Themisto Gaudichaudii Research Articles

Investigating the physiological ecology of mesopelagic zooplankton in the Scotia sea (Southern ocean) using lipid and stable isotope signatures

The mesopelagic zooplankton community plays an important role in the cycling and sequestration of carbon via the biological pump. However, little is known about the physiology and ecology of key taxa found within this region, hindering our understanding of their influence on the pathways of energy and organic matter cycling. We sampled the eight most abundant zooplankton (Calanoides acutus, Rhincalanus gigas, Paraeuchaeta spp., Chaetognatha, Euphausia triacantha, Thysanoessa spp., Themisto gaudichaudii and Salpa thompsoni) from within the mesopelagic zone in the Scotia Sea during a sinking diatom bloom and investigated their physiological ecology using lipid biomarkers and stable isotopic signatures of nitrogen. Data suggest that the large calanoid copepods, C. acutus and R. gigas, were in, or emerging from, a period of metabolic inactivity during the study period (November 15th – December 15th, 2017). Abundant, but decreasing lipid reserves in the predominantly herbivorous calanoid copepods, suggest these animals may have been metabolising previously stored lipids at the time of sampling, rather than deriving energy solely from the diatom bloom. This highlights the importance of understanding the timing of diapause of overwintering species as their feeding is likely to have an impact on the turnover of particulate organic matter (POM) in the upper mesopelagic. The δ15N signatures of POM became enriched with increasing depth, whereas all species of zooplankton except T. gaudichaudii did not. This suggests that animals were feeding on fresher, surface-derived POM, rather than reworked particles at depth, likely influencing the quantity and quality of organic matter leaving the upper mesopelagic. Our study highlights the complexity of mesopelagic food webs and suggests that the application of broad trophic functional types may lead to an incorrect understanding of ecosystem dynamics.

Race to the poles: the thermal response of the transcriptome of two range-expanding pelagic amphipod species

IntroductionHyperiid amphipods of the genus Themisto are a key polar zooplankton group in terms of biomass and play an important role as prey for higher trophic levels. They are prone to undergo changes in abundance and distribution in the course of the ongoing environmental changes. In the Southern Ocean, Themisto gaudichaudii is predicted to expand its distribution poleward. In the Arctic, the boreal-Atlantic T. abyssorum increases in abundance, resulting in an increased competition with a genuine polar congener. It is not known, however, whether T. gaudichaudii and T. abyssorum have the potential to efficiently adapt to changing water temperatures at their current distribution range or whether they will be shifting their ranges poleward.MethodsWe exposed the two Themisto species from different geographic populations to temperature-change experiments, a cold treatment and a heat-shock treatment. After that, we carried out transcriptome sequencing to compare gene expression patterns in the different treatments and species.ResultsWe show that under similar heat conditions, T. gaudichaudii differentially expressed more genes (26-fold change) than T. abyssorum. Furthermore, we observed qualitative differences between genetic clusters in T. gaudichaudii.DiscussionThe differences observed between genetic clusters in T. gaudichaudii suggest that evolutionary divergence can be linked to changes in the regulatory pathways involved in temperature stress. These could influence the capacity of each genetic cluster to cope differently with temperature changes. In contrast to its congeneric species, T. abyssorum showed a pronounced adaptive flexibility to thermal stress; it appears to have the ability to continue its poleward expansion but may also cope with increasing temperatures in its current environments. Our findings contribute to understand the response of two range-shifting Themisto species to thermal stress in view of the environmental gradients they encounter throughout their current or future distribution ranges.

