AMLR 2002 Weekly Report No. 9

 

11 March 2002

 

1. Current position: the R/V Yuzhmorgeologiya is hove-to in Admiralty Bay, King George Island of the South Shetland Islands. The second survey (Survey D) of bio-oceanographic conditions in the vicinity of the South Shetland Islands was completed over the weekend. Weather conditions improved from the beginning of Leg II allowing for the successful completion of survey operations, the past two days have been dedicated to the closing of two field camps. Five personnel were recovered from the camp at Cape Shirreff, Livingston Island, and three persons from the camp in Admiralty Bay of King George Island. The shipboard scientific party currently totals 25 persons. The ship is preparing to steam into the Weddell Sea to conduct two stations to characterize the oceanographic profiles and zooplankton assemblages in the Weddell Sea gyre which feeds into the Bransfield strait. After completing these stations the ship will make way for Punta Arenas, Chile.

 

2. Krill, salps and other zooplankton. Survey D Results: An estimated 57750 krill were collected in 94 Survey D samples. While overall mean krill abundance (281 per 1000 m3) was larger than during Survey A (65 per 1000 m3) the median was substantially lower (0.5 vs. 2 per 1000 m3) due to much patchier distributions. The largest catches, estimated to be 11000 to 22000 individuals (between 7300 and 9300 per 1000 m3), were located over the northern Shetland Island shelf (West area) and in western Bransfield Strait (South area). Here mean abundance values were 694 and 548 per 1000 m3, respectively. Lowest mean abundance (4.3 per 1000 m3) was in the Joinville Island area. With exception of one large catch over the north Elephant Island shelf, krill abundance within the Elephant Island area was relatively low and mean and median abundance (respectively, 10 and 0.4 per 1000 m3) was markedly reduced from January Survey A (39 and 8 per 1000 m3).

 

Krill lengths ranged from 18-59 mm with a modal length of 27 mm; 87% of individuals were < 35 mm. Accordingly, juveniles comprised 73% and immature stages 25% of the total. Immature forms were primarily small (31-35 mm) males and females and, like the juveniles, resulted from a relatively successful 2000/2001 year class.

 

Overall krill demography was highly influenced by large catches in the West and South areas where juveniles comprised 72% and 76% and immatures 27% and 23%, respectively. Size and maturity classes were more heterogeneous in the Elephant Island area where the size distribution was polymodal about 29 mm, 42 mm and 52 mm lengths and juveniles comprised 39%, immatures 17% and mature stages 44% of the total. This composition is more or less similar to that represented in the Elephant Island area during Survey A where 20-30 mm juveniles contributed 46% and > 45 mm mature stages 20% of the total. All length and maturity classes were evenly represented in the Joinville Island area.

 

Larval krill were present in 29% of survey samples, with a mean value of 61 per 1000 m3. The largest catches were primarily offshore of the South Shetland Island shelf and greatest mean abundance in the West area was 134 per 1000 m3. Calyptopis 1 through Furcilia 2 stages were collected and reflected a prolonged spawning season initiated in early December. Calyptopis larvae made up 86% of the total larvae; Calyptopis stage 3 dominated (50%). Mean, median and maximum larval krill abundance values within the Elephant Island area (50, 0 and 730 per 1000 m3) were only slightly elevated over those of Survey A (36, 0 and 360 per 1000 m3).

 

Salpa thompsoni was the fourth most abundant taxon overall following copepods, larval Thysanoessa macrura, and chaetognaths. Chain forming aggregates comprised 95% of the salp catch. Of these, 50% were < 13 mm and resulted from budding over the preceding month. Greatest salp concentrations, and of large mature solitaries and recently budded aggregates, occurred in Oceanic waters of the West and Elephant Island areas. This salp was most frequent (91% of samples) and had greatest median abundance in the Elephant Island area where it also was fourth most abundant taxon. As with larval krill, mean, median and maximum salp abundance values in the Elephant Island area (541, 251 and 2903 per 1000 m3) were only slightly elevated over the January values (410, 86 and 2817 per 1000 m3).

 

Copepods were by far the most frequent (100% of samples) and abundant zooplankton overall and within all four survey areas. Calanoides acutus, Calanus propinquus, Metridia gerlachei and Rhincalanus gigas were the major constituents. Greatest concentrations (>10000 up to 175000 per 1000 m3) generally occurred offshore of the South Shetland and Elephant Island shelf areas. However, several large catches were also made within Bransfield Strait (South and Joinville Island areas) probably due to retention with frontal zones or eddies. Mean and median copepod abundance values in the Elephant Island (17500 and 7600 per 1000 m3) exhibited a 3X seasonal increase over Survey A.

 

2002 Field Season Overview

With respect to the long term AMLR Elephant Island area data set, mean and median copepod abundance values during both 2002 surveys were by far the highest: they were an order of magnitude greater than those during 2001 and 2X the highs observed during February-March 2000. Survey A and D 2002 chaetognath and larval T. macrura abundance values also were the highest recorded, ca. 50% greater than peak values of 2000. Greatly elevated abundance of predatory amphipod species, Themisto gaudichaudii and Primno macrura, relative to previous years further indicates a substantial increase in secondary productivity. Survey A and D results (advanced female maturity stages, krill larval abundance and stage composition) indicate a prolonged, and fairly successful krill spawning season during 2001/2002. Decreased postlarval krill abundance between these surveys probably resulted from seasonal distribution shifts associated with termination of the spawning season rather than a major stock decline. Mean and median January abundance increases over those of 2001 resulted from relatively strong recruitment success of the 2000/2001 year class which offset decreased abundance (i.e., mortality) of the highly successful 1995/1996 year class that has dominated Elephant Island demographics over the past 6 years. Survey A and D larval krill abundance values, while not extraordinarily high as during 1995, were similar to those of 2001 and, like 2001, demonstrated a modest seasonal increase in abundance and development. Given the strong proportional recruitment success of the 2000/2001 year class, there is reason to anticipate at least modest recruitment success from the 2001/2002 season.

