partial at station 100 and 2 complete at station 95. Two graphs showing the distribution of Pleurobrachia by haul and by depth, and distribution by depth in daylight and dark are shown as well. We collected replicate day and night discrete depth samples at a shallow and deep station across the tidal front. Predator abundance and distribution was determined from these samples and will be compared to target species abundance and distribution. Live measurements were made on a subsample of Pleurobrachia, and these wil be compared with sizes of preserved specimens from the same tows. Gut contents of Pleurobrachia collected in the MOC 10 will be examined and compared with those collected in the MOC-1 and with divercollected samples from a previous cruise as their is an indication that the guts are fuller in Pleurobrachia that are hand collected. Several copepod feeding experiments were conducted, which will be discussed in another section
Zooplankton Studies include:
We took 35 pump profiles of zooplankton to provide: (1) detailed prey
field information for the larval fish studies of size, feeding, growth
rate, distribution and condition (collections using the 1-m MOC); (2) detailed
vertical distributions and abundances (including diel changes) to use with
transport and mixing calculations (dye studies, hydrographic data and modeling
components); and (3) abundance estimates to compare with VPR records from
the 1-m MOC and catch rates with the 1/4-m MOC. The VPR attached to the
1-m MOC is a sampling method in development for simultaneous sampling of
prey and larval fish (Greg Lough et al.); the 1/4-m MOC has been used on
numerous GLOBEC cruises for sampling the prey field, and a comparison of
results is needed. The 1/4-m MOC usually samples ~35 m3 from
a (vertical) depth stratum of 10 m, whereas the pump sample comes from
a small volume at a discrete depth. Pump samples from earlier GLOBEC cruises
suggest higher prey field concentrations. We collected a total of 370 samples
during this cruise.
Nutrient samples were taken for Dave Townsend at 23 CTD stations
at near bottom and every 10 meters of the water column .
