Phytoplankton in the Weddell Sea,Antarctica: composition,abundance and distribution in water-column assemblages of the marginal ice-edge zone during austral autumn

At the compacted, north-south line of the ice edge, phytoplankton were sampled during early austral autumn of 1986 in the northwestern Weddell Sea. Cells from discrete water bottle samples from 12 stations on two east-west transects were counted to gain quantitative information on the composition, abundance, distribution, and condition of the phytoplankton in water-column assemblages. Over 70 species were found. The highest numbers of total cells (integrated through the top 150 m) were found in open water, well-separated from and to the east of the ice edge on the southern transect, with 6.01×10¹⁰ cells m^-2. The relative abundance of diatoms was low at ice-convered stations (< 35% of the total phytoplankton in preserved samples) and high at open-water stations (> 80%); however, the relative abundance of the prymnesiophyte Phaeocystis sp. was high at ice-covered stations (> 60%) and low at open-water stations (< 16%), with lower absolute abundances than during a previous austral-spring phytoplankton increase. In the open ocean, the dominants were the pennate diatoms Fragilariopsis cylindrus, Pseudonitzschia prolongatoides, F. curta, and a small form of the centric diatom Chaetoceros dichaeta in chains. Although the three pennate diatoms were frequently dominant in number, they represented less biomass than C. dichaeta in open waters. Mean phytoplankton abundance was low (0.2×10⁶ cells l^-1) but, overall, the diatom cell density (0.14×10⁶ cells l^-1) was similar to that found previously during a northward transect from ice-covered to ice-free water at the Weddell-Scotia Sea ice edge (spring 1983). The phytoplankton spatial patterns in the two autumn transects differed, with the more southerly transect exhibiting a higher abundance of diatoms and dinoflagellates. The ratio of full to empty diatoms was higher on the southern transect, indicating a healthy population, while lower ratios of full/empty frustules on the northern transect suggested a generally declining population. However, Phaeocystis sp. was more abundant on the northern transect.     

Zooplankton biomass in the ice-covered Weddell Sea,Antarctica

Zooplankton was sampled by a Rectangular Midwater Trawl (RMT 1 + 8) in Weddell Sea surface waters (0 to 300 m) between 66 and 78°S during austral summer (February – March 1983). Sixty-nine taxa including different developmental stages were considered and divided into 16 size classes between <1 and >39.5 mm length. Biomass was determined by taxon and size class for three different meso- and macroplankton communities in the oceanic region, on the northeastern shelf and on the southern shelf of the Weddell Sea. The highest biomass of 11.2 mg DW m^–3 (3.4 g DW m^–2) was found in the northeastern shelf community (70 to 74°S), where juvenile and adultEuphausia crystallorophias accounted for 3.7 mg DW m^–3 (1.1 g DW m^–2). Although not quantitatively sampled, early copepodite stages (CI to CIII) ofCalanoides acutus andCalanus propinquus ranked second with 2.7 mg DW m^–3 (0.8 g DW m^–2). Biomass in the northeastern shelf community was concentrated in the size ranges 1 to 4 mm and 19.5 to 39.5 mm. The oceanic community of the central Weddell Sea was dominated by copepods smaller than 5 mm, which made up half of the total oceanic biomass. The tunicateSalpa thompsoni (7.0 to 8.5 mm) was the dominant single species with 1.6 mg DW m^–3 (0.5 g DW m^–2). Euphausiids, mainly juvenile and adult krillEuphausia superba, comprised 1.2 mg DW m^–3 (0.4 g DW m^–2). Total standing stock in the oceanic community was 9.4 mg DWm^–3 (2.8 g DW m^–2). Lowest biomass values were found in the southern shelf community (south of 75°S) with 4.0 mg DW m^–3 (1.2 g DW m^–2), concentrated in the 1 to 4 mm and 14.5 to 34.5 mm size classes. Abundant species were the pteropodLimacina helicina (1 to 2 mm; 0.7 mg DW m^–3; 0.2 g DW m^–2) andE. crystallorophias (24.5 to 39.5 mm; 0.9 mg DW m^–3; 0.3 g DW m^–2). The data reveal that it is essential to distinguish among subsystems in the Southern Ocean. This leads to a better understanding of the structure and function of those pelagic food webs which represent alternatives to the paradigmatic krill-centered system.     

