AMLR 2002 Weekly Report No. 8

 

3 March 2002

 

1. Current position 66 nautical miles north of Elephant Island, conducting a survey of bio-oceanographic conditions in the vicinity of the South Shetland Islands. The west area of the sampling grid is behind us. We are currently sampling the Elephant Island area. Tranquil weather conditions the past seven days have allowed us to progress steadily, with only one minor delay of two hours due to a heavy swell during our first night of the survey.

 

2. All seagoing equipment is functioning, except for the backup salinometer that was shipped from La Jolla, CA to Punta Arenas, Chile to replace our nonfunctioning unit from Leg I. Unfortunately the replacement unit is not providing correct salinity measurements. In this case two water samples from every CTD cast are being saved for later salinity analysis in La Jolla.

 

3. Krill, salps and other zooplankton-West area synopsis. Krill were present in 11 of 24 West area samples. Two extremely large catches over the northern shelf of the South Shetland Islands, containing ca. 10,000 and 20,000 individuals each, dominated the West area demographics. These were mostly (75%) small juvenile and immature stages representing the 2000/2001 year class. Together the other samples contained <100 krill; large mature krill were virtually absent. Larval krill occurred in seven samples offshore of, and over, the outer shelf. The maximum concentration was nearly 2000 larvae per 1000 m3 and overall mean abundance (135 per 1000 m3) was four orders of magnitude greater than that monitored here during January.

 

While mean salp abundance was substantially larger than observed than during January (1200 vs. 93 per 1000 m3) the median value was reduced by >50% (16 vs. 39 per 1000 m3). This was due to a less homogeneous distribution with greatest concentrations (>1000 per 1000 m3) retracted to four stations well offshore in Drake Passage. Most of the salps (75%) were small aggregate forms produced within the past month.

 

Copepods were by far the most numerous taxon. They were present in all samples and had a median abundance of 12500 per 1000 m3. This value represented a 25% increase over that the previous month. Densest copepod concentrations occurred in oceanic water offshore of the island

shelf area.

 

A carnivorous amphipod species, Themisto gaudichaudii, was present in all West area samples with a median abundance of 23 per 1000 m3. Another predatory amphipod, Primno macropa, was in >60% of samples and also relatively abundant (median 2 per 1000 m3), possibly in response to the elevated copepod abundance.

 

4. Krill biomass and dispersion. Preliminary analyses of the West Area acoustical survey indicated a decrease in krill density relative to leg I (10.6-29.5 versus 3.2 g/m2, respectively). Highest krill abundance for the West Area was observed northeast of Livingston Island along the shelf. This region coincided with the two IKMT stations yielding the highest numbers of krill. Net samples also confirmed low numbers of krill observed acoustically throughout the remaining portions of the West Area. There is no obvious correlation between legs I and II for this region of the survey.

 

5. Phytoplankton. Corresponding with the now clearer delineation of water types during this portion of Leg II, chlorophyll at both horizontal and vertical scales are approaching more classical descriptions with notable exceptions. Chlorophyll concentrations at 5 meter depths for Zone I waters (furthest from the South Shetland and Elephant Islands) averaged 0.44 +/- 0.28 mg m-3 (8 stations), Zone II waters averaged 1.15 +/- 0.69 mg m-3 (10 stations), and Zones III (shelf-related) waters averaged 1.34 +/- 0.69 mg m-3 (5 stations). Integrated values of chlorophyll (to 100 meters) were 32.3 +/- 18.2, 67.7 +/- 37.5 and 67.1 +/- 25.9 mg m-2 for Water Zones I, II, and III respectively. Four stations at the southwestern portion of the Bransfield Strait sampled had 1.35 +/- 1.06 mg chl-a m-3 (57.1 +/- 26.2 mg chl-a m-2 integrated to 100 meter). Chlorophyll concentrations for Zone I waters had chlorophyll concentrations that were higher than classically described, and with nearly all lacking maxima at and above the thermocline.

 

6. Oceanography and meteorology. Wind predominantly from the east and northeast except for Thursday and Friday when it blew from the northwest and southeast, respectively. Moderate wind speeds ranging between 0 and 20 knots were experienced for most of the week except on Sunday and Saturday where maximum levels of 30 to 40 knots were recorded. Pressure dropped gradually reaching a minimum of 964 mb on Tuesday before rising steadily to 966 mb. Air temperature fluctuated between 4 and 0.25C, showing a decreasing trend towards the end of the week. Conditions were generally overcast with occasional rain and snow. Water Zones have become more clearly defined during first portion of the standard survey grid for the West Area. Zone I (Drake Passage) waters predominated the outer line of stations above the South Shetland Islands and penetrated as a tongue (Stations 11-03 and 09-03) into Zone II waters (9 stations) north west of Elephant Island. Zone III waters were found to hug the shelf and shelf-break to the north of these islands (9 inner-most stations) with intrusion of Bransfield Strait (Type IV) waters for the three stations between Elephant and King George Islands. Zone II waters were found for the remaining 26 stations and lay between those classed as Zones I and III.

