Small Euphausiids Research Articles

Geographic variation in feeding of Pacific saury Cololabis saira in June and July in the North Pacific Ocean

AbstractGut contents of Pacific saury Cololabis saira during their feeding migration through a transition zone between subtropical and subarctic waters in the North Pacific Ocean are reported. Geographic changes in feeding habits and preferences are described for the months of June and July, for the years 2012, 2016, and 2017. Pacific saury prey comprised 27 taxa. Cluster analysis based on numerical abundance of these prey taxa in guts indicated six Pacific saury feeding types (A to F) existed. Of these, types A–E occurred in low sea surface temperature waters (<ca 14°C). Gut content weight and gut fullness index were greater in types A, B, and E, with copepods Neocalanus plumchrus/flemingeri and/or N. cristatus contributing highly to carbon‐base weight. Although small euphausiids occurred in guts, guts were also often empty, suggesting feeding was less active at higher sea surface temperatures (>ca 14°C). Feeding patterns reveal N. plumchrus/flemingeri tended to be important prey species in areas west of ca 175°E, whereas Pacific saury feed mainly and selectively on N. cristatus in eastern areas, indicating a longitudinal difference in the trophic pathway from zooplankton prey to Pacific saury. These spatial differences in trophic pathways improve our understanding of nutritional intake in the commercially important Pacific saury during its feeding migration, and the effects this might have on fish size and weight, and associated commercial value.

Myctophid Fish (Family Myctophidae) Are Central Consumers in the Food Web of the Scotia Sea (Southern Ocean)

Myctophid fish are the most abundant and diverse mesopelagic fishes in the Southern Ocean. They are a conduit of energy between primary consumers and higher marine predators, and between the upper surface layers and the mesopelagic depths. However, there remain major uncertainties about their ecology, particularly regarding their role in Southern Ocean food webs, which are often regarded as dominated by Antarctic krill in waters south of the Antarctic Polar Front. Limited data on the feeding ecology of myctophids has made it difficult to assess the importance of myctophids as consumers of krill and how they fit in the traditional view of a krill-dominated system (diatom-krill-higher predator). We provide a new assessment of the role of myctophids in Southern Ocean food webs using information from recent trophodynamic studies of myctophids conducted in the Scotia Sea, one of the most productive regions of the Southern Ocean and a region that sustains both major populations of higher predators (sea birds, seals, whales) and important commercial fisheries (krill, toothfish and mackerel icefish). Collectively, these data show that myctophids have a central role in Southern Ocean food webs as both predators and prey. Large myctophid species are prevalent consumers of krill throughout their distributional range and in different seasons in the Scotia Sea. Moreover, best estimates of both myctophid and higher predator consumption of krill to date indicate that large myctophids are the greatest predators of krill in this region, consuming almost as much krill as all other vertebrate predators of krill. Nevertheless, there are several smaller myctophid species that do not eat krill, instead consuming copepods and other small euphausiids. Myctophids therefore link primary producers to higher predators through both krill-dependent and krill-independent trophic pathways, emphasizing their importance in regional food webs. Consequently, myctophid-based trophic pathways are unlikely to be exempt from the direct consequences of a redistribution and/or reduction in krill population biomass. The extent to which myctophids can maintain food web stability and sustain higher predator populations during periods of prolonged reductions in krill abundance is considered further.

Seasonal variation in the predatory impact of myctophids on zooplankton in the Scotia Sea (Southern Ocean)