Distribution and abundance of choanoflagellates (Acanthoecidae) across the ice-edge zone in the Weddell Sea,Antarctica

Choanoflagellates are thought to be an important component of oceanic microbial food webs, but little quantitative data exists on their abundance,, distribution, or relationship to potential food sources. In an Antarctic ice edge zone (northern Weddell Sea, March 1986), choanoflagellate abundance varied over two orders of magnitude in the upper 100 m. The lowest abundances were recorded at the bottom of the water column under ice cover and the highest abundances occurred in the upper 30 m of open water. Species that were predominantly in colonies dominated the open-water samples. Abundances of total choanoflagellates and some individual species were correlated with primary and secondary biomass and production, indicating a response to gradients in potential food sources. This suggests that choanoflagellates are tightly coupled with their food sources and supports the contention that they may an important link between bacteria-sized particles and metazoan grazers.     

Seasonal variations in distribution and population structure of Microcalanus pygmaeus and Ctenocalanus citer (Copepoda: Calanoida) in the eastern Weddell Sea,Antarctica

The abundance, vertical distribution and population structure of two important small calanoid copepod species, Microcalanus pygmaeus (G. O. Sars) and Ctenocalanus citer Heron and Bowman, were studied in the eastern Weddell Sea in summer (January/February 1985), in late winter/early spring (October/November 1986) and in autumn (April/May 1992). The population of Microcalanus pygmaeus consisted mainly of copepodite stages CII and CIII in late winter/early spring and were concentrated between 500 and 200 m depth. In summer, stage CIV was the modal stage and the bulk of the population had ascended above 300 m. In autumn the population structure was bimodal with CI and CV dominating. Most of the population was concentrated between 300 and 200 m. In all investigation periods M. pygmaeus had their maximal concentrations in the thermo-pycnocline. The developmental stages CIII to CV of Ctenocalanus citer formed the bulk of the population in late winter/early spring. In October all developmental stages had their main distribution between 500 and 200 m, except females, which were concentrated in the upper 50 m. In November most of the population occurred between 200 and 50 m. The summer population was concentrated in the upper 50 m, and numbers increased dramatically as the new cohort hatched. Copepodite stages CII and CIII dominated the population at the end of January, while CIV dominated 2 wk later. In autumn, CV was the modal stage. The majority of the population was concentrated in the upper 100 m, but there was an increase in abundance below 300 m compared to summer. Age structure changed with depth with a younger surface population and an older one in deeper water layers. The seasonal change in number of M. pygmaeus is much smaller than that of C. citer; the summer:winter:autumn ratio of the former being about one, whereas the winter:summer/autumn of the latter was about nine. Early copepodite stages and adults of M. pygmaeus occurred throughout all investigation periods. The large proportion of early copepodite stages in April and in mid-October suggests autumn and early to midwinter breeding. Apparently, M. pygmaeus may reproduce and grow year-round or perhaps has a 2-yr life-cycle. In contrast, the dramatic increase in abundance of early copepodite stages of C. citer in summer suggests springtime reproduction.     

Siliceous nanoplankton. II. Newly discovered cysts and abundant choanoflagellates from the Weddell Sea,Antarctica

Siliceous choanoflagellates and previously undescribed, siliceous cyst-like spheroids (in the size range 2.5 to 15 m) were found below sea-ice in the Weddell Sea and studied with scanning electron microscopy. Cell counts from water samples obtained over a 10,000 km area indicated that both spheroids and choanoflagellates were abundant in the upper 100 m of the water column, averaging about 10⁵ cells l^-1. The large numbers of phagotrophic choanoflagellates suggest that bacterioplankton-picoplankton food chains are important and may indicate the presence of longer, more complex trophic networks than previously considered in Antarctic waters. Circumstantial evidence suggests that the spheroids may be the cysts of choanoflagellates; if so, the production of siliceous cysts by members of this enigmatic taxon may be of considerable interest in discussions of the phylogenetic position of the group in relation to the algae and the metazoans. Similar cysts were found simultaneously in the North Pacific Ocean (see preceding paper: Booth et al. 1980), and recently we have discovered the cysts in the upwelling area of the eastern tropical Pacific Ocean. The occurrence of the cyst in such geographically distant localities suggests that they have a worldwide distribution. Since the minute cysts are easily overlooked on the light microscope, their recognition and further study undoubtedly require the electron microscopic examination of plankton samples.     