Mercury concentrations in Antarctic zooplankton with a focus on the krill species, Euphausia superba

The Antarctic is the most isolated region in the world; nevertheless, it has not avoided the negative impact of human activity, including the inflow of toxic mercury (Hg). Hg deposited in the Antarctic marine environment can be bioavailable and accumulate in the food web, reaching elevated concentrations in high-trophic-level biota, especially if methylated. Zooplankton, together with phytoplankton, are critical for the transport of pollutants, including Hg to higher trophic levels. For the Southern Ocean ecosystem, one of the key zooplankton components is the Antarctic krill Euphausia superba, the smaller euphausiid Thysanoessa macrura, and the amphipod Themisto gaudichaudii – a crucial food source for most predatory fish, birds, and mammals. The main goal of this study was to determine the Hg burden, as well as the distribution of different Hg forms, in these dominant Antarctic planktonic crustaceans. The results showed that the highest concentrations of Hg were found in T. gaudichaudii, a typically predatory taxon. Most of the Hg in the tested crustaceans was labile and potentially bioavailable for planktivorous organisms, with the most dangerous methylmercury (MeHg) accounting for an average of 16 % of the total mercury. Elevated Hg concentrations were observed close to the land, which is influenced by the proximity to penguin and pinniped colonies. In areas near the shore, volcanic activity might be a possible cause of the increase in mercury sulfide (HgS) content. The total Hg concentration increased with the trophic position and ontogenetic stage of predation, specific to adult organisms. In contrast, the proportion of MeHg decreased with age, indicating more efficient demethylation or elimination. The Hg magnification kinetics in the study area were relatively high, which may be related to climate-change induced alterations of the Antarctic ecosystem: additional food sources and reshaped trophic structure.

Spatial variations in age-0+ Merluccius hubbsi (Patagonian stock; 45º-47ºS) feeding conditions through fatty acids composition

Fatty acids (FA) play critical roles in trophic interactions in aquatic food webs, transferring from phytoplankton to fish, and being modulated by oceanographic conditions and productivity. FA are a well-established approach for studying fish feeding preferences, and can indicate habitat quality and ecosystem condition. Thus, they provide valuable information in the context of fisheries management. The aim of this research was to determine the FA profiles of age-0 + hake Merluccius hubbsi and their zooplankton prey (Euphausia spp., Munida gregaria, Themisto gaudichaudii), identify FA trophic markers, infer their diet in comparison with stomachs information, and evaluate spatial changes in FA indicators with respect to the austral spring environmental variability in the Patagonian nursery ground (San Jorge Gulf, SJG; 45º-47ºS, 65º30’−67º30’W). We hypothesize that age-0 + hake and zooplankton FA composition relate to frontal structures and chlorophyll-a concentration, revealing diatoms predominance in the SJG spring food web. Major FA in hake and zooplankton were 16:0, 16:1n7, 18:1n9, 20:5n3 and 22:6n3. Age-0 + hake profiles mostly resembled euphausiids FA composition, in agreement with the stomachs analysis. Bacterial (15:0 + 17:0 + 17:1 + 18:1n7) and a mixed signal from both diatoms (16:1n7 + 20:5n3) and dinoflagellates (18:4n3 + 22:6n3) were recorded in the profiles; these suggest the existence of a classical food web in central coastal waters of the gulf and an omnivorous one towards the north and south extremes. Results are useful for a better understanding of the SJG food web and the key role age-0 + hake play in energy transfer, contributing to ongoing M. hubbsi recruitment studies in the region.

Diet and condition of juvenile deep-water hake Merluccius paradoxus on the west coast of southern Africa

The composition of the diet of juvenile deep-water hake Merluccius paradoxus is described and compared for three body-condition groups (below-average, average, and above-average). Diet was analysed using three metrics: occurrence, numeric and gravimetric. Juvenile hakes were caught in January and February of 2012 by means of 28 trawl deployments from the RV Dr Fridtjof Nansen between Orange Banks and Hondeklip Bay on the west coast of southern Africa at depths of 30-232 m. The 300 individuals analysed were a representative sample of all juveniles caught (3 114 fish), ranging in length from 52 to 205 mm (average length 107 mm). Their diet consisted of the euphausiid Euphausia lucens, the hyperiid amphipod Themisto gaudichaudii, the stomatopod Pterygosquilla armata capensis, and mesopelagic fishes Maurolicus muelleri and Lampanyctodes hectoris. The only significant difference in diet between the body-condition groups was for euphausiids and hyperiid amphipods in the numeric metric. A new method of assessing fish condition using otolith weight was tested. Because of the large variability in the data and small sample size, this method could not be applied effectively in this study. However, otolith weight has potential as a new fish condition metric when the variability can be lowered.