Myctophids are the biomass-dominant mesopelagic fishes in the Southern Ocean, but their trophic role within the pelagic food web south of the Antarctic Polar Front is poorly resolved from a seasonal perspective at the ocean-basin scale. In this study, the predatory impact of the predominant Southern Ocean myctophid community (Electrona antarctica, Electrona carlsbergi, Gymnoscopelus braueri, Gymnoscopelus fraseri, Gymnoscopelus nicholsi, Protomyctophum bolini, Protomyctophum tenisoni, Protomyctophum choriodon, Krefftichthys anderssoni and Nannobrachium achirus) on their zooplankton prey was examined during austral spring, summer and autumn in the Scotia Sea, one of the most productive regions of the Southern Ocean. Seasonal variations in diet and predation rates were apparent for all species. Based on the percentage index of relative importance, myctophids had high overlap in their diets, with all species mostly consuming copepods, small euphausiids and amphipods. Myctophid size was a key determinant of diet in the region, with larger species and intra-specific size classes consuming larger prey. Cluster analyses revealed myctophid feeding guilds that appeared to change seasonally, although there was little evidence of dietary specialisation. Myctophid predation on the daily productivity of most copepod species was relatively low across seasons (<7%), except for Calanus simillimus that was predated upon highly in summer (∼26%). From the macrozooplankton component of the prey field, the myctophid community consumed substantial proportions of the euphausiid Thysanoessa spp. in each season (∼7 to 76% daily productivity), particularly in summer. Relatively high proportions of the daily Antarctic krill (Euphausia superba) productivity (∼8–58%) were also consumed by the larger myctophid species, particularly in summer by Electrona antarctica, suggesting increased competition for krill resources during the higher predator breeding season and possible reductions in food web stability during periods of reduced krill availability at this time. The amphipod Themisto gaudichaudii formed an important part of the larger myctophid species’ diet in all seasons, with between 10 and 38% of its daily productivity being consumed. Myctophid predation on the daily productivity of salps was up to 4%, whilst their impact on ostracods and pteropods was negligible (<0.1% of daily productivity) in all seasons. This study demonstrates that Southern Ocean myctophids link secondary productivity to higher predators through both krill-independent and krill-dependent trophic pathways across seasons, with myctophids comprising a more krill-dependent pathway during austral summer.

Trophodynamics of Protomyctophum (Myctophidae) in the Scotia Sea (Southern Ocean).

This study investigated spatial and temporal patterns in distribution, population structure and diet of Bolin's lanternfish Protomyctophum bolini, Tenison's lanternfish Protomyctophum tenisoni and gaptooth lanternfish Protomyctophum choriodon in the Scotia Sea using data collected by midwater trawl during spring, summer and autumn. Protomyctophum bolini was the most abundant species of the genus encountered throughout the Scotia Sea with the greatest concentrations occurring around the Antarctic Polar Front (APF). This species had a life cycle of 2+ years, but spatial differences in population structure were apparent as the I-group was absent from all regions south of the APF, suggesting that the species does not recruit in the Scotia Sea. Protomyctophum tenisoni occurred mostly in waters characteristic of the APF and was absent from the southern Scotia Sea. It had a limited size range, but there was clear size-related sexual dimorphism with males significantly larger than females. The species had a life cycle of c. 2 years, but the I-group (c. 1 year old, 1 November to 31 October the next year) occurred only in regions close to the APF suggesting that recruitment is restricted to these waters. A seasonal southward migration for P. choriodon is likely as the species occurred mostly to the south-west of South Georgia in summer, but extended to the sea-ice sectors in autumn. Protomyctophum choriodon had a life cycle of 4+ years in the Scotia Sea and the population was dominated by age classes >3 years old. Larval stages were absent during the surveys for all species. Diurnal variations in vertical distribution were apparent for all three species. Interspecific variations in diet were evident, but all species were primarily copepod feeders, with Metridia spp., Rhincalanus gigas and Calanus simillimus generally dominating their diet. Small euphausiids, principally Thysanoessa spp., were also an important component of their diets, particularly for P. choriodon which had the largest body size. The spatial and temporal variations in diet for both P. bolini and P. tenisoni were broadly consistent with underlying abundance patterns within the mesozooplankton community.

Composition and structure of macrozooplankton and micronekton communities in the vicinity of free-drifting Antarctic icebergs

Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10 m 2 MOCNESS to collect macrozooplankton and micronekton in the upper 300 m of the water column near free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer). Communities were dominated in all three seasons by Antarctic krill ( Euphausia superba) and salps ( Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37 km from large icebergs vs. 9.26 km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85 km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×10 3 m 3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85 km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean.