Occurrence of large numbers of carcasses of the large,grazing copepod Calanus cristatus from the Japan Sea

Carcasses of Calanus cristatus were discovered in plankton samples collected from the Japan Sea throughout the year from 1970 to 1985. Many carcasses of copepodite Stages IV and V occurred in the layer between 15 and 300 m below a distinct thermocline. The number of copepodite Stage V carcasses also peaked in the layer between 1 500 and 2 000 m. The highest density of copepodite Stages IV and V carcasses was 169 individuals per 1 000 m³ and 1 573 individuals per 1 000 m³, respectively. Carcasses of adults occurred at depths below 500 m and numbers of males and females per 1 000 m³ were 1 to 16 and 1 to 42, respectively. Living males were larger in catch number than living females, but the relationship for carcasses was the opposite. Weight of carcasses was 15 to 25% of living C. cristatus. Carcasses contained about 51% carbon and 8% nitrogen by weight. Carcasses may have been drifting for more than one year in the epipelagic layer under the thermocline because of their slow decomposition rate.     

Deposit-feeding amphipods (Monoporeia affinis) reduce the recruitment of copepod nauplii from benthic resting eggs in the northern Baltic Sea

We experimentally investigated the effect of different densities of the burrowing, deposit-feeding amphipod Monoporeia affinis on the recruitment of zooplankton from benthic resting eggs. Intact sediment cores with in situ density and species composition of zooplankton resting eggs and benthic fauna were collected in the northern Bothnian Sea, part of the Baltic Sea. We removed as many M. affinis as possible from the cores and then added different numbers of M. affinis to the cores to generate a range of densities. The cores were exposed to different densities of M. affinis for either 3 or 40 days, after which the hatched zooplankton was registered. One subset of the cores were initially incubated under low temperature (2–3 °C, to prevent hatching) for 37 days (the resting phase), to allow for effects of M. affinis on unhatched resting eggs. These cores were then incubated under higher temperature (13 °C) for 3 days (the hatching phase), to induce hatching and allow for effects on hatching or hatched specimens. In a second subset of cores with the same time and temperature schedule, the M. affinis density was experimentally reduced at the start of the hatching phase, to evaluate the effect of M. affinis during the hatching phase. To a third subset of cores, we immediately initiated the hatching phase, without an experimental resting phase, to evaluate the effects induced during the resting phase. The most common zooplankton species that hatched was Eurytemora affinis (Copepoda), followed by Bosmina longispina maritima (Cladocera). In all cores that were subjected to a resting phase, the numbers of hatched E. affinis were log-linearly negatively related to density of M. affinis. An increase of M. affinis density from 1,000 to 5,000 individuals m⁻², normal field densities, reduced the hatching by 60–70%. The negative impact was mainly exerted during the hatching phase, suggesting predation on, burial of or physical injury of hatching nauplii or eggs in a late development stage as likely mechanisms. Also, the number of B. longispina maritima that hatched was reduced by M. affinis during the hatching phase, but no clear relation to density of M. affinis could be identified. The results show that M. affinis can reduce recruitment to zooplankton from benthic resting eggs. Such impact by the benthos on resting stages of zooplankton is therefore a potentially significant link between the benthic and pelagic systems. Received: 10 August 2000 / Accepted: 13 November 2000     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Effects of seasonal pack ice on the distribution of macrozooplankton and micronekton in the northwestern Weddell Sea

The presence of mesopelagic organisms in the guts of surface-foraging seabirds feeding in open areas within seasonal pack ice in the Antarctic has given rise to questions regarding the effects of pack ice on the underlying mesopelagic community. Bottom-moored free-vehicle acoustic instruments were used in concert with midwater trawls and baited traps to examine the abundance, size distribution and vertical distribution of pelagic organisms in the uppermost 100 m of the water column during the austral spring of 1992 in two areas of the northwestern Weddell Sea, one covered by seasonal pack icc and the other free of ice cover. Acoustic largets were more abundant and significantly larger at the open-water station than beneath pack ice. However, targets at the ice-covered site exhibited a pronounced diel pattern, with the largest targets detected only at night. Samples from night trawls at the icecovered site contained several species of large, vertically-migrating mesopelagic fishes, whereas these species were absent from trawls taken during the day. In addition, baited traps deployed in pack ice just beneath the ice-water interface collected large numbers of scavenging lysianassoid amphipods, while deeper traps beneath the ice and traps at the open-water station were empty, indicating the presence of a scavenging community associated with the undersurface of the ice. These results sapport the idea that mesopelagic organisms migrate closer to the surface beneath pack ice than in open water, exposing them to possible predation by surface-foraging seabirds.     

