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REPORT ON C.C.G.S. Parizeau CRUISE 98-078

10-16 February 1999

Peter C. Smith and Gary L. Bugden

Ocean Sciences Division

Bedford Institute of Oceanography Dartmouth, Nova Scotia

and

Jackie Spry

Sprytech Biological Services, Inc.

CANADA

March, 1999

BEDFORD INSTITUTE OF OCEANOGRAPHY CRUISE REPORT Parizeau 98-078

Local Cruise Designation: 98-078

Vessel: C.C.G.S. Parizeau

Dates: 10-16 February 1999

Area: Southwest Nova Scotia / Georges Bank

Responsible Agency: Ocean Sciences Division

Maritimes Region, DFO

Ship's Master: Capt. W. English

Scientific Personnel:

P.C. Smith Ocean Sciences

M. Scotney Ocean Sciences

G. Bugden Ocean Sciences

R. Boyce Ocean Sciences

R. Ryan Ocean Sciences

J. Spry Sprytech Biological Services, Inc.

1. PURPOSE

The scientific objectives of this voyage were:

1. Obtain real-time measurements of surface (1 m) temperature and salinity over the eastern flank of Georges Bank in order to monitor for cross-over events from Browns to Georges,

2. Determine the distribution of temperature, salinity, nutrients and biota in the vicinity of SWNS and eastern Georges Bank,

3. Acquire Lagrangian measures of surface drift on Browns and eastern Georges Banks.

The activities planned for the cruise period include:

1. Deploy a sentinel guard buoy mooring carrying an ARGOS transmitter to relay data from a 1-m SeaCat at the NECW site in western Northeast Channel (Figure 1a),

2. Conduct CTD survey of SWNS and eastern Georges Bank,

3. Make a series of biological measurements on Browns, Georges and offshore,

4. Conduct monitoring activities at Halifax Line Station #2.

2. NATURE OF DATA GATHERED

A single mooring (Guard Buoy “V”; Table 1) was deployed during this voyage at the NECW mooring site on the eastern side of Northeast Channel. The buoy, equipped with a SeaCat T,S recorder at 1 m and an ARGOS beacon to transmit data ashore, replaces the original sentinel mooring which failed due to water in the battery compartment.

A total of 57 CTD stations (Fig.1a, Table 2) were occupied at the mooring sites, primarily along the following sections:

1) Section Ia - across Northeast Channel from Browns to Georges Bank on the mooring line, i.e. near the sill (Fig.3),

2) Section Ib - across Northeast Channel from Browns to Georges Bank south of the mooring line, i.e. toward the mouth (Fig.4),

3) Section II - across the eastern portion of Georges Bank proper (Fig.5),

4) Section III - across the eastern tip of Georges Bank, i.e. along the western side of Northeast Channel (Fig.6)

5) Section IV - across the western edge of Browns Bank, i.e. along the eastern side of Northeast Channel (Fig.7), and

6) Section V - across the western Scotian Shelf off Cape Sable (Fig.8).

The quality of the CTD salinity measurements is quite acceptable (Table 3a). The YSI dissolved oxygen sensor was calibrated to match surface saturation conditions at the temperature and salinity measured there by the CTD (Table 3b, Figure 2a). The YSI oxygen sensor membrane was replaced after CTD1 as it was found to be ruptured. No further problems were encountered with this sensor.

Biological measurements were taken at a total of 12 stations in the southwest Nova Scotia-Georges Bank area (Table 4a) and the Halifax Line Station 2 monitoring site was occupied on both departure and return (Table 4b). Nutrient, chlorophyll and salinity samples were variously drawn at roughly standard depths, and plankton samples were taken with double-oblique bongo hauls to 50 m in shallow water (<100m) or a vertical ring net cast in deeper regions. Samples from one of the bongo nets was preserved in formalin; samples from the other were preserved in alcohol for genetic analysis at Dalhousie University. (Chris Taggart).

