Large Krill Research Articles

Target strength measurements of individual sub-Arctic krill have frequency-dependent differences from scattering model predictions.

Target strength (TS) is commonly used to convert acoustic backscatter from marine organisms to numerical abundance estimates. Shipboard, tank-based TS measurements were made on four sub-Arctic krill species (Euphausia pacifica, Thysanoessa spinifera, Thysanoessa inermis, and Thysanoessa raschii) from the eastern Bering Sea and Gulf of Alaska at discrete frequencies between 42 and 455 kHz. These measurements were compared to scattering model predictions parameterized with data from the same (when possible) individual krill. Statistically significant differences between modeled and experimental estimates at 42, 45, 120, and 131 kHz exceeded 2 dB on average. Variability in the signal-to-noise ratio, animal length, and measurements from two separate narrowband and broadband transducer pairs (at those frequencies) did not account for these differences. Scattering predictions at 120 and 131 kHz were consistent with the expected transition from Rayleigh-to-geometric scattering where models become increasingly sensitive to orientation and body shape variability. Disagreement between modeled and measured TS may be due to using scattering models developed for, and validated on, larger krill (i.e., Euphausia superba) rather than smaller species of krill. Acoustic surveys of smaller (15-30 mm) krill may require further validation of both the generalizability and parameterization of applied scattering models.

Evidence for Size-Selective Predation by Antarctic Humpback Whales

Animals aggregate around resource hotspots, but what makes one resource more appealing than another can be difficult to determine. In March 2020 the Antarctic fjord Charlotte Bay included >5× as many humpback whales as neighboring Wilhelmina Bay, a site previously known for super aggregations of whales and their prey, Antarctic krill. We used suction-cup attached bio-logging tags and active acoustic prey mapping to test the hypothesis that whale abundance in Charlotte Bay would be associated with higher prey biomass density, and that whale foraging effort would be concentrated in regions of Charlotte Bay with the highest biomass. Here we show, however, that patch size and krill length at the depth of foraging were more likely predictors of foraging effort than biomass. Tagged whales spent >80% of the night foraging, and whales in both bays demonstrated similar nighttime feeding rates (48.1 ± 4.0 vs. 50.8 ± 16.4 lunges/h). However, whales in Charlotte Bay foraged for 58% of their daylight hours, compared to 22% in Wilhelmina Bay, utilizing deep (280–450 m) foraging dives in addition to surface feeding strategies like bubble-netting. Selective foraging on larger krill by humpback whales has not been previously established, but suggests that whales may be sensitive to differences in individual prey quality. The utilization of disparate foraging strategies in different parts of the water column allows humpback whales to target the most desirable parts of their foraging environments.

The carbon and nitrogen budget of Desmophyllum dianthus-a voracious cold-water coral thriving in an acidified Patagonian fjord.

In the North Patagonian fjord region, the cold-water coral (CWC) Desmophyllum dianthus occurs in high densities, in spite of low pH and aragonite saturation. If and how these conditions affect the energy demand of the corals is so far unknown. In a laboratory experiment, we investigated the carbon and nitrogen (C, N) budget of D. dianthus from Comau Fjord under three feeding scenarios: (1) live fjord zooplankton (100–2,300 µm), (2) live fjord zooplankton plus krill (>7 mm), and (3) four-day food deprivation. In closed incubations, C and N budgets were derived from the difference between C and N uptake during feeding and subsequent C and N loss through respiration, ammonium excretion, release of particulate organic carbon and nitrogen (POC, PON). Additional feeding with krill significantly increased coral respiration (35%), excretion (131%), and POC release (67%) compared to feeding on zooplankton only. Nevertheless, the higher C and N losses were overcompensated by the threefold higher C and N uptake, indicating a high assimilation and growth efficiency for the krill plus zooplankton diet. In contrast, short food deprivation caused a substantial reduction in respiration (59%), excretion (54%), release of POC (73%) and PON (87%) compared to feeding on zooplankton, suggesting a high potential to acclimatize to food scarcity (e.g., in winter). Notwithstanding, unfed corals ‘lost’ 2% of their tissue-C and 1.2% of their tissue-N per day in terms of metabolism and released particulate organic matter (likely mucus). To balance the C (N) losses, each D. dianthus polyp has to consume around 700 (400) zooplankters per day. The capture of a single, large krill individual, however, provides enough C and N to compensate daily C and N losses and grow tissue reserves, suggesting that krill plays an important nutritional role for the fjord corals. Efficient krill and zooplankton capture, as well as dietary and metabolic flexibility, may enable D. dianthus to thrive under adverse environmental conditions in its fjord habitat; however, it is not known how combined anthropogenic warming, acidification and eutrophication jeopardize the energy balance of this important habitat-building species.