Food ingestion by the marine planktonic copepod Paracalanus in relation to abundance and size distribution of food

Juveniles and adult females were presented a food spectrum of three algae of different sizes (4.5, 12 and 20 m cell width). The increase in rate of ingestion of the medium-sized alga with an increase in copepod size was significantly greater than the increase in rate of ingestion of the small alga. It is hypothesized that the perception of chemical signals from the small alga by a copepod decreases as the copepod moults from stage to stage. The rate of ingestion of the large alga by copepod stage V (CV) and adult females was lower than the rate of ingestion of the medium-sized alga at mid- and high phytoplankton concentrations. The amount of nitrogen ingested when the medium-sized alga alone was offered was either higher than (stage C II) or not significantly different from that when the three algae were offered together (stage C IV). Ingestion rates are reduced when there is a multialgal food source. This implies that there is increased stability in the ocean because multiparticle food sources are more slowly depleted than unialgal foods. Weight-specific ingestion rates of copepods fed the three algae simultaneously increased from nauplius to stage C III and then decreased as adulthood approached. The contribution of the small alga to the total amount of nitrogen ingesied was greatest for naupliar stages while the contribution of the medium-sized cells was greater for later stages. The largest alga was readily ingested by stage C V and adult females but never contributed more than 25% of the nitrogen ingested. Eight to 12% of the nitrogen ingested by adult females was from the small alga. It is hypothesized that the algal cell size for maximum nitrogen ingestion in upwelled waters is relatively small, round or square and close to the size threshold below which adult females do not sense individual cells.     

Observations of copepod feeding and vertical distribution under natural turbulent conditions in the North Sea

We present results of simultaneous measurements of turbulent-dissipation rate, zooplankton vertical distribution and copepod gut pigments in the northern North Sea. Analysis shows that some, but not all, copepods (by species, sex and stage) exhibit significant dependence on turbulence in respect to vertical distribution and feeding rate. Oithona similis (female and copepodite stages) exhibits an avoidance of the surface layer when turbulence is strong there. For the range of turbulence (10⁻⁷ to 10⁻³m² s⁻³) and ambient chlorophyll concentration (0.5–0.8 μg l⁻¹) encountered, Calanus spp. and Metridia lucens exhibited a significant negative response in feeding-rate index with increasing turbulence. Centropages typicus and Pseudocalanus spp. also exhibited a negative response but of less significance. Received: 12 October 2000 / Accepted: 11 December 2000     

Variations in the content of trypsin and trysinogen in larval herring (Clupea harengus) digesting copepod nauplii

Trypsin and its proform trypsinogen were quantified by radioimmunoassay in herring (Clupea harengus L.) larvae subjected to different prey densities. During the first weeks of larval life, the enzyme content fluctuated in a threephased pattern. Yolk resorption (Phase 1) was characterized by an increase in enzyme. During the first few days after yolk resorption (Phase 2), there was a sharp decline in enzyme. Older larvae (Phase 3) exhibited a second period of intensive enzyme synthesis. Amounts of trypsin in intestines of feeding larvae were analysed. At first feeding, a basal level of gut enzyme of approximately 30ng was recorded, and the amount of additional enzyme secreted from the pancreatic tissue into the intestine appeared to be dependent upon the numbers of prey items ingested. The enzyme-substrate ratio in the intestine was approximately 1 to 4. Prey availability affected amount of trypsinogen. Larvae experiencing a high prey density had an approximately two-fold higher specific enzyme content in Phase 2 compared to larvae exposed to a low prey density. A proposed nutritional strategy for first feeding herring larvae is discussed.     

