Krill Abundance Research Articles

Changes in Barents Sea ecosystem state, 1970–2009: climate fluctuations, human impact, and trophic interactions

Abstract Johannesen, E., Ingvaldsen, R. B., Bogstad, B., Dalpadado, P., Eriksen, E., Gjøsæter, H., Knutsen, T., Skern-Mauritzen, M., and Stiansen, J. E. 2012. Changes in Barents Sea ecosystem state, 1970–2009: climate fluctuations, human impact, and trophic interactions. – ICES Journal of Marine Science, 69: 880–889. Long time-series of data from the Barents Sea (BS) are analysed to contrast the climate, fishing pressure, plankton, pelagic fish, demersal fish, and interactions between trophic levels in a recent decade (2000–2009) with the period 1970–1999. During the past four decades, fishing pressure and climatic conditions have varied greatly in the BS, and stock levels have fluctuated substantially. Trophic control has changed from mainly bottom–up to top–down, then back to mainly bottom–up. No clear evidence for persistent ecological regimes was found. The past decade has been the warmest on record, with large stocks of demersal and pelagic fish, and increasing abundances of krill and shrimp. Except perhaps for the rather less-studied Arctic species, the short-term effect of the recent warming has been positive for BS stocks. However, as many of the long-established relationships and mechanisms in the BS seem to be changing, the long-term effects of warming are uncertain.

Krill stocks in high latitudes of the Antarctic Lazarev Sea: seasonal and interannual variation in distribution, abundance and demography

The motivation of the LAKRIS project (Lazarev Krill Study) was to investigate the seasonal and interannual variability in distribution, demography, phenology and population dynamics of Antarctic krill Euphausia superba in poorly studied high-latitude areas. The core data set consists of four RMT-net-sampling surveys that were conducted in early summer (December 2005 and 2007), autumn (April 2004) and winter (July to August 2006). Supplementary data were analysed from the 1980s to view the results in a wider geographical context. The major observations are the following: (1) krill abundance and distribution showed geographical, seasonal and interannual fluctuations. Krill abundance was almost one order of magnitude lower compared to the Southwest Atlantic. (2) Krill density showed substantial differences between day and night for autumn and winter surveys. (3) A spatial gradient was observed for krill size groups, with larger sized (adult spawning) krill further south and medium-sized juveniles and immatures mainly between 60° and 67°S. (4) Data on maturity stage composition and larvae occurrence indicate an active and successful spawning in these high-latitude areas even under the conditions of sea-ice cover. At the end of the spawning season, females and males regressed in their external sexual characteristics (‘re-juvenation’) to overwintering immature stages. Major conclusions relate to a conceptual view of the spatial distribution of krill size classes and the occurrence of larvae in the context of major oceanographic features and current systems in the Lazarev Sea and adjacent ocean sectors. The Lazarev Sea obviously does not support a single self-maintaining population, but represents a complex transition zone of stocks with different origins from the Scotia Sea and the Cosmonaut Sea.

The temporal and spatial distribution of krill (Meganyctiphanes norvegica) at the deep seabed of the Faroe–Shetland Channel, UK: A potential mechanism for rapid carbon flux to deep sea communities

The temporal and spatial distribution of Meganyctiphanes norvegica near the seabed (<5 m above the seafloor) was investigated using remotely operated vehicle (ROV) video footage from eight sites in the Faroe–Shetland Channel. Meganyctiphanes norvegica was most abundant near the seabed at 400–600 m depth, which corresponds to a temperature transitional region between shallower warmer water and deeper cold water. Densities of M. norvegica were significantly lower in the warm water, and no krill were detected at 900–1500 m depth in the cold water. Meganyctiphanes norvegica densities declined at night owing to daily vertical migrations. Time-series analysis showed higher krill densities at 06:00–09:00 and 18:00–21:00 at a depth of 400–480 m and at 12:00–15:00 at a depth of 600 m. A great reduction in krill abundance in winter was detected from both ROV surveys and continuous plankton recorder records. Meganyctiphanes norvegica was observed feeding on benthic particulate organic matter on the seafloor and being consumed by benthic and epibenthic predators. The maximum density of M. norvegica at 480 m depth was 596 ± 261 individuals m–3. This represents a standing stock of 12.8 ± 5.6 g C m–3 with an egestion rate of 0.63 ± 0.28 g C m–3 day–1. Meganyctiphanes norvegica potentially provides an important source of carbon for communities in the deep waters of the Faroe–Shetland Channel.

