Eastern McMurdo Sound Research Articles

Sea ice growth rates from tide‐driven visible banding

AbstractIn this paper, periodic tide‐current‐driven banding in a sea‐ice core is demonstrated as a measure of the growth rate of first‐year sea ice at congelation‐ice depths. The study was performed on a core from the eastern McMurdo Sound, exploiting the well‐characterized tidal pattern at the site. It points the way to a technique for determining early‐season ice growth rates from late‐season cores, in areas where under ice currents are known to be tidally dominated and the ice is landfast, thus providing data for a time of year when thin ice prevents direct thickness (and therefore growth rate) measurements. The measured results were compared to the growth‐versus‐depth predicted by a thermodynamic model.

Classification and behavior of free-ranging Weddell seal dives based on three-dimensional movements and video-recorded observations

The goal of this study was to classify free-ranging (FR) dives of Weddell seals (Leptonychotes weddellii) and to compare them to isolated hole (IH) dives. Classification and comparisons were based on 58 descriptors for three-dimensional dive paths computed from data obtained by attaching video and data recorders to the backs of 12 adult Weddell seals that were free-ranging in eastern McMurdo Sound, Antarctica. We then inferred behavioral functions for the dive classes based on video-recorded observations. Three of the four dive types previously identified from seals diving at an IH occurred in FR seals. Although there were differences associated with location, Types 1, 2 and 3 dives clustered in a similar pattern in the discriminant analysis for FR and IH dives. Most prey (79%) captures occurred during Type 1 dives, and the primary (99%) prey was Antarctic silverfish (Pleuragramma antarcticum). Type 1 dives were the deepest (mean maximum depth 324–378m), longest in duration (15.0–27.0min), covered the greatest total distance (1470–2197m), and had the steepest dive angles (descents: −30°; ascents: +27°). Types 2 and 3 dives formed a continuum from short duration (3.6–7.5min), shallow (mean maximum depth 30–66m) dives that were close to the ice hole (farpoint distance 75–130m) and often involved aggressive interactions with other seals for breathing opportunities (Type 2) to progressively longer (7.9–17.2min), deeper (mean maximum depth 81–143m) dives that covered greater total distances (878–1194m) and were associated with transiting between holes, exploring and occasionally foraging (Type 3). Very long distance Type 4 exploratory dives that were identified in the IH study were completely absent in FR seals.

Sea ice salinity and structure: A winter time series of salinity and its distribution

We present a winter time series of Antarctic sea ice salinity from eastern McMurdo Sound, an area close to an ice shelf where a subice platelet layer forms below the sea ice late in winter. This dramatically changes the sea ice structure as the sea ice grows into the subice platelet layer. Every 2 weeks during the 5 months of sea ice formation, salinity profiles were measured, along with detailed measurements of ice structure and growth rates. Once the influence of growth rate on sea ice bulk salinity is removed, the data from 69 cores and the results of a basic parameterization demonstrate that bulk salinity for platelet ice is higher than that for columnar sea ice. We also present measurements of the salinity profile close to the ice‐water interface and use these to investigate the expected regime of fluid flow within the permeable portions of the sea ice, with particular reference to mushy layer and percolation theory. Finally, we provide a new distribution of sea ice salinity from 740 measurements, which can be interpreted as the sum of two spatial fields that we attribute to sea ice samples with and without brine channels and which should be reproduced by any realistic sea ice models. This distribution suggests that two measurements of quantities linearly linked to sea ice salinity must differ by 29% if they are to be considered different with 90% confidence.

Effect of ocean current on the dead-reckoning estimation of 3-D dive paths of emperor penguins

The dead-reckoning technique is a useful method for obtaining 3-D movement data of aquatic animals. However, such positional data include an accumulative error. Understanding the source of the error is important for proper data interpretation. In order to determine whether ocean currents affect dive paths calculated by dead-reckoning, as has previously been hypothesized, we examined the directions of the estimated positions relative to the known real points (error direction) and the relationship between the error direction and the current direction. 3-D dive paths of emperor penguins Aptenodytes forsteri diving at isolated dive holes in eastern McMurdo Sound were recon- structed by dead-reckoning, and the net error and error direction were calculated. The net error cor- related positively with the dive duration. The error directions were not distributed uniformly, and the mean error direction tended to be north of the starting point of dives. Because there was a southward- flowing current in eastern McMurdo Sound, the ocean current was likely to affect the calculated dive paths. Therefore, the method of error correction generally used, in which the net error divided by the dive duration is applied to each estimated position, is realistically appropriate, provided that the cur- rent does not change significantly during a dive.

