E. A. Pakhomov

Long-term decline in krill stock and increase in salps within the Southern Ocean

Antarctic krill (Euphausia superba) and salps (mainly Salpa thompsoni) are major grazers in the Southern Ocean, and krill support commercial fisheries. Their density distributions have been described in the period 1926–51, while recent localized studies suggest short-term changes. To examine spatial and temporal changes over larger scales, we have combined all available scientific net sampling data from 1926 to 2003. This database shows that the productive southwest Atlantic sector contains >50% of Southern Ocean krill stocks, but here their density has declined since the 1970s. Spatially, within their habitat, summer krill density correlates positively with chlorophyll concentrations. Temporally, within the southwest Atlantic, summer krill densities correlate positively with sea-ice extent the previous winter. Summer food and the extent of winter sea ice are thus key factors in the high krill densities observed in the southwest Atlantic Ocean. Krill need the summer phytoplankton blooms of this sector, where winters of extensive sea ice mean plentiful winter food from ice algae, promoting larval recruitment and replenishing the stock. Salps, by contrast, occupy the extensive lower-productivity regions of the Southern Ocean and tolerate warmer water than krill. As krill densities decreased last century, salps appear to have increased in the southern part of their range. These changes have had profound effects within the Southern Ocean food web.

Increasing jellyfish populations: trends in Large Marine Ecosystems

Although there are various indications and claims that jellyfish (i.e., scyphozoans, cubozoans, most hydrozoans, ctenophores, and salps) have been increasing at a global scale in recent decades, a rigorous demonstration of this has never been presented. Because this is mainly due to scarcity of quantitative time series of jellyfish abundance from scientific surveys, we attempt to complement such data with non-conventional information from other sources. This was accomplished using the analytical framework of fuzzy logic, which allows the combination of information with variable degrees of cardinality, reliability, and temporal and spatial coverage. Data were aggregated and analyzed at the scale of Large Marine Ecosystem (LME). Of the 66 LMEs defined thus far that cover the world’s coastal waters and seas, trends of jellyfish abundance after 1950 (increasing, decreasing, or stable/variable) were identified for 45, with variable degrees of confidence. Of those 45 LMEs, the majority (28 or 62%) showed increasing trends. These changes are discussed in the context of possible sources of bias and uncertainty, along with previously proposed hypotheses to explain increases in jellyfish.

Distribution of surface zooplankton and seabirds across the Southern Ocean

Abstract Surface zooplankton and seabird densities and community composition in the Atlantic (between Cape Town and Sanae) and Pacific (between New Zealand and the Ross Sea) sectors of the Southern Ocean are described and related to oceanographic features. Samples were collected during two return voyages aboard the MV Benjamin Bowring as part of the Transglobe Expedition (1979–1981). High abundances of surface zooplankton and seabirds were consistently observed within the main frontal systems of the Southern Ocean. Generally, on a mesoscale significant correlations between surface temperature and the distribution of zooplankton or seabirds were observed. On a macroscale, the geographical positions of the zooplankton and seabird communities coincided with specific water masses. The results of this study suggest that appropriate food availability rather than water temperature is important for the determination of seabird distribution. The ecological importance of the recently described frontal zone associated with the northern boundary of the maximum winter expansion of sea ice is confirmed by biological data obtained in this study.

Salp/krill interactions in the Southern Ocean:spatial segregation and implications for the carbon flux

Available data on the spatial distribution and feeding ecophysiology of Antarctic krill, Euphausia superba, and the tunicate, Salpa thompsoni, in the Southern Ocean are summarized in this study. Antarctic krill and salps generally display pronounced spatial segregation at all spatial scales. This appears to be the result of a clear biotopical separation of these key species in the Antarctic pelagic food web. Krill and salps are found in different water masses or water mass modifications, which are separated by primary or secondary frontal features. On the small-scale (o100 km), Antarctic krill and salps are usually restricted to the specific water parcels, or are well segregated vertically. Krill and salp grazing rates estimated using the in situ gut fluorescence technique are among the highest recorded in the Antarctic pelagic food web. Although krill and salps at times may remove the entire daily primary production, generally their grazing impact is moderate (p50% of primary production). The regional ecological consequences of years of high salp densities may be dramatic. If the warming trend, which is observed around the Antarctic Peninsula and in the Southern Ocean, continues, salps may become a more prominent player in the trophic structure of the Antarctic marine ecosystem. This likely would be coupled with a dramatic decrease in krill productivity, because of a parallel decrease in the spatial extension of the krill biotope. The high Antarctic regions, particularly the Marginal Ice Zone, have, however, effective physiological mechanisms that may provide protection against the salp invasion. r 2002 Elsevier Science Ltd. All rights reserved.