Near surface sampling of temperature, salinity, chlorophyll fluorescence, and solar radiation (PAR) was undertaken along the ship’s track using Biological Oceanography’s flow-through system. Except for occasional computer crashes and periods of high noise during rough weather, this system provided continuous surface data over the entire voyage. The calibration of the flow-through measurements against surface values from the CTD show some offsets (Fig.2b and 3b, Table 3c)

For the Lagrangian experiment, a total of five WOCE drifters with drogues at 10 m were placed in areas off Georges Bank (Table 5). The first three were deployed in a line across the eastern edge of a warm core eddy which had penetrated into the mouth of Northeast Channel. The remaining two drifters were place along the northern flank of Georges to monitor possible flows from the interior Gulf onto the Bank.

3. PROGRAM SUMMARY

Date From (Z) To (Z) Operation

10 Feb. 2100 0230(11) Depart BIO enroute to Hlfx Stn 2 monitoring site

11 Feb. 0230 0400 Conduct monitoring protocol at Hlfx Stn.2, CTD1

2200 0400(12) CTD2-8 on Section Ia

12 Feb. 0530 1100 CTD9-14 on Section Ib; deploy 3 drifters

1300 1400 Mooring placement operations at NECW

1450 2150 CTD15-19 on Section III, biological sampling

2250 0220(13) CTD20,21; deploy 2 drifters

13 Feb. 0230 1900 CTD22-31 on Section II, biological sampling

1950 2200 CTD32,33

2315 0445(14) CTD34-36 on Section III, biological sampling

14 Feb. 0815 2349 CTD37-45 on Section IV, biological sampling

15 Feb. 0036 0205 CTD46,47 toward Cape Sable

0311 1536 CTD48-56 on Section V

1023 1125 CTD57, biological sampling at Hlfx Stn 2

16 Feb. 1500 Arrive BIO

4. MOORING OPERATIONS

After a delay by foul weather and seas on 10 Feb., the single mooring operation at NECW proceeded largely without incident (Table 1 a,b). On the way to the site, a large wave had broken across the foredeck, loosening the bindings on the guard buoy, but there was no apparent damage to the instruments attached to it. The mooring was placed precisely at the same location as the earlier version, whose instruments had failed, so there was no need for a notice to mariners.

5. HYDROGRAPHIC/BIOLOGICAL MEASUREMENTS

Hydrographic measurements, including dissolved oxygen and fluorescence, were made at a total of 57 stations (Table 2) using a Seabird 25 portable CTD system, equipped with a SBE 23Y Yellow Springs Instruments (YSI) dissolved oxygen sensor, a PAR sensor and a SeaTech fluorometer. The data were recorded internally and downloaded periodically to a laptop PC which processed them into ODF files.

In addition, biological measurements, including 1) double-oblique Bongo casts to 50 m in shoal waters (<100m); 2) ringnet casts to the bottom in deeper waters (>100m), and 3) bottle casts for nutrient, chlorophyll, and salinity samples (see Table 4a,b.). The bottle casts generally took samples at some subset of standard depths (1, 30, 50, 70, 100, 150, 200, 300m). All of this work was performed with the hydro winch, because of the inability (due to the PSAC strike) to load the necessary winches and cranes for the VOPC and MVP instruments.

The surface properties of the ocean (T, S, chlorophyll, solar radiation, and nutrients) were monitored underway using Biological Oceanography’s flow through system. This system encountered some problems because of seawater pressure fluctuations associated with the rolling of the ship and loss of suction when the bow was out of the water. This caused a lot of spikes in the signals, especially the salinity. Biological fouling of the seawater intake by mussels also caused unpredictable variations in flow rate. The system computer crashed on several occasions for some unknown reason(s), resulting in the loss of several hours of data in each case. Otherwise the system performed well. Because of the passage of the seawater line through the ship, the temperatures recorded by the flow-through system tended to be a bit higher (~0.5^oC) than those surface values from the SeaBird (Fig.2b; Table 3c).

The portable SeaBird system worked well throughout the cruise, with the minor exception that some water was intermittently found inside the battery case. This did not seem to hamper the operations, however.

Problems/Recommendations:

1. Better pumping for the flow-through surface sampling system is required. Also, the electronics for the system should be trouble shot for greater stand-alone reliability.

2. Return SBE-25 to factory for repair before serious damage is done by the leak.

5a. Processing

The data from the portable SeaBird system were downloaded to a laptop computer after every 3-4 stations, and then processed into ODF files for archiving. The following is a summary of the processing procedure:

1) Convert raw frequency data to binary pressure, temperature, conductivity, etc. using SEABIRD's DATCNV program.

