Langdon B. Quetin

Marine pelagic ecosystems: the West Antarctic Peninsula

The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 2°C increase in the annual mean temperature and a 6°C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.6°C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in ice-dependent Adélie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along the WAP and the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response.

Seasonal variability in the distribution of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula

A unique Antarctic data set from four multidisciplinary cruises (spring, Nov 1991; summer, Jan–Feb 1993; fall, Mar–May 1993; winter, Aug–Sept 1993) was analyzed to provide a description of seasonal variability in the distribution and abundance of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula. Analyses of acoustic observations revealed distinct seasonal variations in estimates of krill abundance, the dimensional parameters used to characterize individual krill aggregations, the geographic area over which krill were acoustically detected, and the depth distribution of krill biomass. Spatially averaged estimates of krill biomass were an order of magnitude higher in spring (32 g m-2) and summer (95 g m-2) compared to fall (12 g m-2) and winter (8 g m-2). In summer, krill were detected throughout the region and most of the krill biomass was associated with aggregations of small cross-sectional area ( 150 g m-3), which were positioned in the upper 50 m of the water column. Winter observations, in contrast, were characterized by the absence of krill throughout most of the region and aggregations occuring deeper than 100 m with large cross-sectional area (>10000 m2) and low mean biomass ( 50 g m-2) at selected locations on the inner shelf and the disproportionate contribution, in terms of total krill biomass (>80%), by a small number of aggregations ( 40 mm) krill positioned offshore of smaller krill (32–38 mm) in all seasons except winter. Synthesis of these data and historical observations suggest that there is a seasonal shift in the primary habitat of krill and that changes in krill behavior are an important factor affecting variations in krill distributions.

Turnover of carbon and nitrogen during growth of larval krill, Euphausia superba Dana: a stable isotope approach

Abstract Using natural abundances of stable isotopes (δ13C and δ15N) as tracers, carbon and nitrogen turnover rates were determined for larval krill, Euphausia superba Dana, maintained in the laboratory. Experimental populations of larvae were reared at +1.5°C and −1.5°C on foods of known isotopic composition and subsampled weekly (8–10 weeks) for a determination of wet weight and isotopic composition. Metabolic turnover of carbon and nitrogen, manifested as temporal shifts in δ13C and δ15N, was tied closely to temperature. Larval krill reared at +1.5°C had replaced 22–29% of their original body carbon at the conclusion of the experiment, but only 13–22% of their original body nitrogen. Larvae reared at −1.5°C exhibited little evidence of carbon turnover and replaced less than 6% of their original body nitrogen. These are the first simultaneous, coupled measurements of long-term carbon and nitrogen turnover for any marine animal, and provide an essential calibration for the interpretation of stable isotope ratios in animals collected from the field. In addition to the feeding experiments, animals were starved for 2 months at +1.5°C and −1.5°C. Starved krill exhibited little isotopic change. This finding suggests that starvation cannot account for large temporal variations observed in the isotopic composition of larval krill collected from the field.

Extreme Anomalous Atmospheric Circulation in the West Antarctic Peninsula Region in Austral Spring and Summer 2001/02, and Its Profound Impact on Sea Ice and Biota*

Exceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South Atlantic coupled with strong negative anomalies in the Bellingshausen-Amundsen and southwest Weddell Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP. In situ, satellite, and NCEP-NCAR Reanalysis (NNR) data are used to examine the profound and complex impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer freshening and widespread ice surface flooding, snow-ice formation, and phytoplankton growth, coincided with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development of an unusually early and rapid (short) retreat season (negative ice extent anomaly). Strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice through summer. Ecological effects were both positive and negative, the latter including an impact on the growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease and lowest reproductive success in a 30-yr Adelie penguin demographic time series. The unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom. Unexpectedly, the initial bloom occurred within compact sea ice and could not be detected in Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data. This analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state/conditions, from both a climatic and ecological perspective; further information is required on thickness and dynamics/deformation.

Thermally protecting cod ends for the recovery of living deep-sea animals

Abstract The use of thermally protecting cod ends for the recovery of living midwater animals is described. The designs and construction details of two such devices are described. These devices have proved effective for the recovery of living animals from depths as great as 2500 m.

Environmental Variability and Its Impact on the Reproductive Cycle of Antarctic Krill 1

SYNOPSIS. ‘‘Recruitment potential’’ in Antarctic krill in the Palmer Long-Term Ecological Research (LTER) study region west of the Antarctic Peninsula varied significantly over the 7-yr time series between January 1993 and January 1999. Timing of ovarian maturation, the percent of the population reproducing, and individual reproductive output (batch volume, embryo diameter) were measured. Indices have been developed to quantify the timing and intensity of reproduction in Antarctic krill. One finding important to estimates of population fecundity for this long-lived species is that the percent of the population reproducing can vary widely, from 10 to 98%. Each season was characterized as having delayed, average or advanced ovarian development. In this study we relate these indices to direct and indirect indicators of spring or annual food availability. The timing of the spring sea ice retreat and the extent of sea ice in the spring (September through November) appear to significantly affect the intensity and timing of reproduction in the population. Intensity of reproduction was highest under ‘‘average’’ conditions, and oöcyte development fastest with conditions of a late retreat and high spring sea ice extent.