Active acoustic examination of the diving behavior of murres foraging on patchy prey

During the 2008 and 2009 breeding seasons of murres Uria spp., we combined visual observations of these predators with active acoustics (sonar), fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We acoustically detected thousands of bubble trails that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the for- aging ecology of diving birds. At the regional scale, the number of acoustically detected bubble trails, which served as a proxy for diving murre abundance, was related to the combined availabil- ity and vertical accessibility of squid, krill, and pollock. There were, however, no clear relation- ships at this scale between diving murres and any individual prey taxon, highlighting the impor- tance of prey diversity to these animals. Individual krill patches targeted by murres had higher krill density and were located shallower than the mean depth of krill patches, but were similar in total krill abundance and overall size. The diving depth of murres within krill patches was highly correlated to the depth of the upper edge of these patches, whereas murres found outside of krill patches showed a depth distribution similar to that of juvenile pollock. Throughout the study area, murres showed strong diel patterns in their diving behavior in response to the diel migrations of their prey. These results suggest that murres select prey with specific patch characteristics imply- ing effective information gathering about prey by murres. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intra- specific local enhancement may facilitate foraging in these predators.

Analysis of abrupt transitions in ecological systems

The occurrence and causes of abrupt transitions, thresholds, or regime shifts between ecosystem states are of great concern and the likelihood of such transitions is increasing for many ecological systems. General understanding of abrupt transitions has been advanced by theory, but hindered by the lack of a common, accessible, and data-driven approach to characterizing them. We apply such an approach to 30–60 years of data on environmental drivers, biological responses, and associated evidence from pelagic ocean, coastal benthic, polar marine, and semi-arid grassland ecosystems. Our analyses revealed one case in which the response (krill abundance) linearly tracked abrupt changes in the driver (Pacific Decadal Oscillation), but abrupt transitions detected in the three other cases (sea cucumber abundance, penguin abundance, and black grama grass production) exhibited hysteretic relationships with drivers (wave intensity, sea-ice duration, and amounts of monsoonal rainfall, respectively) through a variety of response mechanisms. The use of a common approach across these case studies illustrates that: the utility of leading indicators is often limited and can depend on the abruptness of a transition relative to the lifespan of responsive organisms and observation intervals; information on spatiotemporal context is useful for comparing transitions; and ancillary information from associated experiments and observations aids interpretation of response-driver relationships. The understanding of abrupt transitions offered by this approach provides information that can be used to manage state changes and underscores the utility of long-term observations in multiple sentinel sites across a variety of ecosystems.

Geographic variation in beak colouration in gentoo penguins Pygoscelis papua

Ornamental colouration is often due to carotenoid pigments and varies inter- and intra-specifically. This paper reports on variation in beak colour of the gentoo penguin, Pygoscelis papua, corresponding to different geographical locations along a latitudinal gradient in the Antarctic Peninsula (from King George Island (62o15′S–58o37′W) to Ronge Island (64o40′S–62o40′W). The gentoo penguin has a conspicuous red spot on both sides of the beak that indicates the presence of the carotenoid pigment, astaxanthin. Beak colouration was measured with a portable spectrophotometer for 20 individuals in three locations, along the Western coast of the Antarctic Peninsula. In the study area, marked variation can be found in terms of factors such us parasite load, human impact, variations in UV radiation and the abundance of krill; all possibly affecting carotenoid availability for signalling purposes. Colour traits were expected to be more intense, that is more vivid, saturated and pure, in places where there is diminished pressure from factors such as contamination, parasites or diseases, all of which may reduce the availability of carotenoids for other functions, such as antioxidant or immune stimulation involving physiological trade-offs. Likewise, colour traits might be predicted to be more intense where carotenoid sources, krill in the case of gentoo penguins, are more available. However, contrary to this initial expectation, our results indicate that northerly penguins’ populations, which are in the most polluted and parasitized areas, have more saturated beaks. An alternative hypothesis suggests that environmental constraints relating to the variation in abundance of krill may explain the geographical variation in colour expression found among gentoo penguins.