2) Align downcast pressure, temperature and conductivity using SEABIRD's ALIGNCTD program by advancing the conductivity signal by 0.073 sec. Also advance oxygen temperature and oxygen sensor current by 3 seconds.

3) Filter downcast pressure using SEABIRD's FILTER program. This is a low pass filter and we used a time constant of 0.5 seconds.

4) Mark downcast scans where the CTD is moving less than the minimum velocity of 0.01 m/s using SEABIRD's LOOPEDIT program.

5) Compute salinity and dissolved oxygen in ml/l using SEABIRD's DERIVE temperature

6) Produce screen plots of salinity, temperature and dissolved oxygen vs. pressure for initial evaluation of data quality using SEAPLOT program.

7) Produce screen plots of PAR and chlorophyll fluorescence vs. pressure for initial evaluation of data quality using SEAPLOT program.

8) Bin average downcast data to 0.5-dbar intervals using SEABIRD's BINAVG program.

9) Convert the down cast from binary to ASCII using SEABIRD's TRANS program.

10) Convert downcast to ODF format using OSD program SEAODF25.

11) Create IGOSS message using OSD program ODF_IGOS.

Plots and status info displayed during the acquisition are discarded when the program terminates. Plots of T, S, O₂, and s_q vs. pressure and T vs. S were produced from the processed data using SEABIRD’s SEAPLOT program as time permitted. Section plots were produced with Igor Yakashev’s contour package, modified to compute s_q and to accept .ODF files.

5b. Calibration

Because of the nature of this operation (“rapid response”) and difficulties with loading associated with the PSAC strike, the usual calibration standards were not maintained for this mission. Nevertheless, a 1.2-liter bottle was tripped at the bottom of selected CTD casts and a single salinity sample was drawn to be analyzed at BIO (Table 3a). Similarly, selected salinity samples were drawn from the flow-through system to assess its performance (Table 3a). The flow-through properties were also compared to the near-surface CTD measurements (Fig.2b, 2c; Table 3c). The offset of the temperature regression was negative and the slope significantly greater than one, because the coldest intake waters received the most heat from the ship. The slope of the salinity regression was not significantly different from one, so only a simple offset is quoted for calibration purposes. In addition, duplicate nutrient samples were taken from the Nisken bottles to analyze the accuracy of replicates (Data not shown here). Finally, although there were no dissolved oxygen samples taken, the general accuracy of the YSI sensor was assessed by assuming that the surface values are saturated (Fig.3b). The O₂ traces still showed substantial hysteresis. The fluorometer output voltage was fitted to extracted Chl-a values from the almost coincident bottle casts. Results are shown in Table 3d.

Problems/Recommendations:

(1) Efforts should be made to remove the hysteresis between the up and down traces and occasional spiking at steep temperature gradients from the YSI sensor output by application of filters and various time lags.

5c. Sections

CTD sections Ia,b, II, III, IV, and V (Figs. 3-8) depict hydrographic conditions, 1) across the sill in Northeast Channel, 2) across Northeast Channel roughly 10 km seaward of the sill, 3) across the eastern half of the Bank proper, 4) across the eastern peak of the Bank, extending both offshore and into Georges Basin, 5) along the western edge of Browns Bank, and 6) across the western Scotian Shelf off Cape Sable, respectively. Section Ia (Fig.3) shows two temperature maxima in Northeast Channel, with T>15^oC at depths of 50-100m on the eastern side and T>13^oC at 80-120m on the west. The salinity section suggests that the eastern maximum, with levels in excess of 35.5, is derived from a Warm Core Ring (WCR) or the Gulf Stream. Salinities on the western side barely exceed 35, but may represent a diluted product from the same source(s). Near the surface, colder, fresher waters are found over each of the Banks, with T<3^oC(5^oC) in the top 20 m over Browns(Georges), respectively. The low surface salinities over Browns (S<31.5) identify this layer as Scotian Shelf water, whereas the higher salinities over Georges (32<S<33) are characteristic for that Bank. The deep waters (>100m) on this section show S<35 and temperatures of 8<T<10^oC. The T-S plots show interleaving between this water mass and the warm, saline slope water above. The former appears to be a diluted form of Labrador Slope Water (T~7-8^oC; 34.5<S<35); the latter may be identified as Warm Slope Water (T~15^oC, S~35.5) The density section (Fig.3c) reveals that the water over Browns is much more highly stratified due to the intrusion of slope water beneath the cold, fresh surface layer. Furthermore, the sloping isopycnals suggest a strong vertical shear on the east, which may be reversed in the deep layers as the isopycnals slope in the opposite direction there. As expected, the dissolved oxygen section shows high values (>8 ml/l) in the surface layers, but generally reduced levels in the intrusion waters.

On Section Ib (Fig.4), the distribution of high temperatures is much more pervasive than on Ia, both in the NEC and on the Banks. The cold surface layers have virtually disappeared and warm conditions (10-11^oC) prevail at depths of 30-50m, although the extreme temperature maximum (T>15^oC) on Ia is also absent. The minimum surface salinities (32.5-33.0) indicate that Scotian Shelf water is not to be found in this part of the NEC. The density section shows less stratification than on Ia, but also shows some static instabilities that need to be checked (e.g. at CTD14). Again the T-S plots indicate interleaving between Labrador and Warm Slope Waters at CTD10-12.

Section II (Fig.5) across east-central Georges Bank indicates very little contrast in water properties (5<T<6^oC; 32.5<S<33) over the Bank, except near the shelf break front where the water stratifies. There is also a region of low surface salinity (<32.5) near CTD29-30, which may represent diluted Scotian Shelf water that contributes to the stratification on the SE flank. The T-S plot indicates the presence of the two deep water types: Labrador Shelf Water (T~7-8^oC; 34.5<S<35) and Warm Slope Water (T~15^oC, S~35.5). The former is found in Georges Basin (CTD22) while the latter seems to prevail offshore (CTD30-31) but with evidence for strong interleaving. The oxygen section shows a strange maximum at CTD26, but otherwise is reasonably well mixed over the Bank with surface values in excess of 7.5 ml/l.

Section III (Fig.6) clearly shows the proximity to the Bank of a warm, saline WCR or Gulf Stream feature which is overlain by a cold fresh surface layer. Pockets of cold, fresh water (T<4^oC, S<32) found near the surface (depth<30m) at CTD8,15,17, and 19 may be undiluted versions of Scotian Shelf Water arriving at the eastern tip of the Bank. The lightest water on the section is found at CTD8,15. However, most of the section over the Bank is composed of Georges Bank water (32<S<33). A sharp transition in the surface properties occurs between CTD34 and 35, and again the contrast between the two types of slope water is evident in the T-S plot. In particular, there appears to be evidence for Warm Slope Water at CTD18,19, but not so at CTD15. The maximum deep salinities in Georges Basin (34.5<S<35.0) and offshore (>35.5) create a density contrast between the two regions at depths of 50-200 m, which may be, in part, responsible for driving the intrusion through NEC.

Conditions on Section IV (Fig.7) show that the Gulf Stream/WCR water has encroached well onto the shelf on the eastern side of NEC. A sharp front in temperature, salinity and density between coastal (2<T<5^oC, S<31.5) and offshore (T>15^oC, S>35.5) waters lies between CTD41-42 on outer Browns Bank. The T-S plot shows evidence for exceptionally strong interleaving in multiple layers at CTD 41, and Warm Slope Waters with salinities and temperatures in excess of 36 and 16^oC, respectively, at CTD37, just off the mouth of the NEC. Over inner Browns, the salinity rises to >32 throughout the water column, suggesting that its origin may be to the west rather than the east, or it has somehow been diluted by offshore waters. Again the T-S plot indicates that the Scotian Shelf source waters are found inshore at CTD48. Surface oxygens are high (>8.5 ml/l) over the shelf and lower (~6.0 ml/l) in the offshore waters.