Plankton and nekton community structure in the vicinity of the South Sandwich Islands (Southern Ocean) and the influence of environmental factors

The South Sandwich Islands (SSI) are a biologically productive archipelago situated in the eastern Scotia Sea to the south of the eastward flowing Antarctic Circumpolar Current (ACC). The islands support important populations of higher predators, including several penguin species, seals and humpback whales. Despite this, the plankton ecology of the region has been little studied and information on mesoscale structure and environmental forcing of plankton ecology is particularly limited. We conducted a comprehensive oceanographic and net sampling campaign during the CCAMLR Area 48 Survey (January and February 2019), incorporating phytoplankton, mesozooplankton and macrozooplankton/nekton. Satellite chlorophyll-a (chl-a) data showed the development of a large bloom that was initiated two months prior to our study period at the south-eastern edge of the archipelago and propagated northwards along the eastern side, limited to the east by mesoscale features associated with the southern boundary of the ACC (SB). Multivariate cluster analysis revealed distinct mesoscale structure within the plankton community, with four spatially defined groups of phytoplankton and macrozooplankton/nekton, and three cluster groups of mesozooplankton. North of the SB, we found some spatial congruence between the three plankton assemblages, with a distinct, spatially coherent, cluster in each, corresponding to a warmer water community. Here, biomass was dominated by mesozooplankton, particularly calanoid copepods Rhincalanus gigas, Calanus propinquus, C. simillimus and Euchaetidae. The corresponding phytoplankton community was dominated by small diatoms, particularly Thalassionema spp., Pseudo-nitzschia spp., Fragilariopsis spp. and Chaetoceros spp., whilst Themisto gaudichaudii, Euphausia triacantha and myctophids were the major contributors to the macrozooplankton/nekton community. South of the SB, there was some spatial congruence between phytoplankton and macrozooplankton/nekton community structure on the western side of the archipelago, as well as on the eastern side that corresponded to the location of the bloom, but less association with mesozooplankton structure. Macrozooplankton/nekton structure was strongly driven by environmental conditions 1–2 months prior to the survey, including sea-ice distribution, surface phytoplankton concentration and productivity, whilst mesozooplankton was more tightly coupled to in-situ prevailing conditions such as surface temperature and integrated chl-a. Top-down pressure between trophic levels may have also had an influence on spatial patterns although direct evidence is lacking. Antarctic krill (Euphausia superba) was found with relatively low biomass at our net sampling sites (median biomass of 0.04 mg m−3 or <0.01 g m−2) while myctophids and the euphausiid Thysanoessa spp. predominated. We suggest that the highly productive and species rich pelagic community of the SSI supports multiple trophic pathways, and that off-shelf these may operate independently of Antarctic krill.

Swarms of the hyperiid amphipod Themisto gaudichaudii along the False Bay (Western Cape, South Africa) coastline

ABSTRACT Tornado-shaped swarms of the hyperiid amphipod Themisto gaudichaudii were observed along the coast of False Bay, South Africa. This is the first time such swarming behaviour has been recorded in the area. The swarms were confirmed to be monospecific, with the majority of the individuals being female. At least 26% of females were gravid and there was no statistical difference in size among males, gravid females, and non-gravid females.

Who let the young-of-the-year (YOY) up? The pelagic habitat as a nursery and feeding ground area for the YOY Argentine hake Merluccius hubbsi Marini, 1933.