Relative Changes in Krill Abundance Inferred from Antarctic Fur Seal

Antarctic krill Euphausia superba is a predominant species in the Southern Ocean, it is very sensitive to climate change, and it supports large stocks of fishes, seabirds, seals and whales in Antarctic marine ecosystems. Modern krill stocks have been estimated directly by net hauls and acoustic surveys; the historical krill density especially the long-term one in the Southern Ocean, however, is unknown. Here we inferred the relative krill population changes along the West Antarctic Peninsula (WAP) over the 20th century from the trophic level change of Antarctic fur seal Arctocephalus gazella using stable carbon (δ13C) and nitrogen (δ15N) isotopes of archival seal hairs. Since Antarctic fur seals feed preferentially on krill, the variation of δ15N in seal hair indicates a change in the proportion of krill in the seal's diets and thus the krill availability in local seawater. For the past century, enriching fur seal δ15N values indicated decreasing krill availability. This is agreement with direct observation for the past ∼30 years and suggests that the recently documented decline in krill populations began in the early parts of the 20th century. This novel method makes it possible to infer past krill population changes from ancient tissues of krill predators.

Active acoustic examination of the diving behavior of murres foraging on patchy prey

Van Holliday used sound to study nearly every type of organism living in the ocean, contributing to our understanding of acoustics as well as marine ecological processes. To honor Van, I will talk about the one animal group that I could not find in his long publication list—seabirds. We combined visual observations of murres with active acoustics, fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We detected thousands of unique acoustic targets that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the foraging ecology of diving birds. Diving murre abundance was related to the combined availability and vertical accessibility of squid, krill, and pollock. Individual krill patches targeted by murres had higher krill density and were shallower than mean values but were similar in total krill abundance and overall size. Murres found outside of krill showed a depth distribution similar to that of juvenile pollock. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intra-specific local enhancement may facilitate foraging in these predators.

Exploring the effects of reductions in krill biomass in the Southern Ocean on blue whales using a state-dependent foraging model

Many species of baleen whales were hunted to near extinction in the Southern Hemisphere. The recovery of these populations will be affected by the availability of krill, a major dietary component, in the Southern Ocean. We combine a novel energetics model for baleen whales with a state dependent foraging model to explore the impacts of an expanding krill fishery on baleen whales. We parameterize the model for blue whales, but with simple modifications it could be applied to most baleen whales. We predict that an expanding fishery will have a small but significant impact on the blue whale population through decreased birth rates. However, spreading the catch limit throughout the range of krill can reduce these effects. In addition, whales may be able to reduce these impacts through adaptive changes in foraging behavior. The relationship between krill abundance and blue whale foraging and reproductive success is nonlinear, such that larger reductions in krill biomass, potentially following a loss of sea ice due to climate change, could have a much larger negative impact on the recovery of blue whales.

Geospatial variability of krill and top predators within an Antarctic submarine canyon system

The spatial distribution patterns of krill, seabirds (penguin, petrel and albatross), fur seals and baleen whales were mapped in nearshore waters (<50 km from land) to investigate their habitat selection within two adjacent submarine canyons near Livingston Island, Antarctica. Three shipboard surveys were conducted (February 2005–2007), and an echosounder was used to measure the distribution and abundance of krill while simultaneously conducting visual surveys to map seabird and marine mammals. Using a multispecies approach, we test the hypothesis that spatial organization of krill and top predators co-vary according to fine-scale changes in bathymetry in the nearshore marine environment. GAMs are used to examine the effect of sea depth, slope and distance to isobaths on the spatial distribution and abundance of krill and predators. Spatial distribution patterns of krill and predators relate to fine-scale (1–10 km) changes in bathymetry and exhibit cross-shelf gradients in abundance. Krill were concentrated along the shelf-break and abundant within both submarine canyons. Predators exhibited different preferences for locations within the submarine canyon system that relates to their foraging behavior. Penguins concentrated closer to shore and within the head of the east submarine canyon immediately adjacent to a breeding colony. Whales were also concentrated over the head of the east canyon (overlapping with penguins), whereas albatrosses and fur seals were concentrated in the west canyon. Fur seals also showed preference for steep slopes and were concentrated along the shelf-break. Petrels exhibited peaks in abundance throughout both submarine canyons. Owing to their orientation, size and proximity to the coastline, submarine canyons provide important habitat heterogeneity for krill and a variety of predators. This study highlights the multispecies approach for studying spatial ecology of top predators and krill and has implications for marine spatial management of the Scotia Sea.

Super-Aggregations of Krill and Humpback Whales in Wilhelmina Bay, Antarctic Peninsula

Ecological relationships of krill and whales have not been explored in the Western Antarctic Peninsula (WAP), and have only rarely been studied elsewhere in the Southern Ocean. In the austral autumn we observed an extremely high density (5.1 whales per km2) of humpback whales (Megaptera novaeangliae) feeding on a super-aggregation of Antarctic krill (Euphausia superba) in Wilhelmina Bay. The krill biomass was approximately 2 million tons, distributed over an area of 100 km2 at densities of up to 2000 individuals m−3; reports of such ‘super-aggregations’ of krill have been absent in the scientific literature for >20 years. Retentive circulation patterns in the Bay entrained phytoplankton and meso-zooplankton that were grazed by the krill. Tagged whales rested during daylight hours and fed intensively throughout the night as krill migrated toward the surface. We infer that the previously unstudied WAP embayments are important foraging areas for whales during autumn and, furthermore, that meso-scale variation in the distribution of whales and their prey are important features of this system. Recent decreases in the abundance of Antarctic krill around the WAP have been linked to reductions in sea ice, mediated by rapid climate change in this area. At the same time, baleen whale populations in the Southern Ocean, which feed primarily on krill, are recovering from past exploitation. Consideration of these features and the effects of climate change on krill dynamics are critical to managing both krill harvests and the recovery of baleen whales in the Southern Ocean.

Spatial organization of krill and seabirds in the central California Current

AbstractSantora, J. A., Ralston, S., and Sydeman, W. J. 2011. Spatial organization of krill and seabirds in the central California Current. – ICES Journal of Marine Science, 68: 1391–1402. The hypothesis that krill and krill–predator spatial organization and abundance co-vary interannually was tested by investigating the spatial organization of krill and planktivorous seabirds in the central California Current ecosystem over 5 years of varying oceanographic conditions, 2002–2006. To measure the abundance and distribution of krill, data were integrated from large-scale hydroacoustic surveys and station-based net samples, and these data linked to concurrent shipboard visual surveys of seabirds. Acoustically based estimates of the relative abundance of krill were correlated with net samples of Euphausia pacifica, suggesting that acoustic signals mainly reflected the distribution of this numerically dominant species. The distribution and abundance of krill displayed marked changes over years, but the characteristic spatial scale of krill and seabirds remained similar (1–4 nautical miles), confirming the hypothesis of covariance in spatial structure. Krill and the seabird species investigated showed similar habitat associations, i.e. the outer shelf and shelf–slope region, showing that the at-sea distributions of seabirds can provide information on the presence/absence of krill patches. The results also underscore the importance of measuring spatial organization as well as relative abundance in promoting better understanding of predator–prey and marine ecosystem dynamics.

Advection and starvation cause krill (Euphausia pacifica) decreases in 2005 Northern California coastal populations: Implications from a model study

A decrease in krill abundance during 2005 in regions of the California Current has been hypothesized to have had immediate (seabird) and long-term (salmon) negative impacts on upper trophic level predators. We use a suite of coupled models to examine the population biology and spatial and temporal distribution of the krill species Euphausia pacifica during the winter/spring of 2001, a “normal” year, and 2005, an “anomalous” year, to determine if this hypothesis is supported mechanistically. Ocean conditions were simulated using the Regional Ocean Modeling System (ROMS), which forced an individual-based model parameterized to simulate the population biology of E. pacifica. Poleward transport during winter 2005 advected particles north of Cape Mendocino, away from seabirds and salmon feeding in the Gulf of the Farallons region. Few of the particles that were advected north in 2005 returned to their region of release throughout the model run time (200 days). Moreover, the “condition” of those particles remaining within the domain was poor in 2005, with greater mortality from starvation and a decreased mean particle weight. Our results indicate that both physical processes (anomalous northern advection) and biological processes (greater starvation and less weight per individual) contributed to reduced krill availability to predators in the northern California region during 2005, and that the productivity and survival of seabirds and salmonids is dependent on krill during critical life history stages.