Diets and distributions of Leach’s storm-petrel (Oceanodroma leucorhoa) before and after an ecosystem shift in the Northwest Atlantic

The Grand Bank ecosystem has undergone significant shifts during the past two decades owing to oceanographic and fishing effects. Effects on upper trophic level seabirds (dietary shifts, reduced reproductive performance) have been mediated through changes in the biology and behaviour of capelin ( Mallotus villosus (Müller, 1776)), the focal forage species. To explore for effects at lower trophic levels, we combine dietary (1987–1988, 2003–2006) and distributional (1966–1990, 1998–1999) data for Leach’s storm-petrel ( Oceanodroma leucorhoa (Vieillot, 1818)), a small, abundant, and highly pelagic seabird. Fish and crustaceans formed the bulk of nestling diet at two colonies, with fish dominating in all sampling periods and years (occurrence &gt;70%, reconstructed mass &gt;75%). Five families were represented, but mature myctophids (glacier lanternfish ( Benthosema glaciale (Reinhardt, 1837)), horned lanternfish ( Ceratoscopelus maderensis (Lowe, 1839)), Protomyctophum arcticum (Lütken, 1892)) and sandlance (genus Ammodytes L., 1758) dominated. Crustaceans occurred frequently but typically comprised ≤10% by mass; Hyperia galba (Montagu, 1813) dominated this prey class. General diet composition was similar through time with birds relying heavily on myctophid fishes in 1987–1988 and 2003–2006. Crustacean diversity, however, declined with fewer species of hyperiid amphipods and no small euphausiids (genus Thysanoessa Brandt, 1851) consumed in 2003–2006. The latter parallels changes in spring diets of capelin and winter diets of murres (genus Uria Brisson, 1760) in the region. Associations of storm-petrels with deep water are consistent with the predominance of mesopelagic prey in their diets.

Identifying patterns in the diet of mackerel icefish (Champsocephalus gunnari) at South Georgia using bootstrapped confidence intervals of a dietary index

Ontogenetic, inter-annual and regional variations in diet were investigated for mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomach contents from 2239 C. gunnari (130–560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance of prey categories (% IRIDC). Diet varied significantly between years and age classes but there was little regional difference in diet. In general, diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausiids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003–2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length–weight relationships) were significantly lower than in the other summer seasons. A large reduction (>80%) in the estimated annual (2005) biomass of the C. gunnari stock directly followed the season of poor krill availability. This decline was largely because of mortality of 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.

Tidal front affects the size of prey used by a top marine predator, the short‐tailed shearwater (Puffinus tenuirostris)

AbstractOceanographic features are known to influence the distribution of marine predators by affecting the abundance and distribution of their prey. We tested the hypothesis that oceanographic features also affect predator distribution by enhancing the profitability of small‐sized prey. During July and August 1999, short‐tailed shearwaters feeding in Akutan Pass, Alaska (Aleutian Islands) fed upon small (11.6 ± 0.2 mm) euphausiids present in high density near the sea surface. Conductivity–temperature–depth (CTD) casts, hydroacoustic surveys, and net tows revealed that high densities of small euphausiids were associated with a tidal front on the north side of Akutan Pass. At most sites elsewhere in the Bering Sea, away from tidal fronts, shearwaters selected larger (14.2–20.1 mm) euphausiids, even when small euphausiids were present. This study provides evidence that, by promoting high densities of easily accessible prey, oceanographic features can broaden the range of prey sizes taken by marine predators.

DISTRIBUTION OF THE PYGMY RIGHT WHALE, CAPEREA MARGINATA, IN THE AUSTRALASIAN REGION

AbstractThe latitudinal range of all records of sightings (n= 9), and live strandings and carcasses (n= 180) of Caperea marginata in the Australasian region was 32°00′–47°00′S. Records were concentrated on the edge of the South Australian gulfs, around Tasmania, at Stewart Island, in Cook Strait, and in the Auckland area. Limited data suggest that these may be related to planktonrich waters nearby. Body lengths ranged from 198 to 650 cm. Overall sex ratio was 0.70 (males: females). In regions north of about 41°S, juveniles (preweaning) made up 41% of the records and subadults (postweaning) were rare (13%). In regions south of about 41°S subadults (36%) were proportionately more abundant than juveniles (17%). Neonates were recorded from 35°37′ to 47°00′S. Adults were found throughout the latitudinal range of the species in Australasia. The diet of the pygmy right whale included copepods and small euphausiids. Behavior consistent with feeding has been observed in coastal waters of Australia.