Zooplankton structure and grazing in the Atlantic sector of the Southern Ocean in late austral summer 1993 Part 2. Biochemical zonation

The composition, biomass, distribution and grazing impact of dominant components of the meso- and macrozooplankton community were investigated along a transect occupied in the Atlantic sector of the Southern Ocean during the second cruise of the South African Antarctic Ecosystem Study conducted in late austral summer (Jan.–Feb.) 1993. Total zooplankton abundance along the transect ranged from 6 to 161 ind. m−3, and biomass levels ranged from 4. 2 to 80.5 mg DWt m−3. Elevated abundances and biomass were recorded at stations occupied in the vicinity of the Marginal Ice Zone (MIZ), the Antarctic Polar Front (APF) and Subantarctic Front (SAF). Throughout the investigation mesozooplankton, comprising mainly copepods, dominated numerically and by biomass accounting for >80% of the total. South of the APF, the copepods Calanus propinquus, Calanoides acutus and Metridia gerlachei numerically dominated zooplankton counts, while north of the front the zooplankton community was dominated by the copepods Calanus simillimus, Rhincalanus gigas and Lucicutia ovalis. Grazing impact of the six most abundant zooplankton taxa, accounting for up to 90% of all zooplankton counted at each station, demonstrated large spatial variability ranging from 0.01 to 18% of the chlorophyll (chl-a) standing stock or up to 89% of the daily phytoplankton production (DPP). The highest grazing impact along the transect was recorded within the Polar Front Zone where zooplankton removed between 4 and 18% of the chl-a standing stock or between 53 and 89% of DPP daily. Outside this region, zooplankton grazing generally corresponded to <5% of the integrated chl-a or <20% of the daily DPP. The implications of the spatial differences in grazing impact on the efficiency of the biological pump are discussed.

Gut evacuation rates and pigment destruction in the Antarctic krill Euphausia superba

Krill grazing data collected during cruises in the region of the Antarctic Polar Front (S.A. “Agulhas” Voyage 70) and the South Georgia shelf (R.V. “Africana” Voyage 119) during the austral summer of 1993 were analyzed to estimate the variability of crucial parameters of the gut fluorescence technique in relation to food availability and krill feeding history. Gut evacuation rates (k) and passage or throughput times (1/k) varied in the ranges of 0.101 to 0.424 h-1 and 2.3 to 9.9 h and were strongly correlated (p<0.001, r2=0.98) to krill feeding activity (estimated as initial gut pigment content, G0) but not to ambient chlorophyll a concentration. A significant differences was found when k values derived from incubations in filtered seawater and low charcoal particle concentrations (0.4 to 0.8 mgl-1) were compared with values derived from krill fed high concentrations of charcoal (6 mgl-1). The efficiency of gut pigment destruction was among the highest recorded for zooplankton organisms, 58.1 to 98.4%, and did not covary significantly (p>0.05) with ambient food concentration. However, the pigment lost per individual krill was strongly correlated with the total amount of pigment ingested (p<0.001, r2=0.99). We suggest that both gut evacuation rates and pigment destruction efficiency may be realistically estimated only when krill is allowed to continue ingesting particles uninterruptedly. Charcoal particle concentration should be equivalent to the in situ wet weight of total seston per unit volume. An objective criterion for the standardization of the measurement and calculation of k values is also proposed.

Structure and grazing impact of the mesozooplankton community during late summer 1994 near South Georgia, Antarctica

Abstract Mesozooplankton abundance, community structure and grazing impact were determined during late austral summer (February/March) 1994 at eight oceanic stations near South Georgia using samples collected with a Bongo and WP-2 nets in the upper 200-m and 100-m layer, respectively. The zooplankton abundance was generally dominated by copepodite stages C3–C5 of six copepod species: Rhincalanus gigas, Calanus simillimus, Calanoides acutus, Metridia spp., Clausocalanus laticeps and Ctenocalanus vanus. Most copepods had large lipid sacs. All copepods accounted for 41–98% of total zooplankton abundance. Juvenile euphausiids were the second most important component contributing between 1 and 20% of total abundance. Pteropods, mainly Limacina inflata, were important members of the pelagic community at two sites, accounting for 44 and 53% of total abundance. Average mesozooplankton biomass in the upper 200 m was 8.0 g dry weight m−2, ranging from 4.3 to 11.5 g dry weight m−2. With the exception of Calanussimillimus, gut pigment contents and feeding activity of copepod species were low, suggesting that some species, after having stored large lipid reserves, had probably started undergoing developmental arrest. Daily mesozooplankton grazing impact, measured using in situ gut fluorescence techniques and in vitro incubations, varied widely from <1 to 8% (mean 3.5%) of phytoplankton standing stock, and from 5 to 102% (mean 36%) of primary production. The highest grazing impact was found northeast of the island co-incident with the lowest phytoplankton biomass and primary production levels.