Trophic niche overlap between round sardinella (Sardinella aurita) and sympatric pelagic fish species in the Western Mediterranean.

The northward expansion of round sardinella (Sardinella aurita) in the Mediterranean Sea, together with declines and fluctuations in biomass and landings of European sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) observed in recent decades, may suggest potential inter‐specific competition in the pelagic domain. The coexistence of sympatric zooplanktivorous fish species might therefore be exposed in part to trophic niche overlap and competition for food. Combining visual diet characterization under the microscope with DNA metabarcoding from stomach contents of fish collected in spring results show that predation on relatively large krill is equally important for sardinella than for the other two niche overlapping species. Furthermore, an important overlap is found in their isotopic niche, especially with anchovy, using nitrogen (δ15N) and carbon (δ13C) stable isotopes in muscle tissue. In fact, the three fish species are able to feed effectively in the whole prey size spectrum available during the sampled season, from the smallest diatoms and copepods to the larger prey (i.e., decapods and euphausiids), including fish larvae. Moreover, effective predation upon other large prey like siphonophores, which is observed only when multi‐proxy analyses in stomach contents are applied, might also be relevant in the diet of sardinella. The overlapping diet composition in spring, together with the effective use of food resource by sardinella, can be of special interest in potential future scenarios with warmer water temperature leading to lower zooplankton and/or higher jellyfish availability, where sardinella may take advantage over other species due to its feeding plasticity.

Combined effects of ocean acidification and elevated temperature on feeding, growth, and physiological processes of Antarctic krill Euphausia superba

Antarctic krill Euphausia superba is a key species in the Southern Ocean, where its habitat is projected to undergo continued warming and increases in pCO2. Experiments during 2 summer field seasons at Palmer Station, Antarctica, investigated the independent and interactive effects of elevated temperature and pCO2 (decreased pH) on feeding, growth, acid-base physiology, metabolic rate, and survival of adult Antarctic krill. Ingestion and clearance rates of chlorophyll were depressed under low pH (7.7) compared to ambient pH (8.1) after a 48 h acclimation period, but this difference disappeared after a 21 d acclimation. Growth rates were negligible and frequently negative, but were significantly more negative at high (3°C, -0.03 mm d-1) compared to ambient temperature (0°C, -0.01 mm d-1) with no effect of pH. Modest elevations in tissue total CO2 and tissue pH were apparent at low pH but were short-lived. Metabolic rate increased with temperature but was suppressed at low pH in smaller but not larger krill. Although effects of elevated temperature and/or decreased pH were mostly sublethal, mortality was higher at high temperature/low pH (58%) compared to ambient temperature/pH or ambient temperature/low pH (>90%). This study identified 3 dominant patterns: (1) shorter-term effects were primarily pH-dependent; (2) krill compensated for lower pH relatively quickly; and (3) longer-term effects on krill growth and survival were strongly driven by temperature with little to no pH effect.

Application of a Controlled Aquarium Experiment to Assess the Effect of Mesh Sizes and Mesh Opening Angles on the Netting Selectivity of Antarctic Krill (Euphausia superba)

Understanding the interactions between target species and netting is paramount for increasing the sustainability of trawling activities. The selectivity of the utilized netting depends on the sizes and opening angles of the mesh. The effects of the mesh size and mesh opening angle on the fishing selectivity of Antarctic krill (Euphausia superba) were assessed via micro-cosmos experiments. The results show that both the absolute abundance and the incidence of larger krill individuals passing through experimental panels are proportional to the utilized mesh size. Krill individuals larger than 35 mm passed through experimental panels at mesh opening angles larger than 50° for a 15 mm mesh size, 35° for a 20 mm mesh size and 20° for a 30 mm mesh size. Additionally, all L50 values increased with an increasing mesh size and an increasing mesh opening angle at the same mesh size. Furthermore, the selection range increased with an increasing mesh size and with an increasing mesh opening angle at the same mesh size. This paper provides scientific guidance for the choice of liner mesh sizes of krill trawl with the aim to improve fishing efficiency while minimizing fishing losses and potential negative ecosystem impacts from fisheries.