Physiochemical properties influencing biomass abundance and primary production in Lake Hoare, Antarctica

The perennially ice-covered, closed basin lakes in the McMurdo Dry Valleys respond rapidly to environmental changes, especially climate. For the past 15 years, the McMurdo Dry Valleys Long-Term Ecological Research (MCM-LTER) program has monitored the physical, chemical and biological properties of the lakes in Taylor Valley. In order to better assess the physiochemical controls on the biological process within one of these lakes (Lake Hoare), we have used vertical profile data to estimate depth-dependent correlations between various lake properties. Our analyses reveal the following results. Primary production rates (PPR) are strongly correlated to light (PAR) at 12-15 m and to soluble reactive phosphorus (SRP) at 8-22 m. Chlorophyll-a (CHL) is also positively correlated to PAR at 14 m and greater depths, and SRP from 15 m and greater. This preliminary statistical analysis supports previous observations that both PAR and SRP play significant roles in driving plant growth in Lake Hoare. The lack of a strong relationship between bacterial production (BP) and dissolved organic carbon (DOC) is an intriguing result of the analysis.     

Life cycle of the copepod Calanus hyperboreus in the Greenland Sea

The seasonal ontogenetic migration of the Arctic copepod Calanus hyperboreus was described from surface-to-bottom hauls in the central Greenland Sea Gyre (GSG) and in the Westspitsbergen Current (WSC). All stages except females spent the winter below 500 m in the GSG and below 1000 m in the WSC. Seasonal ascent begins in April, and descent in July. For the C.␣hyperboreus population an active downward transport of 8.1 g m⁻² dry weight during 8 months of overwintering was estimated, similar to flux rates of particulate matter in sediment traps. Seasonal distribution of biomass was determined from weight measurements of single stages. Annual means varied from 4.0 to 9.2 g m⁻² in two different years in the GSG and were 1.1 in 1 year in the WSC. The life cycle in the Greenland Sea was reconstructed from field data on stage composition, vertical distribution, reproduction, and moult cycle phase from tooth development of CV. Laboratory experiments were conducted on the moulting of CIV and CV in fall. A 3-year (males) and 3- to 4-year (females) life cycle is proposed for the GSG and 2 to 3 years for the WSC. However, the small number of young larvae and the incomplete spring ascent by older copepodites observed in the WSC cast doubt on the reproductive success in the WSC. A suite of physiological strategies and adaptations performed by the developmental stages support survival of this species in harsh environments. Received: 25 January 1997 / Accepted: 11 February 1997     

Reproduction patterns of four Antarctic octocorals in the Weddell Sea: an inter-specific,shape, and latitudinal comparison

The reproductive patterns of four Antarctic gorgonian species have been investigated. Two of them, Dasystenella acanthina and Thouarella sp., present the bottle-brush-shape type; the other two, Fannyella rossii and Fannyella spinosa, are fan-shaped. Two different reproductive patterns have been observed in D. acanthina and Thouarella sp., which point to two size classes in the frequency distribution of oocytes. This feature indicates a reproduction cycle with overlapping generations, being each of them of more than one year, probably with seasonal spawning. F. rossii and F. spinosa show only one size class of oocytes, which could point towards an annual reproductive cycle. The presence of larvae in the gastrovascular cavities in both Fanyella species and Thoaurella sp. is a sign of a possible larvae release during austral summer. The number of oocytes per polyp ranges from 1.1 ± 0.10 SE to 1.5 ± 0.06 SE, and the size ranges from 50 to 1200 μm. The number of spermatic cysts ranges from 2.6 ± 0.19 SE to 5.0 ± 0.21SE, and their sizes range from 50 to 800 μm. These values are comparable to the reproductive trends and features found in gorgonians from other latitudes, which correspond in some cases with similar morphotypes. These coincident strategies could be related to morphological similarities rather than with latitude or water temperature. The results of this study indicate that octocoral morphology may play a crucial role in determining the reproductive output of these organisms.     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks     

Vertical distributions of eggs,nauplii and copepodites of Calanus helgolandicus (Copepoda: Crustacea) in the Celtic Sea