The young-of-the-year (YOY) Argentina hake Merluccius hubbsi remained at particular distances off the seabed at pelagic habitat, in a nursery area located in the San Jorge Gulf off south-western Atlantic. Older specimens were daylight distributed near the bottom in the demersal habitat. In this paper the authors show that the pelagic habitat is favourable for YOY hake compared to the surrounding demersal habitat by decreasing the probability of encounter with larger hake protecting the YOY against conspecific predation, and providing a better food supply. From 303 stomachs of pelagic YOY Argentine hake analysed (60-250 mm), 274 (90.43%) contained prey. Pelagic Argentine YOY hake fed almost exclusively on zooplanktonic crustaceans (Euphausia spp. and Themisto gaudichaudii). From 980 demersal specimens (14-82 cm), 572 (58.36%) contained prey, mainly the lobster krill Munida gregaria, followed by other hakes by cannibalism. The intensity of feeding was higher on pelagic layers. The allometric weight-length relationship revealed that the YOY Argentine hake find sufficient food in the pelagic habitat to live and develop, by increasing their relative body thickness. The cannibalism increases from close to zero when the YOY hake are c. 20 m off the seabed, to between 15% (cold season) and 20% (warm season) when they are 10 m off the seabed. These results suggest that the pelagic habitat is a feeding ground for YOY hake, and it is a favourable one compared to the surrounding demersal habitat by protecting the YOY from cannibalism. Pelagic YOY hakes were less abundant and more distant from the bottom during the cold season (14.3 m) than during warm one (11.4 m), probably because of natural mortality and progressive recruitment to demersal habits.

Macrozooplankton and micronekton community structure and diel vertical migration in the Heard Island Region, Central Kerguelen Plateau

The Kerguelen Plateau is the largest topographic barrier to the eastward flowing Antarctic Circumpolar Current (ACC) in the Indian Sector of the Southern Ocean. The plateau is separated into a northern and southern plateau by the Fawn Trough. The northern plateau has a shallow bathymetry (<700 m) with two prominent island systems, the Kerguelen Islands on the northern end and Heard and McDonald Islands on the southern end. The interaction of the northern and southern branches of the Polar Front with the northern plateau results in a complex oceanographic environment, with potential for dynamic mixing of sub-Antarctic and Antarctic fauna. Furthermore, the High Nutrient Low Chlorophyll (HNLC) water that flows over the northern plateau is subject to natural iron fertilization from the shallow bathymetry (< 700 m), leading to extensive phytoplankton blooms each summer, particularly to the southeast of the Kerguelen Islands. The Heard Island region on the central plateau has been relatively underexplored. In January 2004, the Heard Island Predator-Prey Investigation and Ecosystem Study (HIPPIES) conducted macrozooplankton (RMT-8 trawl) and micronekton (IYGPT trawl) sampling in foraging areas used by top predators in the vicinity of the island. Macrozooplankton and micronekton were sampled day and night from three depth strata (0–100 m, 100–300 m and 300–600 m). In the area south and east of Heard Island, over bathymetry ≥1000 m depth, the water column had characteristics of the southern branch of the Polar Front (S-PF) and the macrozooplankton and micronekton were dominated by Antarctic Zone fauna (e.g., the euphausiids Euphausia triacantha and Euphausia frigida, and the myctophids Krefftichthys anderssoni, Electrona antarctica, Gymnoscopelus braueri and Gymnoscopelus nicholsi). Shallow slope and shelf stations were located north of the S-PF and the macrozooplankton assemblages were influenced by Polar Frontal Zone species (e.g., Euphausia vallentini), while the shelf micronekton were dominated by the bentho-pelagic channichthyids Champsocephalus gunnari and Channichthys rhinoceratus, together with G. nicholsi. Most macrozooplankton migrated from the deep epipelagic (100–300 m) into the surface epipelagic (<100 m) at night, although some macrozooplankton occurred at high densities in the surface epipelagic during the day (e.g., Rhincalanus gigas, Themisto gaudichaudii). Most micronekton species occurred in the mesopelagic layers (300–600 m) during the day, and migrated into the deep and surface epipelagic at night. Only the small but very abundant K. anderssoni utilized the top 300 m during the day. The vertical distributions of macrozooplankton and micronekton are discussed with respect to the foraging behavior and diets of top predators.