Ammonium regeneration by Antarctic mesozooplankton: an allometric approach

Given the importance of copepods in the Southern Ocean food web, there are few assessments of their N budgets or their role in regenerating N. In this study we measured elemental composition and ammonium-excretion rates of copepods and small euphausiids, and estimated the role of metazoans in recycling ammonium in the South Georgia region. Measurements were made during summer on animals ranging over about two orders of magnitude in body mass. A phytoplankton bloom extended throughout the study area, and high C and dry masses of late-stage copepodites suggested good recent feeding conditions. Excretion rates declined roughly exponentially during the ~1 day incubations in filtered sea water. The patterns observed suggested that the onset of starvation rather than the stress of capture caused this. Allometric relationships between body mass m and excretion rate R were derived using the equation R=amb. Large compilations of literature data produce a value of b (the body-mass scaling coefficient) of 0.7–0.8. However, in this study, b ranged from 0.57 (for C as the unit of body mass) to 0.71 for N as the unit of body mass. Such low values are also common to previous studies of feeding and excretion among Antarctic copepods. We suggest that this reflects peculiarities of polar environments; namely, lipid storage and diapause in the largest copepods. Previous studies have suggested that ammonium is a preferred N source for algae at South Georgia. Based on the monitoring of a region to the north-east of South Georgia and on zooplankton abundance and excretion rates from this study, we estimate that within the upper mixed layer the copepods and small euphausiids excrete at least one third of the ammonium potentially required by phytoplankton. Krill excretion in this area was measured in a previous study, and it appears that mesozooplankton and krill are together significant regenerators of N in parts of the South Georgia pelagic system.

Production, metabolism and production/biomass (P/B) ratio of Themisto japonica (Crustacea: Amphipoda) in Toyama Bay, southern Japan Sea

The production and metabolism of the amphipod <it>Themisto japonica</it> in Toyama Bay, southern Japan Sea, were estimated based on their biomass and population structure data collected from every 2-week samplings from 1 February 1990 through 30 January 1991 (363 days). Over the sampling period, the mean biomass (<it>B</it>) was 370 mg C m^-2. Production (<it>P</it>) was calculated as the sum of somatic (<it>P</it>g) and molt (<it>P</it>e) production (<it>P = P</it>g +<it>P</it>c), and metabolism (<it>M</it>) as the sum of routine metabolism (<it>M</it>rtn) and diel vertical migration (<it>M</it>dvm). Integrating over the entire sampling period, <it>P</it>g and <it>P</it>e were 1934 and 176 mg C m^-2, respectively, and<it>M</it>rtn and <it>M</it>dvm were 4100 and 1778 mg C m^-2, respectively. Mean daily<it>P/B</it> and <it>P</it>g/<it>B</it> ratios were 0.016 and 0.014, respectively, and mean<it>P</it>g/<it>M</it> and <it>P/M</it> ratios were 0.33 and 0.36 respectively. Assuming assimilation efficiency of 0.904, ingestion was computed as 8837 mg C m^-2 per 363 days. For the daily maintenance of growth and metabolism, the<it>T.japonica</it> population needs to ingest an amount of prey which equates to 6.6% of their biomass, or 30% of possible total production of their prey animals (copepods and small euphausiids) in Toyama Bay.

The summer zooplankton community at South Georgia: biomass, vertical migration and grazing

Zooplankton abundance and biomass were determined during January 1990 at two stations to the north-west of South Georgia using a Longhurst Hardy Plankton Recorder (LHPR). At both shelf and oceanic station sites, zooplankton biomass, (excluding Euphausia superba), was found to be ca. 13 g dry mass m−2. Copepods and small euphausiids dominated the catches. These estimates are over 4 times higher than values generally reported for the Southern Ocean and may reflect firstly, the high productivity of the study area, secondly, the time of year, summer, when biomass for many species is maximal, and thirdly, the high sampling efficiency of the LHPR. Principal components analysis disclosed similarities and differences between adjacent depth strata in terms of abundance, biomass and species composition. At both stations most variability occurred in the mixed layer (0–60 m) and thermocline (60–120 m) with depth horizons below this being more homogeneous. Diel migrations were observed for most taxa with abundance increasing in the mixed layer at night. At the oceanic station, species and higher taxa belonging to the mesopelagic community were generally well spread throughout this domain and, with the exception of Pleuromamma robusta and Metridia curticauda, showed little evidence of migration. The grazing impact of the epipelagic community (copepods and small euphausiids) was estimated to remove 3–4% of the microbial standing stock day−1 and a conservative 25% and 56% of daily primary production at the oceanic and shelf stations respectively.