Classification of Southern Ocean krill and icefish echoes using random forests

Abstract Target identification remains a challenge for acoustic surveys of marine fauna. Antarctic krill, Euphausia superba, are typically identified through a combination of expert scrutiny of echograms and analysis of differences in mean volume backscattering strengths (SV; dB re 1 m−1) measured at two or more echosounder frequencies. For commonly used frequencies, however, the differences for krill are similar to those for many co-occurring fish species that do not possess swimbladders. At South Georgia, South Atlantic, one species in particular, mackerel icefish, Champsocephalus gunnari, forms pelagic aggregations, which can be difficult to distinguish acoustically from large krill layers. Mackerel icefish are currently surveyed using bottom-trawls, but the resultant estimates of abundance may be biased because of the species' semi-pelagic distribution. An acoustic estimate of the pelagic component of the population could indicate the magnitude of this bias, but first a reliable target identification method is required. To address this, random forests (RFs) were generated using acoustic and net sample data collected during surveys. The final RF classified as krill, icefish, and mixed aggregations of weak scattering fish species with an overall estimated accuracy of 95%. Minimum SV, mean aggregation depth (m), mean distance from the seabed (m), and geographic positional data were most important to the accuracy of the RF. Time-of-day and the difference between SV at 120 kHz (SV 120) and that at 38 kHz (SV 38) were also important. The RF classification resulted in significantly higher estimates of backscatter apportioned to krill when compared with widely applied identification methods based on fixed and variable ranges of SV 120–SV 38. These results suggest that krill density is underestimated when those SV-differencing methods are used for target identification. RFs are an objective means for target identification and could enhance the utility of incidentally collected acoustic data.

Species diversity affects ecosystem structure and mass flows in fjords

Latitude and temperature have been hypothesized to influence species richness, diversity and trophic control (top-down vs. bottom-up) in marine ecosystems. Ecosystem structures and mass flows of two moderately exploited fjord ecosystems with different temperatures and species diversity were compared. The Ullsfjord and Sørfjord systems (69°–70° N) are located between the relatively simple, low-diversity arctic Barents Sea in the north (71°–80° N) and the more species-rich and temperate North Sea (51°–62° N) in the south. Ullsfjord is the outer part of the fjord system and is deeper, warmer and more diverse than Sørfjord. Ecopath mass-balance models containing 40 ecological groups were developed for Ullsfjord and Sørfjord for the time period 1993–96. To obtain input data, abundance and diet of top-predators, fish, pelagic and benthic invertebrates were investigated. In the more diverse Ullsfjord system, large krill and pelagic shrimps were abundant and lower trophic level groups (TL<3) had the highest keystoneness, suggesting importance of bottom-up control. In contrast, large cod had the highest keystoneness and a large top-down effect as predator on small fishes and the larger crustacean groups in Sørfjord. For the diverse benthic invertebrates, the warmer and faster system (Ullsfjord) had higher mortality rates, shorter life spans and lower biomass (6.3 vs. 9.2 g C m−2) than the colder system (Sørfjord), but production (3.2 vs. 3.6 g C m−2 year−1) was similar in the two systems suggesting bottom-up control of benthic invertebrates.

Distribution and abundance of Antarctic krill (Euphausia superba) along the Antarctic Peninsula