Energetic cost to develop to the first feeding stage of Euphausia superba Dana and the effect of delays in food availability

C and N lost or gained during development and the substratum utilized were determined from measurements of the C and N content of laboratory-reared larvae of the Antarctic krill Euphausia superba Dana from hatching through mid-Calyptopis 1 (Cl, the first feeding stage) at 0, 1 and 2 °C. The relative and absolute use of C and N during early development supported the hypothesis that lipid stores were the primary catabolic substratum during development to Cl and mid-way through Cl. Both measured and calculated estimates of the C required to develop from hatching to Cl indicated that larvae developing at higher temperatures used less energy to reach the Cl stage. After metamorphosis to the Cl stage, internal C reserves continued to satisfy ≈90% of calculated C requirements of fed larvae but the availability of food during this time may be important for development of the digestive enzyme system or of efficient feeding behavior. Development, growth, survival and elemental composition of larvae that experienced delays in food availability after metamorphosis to Cl were observed at 0 and 2 °C. Results of these experiments were used to determine the point-of-no-return (PNR) and quantify the importance of energy reserves to the first feeding stage. If initial delays in food availability exceed a certain point (PNR), starved larvae cannot recover when refed and eventually die. Short delays in food availability affected the timing of development, not the ability to continue to develop, but after iong delays the larvae lost the ability to metamorphose. The PNR depends not only on the initial energy reserves of the embryo but also on the amount of energy needed to reach Cl which in turn depends on the effect of temperature on O2 consumption rates and developmental times. The PNR of 10–14 days from the food availability experiments was similar to the calculated PNR from metabolic requirements and known lipid reserves. The calculated PNR increased with increasing temperature, from 9 to 15 days within the environmental range. Despite the narrow temperature range in the Southern Ocean, both temperature and food availability will affect the survival and recruitment of the early larvae of E. superba.

Diet of the Antarctic krill (Euphausia superba Dana): II. Selective grazing in mixed phytoplankton assemblages

Abstract Diatoms, prymnesiophytes, and cryptophytes are the major taxonomic classes of phytoplankton available to Antarctic krill as primary food resources. However, the relative contribution of each class to the diet of krill is unknown. In this study, selectivity for different phytoplankton taxa by Euphausia superba was examined during laboratory experiments with E. superba grazing on mixed phytoplankton assemblages from the wild and from laboratory cultures. Clearance rates and changes in relative concentrations of diatoms, prymnesiophytes and cryptophytes were measured by analysis of taxon-specific accessory photopigments with high performance liquid chromatography (HPLC). E. superba grazed diatoms at higher rates than Phaeocystis and cryptophytes. Grazing was negligible in cryptophyte-dominated assemblages. Increases of up to 369% in 19′-hexanoyloxyfucoxanthin (HF)/fucoxanthin (FUCO) (prymnesiophyte/diatom) ratios indicated a high level of selectivity for diatoms over Phaeocystis . This selectivity occurred even for Phaeocystis colonies of similar size to diatoms, and thus cannot be entirely attributed to differential sieving efficiency based on particle size. The results suggest that krill actively select diatoms in phytoplankton mixtures.

How Productive Are Antarctic Krill?Recent research is challenging some long-held assumptions about the fecundity and development of Euphausia superba

T he immensity of the Southern Ocean and the abundance of its living resources guarantee that it will be a focal point of international research well into the future. A key component of the Southern Ocean ecosystem is the shrimplike Antarctic krill, Euphausia superba,l which dominates the waters south of the Antarctic Convergence, a circumpolar front where northward-flowing Antarctic surface water meets warmer, southward-flowing subantarctic water (see map, p. 240). Krill predominate in the diets of many species, and all Antarctic marine vertebrates

Spawning frequency and fecundity of the Antarctic krill Euphausia superba

The spawning frequency of Euphausia superba was estimated from observations of egg production by live individual females in captivity on board ship. The females were collected during the austral summer of 1982 near the South Shetland Islands and Elephant Island, Antarctica. On the average, 15.6% of the females spawned each day, which indicates an interval between spawnings of 6.7 d. This evidence contradicts earlier descriptions of the spawning behavior of E. superba. The females appear to be continually producing eggs and releasing them periodically over the entire spawning season, and thus have much higher fecundities than previously estimated.