Sources of acoustic backscattering at 87.5 kHz

Sources of acoustic backscattered energy at 87.5 kHz were identified using three independent approaches. The first approach combined the Johnson fluid sphere scattering model [R. K. Johnson, J. Acoust. Soc. Am. 61, 375–377 (1977)] with a hypothesized distribution of scatters. From these models the percentage of total scattering due to organisms of a particular size range was determined. The second approach inferred scatterer abundance from probability distributions of acoustic scattering strength for insonified volumes of different sizes. The third approach consisted of comparing levels and patterns of scattering strength observed with a sonar to biological samples taken in the same area at the same time. All three approaches indicated that the sources of backscattered acoustic energy came from large zooplankton and small nekton. However, the first approach underestimated the importance of large scatterers at night. Day scattering was dominated by large copepods and small euphausiids. When small fish were present, they dominated the scattering. Night samples were dominated by mesopelagic fish, squid, and euphausiids. The fish and squid usually dominated the scattering early in the evening while euphausiids tended to dominate the scattering later at night.

Further studies of plankton ecosystems in the eastern Indian Ocean. VI. Ecology of the Euphausiacea

Examination of 26 427 euphausiids from 175 samples taken by vertical hauls in the upper 200-m stratum along the 110°E. meridian during 1962-63 indicated that day samples contained mainly larvae and juveniles whereas night samples contained more adults. The following six species of small euphausiids were the most abundant and ubiquitous: Euphausia mutica, E. tenera, E. brevis, Stylocheiron carinatum, S. suhmii, and S. Affine. Their distribution overlapped according to the distribution, in the upper 150 m, of the tropical and subtropical water masses and their mixtures. These species were commonly observed in breeding condition. Their larval distribution was related to the distribution of the breeding adults. The rarer and less ubiquitous species, which were mainly younger stages and deeper living species, showed more pronounced relationships with seasonal water movements. Food resources appeared more important than temperature and salinity in influencing distribution, abundance, and association of species.

Radioactivity in Large Marine Plankton as a Function of Surface Area

Radioisotopes from Hanford reactors and from fallout are available to marine organisms in the north-east Pacific Ocean. The radioactivity in large marine plankton was measured as a function of surface area. Data are included from measurements of the content of K/sup 40/, Zn/sup 65/, Zr/sup 95/, Nb/sup 95 p 141/ in two species of pelagic crustaceans of different size taken from the same sample. Comparisons were also made between large adult and small adult or juvenile Euphausiids. The radioactivity of internal organs and external body parts was compared in the pelagic tunicate, Salpa. Results led to the conclusion that surface adsorption plays a relatively insignificant pant in the accumulation of these radioisotopes by macroplankton. (C.H.)

Bol. Inst. Oceanogr. Univ. Oriente, Cumana, : Zoppi E., 1961. Distribution vertical del zooplancton en el golfo y extremo este de la fosa de Cariaco., Venezuela, 1 (1): 219–247

Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10 m2 MOCNESS to collect macrozooplankton and micronekton in the upper 300 m of the water column near free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer). Communities were dominated in all three seasons by Antarctic krill (Euphausia superba) and salps (Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37 km from large icebergs vs. 9.26 km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85 km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×103 m3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85 km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean.

Mezhd. Geofiz. Komitet, Prezidiume Akad. Nauk, SSSR, Rezult. Issled. Programme Mezhd. Geofiz. Goda, Okeanol., Issled. : Zhivago A.V., 1962. Investigation methods of floor topography in the Antarctic. 5: 60–66

Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10 m2 MOCNESS to collect macrozooplankton and micronekton in the upper 300 m of the water column near free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer). Communities were dominated in all three seasons by Antarctic krill (Euphausia superba) and salps (Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37 km from large icebergs vs. 9.26 km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85 km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×103 m3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85 km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean.