May 19, 1997
Miller Freeman Cruise MF97-05, Leg 2 Report
FOCI No: 6MF97
Operating Area: Eastern Bering Sea shelf
NOAA - Alaska Fisheries Science Center (AFSC)
University of Texas (UT)
Edward D. (Ned) Cokelet , Ph. D. PMEL
Debbie Blood AFSC
The purpose of this cruise was to investigate spring-bloom physical, chemical and biological oceanographic processes in the southeastern Bering Sea, a major center for primary production that supports the upper trophic levels in the region. The renewal and dispersal of nutrients by physical processes and regeneration by organic decomposition is balanced by physical stratification and phytoplankton uptake as the seasons progress from winter to summer. This Southeast Bering Sea Carrying Capacity (SEBSCC) May cruise represented the major bloom time period where phytoplankton utilization of nutrients is highest throughout the year, and conditions tend toward nutrient depletion in the euphotic zone. Only occasional mixing events provide any new nutrients to the upper water column. Within this context, nutrients were sampled in conjunction with physical sampling for water mass identification of source water that enriches shelf processes. Biological samples were also analyzed for nutrient content to provide the basis for experimental measurements of nutrient uptake for the phytoplankton populations. At productivity study sites the rates of photosynthesis and nitrogen uptake (NO3, NH4 and urea) were measured in the euphotic zone.
A study of the effects of low temperatures on the incubation rate of pollock eggs began on cruise MF97-04 and continued on board during this leg.
This FOCI (Fisheries Oceanography Coordinated Investigations) cruise was part of the NOAA Coastal Ocean Program SEBSCC project. FOCI is a basic research effort by NOAA scientists to understand the physical and biological processes that determine recruitment variability of commercially valuable fin fish and shellfish stocks in Alaskan waters. SEBSCC is a collaborative effort by NOAA and academic scientists to understand the effects of abiotic and biotic variability on the SE Bering Sea ecosystem.
Physical oceanographic measurements were made on conductivity-temperature-depth (CTD) casts and via acoustic Doppler current profiler (ADCP) transects. Water samples were collected with bottles attached to the CTD profiler rosette. Bongo and ring net tows provided zooplankton and fish larval samples.
Water samples from all 79 hydrographic stations were collected at approximately 850 depth horizons sampled by the CTD/rosette profiling system for a total of about 4300 analyses. The biogenic nutrients (phosphate, silicate, nitrate, nitrite and ammonium) were analyzed on board ship in near real time to provide a basis for further sampling and definition of biological patterns. An additional 400 frozen water samples from the February 1997 Miller Freeman current meter mooring deployment cruise (MF97-01) were also analyzed to provide an estimate of the late winter nutrient conditions of the study area. Thirty nutrient calibration samples were analyzed in conjunction with a collaborative NOAA investigator from the April cruise (MF97-05, Leg 1), and about 30 additional replication samples were collected on the present cruise.
Nutrient samples were analyzed from 16 productivity stations at six sampling depths in collaboration with phytoplankton uptake experiments. The samples were analyzed fresh for nutrient concentrations and frozen for later analysis of dissolved organic nitrogen and urea concentrations. Photosynthetic and nitrogen uptake rates were estimated by the addition of H13CO3, 15NO3, 15NH4 and 15N-urea to euphotic zone water collected at depths corresponding to the 100%, 50%, 30%, 12%, 5% and 1% surface light levels as determined from Secchi disk or underwater PAR (photosynthetically available radiation) sensor measurements. After the addition of 13C- and 15N-enriched compounds, the uptake samples were incubated on deck for about 4 hours in a surface-sea-water-cooled tank. In situ light levels were produced artificially by fitting the incubation bottles with neutral density stainless steel screens.
Chlorophyll and pigment samples from the upper mixed layer were filtered on most hydrographic stations and at all depths of the productivity stations for a total of more than 400 samples. Several samples from surface water were also filtered for HPLC pigment analysis to obtain a profile of phytoplankton groups present.