Long-term variation in the breeding diets of macaroni and eastern rockhopper penguins at Marion Island (1994–2018)

Populations of the macaroni penguin Eudyptes chrysolophus and the eastern rockhopper penguin E. filholi breeding at Marion Island (Prince Edward Islands) in the sub-Antarctic Indian Ocean decreased from 1994 to 2018. We examined their diets when rearing chicks during this period. There was substantial overlap in the diets of the two species, with crustaceans, dominated by the euphausiids Thysanoessa vicina and Euphausia vallentini, making up >80% of the diets by number and >60% by mass over the study period. The lanternfishes Krefftichthys anderssoni and Protomyctophum tenisoni were the most-commonly consumed fish in the diet of breeding macaroni penguins and dominated in their diet in three of the 25 years (1994, 1998 and 2014); they were also the most commonly consumed fish by eastern rockhopper penguins. Macaroni penguins consumed more of the amphipod crustacean Themisto gaudichaudii and the lanternfish Electrona carlsbergi as well as a greater diversity of and larger-sized fish than did eastern rockhopper penguins. The horsefish Zanclorhynchus spinifer was found in substantial amounts in the diet of eastern rockhopper penguins in 1996 and 1997. Despite annual variations in relative prey contributions to the diets, there were no significant long-term changes in the diet of either penguin species over the study period or when compared with an earlier assessment in 1982. We conclude that changes in the relative proportions of prey in the diets of these penguin species during breeding are unlikely to account for the recent declines in these penguin populations.

Are hakes truly opportunistic feeders? A case of prey selection by the Argentine hake Merluccius hubbsi off southwestern Atlantic

Prey selection and feeding strategy of the Argentine hake Merluccius hubbsi were investigated by analyzing the diet and prey availability in the main nursery area of the Patagonian stock (42.4°–46.4 °S, 70–102 m depth). Stomach contents from fish captured using a bottom trawl net, and zooplankton samples collected with a Bongo net, were analyzed. Acoustic data were examined to identify pelagic aggregations and quantify relative prey abundance. The Argentine hake M. hubbsi fed mainly on crustaceans (92.7%IRI), followed by fish (7.1%IRI) and cephalopods (0.2%IRI). Among crustaceans, the diet was dominated by euphausiids (Euphausia spp.), the lobster krill Munida gregaria, and the hyperiid amphipod Themisto gaudichaudii. The most consumed fish was other hake by cannibalism. Prey availability was also numerically dominated by the euphausiids (Euphausia spp.), followed by the hyperiid amphipod T. gaudichaudii and the lobster krill M. gregaria. Ontogenetic dietary changes were detected: consumption of euphausiids and amphipods decreased with predator´s total length, and the consumption of the lobster krill M. gregaria was higher in adult specimens. The Argentine hake M. hubbsi showed positive prey selection for lobster krill M. gregaria. The consumption of Euphausiids decreased with the increasing acoustic abundance of the lobster krill M. gregaria. The cannibalism was denso-independent. The Argentine hake appeared to be opportunistic when the proportion of prey eaten to the proportion present in the environment was compared, but deeper analyses confirmed that zooplankter with increased abundances and higher energy density was the preferred prey.

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.

Seasonal prey switching in non-breeding gentoo penguins related to a wintertime environmental anomaly around South Georgia

Information is needed on how anomalous environmental conditions affect important Antarctic ecological sentinels during the winter. Using a non-invasive method (scats), the diet of non-breeding gentoo penguins Pygoscelis papua at Bird Island, South Georgia, was examined during the winter of 2009 when local environmental conditions were abnormal (e.g. warmer sea surface waters). Scats were collected every 2 weeks from May until October 2009 (N = 168); the diet was dominated by the amphipod Themisto gaudichaudii by frequency of occurrence (77.5% of the samples) and by number (70.5% of the total individuals). By mass, different species of crustaceans (T. gaudichaudii and Antarctic krill Euphausia superba) and fish (Champsocephalus gunnari, Lepidonotothen larseni and Pseudochaenichthys georgianus) were the most important in different periods. Numerically, prey switching occurred at the end of winter, when E. superba became more abundant in the penguins’ diets in late September and early October (austral spring), when sea surface temperatures became colder. T. gaudichaudii is sub-optimal prey for gentoo penguins and its consumption most likely reflects a lack of E. superba in local waters. The consumption of T. gaudichaudii and fish, in place of E. superba, was probably insufficient for the penguins to maintain a body condition suitable for reproduction, contributing to the observed late commencement of breeding (i.e. 2–3 weeks) that year. Our results highlight the sensitivity of gentoo penguin populations to variations in environmental conditions, particularly in terms of how these conditions affect the availability of preferred dietary items and potential follow-on effects in the subsequent breeding season.

