Section 1.0 - Background

Section 2.0 - Cruise Narrative

Section 3.0 - Physical and Chemical Oceanographic Measurements

Section 4.0 - Bio-Optical Characterization of Bering Sea Waters

Section 5.0 - Productivity Studies

Section 6.0 - Acknowledgements

Section 7.0 - Figures

Section 8.0 - Tables
 

Ship:

Area of Operations:

Itinerary:

Chief Scientist:

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1.0 Background

1.1 Program Description

1.2 Cruise Objectives

The purpose of the NPMR project is to understand the influence of mesoscale eddies on continental slope-shelf exchange in the Southeastern Bering Sea. The objectives are to

(1) Detect movements of nutrient-rich slope water onto the shelf and relate them to temporal and spatial variations in chlorophyll,

(2) Identify physical mechanisms that create slope-water fluxes onto the continental shelf,

(3) Detect ocean-color variability in relation to physical processes,

(4) Use shipboard measurements of near-surface optical and biological parameters to validate and extend bio-optical algorithms for use in autonomous sampling and remote sensing, and

(5) Investigate the effects of on-shelf flow on phytoplankton biology.

1.3 Operating Area

1.4 Participating Organizations

University of Alaska-Fairbanks (UAF)
School of Fisheries & Ocean Sciences
P.O. Box 757220
Fairbanks, AK 99775-7220

Dalhousie University (Dal)
Department of Oceanography
Halifax, NS B3H 4J1
Canada

Smithsonian Environmental Research Center (SERC)

1.5 Personnel

CTD Watches: Salo and Kinoshita midnight-noon (ADT = GMT - 8 hr)
                        Dobbins and Curran noon-midnight
 

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2.0 Cruise Narrative

After station 16 the ship left the SEBSCC line and proceeded to the slope-shelf-exchange study area. Two separate tools provided a way to locate eddies there - sea-surface altimetry observations from the TOPEX/Poseidon satellite and satellite-tracked drifting buoy trajectories. The former were available from World Wide Web pages http://www-ccar.colorado.edu/~leben/research.html provided by Dr. Robert Leben at the University of Colorado which we were able to download throughout the cruise via Inmarsat-A satellite-telephone modem connections. Unfortunately the altimetry showed only weak eddy activity and suggested no likely site for our study. On the other hand, the trajectory of a satellite-tracked drifter drogued at 40 m and launched a few months earlier showed a cyclonic eddy of ~50 km diameter off the continental shelf. We opted to study this eddy for two reasons: (1) it was potentially near enough to the continental shelf to interact with it and push basin water landward, and (2) it rotated cyclonically (counterclockwise) which is of the opposite sense to the anticyclonic eddies usually detected in the SE Bering Sea. This gave the first opportunity to study a persistent, cyclonic eddy in this region.

The ship began a rectangular grid pattern of CTD casts (stations 17-71 of Fig. 1b, casts 15-75 of Fig. 2b). Temperature and salinity measurements allowed us to compute water density, dynamic height and geostrophic velocities (usually referenced to 1500 dbar) normal to CTD sections. Graphs of the transects, available within ~1 hour of transect completion, showed cyclonic eddy flow with maximum velocities of ~25 cm/s, a velocity-maximum core at ~300 m depth, and a diameter of ~80 km. We seeded the eddy with one satellite-tracked drifter that measured ocean color (a color drifter, stn. 29). After the initial slices through the eddy, we filled in the sampling grid toward the continental shelf break, looking for signs of on-shelf flow. Geostrophic velocity sections became less useful in shallow water where it was too shallow to refer them to 1500 dbar, and a level of no motion assumed to be at the sea bottom seemed untenable. After several days of sampling, we deployed two more drifters at the north (stn. 70) and south (stn. 71) ends of the shoreward side of the eddy study region. We planned to return to the drifter showing the greatest on-shelf flow and continue sampling.

SEBSCC Line sampling resumed at stations 72-74 which repeated stations 14-16 of several days earlier (Fig. 1a). Mooring 3's Argos transmitter had ceased transmitting a few weeks before the cruise, and we could not locate the mooring on the surface. Attempts to enable and range upon its underwater acoustic release failed, and we presumed the mooring lost. Intensive bio-optical sampling was conducted near SEBSCC Mooring 3's deployment site (Table 1 and Fig. 1a).

At dawn on 7 September near station 89, the sea was aquamarine in color indicating the presence of a coccolithophorid bloom. Owing to darkness, it was not known where that condition was first encountered, but it probably occurred somewhere between stations 83 and 89. Extra bio-optical sampling efforts were laid on to take advantage of the situation. Mooring 2 was in good condition based upon a visual inspection.

We made periodic attempts to contact Mooring 3's acoustic release. Surprisingly, we got a positive response at a range of ~6 km near station 97. A search pattern gave more responses at ~2 km range, and the ship was able to range within ~200 m. The position was noted and relayed to PMEL for future mooring recovery.