Horizontal and vertical distribution of euphausiid species on the Western Antarctic Peninsula U.S. GLOBEC Southern Ocean study site

The Western Antarctic Peninsula (WAP) is a site of high krill abundance and a likely source region for krill populations found to the north and east of the area. The U.S. GLOBEC Southern Ocean program studied factors that contribute to the overwintering success of krill in the region of Marguerite Bay, WAP. A MOCNESS net system was used to sample the vertical distribution and abundance of zooplankton relative to physical features (hydrography and circulation) during four broad-scale survey cruises in the fall and winter of 2001 and 2002. Four species were found throughout the study area on all four cruises: Euphausia superba, Euphausia crystallorophias, Euphausia triacantha, and Thysanoessa macrura. The species had significantly different horizontal and vertical distributions. Both E. superba and T. macrura were broadly distributed throughout the area, but the central 50% of their vertical distributions were distinct with E. superba most abundant in the upper 100 m in the coldest, freshest water (average temperature and salinity: −1.13 °C; 33.80) and T. macrura occurring between 100 and 250 m (at 0.01 °C; 34.228). E. crystallorophias had a more coastal distribution and was usually found deeper in slightly warmer and saltier water (−0.44 °C; 33.9982) than E. superba and either overlapped or was above T. macrura in depth. E. triacantha was much rarer and sporadically distributed in the study area and was found substantially deeper (center of distribution about 300 m) in the warmest saltiest water (1.40 °C; 34.65) than the other three euphausiid species. Larval distributions for E. superba indicated that at least some proportion of the populations resulted from reproduction and development on the continental shelf, and not solely from offshore reproduction and transport onto the shelf. A neutral particle tracking model was used to gain insight into the relative importance of shelf versus off-shelf origins for the larvae. The results suggest that a combination of offshore and onshore reproduction can account for the observed E. superba larval distributions in the U.S. SO GLOBEC study site.

Abundance and distribution of Antarctic krill (Euphausia superba) nearshore of Cape Shirreff, Livingston Island, Antarctica, during six austral summers between 2000 and 2007

Abundance and distribution of Antarctic krill ( Euphausia superba ) in the nearshore waters north of Livingston Island, Antarctica, were characterized from six small-boat surveys conducted in late January or early February from 2000 to 2007. The first three surveys (2000, 2002, 2004) were conducted using a 120 kHz split-beam echosounder to measure water column acoustic backscatter. The last three surveys (2005–2007) were conducted using 38 kHz and 200 kHz single-beam echosounders. A portion of the acoustic backscatter was attributed to Antarctic krill based on the results of net tows, underwater video observations, and a multiple-frequency acoustic classification algorithm. The annual mean krill biomass density in the survey area ranged from 11 to 84 g·m–2. Results are compared with the western Scotia Sea area of the US Antarctic Marine Living Resources (AMLR) program’s acoustic surveys of krill biomass density for the same years. Nearshore krill biomass densities were significantly larger (t test, p &lt; 0.05), more stable, and the coefficients of variation were smaller than the much larger AMLR surveys. Increased competition between seals, penguins, and humans for the nearshore krill resource, especially during the austral summer months, could impact the recruitment success of these land-based krill predators. Implications of nearshore krill biomass on small-scale management units are discussed.