Conventional and molecular analysis of the diet of gentoo penguins: contributions to assess scats for non-invasive penguin diet monitoring

There is a growing search for less invasive methods while studying the diet of Antarctic animals in the wild. Therefore, we compared the diet of gentoo penguins from stomach contents (i.e. through visual identification of prey remains) and scats (i.e. faeces), and further compared prey DNA assay in fresh and old scats. Prey remains identified visually in stomach contents and scats were broadly comparable: the crustaceans and fish were the most important components, with Themisto gaudichaudii clearly being the most frequent and numerous prey species in both sampling methods. By mass, differences in species frequency were observed in stomach contents (Parachaenichthys georgianus) and scats (Champsocephalus gunnari), with the former fish species absent in scats. Differences were detected in the most frequent prey (T. gaudichaudii and Euphausia superba) and in various fish species, most with bigger sizes in scats. Allometric equations to estimate most crustacean’s sizes (i.e. relationships between carapace and mass/total length) are needed. For DNA studies, when comparing DNA from fresh and old scats, both provided similar results that, in general, were also similar to the visual analysis. In order to use penguin scats (along with the use of DNA analyses) for monitoring purposes, allometric equations to estimate mass and size of prey (most crustaceans) and better designed species-specific primers are needed for targeting key prey species (e.g. Euphausia superba, T. gaudichaudii). These DNA methodologies can complement other methods (i.e. visual analyses and stomach contents analyses) in monitoring programs of penguins.

Iron partitioning during LOHAFEX: Copepod grazing as a major driver for iron recycling in the Southern Ocean

eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~20 μM), low dissolved iron (DFe ~0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean.

Trace elements in invertebrates and fish from Kerguelen waters, southern Indian Ocean

Given the lack of background data on essential and non-essential trace elements in invertebrates and fish known to be the predominant prey of marine mammals and seabirds breeding at the Kerguelen Islands, this study intends to provide these results of great influence for predators in higher trophic levels. To this end, plankton organisms (9 species/4 phyla), mollusks (2 bivalves and 2 squid species) and fishes (8 benthic and 10 pelagic species) from Kerguelen waters were analysed for cadmium (Cd), copper (Cu), mercury (Hg), and zinc (Zn). Individual concentrations of non-essential elements (particularly Cd) showed larger variation in comparison with essential ones due to homeostasis. Cd ranged over 4 orders of magnitude; however, Hg ranged only 1, without significant correlation to trophic level. Instead, ecological parameters (benthic/mesopelagic habitat and feeding ecology) showed a more important influence on the results. Concerning seashore organisms, bivalves collected inside the Gulf of Morbihan had higher Cd concentrations compared to those from the Kerguelen shelf, suggesting a local source of Cd, such as runoff water from bird colonies. Comparison with literature showed metal concentrations in invertebrates and fishes from Kerguelen Islands somewhat lower than in the Antarctic area, with Hg prevailing in benthic species and Cd in pelagic ones. In contrast to Hg, Cd values of squids, jellyfish and the amphipod Themisto gaudichaudii were significantly higher than all other species. Finally, top predators foraging in this area that can be subject to potentially high Hg and Cd exposure through their diet at Kerguelen are reviewed.

Sexual and individual foraging segregation in Gentoo penguins Pygoscelis papua from the Southern Ocean during an abnormal winter.

Knowledge about sexual segregation and gender-specific, or indeed individual specialization, in marine organisms has improved considerably in the past decade. In this context, we tested the “Intersexual Competition Hypothesis” for penguins by investigating the feeding ecology of Gentoo penguins during their austral winter non-breeding season. We considered this during unusual environmental conditions (i.e. the year 2009 had observations of high sea surface and air temperatures) in comparison with the long term average at Bird Island, South Georgia. Through conventional (i.e. stomach contents) and stable isotopic values from red blood cells, plasma and feathers of both male and female Gentoo penguins, we showed that there were significant differences between sexes, with males feeding mainly on fish (54% by mass) followed by crustaceans (38%) whereas females fed mainly on crustaceans (89% by mass) followed by fish (4%). Themisto gaudichaudii was the most important crustacean prey for males (64% by mass; 82% by number; 53% by frequency of occurrence) and females (63% by mass; 77% by number; 89% by frequency of occurrence), contrasting with all previous studies that found Antarctic krill Euphausia superba were generally the main prey. Stable isotopic data showed that, in terms of habitat use (based on δ 13C), there were significant differences in short-term carbon signatures between males and females (based on plasma and red blood cells), suggesting that both sexes explored different habitats, with females exploring more offshore pelagic waters and males feeding more in coastal benthic waters. Based on δ 15N, males fed on significantly higher trophic level than females (based on plasma and red blood cells), in agreement with our diet results., Thus, Gentoo penguins behave in a similar manner to other non-breeding penguins species (e.g. king, macaroni and rockhopper penguins), albeit at a smaller spatial scale (as they do not disperse as these other penguins do), in that they have a wider habitat and trophic niche during the Antarctic Winter (in comparison to Summer). We also detected individual specialization in feeding/trophic levels for each gender, with certain males feeding mainly on fish and certain females mainly on crustaceans, which may be driven the prevailing environmental conditions that lead individuals to search for alternative prey, and cause sexual diet segregation. Our results provide further information to help improve understanding about sexual segregation and individual specialization of marine organisms, while contributing valuable information on the winter diet for Antarctic monitoring programs and for modelling Antarctic marine food webs.

Humpback whale "super-groups" - A novel low-latitude feeding behaviour of Southern Hemisphere humpback whales (Megaptera novaeangliae) in the Benguela Upwelling System.

Southern Hemisphere humpback whales (Megaptera novaeangliae) generally undertake annual migrations from polar summer feeding grounds to winter calving and nursery grounds in subtropical and tropical coastal waters. Evidence for such migrations arises from seasonality of historic whaling catches by latitude, Discovery and natural mark returns, and results of satellite tagging studies. Feeding is generally believed to be limited to the southern polar region, where Antarctic krill (Euphausia superba) has been identified as the primary prey item. Non-migrations and / or suspended migrations to the polar feeding grounds have previously been reported from a summer presence of whales in the Benguela System, where feeding on euphausiids (E. lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp (Pterygosquilla armata capensis) and clupeid fish has been described. Three recent research cruises (in October/November 2011, October/November 2014 and October/November 2015) identified large tightly-spaced groups (20 to 200 individuals) of feeding humpback whales aggregated over at least a one-month period across a 220 nautical mile region of the southern Benguela System. Feeding behaviour was identified by lunges, strong milling and repetitive and consecutive diving behaviours, associated bird and seal feeding, defecations and the pungent “fishy” smell of whale blows. Although no dedicated prey sampling could be carried out within the tightly spaced feeding aggregations, observations of E. lucens in the region of groups and the full stomach contents of mantis shrimp from both a co-occurring predatory fish species (Thyrsites atun) and one entangled humpback whale mortality suggest these may be the primary prey items of at least some of the feeding aggregations. Reasons for this recent novel behaviour pattern remain speculative, but may relate to increasing summer humpback whale abundance in the region. These novel, predictable, inter-annual, low latitude feeding events provide considerable potential for further investigation of Southern Hemisphere humpback feeding behaviours in these relatively accessible low-latitude waters.

Predatory impact of the myctophid fish community on zooplankton in the Scotia Sea (Southern Ocean)

Myctophids are the most abundant mesopelagic fishes in the Southern Ocean, although their trophic role within the predominantly krill-based food web in regions south of the Antarctic Polar Front (APF) is poorly resolved. This study therefore examined the diets of 10 species of myctophid fishes: Electrona antarctica, E. carlsbergi, Gymnoscopelus braueri, G. fraseri, G. nicholsi, Krefftichthys anderssoni, Protomyctophum bolini, P. tenisoni, P. choriodon and Nannobrachium achirus, in the Scotia Sea, together with their predatory impact on the underlying zooplankton community. Myctophids and their prey were sampled in different seasons by scientific nets deployed across the Scotia Sea from the sea-ice zone to the APF. Based on the percentage index of relative importance, myctophids had high overlap in their diets, although the data indicate dietary specialisation in some species. There was also a distinct switch in diet, from copepods to euphausiids and amphipods, with increasing myctophid size. Myctophid predation impacted daily copepod production by between 0.01 and 5%, with Calanus simillimus being most impacted. Total annual consumption of copepods was around 1.5 million t (Mt) per year. All myctophids preyed upon the euphausiid Thysanoessa spp., consuming ~12% of its daily productivity and around 4 Mt per year. However, only larger myctophid species preyed upon Antarctic krill Euphausia superba, consuming 2% of its daily productivity, which could amount to as much as 17 Mt per year. Themisto gaudichaudii was also an important dietary component, with 4% of its daily productivity being consumed, amounting to around 2 Mt per year. This study demonstrates that myctophids link secondary productivity to higher predators both through krill-dependent and krill-independent trophic pathways.

Assemblages of micronektonic fishes and invertebrates in a gradient of regional warming along the Western Antarctic Peninsula

Micronektonic fishes and invertebrates were sampled with 32 midwater trawls at nine sites along the Western Antarctic Peninsula (WAP) in the austral fall (March–April) of 2010. Study sites were located within four hypothesized hydrographic regions: near Joinville Island in Region I, at Croker Passage, near Anvers Island, and near Renaud Island in Region II, within Marguerite Bay and the Marguerite Trough in Region III, and near Charcot Island in Region IV. A total of 62 taxa representing 12 taxonomic groups of pelagic invertebrates and 9 families of fish were captured, but assemblages were dominated by only a few species. The most numerically abundant taxa were the euphausiids, Thysanoessa macrura, Euphausia superba, and E. crystallorophias, combining to contribute nearly 79% of the total catch. Biomass dominants included E. superba, which contributed more than 44% of the total catch, the notothenioid Pleuragramma antarctica, and the salp, Salpa thompsoni. A comparison of total catches among sites revealed that the largest volumetric abundances and biomasses were captured at the Marguerite Bay site.Cluster analysis of abundance data identified distinct multispecies assemblages at Joinville Island in Region I, Croker Passage in Region II, Marguerite Bay in Region III, and Charcot Island in Region IV. A fifth distinct assemblage included samples from sites near Anvers and Renaud Island in Region II, and from the Marguerite Trough in Region III. Assemblages at Joinville Island and Croker Passage were both dominated by E. superba and S. thompsoni, but hydrographic conditions at Joinville Island favored a neritic assemblage, underscored by substantial numbers of P. antarctica. The assemblage at Croker Passage was more oceanic in nature with major inputs from the myctophid, Electrona antarctica and the hyperiid amphipod, Themisto gaudichaudii. Marguerite Bay and Charcot Island were well-mixed assemblages with strong representation by both neritic and oceanic fauna. The mid-peninsula assemblage was oceanic in character, being overwhelmingly dominated by Thysanoessa macrura and T. gaudichaudii.Pleuragramma antarctica were captured at five sites: Joinville Island, Croker Passage, Marguerite Bay, and the two sites near Charcot Island. They were completely absent at the two sites near Anvers Island, at Renaud Island, and in the Marguerite Trough. One fish was captured in Croker Passage. The majority of fish captured in Marguerite Bay were larger than 150 mm standard length (SL), with very few fish of smaller size present. If resident populations of Pleuragramma reproduce and recruit locally rather than being sustained by larval advection, those populations will be highly susceptible to local disappearance. This may be the causative factor behind the absence of Pleuragramma from the mid-peninsula region. Continued warming and subsequent sea ice reductions may not only cause Pleuragramma population collapses in the Marguerite Bay and Charcot Island regions, but may also change the character of the faunal assemblages along the WAP to those of an oceanic system.