Antarctic neritic krill Euphausia crystallorophias : spatio-temporal distribution, growth and grazing rates

Abstract Dynamics of distribution, growth, life span and feeding were studied in the endemic Antarctic euphausiid Euphausia crystallorophias in the central part of the Indian sector of the Southern Ocean during the austral summer, from 1977 to 1990, and in the Lazarev Sea during the summer 1990/91. Both larvae and adult E. crystallorophias were found in abundance in shelf waters of 100–500 m depth. Maximum abundances of larvae (up to 23,969 ind. 1000 m −3 ) and adults (up to 12,670 ind. 1000 m −3 ) were found in the Prydz Bay region. Size frequency analysis indicated that the generation time was ≈ 3 years in the Indian sector of the Southern Ocean. Total life span of E. crystallorophias may reach 5 years in the Prydz Bay region and 4 years in the Cosmonaut Sea. Analysis of size distribution by sex suggests that male E. crystallorophias may have a shorter life span than females. Growth for only 180 days per year is assumed, mean growth rates range from 0.070–0.075 to 0.019–0.022 mm per day during the first and the fourth year, respectively. Von Bertalanffy growth curves calculated for different areas were similar to those obtained in the Antarctic Peninsula region. In the Prydz Bay and the Cosmonaut Sea, spawning of E. crystallorophias appears to peak from the end of November to early December and may extend to the beginning of January. Most larvae were in the stage of metanauplius/calyptopis I at the beginning of January and calyptopis III/furcilia I during February. The year-1 cohort was identified in January-February and exhibited a mean length in the range 10.8–15.8 mm. In situ feeding rates were estimated in the Lazarev Sea using the gut fluorescence method. Ingestion rates during austral summer 1990/91 ranged from 52 to 471 ng (pigm) ind −1 h −1 in adults and from 2.5 to 25.2 ng (pigm) ind −1 h −1 in calyptopis II larvae. Total population impact on the phytoplankton stock varied between 160–2860 and 215–652 μg (C) m −1 day −1 for adults and larvae, respectively. This is equivalent to 0.06–1.12% and 0.02–0.07% of total daily production. In areas with dense E. crystallorophias swarms, however, daily consumption rates may attain levels as high as 13.6–96.5% of daily primary production. The long-term monitoring of different populations of E. crystallorophias allowed the identification of similar patterns between its abundance and spawning success and the formation of coastal polynyas, especially in the Prydz Bay region.

Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

Abstract The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (April/May) 1989 are presented. During the investigation, the Subantarctic Front, upstream of the islands, was shown to lie initially south at 46°38′S, while downstream, the front remained in a northern position of approximately 46°S. Surface expressions of the front show that the Subantarctic Front forms a zonal band, while the subsurface expressions (200 m) show a distinct meander in both regions. In the upstream region of the islands, the northern branch of the Antarctic Circumpolar Current, the Subantarctic Front, influenced by the shallow bathymetry, was deflected around the northern edge of the islands. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (7°C, 33.75) to Antarctic Surface Water (5°C, 33.70) as the Polar Frontal Zone was crossed. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surface Water from north of the Subantarctic Front was advected southwards across the Polar Frontal Zone, while cooler waters, which had been modified in the transitional band of the Polar Frontal Zone, were advected northwards. In the downstream region a warm eddy consisting of Subantarctic Surface Water was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution patterns during the investigation were consistent with the prevailing oceanographic regime. Four distinct groupings of stations were identified by numerical analysis. These corresponded to stations found north of the Subantarctic Front, within the warm eddy, located in the Polar Frontal Zone, and those stations associated with the meander. The groupings were separated by the Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddies in the downstream region of the islands may represent an effective mechanism for transporting warm water species across the Subantarctic Front.

Poles Apart: The “Bipolar” Pteropod Species Limacina helicina Is Genetically Distinct Between the Arctic and Antarctic Oceans

The shelled pteropod (sea butterfly) Limacina helicina is currently recognised as a species complex comprising two sub-species and at least five “forma”. However, at the species level it is considered to be bipolar, occurring in both the Arctic and Antarctic oceans. Due to its aragonite shell and polar distribution L. helicina is particularly vulnerable to ocean acidification. As a key indicator of the acidification process, and a major component of polar ecosystems, L. helicina has become a focus for acidification research. New observations that taxonomic groups may respond quite differently to acidification prompted us to reassess the taxonomic status of this important species. We found a 33.56% (±0.09) difference in cytochrome c oxidase subunit I (COI) gene sequences between L. helicina collected from the Arctic and Antarctic oceans. This degree of separation is sufficient for ordinal level taxonomic separation in other organisms and provides strong evidence for the Arctic and Antarctic populations of L. helicina differing at least at the species level. Recent research has highlighted substantial physiological differences between the poles for another supposedly bipolar pteropod species, Clione limacina. Given the large genetic divergence between Arctic and Antarctic L. helicina populations shown here, similarly large physiological differences may exist between the poles for the L. helicina species group. Therefore, in addition to indicating that L. helicina is in fact not bipolar, our study demonstrates the need for acidification research to take into account the possibility that the L. helicina species group may not respond in the same way to ocean acidification in Arctic and Antarctic ecosystems.