Net-based data on the abundance, distribution, and demographic patterns of Antarctic krill are quantified from a contemporaneous two ship survey of the Antarctic Peninsula during austral summer 2011. Two survey areas were sampled focussed on Marguerite Bay in the south, and the tip of the Antarctic Peninsula in the north. Data from 177 stations showed that the highest concentrations of krill were found in the southern sampling area. Differences between areas were associated with a few large catches of one year old krill found in anomalously warm and productive waters in Marguerite Bay, and small krill catches in the less-productive, offshore waters in the north. Estimated krill density across the survey area was 3.4krillm−2, and was low compared to the long-term average of 45krillm−2 for the Elephant Island area. Overall recruitment between the two survey regions was similar, but per capita recruitment was about 60% lower than historical mean recruitment levels measured at Elephant Island since the late 1970s. Demographic patterns showed small krill concentrated near the coast, and large krill concentrated offshore on the shelf and slope all along the survey area. The offshore distribution of adult krill was delineated by the warm (∼1°C), low salinity (33.8) water at 30m, suggesting that most krill were present shoreward of the southern boundary of Antarctic Circumpolar Current Front. Distributions of larvae indicated that three hotspot areas were important for the production of krill: slope areas outside Marguerite Bay and north of the South Shetland Islands, and near the coast around Antarctic Sound. Successful spawning, as inferred from larval abundance, was roughly coincident with the shelf break and not with inshore waters. Given the rapid changes in climate along the Antarctic Peninsula and the lower per capita recruitment observed in recent years, studies comparing and contrasting production, growth, and recruitment across the Peninsula will be critical to better understand how climate change will impact krill populations and their dependent predators in the Scotia Sea.

Acoustic determination of the distribution of fish and krill across the Scotia Sea in spring 2006, summer 2008 and autumn 2009

Multifrequency acoustic backscatter data were examined from transects at eight stations across the Scotia Sea, from the South Orkneys to the north west of South Georgia. These transects were repeated in austral spring (November 2006), summer (December 2007/January 2008) and autumn (March 2009). A dB identification window ( S v120-38 ) identified two types of scatterers dominating the acoustic backscatter; a positive S v120-38 indicative of Antarctic krill ( Euphausia superba) and a negative S v120-38 indicative of mesopelagic fish with swim bladders, such as myctophids. Acoustic backscatter attributed to schooling myctophid fish was greater at stations in the north of the Scotia Sea across all seasons and found to increase significantly with increasing mean sea temperature and absolute dynamic height. A schools analysis identified more fish schools at the stations with high fish acoustic backscatter. The number of fish schools increased, were distributed further south, and occurred deeper from spring through to autumn. Krill density, calculated using a target strength model, was similar in spring (27.7 g m −2) and summer (27.2 g m −2), but lower (2.2 g m −2) in autumn 2009. An analysis of swarm characteristics showed that during the spring cruise the high mean krill density resulted from a small number of swarms containing a large number of small krill, particularly in the south of the survey region. In contrast in the summer cruise the high mean krill density resulted from a large number of swarms containing a low number of large krill. Finally, in autumn the low krill density resulted from a small number of swarms containing a small number of large krill. Krill swarms were located at a mean depth of 58 m in spring, 94 m in summer and 123 m in autumn and were generally further apart, deeper, contained fewer krill and smaller in the north of the survey region. Large-scale shifts in the latitudinal and vertical distribution of krill and schooling mesopelagic fish in the Scotia Sea were observed. This seasonal reorganisation in both horizontal and vertical distribution will alter the role of these prey items in the ecosystem.

Individual Variation of Foraging Behavior and Food Provisioning in Adélie Penguins (Pygoscelis adeliae) in a Fast-Sea-Ice Area

ABSTRACT. To understand the effects of foraging behavior on the amount of food provided to chicks (meal mass and frequency), we monitored nest attendance and diving behavior of 20 Adelie Penguin (Pygoscelis adeliae) pairs that were rearing chicks. Because ice forms on the sea surface at night, parents foraged through cracks or within leads in the sea ice mainly during 0800–2400 hours. Birds that departed the colony in the afternoon and returned in the evening of the same day fed their chicks more frequently than those that made longer trips (i.e., those that departed in the afternoon and returned the next day or departed in the early morning and returned that evening). When the duration of the longest dive bout during each trip was longer, parents brought back heavier meals that contained larger krill. On average, birds made longer dive bouts when diving to greater depths. Thus, daily foraging pattern and foraging depth affected the provisioning rate, but the proportion of time spent foraging during the p...