Process environments on modern and raised beaches in McMurdo Sound, Antarctica

Modern beaches in McMurdo Sound can be divided into three process regimes. Beaches on Ross Island (eastern McMurdo Sound) are characterised by normal marine processes with little ice modification. On ice-bound western McMurdo Sound, coastal orientation is of paramount importance. Ice thrust features are prominent on south facing beaches, which are open to the predominant wind direction, while more marine dominated beaches occur on north facing coasts where sea ice is blown offshore. In contrast to the modern beaches, raised beaches are similar all around McMurdo Sound. These are inferred to be storm ridges. The size and frequency of the ridges is a product of the local wave climate. The number of raised beaches at any site is a useful indicator of the paleo-wave climate. More ridges occur in sheltered south facing locations, because they are more protected from open marine conditions, than on beaches in ice-free or north facing locations. This is important for rebound reconstructions in the Sound as marine limits have been incorrectly identified in the past.

Nano- and Microplankton Dynamics during the Spring Phaeocystis Sp. Bloom in McMurdo Sound, Antarctica

The seasonal development of the microbial food web in eastern McMurdo Sound, Antarctica, was investigated during and immediately after the 1990–1991 bloom of Phaeocystis sp. (Prymnesiophyceae: Prymnesiales). From 23 November to 7 December, which was before the appearance of macroscopic colonies of Phaeocystis, both phytoplankton and Protozoa were low in abundance. During the Phaeocystis bloom (~10 December to 7 January), phytoplankton biomass was high and was dominated by colonial and singlecelled Phaeocystis, but other phytoplankton taxa, including diatoms and photosynthetic dinoflagellates, co-occurred. Heterotrophic nanoplankton and protozoan microzooplankton increased dramatically in biomass during the bloom. Non-thecate heterotrophic dinoflagellates were the most abundant microzooplankters. There was no evidence for inhibition of the microbial food web by Phaeocystis. By the post-bloom period, macroscopic colonies were rare and phytoplankton biomass had declined markedly. Microzooplankton biomass continued to rise until the end of the sampling period (23 January). Microfaecal pellets (median size range, 10–30 μm) were abundant during the bloom and post-bloom periods. In the post-bloom period, the heterotrophic protistan assemblage became very diverse, with numerous trophic linkages within the microbial food web. The abundance and diversity of the heterotrophic protist assemblage suggests that there was little control of protists by metazoans and that the microbial food web, consisting of bacteria, algae and Protozoa, was poorly coupled to metazoan zooplankton.

Population structure and feeding deterrence in three shallow-water antarctic soft corals

Alcyonium paessleri and Clavularia frankliniana are numerically abundant soft corals in the nearshore (12 to 33 m depth) benthic communities of eastern McMurdo Sound. They are much less abundant in western McMurdo Sound where a third species, Gersemia antarctica, co-occurs in low numbers. The body tissues of these three species are comprised mainly of organic material (53 to 70% dry wt), which is primarily dervied from NaOH-soluble protein and refractory material. The energetic contents of the whole-body tissues of A. paessleri, C. frankliniana and G. antarctica are 15.9, 17.3, and 14.5 kJ g-1 dry wt, respectively. The mean biomass per individual is 1.81, 0.008, and 45 g dry wt for each respective species. Based on population densities of 7.3, 1337.3, and 0.04 soft corals m-2 for A. paessleri, C. frankliniana and G. antarctica, respectively, the population energetic densities are estimated to be 210.1, 185.1, and 26.1 kJ m-2. Despite the relatively rich energetic content of the tissue and apparent vulnerability to predators, very little predation occurs on these soft corals. Two potential predators, the antarctic sea stars Perknaster fuscus and Odontaster validus, exhibited significant chemotactic defensive tube-foot retractions to hexane, chloroform, methanol, and aqueous methanol extracts of each soft coral. In addition, wholebody tissue of each soft coral was rejected by the demersal fish Pseudotrematomus bernacchii and the cryopelagic fish Pagothenia borchgrevinki. In contrast, whole soft-coral tissues sequentially extracted in four increasingly polar solvents were readily ingested by these antarctic fishes, indicating that sclerites do not play a significant role in deterring predators. Our results indicate that these antarctic soft corals contain bioactive compounds which deter common predatory seastars and fishes.