 

7. Predator diet studies. Lipids have been extracted from an additional 24 fur seal milk samples and frozen. During the ongoing large area survey excess krill and myctophids caught in the net tows continue to be frozen for processing at the end of the leg. An additional twelve fur seal scats were also processed and krill carapaces were measured and stored in ethanol. Scats fell into two categories this week: those containing large amounts of krill chitin and few otoliths, and those that were almost entirely fish remains with close to zero chitin. Unlike week 5 scats, week 6 otoliths show a predominance of G. nicholsi, with E. antarctica appearing less often and E. carlsbergi nearly absent. Two more squid beaks were found and preserved in ethanol for later identification to species. After radio contact with the Cape Shirreff, it was learned that squid beaks are also appearing in current island scats.

 

8. Advanced Survey Technologies.

A. Near shore survey. The data sets from the near-shore survey were preliminarily analyzed aboard ship to study the relationships between the oceanography and biology of the area. Preliminary results of the ship and R/V Ernest collected bathymetric data for this region indicate the presence and effect of two large submarine canyons that flank Cape Shirreff. As was seen in the 2000 survey effort, the highest concentrations of krill were found in the near-shore region southeast of Cape Shirreff. However, this year's survey also found high densities of krill in the near-shore region southwest of the Cape. Based on the 2000 near-shore survey and the 2002 net tow data from Yuzhmorgeologiya, the acoustical targets are believed to be euphausiids, Thysanoessa macrura and Euphausia superba. Individual target strengths (TS) recorded between 10 and 40 m depth with along- and athwart-ship angles less than 3 had a bimodal distribution with a major mode centered at approximately -68 dB. This value is consistent with the results from the near shore survey in 2000, and is believed to indicate that the scatterers are large krill (length > 5 cm). The higher TS values are likely from small fish.The results of the IKMT net samples show that juvenile krill had a higher concentration offshore (water deeper than 500 m) while adults were generally found in waters shallower than 500 m. The most abundant species at each station varied between juvenile and adult stages of Euphausia superba, Thysanoessa macrura, and Euphausia frigida. Copepods were also abundant and had a similar distribution to that of the krill. The distribution of Thysanoessa macrura appeared to follow the bathymetry of the region to some extent with higher densities of these animals found in regions in or near the two submarine canyons that flank Cape Shirreff. An analysis of the CTD and oxygen profiles collected from Yuzhmorgeologiya also shows a relationship between the physical oceanography of the region and the bathymetry, particularly the submarine canyons. Alongshore profiles show two elevated regions of temperature and oxygen in the near surface water, which may possibly be from Antarctic Circumpolar Current water that is upwelling through the submarine canyons. This water would provide nutrients to the near-shore region that would stimulate primary and secondary production, and increased oxygen. This is a possible explanation for the elevated regions of acoustic scattering that were observed during this survey, higher oxygen levels, and why these particular near-shore regions are the primary foraging grounds for the penguin and seal populations of Cape Shirreff.

 

B. Multi-instrumented buoys. One challenge of investigating the responses of the land-based predators to changes in food availability is to temporally and spatially match the observations of penguins, seals, krill and fish. Towards this end, multi-instrumented, remotely-monitored, oceanographic buoys were developed to provide long time-series measurements of relative krill abundance in the near-shore area of Cape Shirreff. The first buoy was fitted with a 300 kHz acoustic Doppler current profiler to measure current vectors, acoustical volume backscatter, water temperature, pitch, roll, and bearing. The second buoy was fitted with 38 and 200 kHz scientific echosounder. The buoys also include a data logging computer, GPS, radar reflector, strobe, radio-modem, and power management circuit. Remote control of the instrumentation and real-time monitoring of data is accomplished by radio-telemetry between the buoy and a control station at Cape Shirreff. The first buoy was deployed during the near-shore survey, approximately 5 n.mi. east of Cape Shirreff near the head of a submarine canyon. The mooring location was chosen for its consistent association with krill aggregations and predator foraging activity. Preliminary results have identified a shoreward current in the canyon, possibly causing upwelling of deep water into the neritic zone. Also identified is a strong diel variation of krill abundance in the area. The second buoy continues to collect data in the same location and will be recovered at the conclusion of the large-area survey.

 

C. Acoustical characterization of zooplankton and nekton. Using a new multi-scattering technique, acoustical total target strength (TTS) measurements have continued to be made aboard the ship over a wide-bandwidth for Euphausia superba, Electrona Antarctica, and Cyllopus. These first-of-their-kind measurements are preliminary to the development of an improved classification method for the three-frequency echo sounder data. In the experiment, 200 sound pulses at each frequency are sequentially transmitted into a highly-echoic tank containing swimming animals of a single species. For each pulse, the animals take different positions within the fixed-boundaried-tank and the modulated reverberation is recorded. The coherent energy in 200-pulse ensembles identifies sound scattered from the echoic tank. Because the positions of the animals are uncorrelated from ping-to-ping, the incoherent energy describes sound scattering from the animals. Thus, the TTS at each frequency is extracted from an analysis of the coherent and incoherent energy reverberated in the tank. Initial analyses indicate good agreement between the empirical TTS data and model predictions.

 

A. Jenkins sends