SEBSCC Line sampling proceeded to Mooring 4 where it was completed at stn. 102 (Fig. 1a). Then the ship returned to the eddy study area. The two drifters deployed over 3 days earlier at stns. 70 and 71 did not show much on-shelf flow, so we began sampling the seaward side of the cyclonic eddy at stn. 103 (Fig. 1b). CTD casts were taken to 1500 m filling in the 10 x 10 km grid begun earlier. Bio-optical work focussed on obtaining samples within and outside of the eddy to draw contrasts. Two drifters were deployed near the southwestern edge of the eddy at stns. 134 and 139. This work continued to the last station (stn. 141) when we returned to Dutch Harbor as scheduled.
 

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3.0 Physical and Chemical Oceanographic Measurements

A 150-kHz RDI acoustic Doppler current profiler (ADCP) ran continuously measuring the velocity of the water relative to the ship. Auxiliary input from a Trimble P-code GPS receiver, Sperry ring-laser-gyro and Seatex Seapath 200 GPS-based attitude determination unit allow for the determination of absolute current velocity relative to the sea bottom.

The ship carried a thermosalinograph to measure the temperature and salinity of near-surface waters and a flow-through Turner fluorometer to measure near-surface fluorescence on a continuous basis.

Nutrient samples were collected from the majority of the CTD casts and were analyzed on board using an Alpkem RFA300 Auto-Analyzer. A total of 1032 samples were analyzed. There were 355 analyzed from the SEBSCC Line, 631 from the eddy transects and 46 for C/N productivity studies.
 

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4.0 Bio-Optical Characterization of Bering Sea waters

We collected measurements of biological and optical properties of near-surface waters throughout the cruise. This section outlines some of the preliminary results and explains types of data collected. Emphasis is on aspects directly related to NPMR research.

4.1 Solar Radiation

4.2 Upwelling Radiance

The ratio of L_u to E_d gives the reflectance spectrum (R; sr^-1) which shows the biologically determined optical variability associated with the coccolithophore bloom as encountered on the middle shelf. Figure 3 contrasts deployments inside and outside of the bloom. The reflectance spectra have been normalized to R(490).

4.3 Chlorophyll a

An objective of our study was to relate measures of near-surface reflectance to surface chlorophyll. These data are consistent with many studies of remote sensing that identify log(L_u(490)/L_u(555)) as a measure of chlorophyll. We report reflectance here because variability in solar E_d(490)/E_d(555) was measured, and will be incorporated into our model.
 
 

A major task of this cruise was to develop an algorithm for interpretation of optical drifter data. This requires comparison of surface chlorophyll with ocean color measurements. The preliminary analysis of Figure 4 indicates that a robust algorithm can be developed for chlorophyll concentrations > 1 mg m^-3. At this time, the relationship for bluer waters looks weak. Caution is needed, however, because these data are for bucket samples only, and vertical gradients near the surface can affect the signal in clearer waters.
 

4.4 Penetration of Spectral Irradiance

4.5 Bio-Optical Profiling Package

Absorption (a) and attenuation (c) of the upper water column were measured at 9 wavelengths (412, 440, 488, 510, 532, 555, 650, 676, 715 nm) using a WETLabs ac-9. This instrument has a 25 cm path length. Two profiles were performed at each station. The first was an unfiltered drop to measure a and c for all constituents in the water. A second filtered cast (0.2 mm) was performed so that a and c of dissolved material only was measured. Daily calibrations of the instrument were performed with ship water run through an organic-removal filter, an ultrapure filter, and finally a 0.2 mm filter.

Absorption and attenuation within the coccolithophore bloom were quite high, with c values over 4 m^-1. Elsewhere attenuation values were an order of magnitude lower at 0.4 m^-1.

A Chelsea Instruments FASTtracker Fast Repetition Rate (FRR) fluorometer was used to evaluate the vertical distributions of phytoplankton abundance and photosynthetic performance. The FRR fluorometer (FRRF), like all fluorometers, measures the fluorescence signal emitted from chlorophyll contained in a small volume of water. The FRRF delivers a rapid sequence of shortly spaced, high intensity flashes (usually at a rate of 100-200 kHz) and measures the transient emissions from this sequence. These flashes gradually saturate the photosystem of algal cells contained in the sample volumes of 2 similar chambers. Under in situ conditions, one chamber measures the response of algal cells under ambient irradiance conditions. The other chamber is shielded from light and measures the fluorescence immediately after transfer of the algae to darkness.

The fluorescence measurements that are made from the 2 chambers can be used in isolation or in combination to provide values of biophysical parameters associated with the flash stimulus and emission. These parameters can be further incorporated into mechanistic models of photosynthesis and ultimately derive rates of photosynthetic production.