Biogenic sedimentation in McMurdo Sound, Antarctica

Surface sediments from eastern McMurdo Sound and fjord basins of the Victoria Land Coast are enriched in organic carbon and opal. At depths below 600 m, opal contents commonly exceed 30%; organic carbon contents average 1.5% and are as high as 3.5%. Opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern Ross Sea. The distribution of organic carbon and opal at the sea floor is consistent with cyclonic (clockwise) water circulation in McMurdo Sound. Advective transport from the Ross Sea supplies biogenic sediment to eastern and northern McMurdo Sound; the southwestern shelf is bathed by waters derived in part from beneath the Ross Ice Shelf which transport very little allochthonous carbon. The supply of biogenic debris in southwestern McMurdo Sound is further curtailed by sea-ice conditions, e.g., more prevalent multi-year sea ice which reduces photosynthesis, and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open-water production to the sea floor. Consistent with this hypothesis, organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western McMurdo Sound. Pronounced cross-sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east-west variations in productivity. Our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of McMurdo Sound. Pervasive resuspension is documented by pronounced near-bottom flux increases measured by sediment trapping. Resuspension events, which may be more common during the austral winter, act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions (< 400 m) to protected shelf basins. Despite oxidative loss within the water column, the accumulation rate of organic carbon in the deep basins of McMurdo Sound averages 45 mg C m −2 day −1, more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings. Preservation efficiency of opal during transport and deposition appears anomalously high and the most opal-rich sediments of McMurdo Sound are depleted in organic carbon relative to mid- and low-latitude siliceous deposits. Despite this observation, if similar patterns of biogenic sedimentation exist on many other parts of the shelf, the Antarctic continental margin is an important sink for sedimentary organic carbon, as has been previously suggested for the silica system.

Juvenile survival and recruitment in a population of Weddell seals (Leptonychotes weddellii) in McMurdo Sound, Antarctica

Survival and recruitment of Weddell seal pups were studied in eastern McMurdo Sound, Antarctica. Pups were marked and their apparent survival estimated by mark–recapture methods. The resulting estimates were used together with published estimates of adult survival, yearly sighting probabilities, and direct counts of pup production to simulate the dynamics of the population and evaluate the assumption that it is closed to immigration. Estimates derived from census data in 1982 and 1983 were over five times larger than those simulated. This discrepancy was due to the extremely low juvenile survival rates calculated from marked seals. Since few animals born in Erebus Bay return to breed, the large adult breeding population must be the result of substantial immigration, indicating an important role for juvenile dispersal in the population dynamics of Weddell seals.

Scavenging and other feeding habits of lysianassid amphipods (Orchomene spp.) from McMurdo Sound, Antarctica

The distribution and scavenging habits of the two most abundant lysianassid amphipods in McMurdo Sound differ markedly. Orchomene plebs lives primarily in deep water (>100 m), where planktonic and benthic food is sparser and scavenging events are less common and predictable than in shallower water. Orchomene plebs is common in shallow areas (<100 m) only under the Ross Ice Shelf and along the western McMurdo Sound. Here Weddell seals frequent tidal cracks in which they discard carrion and defecate; otherwise food is scarce. Orchomene pinguides lives on shallow (<10 m) wave-cut benches that are rich in food along the eastern McMurdo Sound. They, along with other omnivorous invertebrates which scavenge the food-rich eastern sound benches, are rare from shallow water along the western sound. The eastern benches are bathed by dense plankton blooms and harbor a high biomass of benthic diatoms and invertebrates. Scavenging events there were observed throughout the year. Orchomene plebs is larger and more motile, and came to laboratory carrion and baited field traps more rapidly and in greater numbers than O. pinguides. The crop contents of O. plebs contained only amorphous organic matter that suggested a scavenging habit. Crops of O. pinguides contained not only amorphous organic matter but also invertebrate prey, especially planktonic copepods that impact the bottom during winter.

Regulation of Numbers of an Apparently Isolated Population of Weddell Seals (Leptonychotes weddelli)

A population of Weddell seals ( Leptonychotes weddelli ) at White Island on the McMurdo Ice Shelf, Antarctica, appears to be geographically isolated. Its origin is still uncertain but three explanatory theories are mentioned. The breeding and nonbreeding fractions of the population separate during the pupping season (October to mid-December) and mix later as do their counterparts in eastern McMurdo Sound. Despite a greater reproductive potential, only two or three pups have been born annually for the past five summers. Natality clearly exceeds mortality of adults and subadults, but known survival of pups is of the same order as known mortality of adults and subadults. Censuses of seals visible on the sea ice in 1961 and 1968 were identical (11 seals), which also indicate stability in numbers of the population. The extreme fatness of all seals at White Island suggests that availability of food does not regulate numbers. Probably the availability of breathing holes in winter does. Intrasexual social strife between females also appears to lower the reproductive rate below its theoretical potential.

Observations on a stabilizing population of weddell seals

The Weddell seal population of eastern McMurdo Sound, after a period of increase and subsequent decline, is now stabilizing in terms of total numbers and pup production. Increased intrasexual strife resulting mainly from competition for access to the best pupping and breeding colonies appears to be the major factor causing stabilization.