The vertical distributions of eggs, nauplii, copepodites and adults of Calanus helgolandicus (Claus) from five oblique plankton-net hauls taken in May (1980), March and September (1981) and January (1982) at a site in the shelf sea to the south-west of the United Kingdom are described. The water depth is approximately 95 m and becomes thermally stratified during the summer months when a thermocline of ∼6 C° develops. In early spring when the water column was isothermal (∼8 °C), the development of the eggs and nauplii took place below 60 m and a single ontogenetic migration was observed between Nauplius VI (NVI) and Copepodite I (CI). As the temperature of the water increased, this migration occurred in progressively earlier naupliar stages. The eggs were distributed throughout the water column in the profile taken in early May when a 1 C° thermocline occurred between 30 to 40 m. The majority of the NI to NIV stages occurred below 40 m, with the ontogenetic migration taking place in the NIV stage; the NV and NVI stages were found above the thermocline. In September, the eggs were again distributed throughout the water column (101 490 m^-2), with a maximum number of >4 500 m^-3 occurring in the surface to 5 m depth interval. Nauplius I and II were found at all depths, demonstrating that hatching occurred throughout the water column. The ontogenetic migration in these late-summer profiles took place between the NII and NIII stages, the remainder of the nauplii being found above the thermocline in the top 20 m. This is the first time that an ontogenetic migration, similar to the developmental ascent observed in the naupliar stages of the euphausiid Euphausia superba in the deep ocean, has been shown for a copepod nauplius.     

Competition among penguins and cetaceans reveals trophic cascades in the western Ross Sea, Antarctica

An apparent trophic cascade that appears during summer in the western Ross Sea, Antarctica, explains why the Antarctic silverfish (Pleuragramma antarcticum) there becomes cannibalistic; its principal prey, crystal krill (Euphausia crystallorophias) becomes scarce; and the diatom community is minimally grazed compared to adjacent areas. The krill is the major grazer of diatoms. On the basis of fieldwork at Ross Island, we suggest that the cascade results from foraging by unusually numerous Adélie Penguins (Pygoscelis adeliae), minke whales (Balaenoptera bonaerensis), and fish-eating killer whales (Orcinus orca). These species and other top predators apparently deplete the krill and silverfish. In drawing our conclusions, we were aided by two "natural experiments." In one "experiment," large, grounded icebergs altered the seasonal pattern of change in regional sea-ice cover, but not the seasonal change in penguin diet and foraging behavior that was also detected during the pre-iceberg era. In the other "experiment," a short-term polynya (opening in the ice) brought penguins and whales together in a confined area, this time altering both penguin diet and foraging behavior. We conclude that the foraging of penguins and whales, and not a formerly hypothesized seasonal decrease in sea-ice cover, explains (1) the annual switch in the penguins' prey from krill to silverfish, (2) the subsequent lengthening of penguin foraging trips, and (3) a marked decline of cetaceans in the area later in the season. Reduction in the middle-trophic-level prey is expressed in the relaxed grazing pressure on phytoplankton.     

Reproduction in the externally brooding sea anemone Epiactis georgiana in the Antarctic Peninsula and the Weddell Sea

External parental care is uncommon among actiniarians but common in Epiactis species. Here, several aspects of reproduction are analyzed for of one of them, Epiactis georgiana. Samples were collected in December, January, February, March, and April in the Antarctic Peninsula and the eastern Weddell Sea, during 1998, 2000, 2002, and 2003. Most sexually mature individuals of E. georgiana are male or female, but some are hermaphrodites. This is the first report of hermaphroditism in E. georgiana, which is the third species of the genus with this sexual pattern. The results suggest that oogenesis starts in December and that at least two generations of oocytes overlap; a third generation is often brooded externally. Putative fertilization is likely internal, and larvae and/or embryos are externally brooded on the distal part of the adult column until an advanced developmental stage. Apparently E. georgiana reproduces seasonally, probably releasing the embryos/larvae in the last months of the austral spring (December). Inter-individual variability was observed in gametogenesis. In addition, specimens from the Antarctic Peninsula were larger than those from the Weddell Sea. This study represents the first step in understanding the reproductive mode of E. georgiana.     

Calanoid copepod eggs in sea-bottom muds. II. Seasonal cycles of abundance in the populations of several species of copepods and their eggs in the Inland Sea of Japan

The seasonal cycles of abundance of populations of dominant calanoid copepods in the water column and of their eggs recovered from the bottom sediment in the central part of the Inland Sea of Japan are described. The numbers of both copepods and eggs fluctuated markedly with season in an essentially similar pattern among the 6 species studied (Tortanus forcipatus Giesbrecht, Calanopia thompsoni A. Scott, Acartia erythraea Giesbrecht, A. clausi Giesbrecht, Centropages abdominalis Sato, C. yamadai Mori). The density of eggs in the sea bottom was highest shortly before the population of adults and late copepodids disappeared from the plankton; the numbers of eggs then gradually decreased until the appearance of the next planktonic population.