Behaviour affects local-scale distributions of Antarctic krill around South Georgia

Antarctic krill Euphausia superba are key to most Antarctic ecosystems, particularly at South Georgia, where penguins and seals rely on them for their breeding success each summer. Changes in krill behaviour and local-scale distribution during this time can have large consequences for these predators; however, we currently have little understanding of the behaviour of Antarctic krill in response to changing conditions of predation and food. We used a stochastic dynamic programming model to investigate the behaviour of krill within a region close to South Georgia that is known to be heavily foraged by penguins and seals. In the model, krill responded to changing conditions by adjusting their depth, density of swarm and swimming behaviour. We have shown the optimal behaviour of krill in 3 biologically distinct regions: the on-shelf region, shelf-break region and off-shelf region. We predict significantly higher concentrations of krill will result at the shelf-break region from krill choosing to swim slower and turn more often in a favourable zone. In addition, we predict a diel pattern in swarm density in most conditions of the model, with small krill generally forming lower density swarms than large krill, particularly on-shelf. This work is the first prediction of the effects of krill swarming and swimming behaviour on local-scale distribution.

Improved parametrization of Antarctic krill target strength models

There are historical discrepancies between empirical observations of Antarctic krill target strength and predictions using theoretical scattering models. These differences are addressed through improved understanding of key model parameters. The scattering process was modeled using the distorted-wave Born approximation, representing the shape of the animal as a bent and tapered cylinder. Recently published length-based regressions were used to constrain the sound speed and density contrasts between the animal and the surrounding seawater, rather than the earlier approach of using single values for all lengths. To constrain the parameter governing the orientation of the animal relative to the incident acoustic wave, direct measurements of the orientation of krill in situ were made with a video plankton recorder. In contrast to previous indirect and aquarium-based observations, krill were observed to orient themselves mostly horizontally. Averaging predicted scattering over the measured distribution of orientations resulted in predictions of target strength consistent with in situ measurements of target strength of large krill (mean length 40-43 mm) at four frequencies (43-420 kHz), but smaller than expected under the semi-empirical model traditionally used to estimate krill target strength.

Региональные особенности биологического состояния криля Euphausia superba Dana, обитающего в шельфовых водах острова Южная Георгия в июне - августе 2004 года

The data collected in the shelf waters of the northern South Georgia allowed determining the&nbsp;morpho-physiological state of Antarctic krill (Euphausia superba) in the winter months of the Southern Hemisphere (June-August, 2004). The analysis of variation curves permit distinguishing three krill size groups: the first group is small krill (of modal size 32-40 mm), the second group consists of&nbsp; comparatively large crustaceans (of modal size 34-44 mm), and the third – large krill (of modal size 44-50 mm). Changeability of their distribution was found: on the north-western shelf small crustaceans prevailed (Group I). As the fishery was moving to the east the rate of juveniles decreased considerably, and the abundance of the second group krill increased. During winter krill groups of the same size had similar morpho-physiological state both on the north-western and north-eastern ground. The differences were recorded in the second half of the winter. They dealt with maturity stages of females and males: in June males and females were at I and II stages, and in July-August – at stages II and II-III, correspondingly. As it was before (2001-2003), among large krill individuals the affection of carapace soft tissues and its chitinous cover was found. The percentage of the affected krill on the north-western ground reached 7.5, and on the north-eastern one – 5.5.

Distribution of zooplankton on the continental shelf off Marguerite Bay, Antarctic Peninsula, during Austral Fall and Winter, 2001

Abstract The Marguerite Bay region of the Western Antarctic Peninsula is known to support a large population of krill during the summer and is hypothesized to be a site of successful overwintering of krill. The distributions of zooplankton biomass, abundance, and taxonomic composition at six locations on or near the continental shelf at Marguerite Bay were determined for fall and winter, 2001, as a part of the Southern Ocean GLOBEC program. Water column zooplankton were sampled using a 1-m 2 MOCNESS. Samples were analyzed for taxonomic composition, size, and estimated biomass using silhouette analysis. Characteristics of four categories (small and large copepods, small and large krill) were described in detail. Total biomass and abundance were markedly reduced in winter relative to fall. The vertical distributions of total abundance and biomass, and of taxonomic composition, were associated with the hydrographic structure of the water column. Larger larval krill were observed at all locations during winter than during fall. Krill were relatively unimportant to total biomass and abundance at most locations, while copepods frequently dominated both biomass and abundance. In fall, greatest abundance and biomass of larval krill were observed on the continental shelf in association with a persistent clockwise gyre located to the west of Adelaide Island. During winter, much lower abundances of larval krill were present in the water column throughout the study region. High abundance and biomass of juvenile and adult krill were observed only in Laubeuf Fjord (Marguerite Bay) during fall.

