Effects of sea ice cover on the swarming behaviour of Antarctic krill,Euphausia superba

first_imgMuch of the distribution range of Antarctic krill, Euphausia superba, is covered by permanent or seasonal sea ice. Sea ice extent has been implicated as a major factor affecting reproductive success of krill and krill dispersal, but little is known of the way in which ice cover may influence krill behaviour. This is largely because the under-ice environment is difficult to study. Ship-borne echosounders have, however, detected krill aggregations in midwater in ice-covered regions. We used 120-kHz echograms collected underway during three cruises that crossed ice-covered and adjacent open waters in the Bellingshausen, Weddell, and Scotia seas to compare morphological and next-neighbour characteristics of krill swarms within and without ice cover. No significant differences were detected between the horizontal and vertical extent of swarms or swarm next-neighbour distance in ice-covered or open waters. Distributions of swarm mid-depths did, however, differ significantly between ice-covered and open areas in all three seas, although the direction of difference was not the same in each instance: swarms in the Weddell and Scotia seas were generally shallower under ice than in open water, whereas in the Bellingshausen Sea the opposite prevailed.last_img read more

Modeling tidal current profiles and vertical mixing beneath Filchner-Ronne Ice Shelf, Antarctica

first_imgOne of the warmest water masses beneath Filchner-Ronne Ice Shelf (FRIS) is dense, high salinity shelf water (HSSW) that flows into the sub-ice-shelf cavity from the ice front and occupies the lower portion of the water column. A one-dimensional turbulence closure ocean model has been applied to this high latitude sub-ice-shelf environment to demonstrate that tidal currents mix HSSW vertically through the water column and cause melting at the bottom of the ice shelf. Significantly FRIS lies near the critical latitude for the semidiurnal tide, where the Coriolis frequency equals the tidal frequency, resulting in a strongly depth-dependent tidal current and thick boundary layers. Using the model, the effect of the critical latitude, stratification, and the polarization of the tidal current ellipse on boundary layer structure and subsequent vertical mixing are examined. The model shows that stratification significantly affects how the shape of the tidal current ellipse varies with depth and that both the depth to which the pycnocline initially develops and the longer term melt rates are highly dependent on tidal current ellipse polarization. The sensitivity to both the stratification and the polarization are due, in large part, to the proximity of the critical latitude. Positive polarizations (anticlockwise rotating current vectors) quickly develop deeper pycnoclines and maintain higher melt rates than negative polarizations (clockwise rotating current vectors). For many areas beneath FRIS the polarization ranges from -0.3 to +0.3; here the modeled pycnocline development is sensitive to polarization, though the effect on the time-averaged melt rate is suppressed for positive polarizations. However, in key areas where the polarization exceeds +/-0.3 and the ellipses are more open and circular, the effects of polarization are significant. Levels of tidal mixing and associated melting vary by more than an order of magnitude over the whole tidal ellipse polarization range, showing that very different mixing and melting regimes are present beneath FRIS.last_img read more

An investigation into the correlation of geomagnetic storms with tropospheric parameters over the South Pole

first_imgWe test the proposal that the Sun’s magnetic activity,communicated via the solar wind, provides a link betweensolar variability and the Earth’s climate in the Antarctictroposphere. The strength of a geomagnetic storm is oneindicator of the state of the solar wind; therefore, we use the dates of 51 moderate to strong winter geomagnetic stormsfrom the period 1961–1990 to conduct a series of superposedepoch analyses of the winter South Pole isobaric height andtemperature, at pressures of between 100–500mbar. UsingStudent’s t -test to compare the mean value of the pre- andpost-storm data sets, we find no evidence to support the hypothesis that there is a statistically-significant correlation between the onset of a geomagnetic storm and changes in the isobaric temperature or height of the troposphere and lower stratopshere over the South Pole during winter months. This concurs with a similar study of the variability of the troposphere and lower stratosphere over the South Pole (Lam and Rodger, 2002) which uses drops in the level of observed galactic cosmic ray intensity, known as Forbush decreases, as a proxy for solar magnetic activity instead of geomagnetic storms.last_img read more

Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters

first_imgDuring the austral summer of 2002, a large and persistent phytoplankton bloom was detected with SeaWiFS imagery in the Georgia Basin to the north-west of South Georgia, while waters to the east of the island were relatively unproductive. A British Antarctic Survey research cruise in January 2002 confirmed this west/east difference with production values of up to 2.5 g C m(-2) d(-1) and chlorophyll a (chl a) values up to 15 mg m(-3) at stations to the northwest of the island and 0.17 g C m(-2) d(-1) and 1.3 mg chl a m(-3) to the northeast. These differences were not attributable to light limitation as mixed layer depth never exceeded critical depth. Instead, substantial nutrient depletions to the northwest of the island compared with the northeast suggested a difference in nutrient use between the two regions. The exceedingly high nutrient depletions (to < 6.0 and 0.3 mmol m(-3) for NO3-N and PO4-P, respectively) measured to the northwest were associated with an anticyclonic eddy situated over the Northwest Georgia Rise. Furthermore, differences in NO3-N:PO4-P depletion ratios suggested a greater ability in the northwest phytoplankton to utilise NO3-N, and a greater dependence on NH4-N at the northeast stations. Three distinct station groups were identified around the island based on watermass and size-fractionated chlorophyll. To the east, waters were characterised by a high proportion of microplankton and low NO3-N:PO4-P depletion ratios, to the west, by either a high proportion of microplankton and high NO3-N:PO4-P depletion ratios, or a high proportion of nanoplankton and moderate NO3-N:PO4-P depletion ratios. We consider this to be indicative of greater Fe availability, promoting NO3-N use, to the northwest of South Georgia. However, an absence of microplankton over the western shelf regions may be due to size selective grazing by krill. Our field data, in conjunction with SeaWiFS imagery, indicated that the Georgia Basin phytoplankton most likely originated upstream of South Georgia. Subsequent interactions with the Northwest Georgia Rise and South Georgia's south-western shelf promoted increased growth that converged to the west of the island to form a large bloom in the Georgia Basin.last_img read more

Errors in juvenile copepod growth rate estimates are widespread: problems with the Moult Rate method

first_imgThe ‘Moult Rate’ (MR) method has been used widely to derive stage-specific growth rates in juvenile copepods. It is the most common field-based method. Unfortunately, the equation underlying the method is wrong and, consequently, large errors in juvenile growth rate estimates are widespread. The equation derives growth from the mean weight of 2 consecutive stages (i and i + 1) and the duration of stage i. The weight change and the period to which this change is attributed are, therefore, offset. We explore this potential source of error in the MR method critically. Errors arise as a result of 2 primary factors: (1) unequal durations of successive stages and (2) unequal rates of growth of successive stages. The method of deriving the mean weight (arithmetic or geometric) also has an impact and is examined. Using a steady-state assumption, a range of scenarios and the errors that arise are examined. The literature is then reviewed and the size of errors resulting from MR method application in both field and laboratory situations is estimated. Our results suggest that the MR method can lead to large errors in growth estimation in any stage, but some stages are particularly prone. Errors for the C5 stage are often large because the following stage (the adult) does not moult, and has a different rate of body weight increase. For the same reason, errors are also great where the following stage is not actively moulting (e.g. when diapausing). In these circumstances, published work has commonly greatly underestimated growth. For example, MR growth ranges from 11 to 47% of the value derived correctly for this stage, gi_corr (calculated assuming the non-moulting stage does not grow). In late stages that are followed by actively moulting stages, the MR method has commonly given values in excess of 150% of gi_corr, but underestimation also occurs, with values <90% of gi_corr. We propose new methods and equations that overcome these problems. These equations are written with and without within-stage mortality included. The equations are relatively insensitive to mortality rates within the range found in the field, but only provided that the stage duration is not determined from moult rate. Stage duration estimates obtained from measuring moulting rates of field-collected animals are very sensitive to mortality rates of the animals prior to capture, and field mortality rates are often high enough to produce dramatic over-estimation of stage duration.last_img read more

Breeding latitude drives individual schedules in a trans-hemispheric migrant bird

first_imgDespite clear benefits of optimal arrival time on breeding grounds, migration schedules may vary with an individual bird’s innate quality, non-breeding habitat or breeding destination. Here, we show that for the bar-tailed godwit (Limosa lapponica baueri), a shorebird that makes the longest known non-stop migratory flights of any bird, timing of migration for individual birds from a non-breeding site in New Zealand was strongly correlated with their specific breeding latitudes in Alaska, USA, a 16,000-18,000 km journey away. Furthermore, this variation carried over even to the southbound return migration, 6 months later, with birds returning to New Zealand in approximately the same order in which they departed. These tightly scheduled movements on a global scale suggest endogenously controlled routines, with breeding site as the primary driver of temporal variation throughout the annual cycle.last_img read more