The values of minimum fluorescence (F_o), maximum fluorescence (F_m) and variable fluorescence (F_v) provide an index of chlorophyll concentration and the concentration of functional photosystem II reaction centres. The ratio of variable to maximum fluorescence provides an index of the efficiency of photosystem II. Values of F_v/F_m of 0.6 to 0.65 indicate highly efficient, nutrient-replete cells. Low values of F_v / F_m (<0.3) in surface waters are indicative of severe nutrient limitation. However, fluorescence signals need to be interpreted with caution because bright light can significantly depress fluorescence yields.

During RB-00-07 F_v/F_m values were typically 0.5, with lower values observed near the surface.

A HOBI Labs HydroScat -6 was used to measure backscattering at 6 wavelengths (442, 470, 510, 589, 620, and 671 nm). Data from this instrument were not processed on the cruise.

The objective of this study was to quantify the influence of in situ particle size and concentration on backscatter of light. An in situ digital silhouette floc camera was deployed at approximately 10 stations, including the coccolithophore bloom stations on the shelf and outer shelf eddy stations. The camera suffered from an early setback when it was damaged but was able to be repaired. The camera is capable of resolving particles down to 100mm in diameter. Camera images will be processed back in the lab using Image Pro Plus imaging software and compared to the backscatter coefficients measured with the HydroScat.
 

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5.0 Productivity Studies

Productivity studies were conducted during the cruise. Photosynthetic and nitrogen uptake rate measurements were estimated by the addition of heavy carbon and nitrogen isotopes to euphotic zone water collected in Niskin bottles at light levels measured by the Dalhousie group's underwater PAR sensor. Seawater was apportioned into incubation bottles with neutral density metal screens to simulate the light levels in the euphotic zone of 100%, 50%, 30%, 12%, 5%, and 1% of surface values. After the addition of ¹³C and ¹⁵N enriched compounds (H¹³CO₃^-, ¹⁵NO₃^-, ¹⁵NH₄⁺ and ¹⁵N-Urea) to each incubation bottle, they were placed in child-sized swimming pools on deck and cooled with running surface sea water. Incubation was terminated by filtering seawater using GF/F glass fiber filters after 4 hours of incubation. The filter samples were frozen and will be analyzed using mass spectrometric methods at the stable isotope laboratory of UAF.

In addition to the productivity measurements in the euphotic zone, several additional experiments were conducted. First, we measured surface carbon and nitrogen (NO₃^- and NH₄⁺) uptake rates at the eddy stations and on the SEBSCC line. Surface sea water was transferred from Niskin bottles to 250 ml polycarbonate bottles and spiked with ¹³C and ¹⁵N enriched compounds. The bottles were placed in the incubation tank at the surface light level. During these surface productivity measurements, we took surface and water column chlorophyll samples (0, 10,20, and 50m) along the SEBSCC line from the shelf break station to Mooring 4.

Second, we measured how the mixing of slope and shelf water affects the productivity of the mixture. For this experiment, we placed surface water from the 140-m-depth station (stn. 16) into three 10-L jugs. One was placed into an incubation tank without any treatment (control 1). Water in the others was filtered through GF/F glass fiber filters, and the filtrate was collected and refrigerated. At the next open-ocean station (stn. 17), we also collected surface water for the mixing experiments. We mixed 9 L of unfiltered open-ocean water with 4 L of filtered shelf water (treatment 1). We also used 100% open-ocean water for another control (control 2). Every day we subsampled each jug and measured the ¹³C and ¹⁵N (nitrate and ammonium) uptake rates. Chlorophyll and nutrient samples were also taken daily. To investigate the effects of the onshore transport of nutrient-rich slope water onto the shelf, we collected surface water from station 56, 57 and 58. Filtrates were collected through GF/F glass fiber filters and refrigerated. 100 ml of filtered seawater were mixed with these and sampled for ¹⁵N-NO₃^- uptake.

Third, we performed nutrient-addition experiments to coccolithophore bloom water. We collected surface water (5 m) at Mooring 2 and transferred it to five 10-L jugs for nutrient treatments (control, 20 mM NO₃ addition, 10 mM NH₄ addition, 20 mM NO₃ + 5 mM PO₄ addition, 10 mM NH₄ +5 mM PO₄). We took initial samples for ¹³C and ¹⁵N (NO₃ and NH₄) uptake, chlorophyll and in vivo fluorescence. We collected those samples every day for 5 days. We also made one measurement of nitrate and ammonium uptake kinetics in the coccolithophore bloom.
 

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6.0 Acknowledgements

7.0 Figures

Figure 1b -- RB-00-07 Leg 1 Eddy Station Locations

Figure 2a -- RB-00-07 Leg 1 CTD Cast Locations

Figure 2b -- RB-00-07 Leg 1 Eddy CTD Cast Locations

Figure 3 -- Normalized reflectance as a function of wavelength.

Figure 4 -- Chlorophyll concentration as a function of reflectance ratio.  1996 and 1997 cruise data are shown along with preliminary data from this cruise.
 

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8.0 Tables