Widening the net: spatio-temporal variability in the krill population structure across the Scotia Sea

Resolving the spatial variability in the population structure of Antarctic krill (Euphausia superba) requires a synoptic sample, as in the design of the CCAMLR 2000 Survey. However, this approach is not appropriate for assessing temporal variability. The size of krill in the diet of Antarctic fur seals (Arctocephalus gazella) has been shown to provide a good representation of the temporal changes in the krill population structure from within their foraging area. At Cape Shirreff, South Shetland Islands, krill in nets had modal size classes of 46-48 mm and 52-54 mm in length and appeared to reflect the presence of larger krill offshore and smaller krill inshore; krill in the diet of fur seals contained both modes, indicating that the foraging area of fur seals integrated the spatial variability in the krill population. At Signy Island, South Orkney Islands, krill in nets and fur seal diets had a modal size class of 52 mm when sampled simultaneously; however, krill in the diet of seals showed a decline in size later in the season with an overall mode of 48 mm. At Bird Island, South Georgia, there was little overlap between net samples, with a modal size class of 30-40 mm, and fur seal diets, with distinct modes of 44 and 54 mm; and there was also much greater spatial variability in the size of krill in these net samples than in those from the other two locations. Extending the comparison of krill size in the diet of seals, to include spatially congruent net samples collected immediately prior to the CCAMLR 2000 Survey, showed almost complete overlap and indicated an even greater spatial variability in the krill population structure at South Georgia. Interactions between the oceanographic transport and enhanced growth rates of krill at South Georgia may combine to produce a higher degree of spatial variability in the krill population compared to other locations and this may limit the use of differences in krill length as an indicator of their provenance. This study underlines the importance of using data from multiple sources when considering large-scale krill population dynamics; information that is crucial to the effective management of the commercial exploitation of krill.

Using a high-powered strobe light to increase the catch of Antarctic krill

Adult euphausiids are difficult to capture with the nets typically used by oceanographers due in part to avoidance of the net. A cruise of the Southern Ocean GLOBEC program (NBP0103) in April/May 2001 permitted an evaluation of the efficacy of a high-powered strobe light (150 W) mounted on a 1-m2 multiple opening/closing net and environmental sampling system (MOCNESS) in reducing net avoidance. Three horizontal tows were made in Laubeuf Fjord, Marguerite Bay (western Antarctic Peninsula, 67.89°S; 68.30 W) on 28–29 May in an area that had a high abundance of adult krill (mostly Euphausia superba, Dana and a few E. crystallorophias, Holt and Tattersall). Each tow consisted of a series of paired down and up casts through a set depth interval (e.g. 50–90 m), with each successive net (335 µm mesh) sampling both a down and up cast. The strobe light was either “on” or “off” while each net was open, and when on, the light flashed at 4-s intervals. During a tow, four of the eight nets sampled with the strobe flashing and four sampled with the strobe off, in a random sequence. Total zooplankton displacement volume was significantly higher (P<0.05), on average by a factor of ~1.5, when the strobe light was on. The increased biovolume was due to the enhanced catch (factor of ~2) of adult krill (15–60 mm length). There was no enhanced catch of smaller krill (5–15 mm length: a mixture of E. superba, E. crystallorophias, and Thysanoessa macrura, GO Sars). In addition, the average length of the large krill fraction was not changed substantially with the strobe light on. These results suggest that krill avoidance of nets can be overcome by intense strobe lighting.