Using ecosystem monitoring data to detect impacts

first_imgThe purpose of ecosystem monitoring programs is to indicate the state of ecosystems and whether they have been impacted by activities such as fishing. This paper discusses a rangeof methods for inferring such impacts using monitoring data with no control sites. These methods assess either (i) the expected probability of an observed value in an unimpactedsystem, or (ii) the frequency of values below a fixed reference point. The second approach allows inference criteria based on changes in this frequency rather than by reference to a critical probability. All methods would have provided a sustained indication of a significantdecline in Antarctic fur seal (Arctocephalus gazella) pup production at South Georgia from the early 1990s within a few years of its onset, but a fixed reference point method could have provided this sustained indication from the onset. Furthermore, simulation of all methods suggests that the total probability of error (false positives and false negatives combined) is lowest with fixed reference point methods. The probabilities of Type I andType II error can be evaluated analytically for these methods, which facilitates decision-making based on attitudes to risk.last_img read more

A record of Holocene glacial and oceanographic variability in Neny Fjord, Antarctic Peninsula

first_imgAnalyses of a 12 m marine sediment core from Neny Fjord, Marguerite Bay, Antarctic Peninsula (68.2571°S, 66.9617°W), yield a high-resolution record of Holocene climate variability. The sediments preserve signals of past glacial and marine environments and offer a unique insight into atmospheric and oceanic forcings on the western Antarctic Peninsula climate. Dating of basal material reveals that deglaciation of the fjord occurred prior to 9040 cal. yr BP and provides a minimum constraint on the timing of deglaciation close to the southern Antarctic Peninsula ice-divide. Continuous deposition of ice-distal sediments and seasonally open-water diatoms indicates that the site has not been over-ridden by glacier ice during the Holocene. A facies of sand-rich material offers the only evidence of a localized glacier advance, during the mid Holocene. Statistical analysis of diatom assemblage data reveals several climatic episodes of varying magnitude and duration. These include an early-Holocene warm period (~9000 and ~7000 cal. yr BP), potentially associated with influx of Circumpolar Deep Water onto the continental shelf and coinciding with widespread glacial retreat and Holocene collapse of the George VI Ice Shelf. The mid-Holocene (~7000 to ~2800 cal. yr BP) sediments are characterized by diatom assemblages indicative of less pervasive sea-ice cover and prolonged growing seasons with evidence of increased meltwater discharge from ~4000 cal. yr BP. The youngest sediments (~2800 cal. yr BP to present) contain a record that is consistent with the widely documented ‘neoglacial’ period followed by an abrupt reversal and climate amelioration from sometime after ~200 cal. yr BP.last_img read more

East Antarctica’s Achilles heel

first_imgThe East Antarctic ice sheet is believed to be Earth’s most stable ice sheet. Changes in geochemical composition of offshore sediments suggest that its margin repeatedly retreated by at least 350–550 kilometres inland between 5.3 and 3.3 million years ago.last_img

Revealing higher than expected meiofaunal diversity in Antarctic sediments: a metabarcoding approach

first_imgAn increasing number of studies are showing that Antarctic mega- and macrofauna are highly diverse, however, little is known about meiofaunal biodiversity in sediment communities, which are a vital part of a healthy and functional ecosystem. This is the first study to analyse community DNA (targeting meiofauna) using metabarcoding to investigate biodiversity levels in sediment communities of the Antarctic Peninsula. The results show that almost all of the meiofaunal biodiversity in the benthic habitat has yet to be characterised, levels of biodiversity were higher than expected and similar to temperate regions, albeit with the existence of potentially new and locally adapted species never described before at the molecular level. The Rothera meiofaunal sample sites showed four dominant eukaryotic groups, the nematodes, arthropods, platyhelminthes, and the annelids; some of which could comprise species complexes. Comparisons with deep-sea data from the same region suggest little exchange of Operational Taxonomic Units (OTUs) between depths with the nematodes prevalent at all depths, but sharing the shallow water benthos with the copepods. This study provides a preliminary analysis of benthic Antarctic Peninsula meiofauna using high throughput sequencing which substantiates how little is known on the biodiversity of one of the most diverse, yet underexplored communities of the Antarctic: the benthos.last_img read more