 

The late season spurt of aggregate salp production during 2002 is reminiscent of the 1997 season which preceded a major salp year in 1998. This could be reason for concern were it not for the facts that (a) this was associated with only a modest seasonal abundance increase (there was an order of magnitude increase between the two surveys in 1997) and (b) most of this late season production was located in Oceanic waters and most likely not retained as overwintering seed populations within the survey area. Salp abundance within the Elephant Island area has remained fairly stable over the past three years; no major changes are anticipated for 2003.

 

3. Krill biomass and dispersion. Highest krill densities were mapped north of Elephant Island at the shelf break. Krill was also observed in great abundance along the shelf northeast of Livingston Island and south of Snow Island. Overall krill biomass densities were calculated to be 11 to 18 g m-2 during leg I and 5 g m-2 during leg II. Average krill biomass densities for the West, Elephant Island, Joinville Island and South Areas were 3, 6, 4 and 6 g m-2 and have declined with respect to leg I (11-29, 19-40, 7 and 16 g m-2, respectively).

 

4. Phytoplankton. 5m chlorophyll averages for the Elephant Island Sector were 0.97 +\- 0.58 mg m-3, and integrated (100 meters) averages 67 +\- 44 mg m-2 for the entire Elephant Island section (37 stations). These surface values are about 80% higher, while integrated values about 50% higher, than those found during Leg I (January). For this sector, chlorophyll concentrations for February are slightly higher than the 11 year Leg II averages (420 stations) of 0.89 +\- 0.98 mg chl m-3 for 5 meters and 50 +\- 45 mg chl m-2 for 100 meter integrated values. Phytoplankton biomass also increased over January values for the Bransfield Strait (27 stations), with surface chlorophyll values of 1.00 +\- 0.58 mg m-3 (a 45% increase) and integrated values of 58 +\- 34 mg chl m-2 (about 18% higher). The Bransfield Strait phytoplankton biomass for February is less than the 11-year average of 1.6 +\- 1.9 mg chl m-3 for 5 meters and 85 +\- 94 mg m-2 for integrated (100m) chlorophyll. For the Joinville Island area (6 stations), phytoplankton biomass increased as well having surface values of 0.88 +\- 0.55 mg chl m-3 and integrated chlorophyll of 65 +\- 26 mg m-2. This is a new section so no comparison to historical averages might be made. All equipment fairs well, with only replacement of cables for the profiling fluorometer and profiling transmissometers as needed.

 

5. Oceanography and meteorology. Wind varied between northeast and northwest for the greater part of the week, changing to the southwest on Tuesday and then gradually swinging back to the north over the following two days. Wind speeds were variable reaching a maximum level of approx. 40 knots from the southwest on Tuesday. Pressure cycled around 988 mb until Wednesday and then decreased steadily to 972 mb by the end of the week. Temperatures varied between approx. 4.5 and 1C, the minimum being recorded in midweek. Conditions were generally overcast with drizzle clearing partially on Sunday, the 3rd. Light snowfalls were also experienced.

 

Water Zone II dominated the northern portion of the Elephant Island area with Zone III water passing through the northern shelf region between King George and Elephant Islands. Water Zone I penetrated as a tongue, from the west (Stations D11-03 and D09-03), into Zone II waters. Water Zone IV lay directly south, extending from just below Elephant and Clarence Islands through the deeper portions of the Bransfield Strait to southwest of Deception Island. A thin band of Water Zone III extended along the southern coastline of the South Shetland Islands. Water Zone V extended from north of Clarence Island (Station D02-05) along the east-most transect to the Weddell Sea, and continued in a southwesterly direction hugging the Antarctic Peninsula along its northern continental shelf (to station D15-15).

 

6. Predator diet studies. Both the milk lipid extraction and the scat sorting portions of the diet studies were completed. Lipids were extracted from the remaining 25 prepared fur seal milk samples and were frozen. An additional 9 whole milk samples were aliquoted in duplicate, one set undergoing lipid extraction while the other was frozen. The final eleven scats and three vomits were sorted, and krill carapaces were measured and stored in ethanol. Scats from this week were mostly comprised of krill chitin and contained far fewer otoliths, predominantly G. nicholsi. However, one additional squid beak was found and also stored in ethanol. Krill and myctophids caught during the last portion of the large area survey were again saved. Approximately five hundred krill and all of the myctophids will be sent back to the lab for extra processing there.

 

7. Advanced Survey Technologies. The second multi-instrumented buoy was retrieved following the completion of the large-area survey. The buoy, fitted with the dual-frequency echosounder, was moored near the head of a canyon approximately 4.5 n.mi. offshore to the east of the field camp at Cape Shirreff. A cursory look at the multi-day time-series indicates a strong diel migration pattern for krill residing in the area. These data allow a two-frequency krill identification technique to be implemented. The preliminary results are consistent with a previous interpretation of backscatter data from the ADCP mounted on the first buoy that was deployed in the area. Total target strengths (TTS) were measured over a wide acoustical bandwidth (36-202 kHz) for multiple size classes of Antarctic krill (Euphausia superba), three species of myctophid (Electrona antarctica, Gymnoscopelus nicholsi, and G. braueri), and a squid. The TTS data for krill are remarkably consistent with predictions based on a distorted wave born approximation.

 

A. Jenkins sends