The following tables show the total numbers of samples taken and gear types employed as summarized from the Discrete Sample Data Base (DSDB).
The following table summarizes the productivity studies which will provide information to estimate photosynthetic carbon uptake and the proportion of new productivity (NO3 uptake) and regenerated productivity (NH4 and urea uptake) during the spring bloom.
Table 2. Productivity measurement stations.
Summary of Cruise
The first scientific sampling began along a line of CTD stations crossing the Aleutian north-slope flow west of Dutch Harbor. A satellite-tracked drifting buoy was deployed at station 6 (Fig. 1) near a hypothesized eddy center as determined from sea-surface topographic anomalies measured by satellite. The drifter remained near the point of deployment and probably was not in an eddy.
During the cruise, CTD and water bottle casts were to within 10 m of bottom or 1500 m, whichever was in shallower. Nutrient samples were gathered on nearly every cast usually with 11 bottles spread over the depth range.
Owing to the depth limitations of the instruments sampling biological variables on the CTD package, we had to remove the light meter, fluorometer and CHLAM (chlorophyll light absorbance meter) for the deep casts in the Bering Sea basin. It was not practical to take double casts (one deep and one shallow) at each station because the instruments could not be removed and remounted easily and reliably. Therefore we failed to obtain as comprehensive a data set as potentially possible at these sites. FOCI should upgrade these instruments to provide greater depth capability - not so much to measure at the deeper depths, but to optimize the use of ship time on multi disciplinary cruises.
The ship proceeded NE along a transect over the basin, across the continental slope and onto the shelf with CTD and water bottle casts at each station (Fig. 1). The northeastward station (stn. 32) was in the inner shelf domain as shown by water properties which were nearly independent of depth. The ship then turned NW along a 70-m-isobath transect from Moorings 2 to 4 (stns. 34-40, Fig. 1). X-shaped sections were occupied at Moorings 3, 2 and 4 to provide information on horizontal gradients.
In water depths less than 300 m the biological instruments were added to the CTD package and remained attached while on the shelf. Water samples for phytoplankton productivity studies were taken nearly every day on the first morning cast. They were incubated in screened water bottles simulating different light levels and placed in a rubber wading pool on the fan tail. Samples were filtered and preserved for later mass spectrometry.
Bongo tows for zooplankton and fish larvae were taken to 300 m or near-bottom on the shelf and at some basin sites. Ring tows for live samples were made once a day on the shelf.
From the Mooring-4 cross (stns. 39-44) we steamed SE to recover and redeploy 2 crab-study moorings (KC96-2 & KC96-1). Mooring KC96-2 would not respond to acoustic interrogation and is presumed lost. We deployed mooring KC97-2 at that site and moved on to KC96-1. It was released, recovered and replaced by a new mooring (KC97-1). Each crab mooring contained just 1 MTR (Miniature Temperature Recorder).
The Miller Freeman steamed to Unimak Pass where we sampled 4 transects in a rectangle (Fig. 1). From there we moved to Mooring 3 and sampled chlorophyll concentrations in triplicate at the depths of the deployed bio-optical sensors. A part of the cross-shelf transect was reoccupied heading seaward.
With a new potential eddy location as determined from satellite sea-surface topography communicated from PMEL, we turned NW to sample across the eddy. We intersected it with a line of CTD casts to 1500 m and computed the dynamic topography. From this we deduced the updated location of the eddy center and deployed a drifter. We backtracked with 300-m CTD casts and all biological instruments attached. Nutrient samples, zooplankton bongo tows and productivity casts at each site should provide insight into the biological activity there if this was indeed an eddy. The ship returned to Dutch Harbor on the morning of May 13.
AFSC investigations of the effects of low temperatures on pollock egg development continued throughout the cruise. Periodically, the water in which the eggs were kept was replenished with fresh sea water.