Spatial associations between large baleen whales and their prey in West Greenland

This study combined data on fin whale Balaenoptera physalus, humpback whale Megaptera novaeangliae, minke whale B. acutorostrata, and sei whale B. borealis sightings from large-scale visual aerial and ship-based surveys (248 and 157 sightings, respectively) with synoptic acoustic sampling of krill Meganyctiphanes norvegica and Thysanoessa sp. abundance in September 2005 in West Greenland to examine the relationships between whales and their prey. Krill densities were obtained by converting relationships of volume backscattering strengths at multiple frequen- cies to a numerical density using an estimate of krill target strength. Krill data were vertically inte- grated in 25 m depth bins between 0 and 300 m to obtain water column biomass (g m -2 ) and trans- lated to density surfaces using ordinary kriging. Standard regression models (Generalized Additive Modeling, GAM, and Generalized Linear Modeling, GLM) were developed to identify important explanatory variables relating the presence, absence, and density of large whales to the physical and biological environment and different survey platforms. Large baleen whales were concentrated in 3 focal areas: (1) the northern edge of Lille Hellefiske bank between 65 and 67° N, (2) north of Paamiut at 63° N, and (3) in South Greenland between 60 and 61° N. There was a bimodal pattern of mean krill density between depths, with one peak between 50 and 75 m (mean 0.75 g m -2 , SD 2.74) and another between 225 and 275 m (mean 1.2 to 1.3 g m -2 , SD 23 to19). Water column krill biomass was 3 times higher in South Greenland than at any other site along the coast. Total depth-integrated krill biomass was 1.3 × 10 9 (CV 0.11). Models indicated the most important parameter in predicting large baleen whale presence was integrated krill abundance, although this relationship was only significant for sightings obtained on the ship survey. This suggests that a high degree of spatio-temporal synchrony in observations is necessary for quantifying predator-prey relationships. Krill biomass was most pre- dictive of whale presence at depths >150 m, suggesting a threshold depth below which it is energet- ically optimal for baleen whales to forage on krill in West Greenland.

Krill population dynamics at South Georgia: implications for ecosystem-based fisheries management

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 399:243-252 (2010) - DOI: https://doi.org/10.3354/meps08356 Krill population dynamics at South Georgia: implications for ecosystem-based fisheries management Keith Reid1,2, Jon L. Watkins1,*, Eugene J. Murphy1, Phil N. Trathan1, Sophie Fielding1, Peter Enderlein1 1British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK 2Present address: Commission for the Conservation of Antarctic Marine Living Resources, PO Box 213, North Hobart, Tasmania 7002, Australia *Corresponding author. Email: jlwa@bas.ac.uk ABSTRACT: The South Georgia region supports a large biomass of krill that is subject to high inter-annual variability. The apparent lack of a locally self-maintaining krill population at South Georgia means that understanding the mechanism underlying these observed population characteristics is essential to successful ecosystem-based management of krill fishery in the region. Krill acoustic-density data from surveys conducted in the early, middle and late period of the summers of 2001 to 2005, together with krill population size structure over the same period from predator diet data, were used with a krill population dynamics model to evaluate potential mechanisms behind the observed changes in krill biomass. Krill abundance was highest during the middle of the summer in 3 years and in the late period in 2 years; in the latter there was evidence that krill recruitment was delayed by several months. A model scenario that included empirically derived estimates of both the magnitude and timing of recruitment in each year showed the greatest correlation with the acoustic series. The results are consistent with a krill population with allochthonous recruitment entering a retained adult population; i.e. oceanic transport of adult krill does not appear to be the major factor determining the dynamics of the adult population. The results highlight the importance of the timing of recruitment, especially where this could introduce a mismatch between the peak of krill abundance and the peak demand from predators, which may exacerbate the effects of changes in krill populations arising from commercial harvesting and/or climate change. KEY WORDS: South Georgia · Antarctic krill · Euphausia superba · Population dynamics · Recruitment · Match-mismatch Full text in pdf format PreviousNextCite this article as: Reid K, Watkins JL, Murphy EJ, Trathan PN, Fielding S, Enderlein P (2010) Krill population dynamics at South Georgia: implications for ecosystem-based fisheries management. Mar Ecol Prog Ser 399:243-252. https://doi.org/10.3354/meps08356 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 399. Online publication date: January 28, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean

Abstract. As part of the US-AMLR program in January-February of 2006, 99 stations in the South Shetland Islands-Antarctic Peninsula region were sampled to understand the variability in hydrographic and biological properties related to the abundance and distribution of krill in this area. Concentrations of dissolved iron (DFe) and total acid-leachable iron (TaLFe) were measured in the upper 150 m at 16 of these stations (both coastal and pelagic waters) to better resolve the factors limiting primary production in this area and in downstream waters of the Scotia Sea. The concentrations of DFe and TaLFe in the upper mixed layer (UML) were relatively high in Weddell Sea Shelf Waters (~0.6 nM and 15 nM, respectively) and low in Drake Passage waters (~0.2 nM and 0.9 nM, respectively). In the Bransfield Strait, representing a mixture of waters from the Weddell Sea and the Antarctic Circumpolar Current (ACC), concentrations of DFe were ~0.4 nM and of TaLFe ~1.7 nM. The highest concentrations of DFe and TaLFe in the UML were found at shallow coastal stations close to Livingston Island (~1.6 nM and 100 nM, respectively). The ratio of TaLFe:DFe varied with the distance to land: ~45 at the shallow coastal stations, ~15 in the high-salinity waters of Bransfield Strait, and ~4 in ACC waters. Concentrations of DFe increased slightly with depth in the water column, while that of TaLFe did not show any consistent trend with depth. Our Fe data are discussed in regard to the hydrography and water circulation patterns in the study area, and with the hypothesis that the relatively high rates of primary production in the central regions of the Scotia Sea are partially sustained by natural iron enrichment resulting from a northeasterly flow of iron-rich coastal waters originating in the South Shetland Islands-Antarctic Peninsula region.

Long-Term Relationships between the Marine Environment, Krill and Salps in the Southern Ocean

Long-term variations (1975–2002) in climatology of marine environmental parameters, Antarctic krill,Euphausia superba, and the pelagic tunicate,Salpa thompsoni, were compared within the Atlantic Sector of the Southern Ocean. Sea water temperature in the top 400 m increased at a rate of 0.020–0.030°C ⋅ yr−1, which was accompanied by the dissolved oxygen decline. Top 100 m water layer became fresher with lower concentrations of phosphates and nitrates, while at subsurface layers (200–400 m) both salinity and nutrients showed small increasing trend. Unlike phosphates and nitrates, silicate concentrations decreased in the entire water column. Shorter-term water temperature dynamics closely correlated with the El Nino events expressed as the Southern Oscillation Index which in turn was linked to the propagation of the Antarctic Circumpolar Wave (ACW). The variations of sea-ice extent matched well the changes in both air and water temperatures. In general, abundance of krill and salps showed opposite to each other trends. Due to large area considered in this study, no significant relationships between abiotic factors and both krill and salps were found. However, our analysis demonstrated that krill abundance was greater in years with lower sea water temperature, greater sea-ice extent and higher nutrient concentration, while salps showed the opposite pattern.

Acoustic characterisation of the broad-scale distribution and abundance of Antarctic krill (Euphausia superba) off East Antarctica (30-80°E) in January-March 2006

Abstract A large-scale oceanographic survey (BROKE-West) was undertaken off East Antarctica in the austral summer of 2005/06. Throughout the survey, multi-frequency echosounder data and ancillary environmental data were collected to determine the distribution and abundance of Antarctic krill (Euphausia superba) and to explore its broad relationship with the bio-physical environment. The acoustic data were analysed using three different methods to provide measurements of krill abundance that can be set in context with previous studies. Based on the most recently developed acoustic method, the mean biomass-density of krill across the survey area (1.3 million km2) was estimated to be 24 g m−2. Total biomass was estimated to be 28.75 million tonnes (Mt) with a coefficient of variation (CV) of 16.18%. This biomass estimate has been used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to update the precautionary catch limit for krill in this area (CCAMLR Division 58.4.2) from 0.450 to 2.645 Mt. Overall, krill were widely distributed at relatively low densities: 25% of the 2-km along-track echo-integration intervals were devoid of krill, 50% registered densities of 1 g m−2 of krill or less and 75% registered densities of 12 g m−2 or less. Mean densities were highest in the waters to the south of the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC), while the waters to the north of the Southern ACC Front (SACCF) were almost devoid of krill. Half of the cumulative krill density across the survey was found within 80 km of the 1000 m isobath (the shelf break), and 40% within 40 km. This was mostly due to particularly high densities (up to 4400 g m−2) around the shelf break on 3 of the 11 transects surveyed. The majority of acoustic krill detections were in the top 100 m of the water column, centred around 50 m depth.