The linkage between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages on the west Antarctic Peninsula continental shelf

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from the Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identified in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identified. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly flowing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient silicate:nitrate ratio relative to the corresponding value for unmodified UCDW (1.5 versus 3.0 for UCDW). The magnitude of the nutrient drawdown in areas of topographically-induced upwelling suggested that diatom growth had been elevated in response to recent upwelling but that the resulting increased algal biomass was either dispersed by advective processes and/or consumed by the larger krill that were observed to be associated with each offshore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf may not be recognized based on elevated biomass and/or rates of carbon fixation. It was likely that similar physical forcing of significant phytoplankton growth, especially diatoms, may occur but be undetected in regions where the southern boundary of the ACC nears the Antarctic continental shelf edge. Our analyses from the west Antarctic Peninsula demonstrate coupling of the structure of the physical environment with nutrient distributions and phytoplankton assemblages and through to the higher trophic levels, such as Antarctic krill. This environment-trophic coupling may also occur in other regions of the Antarctic, as suggested by correspondences between the distribution of Southern ACC boundary and regions of high concentrations of Antarctic krill. The many mechanisms underlying this coupling remain to be determined, but it was clear that the ecology and biology of the components of the marine food web of the Antarctic continental shelf cannot be studied in isolation from one another or in isolation from the physical environment.

Variation in the distribution of Antarctic krill Euphausia superba around South Georgia

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 188:149-160 (1999) - doi:10.3354/meps188149 Variation in the distribution of Antarctic krill Euphausia superba around South Georgia J. L. Watkins*, A. W. A. Murray, H. I. Daly British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, United Kingdom *E-mail: j.watkins@bas.ac.uk ABSTRACT: There is considerable spatial structure within the South Georgia krill-based ecosystem in terms of predator demand, commercial fishery pressure and krill abundance. Here we investigate the hypothesis that there is also spatial structure in the krill population composition, finding differences in length-frequency histograms between the eastern and western ends of the island. Between 1981 and 1997 the British Antarctic Survey carried out 6 major cruises where krill were taken from both ends of South Georgia. Cluster analysis revealed that the length-frequency histograms could be grouped into 4 main types with relatively simple biological characteristics: Cluster 1 contained small krill (mean size 23.9 mm) up to 2 yr old (1+ yr class); Cluster 2 (mean size 31.1 mm) contained a mixture of 1+ with some 2+ and 3+ yr classes; Cluster 3 contained medium-sized krill (mean size 41.4 mm) probably 2+ and 3+ yr classes; Cluster 4 contained large krill (mean size 50.3 mm) likely to be 3+ or older. Principal components analysis (PCA) provided good separation of these clusters using the first 2 axes (80% of the total variance). There were no obvious differences in the length composition of krill sampled from different water depths, although there were some indications that differences did occur between different water masses. Detailed inspection of the individual cruises revealed that the length-frequency histograms at the western end of the island contained more large krill than those from the eastern end of the island and also that when Weddell Sea water was found within a cruise then this contained the smallest krill. We consider that such differences may arise not only because krill may experience different conditions at each end of the island but also may originate from 2 separate sources which may have different population structures. KEY WORDS: Antarctic krill · Euphausia superba · South Georgia · Population structure Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 188. Publication date: November 03, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Krill population dynamics at South Georgia 1991-1997, based on data from predators and nets

Central to understanding krill population dynamics is knowledge of their population structure. To examine this we used length-frequency distributions from 142 wk of sampling (n = 23996 krill) of 3 predator species breeding at South Georgia and 12 wk of sampling (n = 10252 krill) from scientific nets from the same area over the summers of 1991-1997. In comparing the 5 years for which both predator and net samples were available, despite differing selectivities and spatio-temporal circumscriptions, both predators and nets were sampling the same overall krill population. Greatest similarity resulted from comparing net samples with samples from Antarctic fur seals and macaroni penguins combined; least temporal variation occurred in predator samples from late summer (March). From the 7 yr predator time series, within-year variation was greatest in 1991 and 1994, both years of low krill biomass at South Georgia. In both of these years large krill dominated during December but were completely replaced by small krill by February. The mean length of the March krill population showed a regular increase from 1991 to 1993, fell to a minimum in 1994 and thereafter increased steadily to 1997. Using these data in conjunction with putative size/age-group cohorts in the length-frequency distribution, we suggest that years of high mean krill length reflect failure of small krill to recruit into the population, producing a period of low krill biomass in the following year. Similar recruitment failure in the same years was evident in krill populations in the Antarctic Peninsula region to the south, indicating large-scale events. This supports suggestions of periodic fluctuations in krill production and recruitment which may relate directly to physical phenomena such as cycles in the distribution and extent of sea ice.