Conditions on Section V (Fig.8) across the western Scotian Shelf are similar to those on Section IV, except that the Gulf Stream/WCR water has not penetrated beyond the shelf break, where a very sharp front exists. Scotian Shelf waters from the east (S<31.5) are pervasive in the surface layers over the shelf, with the major transition in properties occurring at CTD54 (Fig.8e). The T-S plot also indicates some mixing with a water mass other than that offshore at CTD53, perhaps a remnant of the cold intermediate layer on the shelf or Labrador Slope Water. Again the surface oxygens differ between inshore and offshore waters.

6. DRIFTER DEPLOYMENTS

A complement of five WOCE-style drifters (spherical surface flotation ball encasing ARGOS transmitter, holey sock drogue centred at 10 m) were deployed during the mission (Table 5). After an eddy-like feature in Northeast Channel was crudely mapped, three of the drifters were placed in a line across the eastern front of the eddy (between CTD13-14; see Table 2, Fig.1a) to determine whether the eddy flow field would sweep them across the Channel.

The remaining pair of drifters was deployed off the northern flank of the Bank (at CTD21,22; see Table 2, Fig.1a) to determine whether there was direct flux across the Bank from Georges Basin.

Drifter tracks (as of March 22, 1999; Fig.9) indicate that the three buoys deployed on the eastern side of the Channel were indeed swept across to the west, then were drawn off into the slope water by a Gulf Stream warm-core ring.(Fig.9a). Of the two buoys deployed off the northern flank, they converged at first, then split, with the inner one passing onto Northeast Peak, and the outer executing a circuit counter-clockwise around Georges Basin (Fig.9b).

7. FLOW-THROUGH SYSTEM

Surface fields based on the flow-through system (Fig.10a,b) reveal cold, fresh water (T»2-3^oC. S<31.5) over the Scotian Shelf to its edge, beyond which lies a warm-core ring (T>17^oC. S>36.0) bounded by a sharp front near the 1000m isobath. Conditions over inshore Browns Bank and across Georges Basin are somewhat warmer and saltier (T»3-4^oC. S<32), while on Georges Bank, conditions are warmer and saltier still (T»5-6^oC. S~32.5). Surface properties over Northeast Channel and the eastern tip of Georges Bank are quite variable where an intruding tongue of slope water meets the Scotian Shelf surface water near the mooring line (Section Ia; Fig.3). There is also some evidence for the presence of Scotian Shelf water over the eastern and southeastern flanks of Georges Bank near the 100m isobath.

Surface nutrient fields (Fig.10c-e) appear to have low concentrations for this time of year, based on historical observations (B. Petrie, personal comm.). In particular, surface values of nitrate are expected to be in the 8-12mM range over the western Scotian Shelf in mid-February, compared to the observed values of 4-6mM (Fig.10c). Similar levels are expected for the Gulf of Maine surface waters, compared with the observed values of 7-8mM over Georges Bank.

Acknowledgements:

We are greatly indebted to the officers and crew of the C.C.G.S. Parizeau for their skilled assistance and friendly cooperation, which was vital to the success of this mission. We also thank Erica Head, Jim Reid and Jeff Anning for their advice and support with the biological sampling systems.

LIST OF TABLES

Table 1. Moorings Deployed During Parizeau Cruise 98078, 10-16 February, 1999

Table 2. CTD Stations During C.C.G.S. Parizeau 98-078, 10-16 February 1999

Table 3. a) Salinity and Nutrient Calibration Results for Parizeau 98-078

b) Surface Dissolved Oxygen Regression Results for Parizeau 98-078

c) 5 dBar Corrected CTD vs. Flow-Through System for Parizeau 98-078

d) Extracted Chl-a vs. CTD Fluorometer for Parizeau 98-078

Table 4. a) Cruise 98078 Sample Information-Biological Stations

b) Long-Term Monitoring Station Information

Table 5. Drifter Deployments during C.C.G.S. Parizeau 98-078

TABLE 1. Moorings Deployed During Parizeau Cruise 98078, 10-16 February, 1999

Mooring Site N. Lat. Placement Instrument

No. (Depth,m) W. Long. Time(Z),Date (Depth,m)

Guard Buoy “V” NECW 42^o07.38’ 1325, Feb.12 SC2325(1)

(213) 66^o01.08’

_________________________________________________________________________________

TABLE 2. CTD Stations During C.C.G.S. Parizeau 98-078, 10-16 February 1999