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

Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage

We used the instantaneous growth rate method to determine the effects of food, temperature, krill length, sex, and maturity stage on in situ summer growth of krill across the southwest Atlantic sector of the Southern Ocean. The main aims were to examine the separate effects of each variable and to generate a predictive model of growth based on satellite-derivable environmental data. Both growth increments in length on moulting (GIs) and daily growth rates (DGRs, mm d-1) ranged greatly among the 59 swarms, from 0.58–15% and 0.013–0.32 mm d-1. However, all swarms maintained positive mean growth, even those in the low chlorophyll a (Chl a) zone of the central Scotia Sea. Among a suite of indices of food quantity and quality, large-scale monthly Chl a values from SeaWiFS predicted krill growth the best. Across our study area, the great contrast between bloom and nonbloom regions was a major factor driving variation in growth rates, obscuring more subtle effects of food quality. GIs and DGRs decreased with increasing krill length and decreased above a temperature optimum of 0.5°C. This probably reflects the onset of thermal stress at the northern limit of krill’s range. Thus, growth rates were fastest in the ice edge blooms of the southern Scotia Sea and not at South Georgia as previously suggested. This reflects both the smaller size of the krill and the colder water in the south being optimum for growth. Males tended to have higher GIs than females but longer intermoult periods, leading to similar DGRs between sexes. DGRs of equivalent-size krill tended to decrease with maturity stage, suggesting the progressive allocation of energy toward reproduction rather than somatic growth. Our maximum DGRs are higher than most literature values, equating to a 5.7% increase in mass per day. This value fits within a realistic energy budget, suggesting a maximum carbon ration of ~20% d-1. Over the whole Scotia Sea/South Georgia area, the gross turnover of krill biomass was ~1% d-1

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

Life cycle, distribution and trophodynamics of the lanternfish Krefftichthys anderssoni (Lönnberg, 1905) in the Scotia Sea

Myctophid fish play an important role in the Southern Ocean pelagic food web. The lanternfish Krefftichthys anderssoni is one of the most common myctophids in the region, but its ecology is poorly known. This study examines spatial and temporal patterns in the species distribution of density, life cycle, population structure and diet using samples collected by mid-water trawl nets deployed in different seasons across the Scotia Sea. Virtually absent from the sea-ice zone, the species was most abundant in the northern Scotia Sea around the Georgia Basin at depths below 400 m that are associated with the Circumpolar Deep Water. The species migrated during night from waters deeper than 700 m to waters above the 400 m following their main prey species: the copepods Rhincalanus gigas and Calanoides acutus and euphausiids of the Thysanoessa genus. Larvae length distribution and post-larvae length-frequency analyses suggested a life cycle of ~3 years with spawning and recruitment strongly connected with APF and the South Georgia shelf. Our results show that species spatial distribution, population structure and diet changed both seasonally and ontogenetically. This study is the most comprehensive examination of the ecology of K. anderssoni in the Southern Ocean to date and contributes to resolving how pelagic food webs and ecosystems operate in the region

Egg hatching times of Antarctic copepods

Egg hatching times were determined at a range of temperatures for four species of commonly occurring Antarctic copepods. At a given temperature the eggs of Rhincalanus gigas took longest to hatch, up to 9 days at 0°C, followed by those of Calanoides acutus, Calanus propinquus and Calanus simillimus. A Bělehrádeks temperature function with the parameter b fixed at −2.05 accounted for >95% of the variance for each species. There was an approximate doubling in hatching times between 5°C and 0°C for R. gigas and for the other species the increase in embryonic duration was 40–50% at the lower temperature

Egg production in three species of Antarctic Calanoid Copepods during an austral summer

Egg production in three species of calanoid copepods Rhincalanus gigas, Calanoides acutus and Calanus simillimus was investigated via incubations of females and recovery of eggs from net hauls made around South Georgia during January 1993. Average daily egg production was highest for the sub-Antarctic C. simillimus, (15.5 eggs female−1 d−1). This species normally spawns in the spring in the central part of its geographical range but was apparently delayed by the colder waters found around South Georgia. For R. gigas and C. acutus egg production averaged 8.9 and 6.0 eggs female−1 d−1, respectively. The former species appeared to be undergoing protracted recruitment while the population of the latter was preparing to overwinter. Considerable interstation variability existed, although no relationships were apparent between surface chlorophyll concentrations and either egg production in experiments or in the numbers of eggs recovered by the nets. Clutch size (eggs produced spawning female−1 d−1) did not differ significantly between the three species although the maximum clutch size recorded for R. gigas (94 eggs) was almost twice that of C. simillimus. Samples taken from the Bellingshausen Sea during the latter part of 1992 indicated that recruitment of R. gigas and C. acutus commenced in early December in this region when adult females were concentrated in the surface 250 m and a diatom bloom was developing. Egg numbers were highest in the surface 50 m (up to 350 m−-3) at both the Bellingshausen and South Georgia stations. At the latter site females migrated into these surface layers at night; thus it would appear that spawning is largely nocturnal and linked to diurnal migratory behaviour

Moulting and growth of the early stages of two species of Antarctic calanoid copepod in relation to differences in food supply

Instantaneous measurements of moulting and growth of the early copepodite stages of 2 species of Antarctic copepod, Rhincalanus gigas and Calanoides acutus, were made at 4 regions around South Georgia during austral summer 1996/1997. Sea surface temperature was ~3°C across the study area whereas chlorophyll a concentrations were considerably higher towards the western end of the island. Despite this, moulting rate experiments showed that stage durations of both species were invariably short with no significant regional differences. Stage durations of R. gigas CI, CII and CIII averaged 9, 28 and 15 d respectively, and those of CII, CIII and CIV C. acutus were 4, 7, and 16 d respectively. Daily mass-specific growth rates were lower and less variable in R. gigas (mean 0.05 d-1) than in C. acutus (mean 0.14 d-1), and showed no measurable regional differences. Those for C. acutus however, were higher off-shelf at the western end of the island where the copepodites were heavier than elsewhere. In addition to variations in concentration of chlorophyll a, qualitative differences in the microplankton food supply may also have influenced growth rates. Large diatoms were far more abundant off-shelf at the western end of the island compared to elsewhere, where micro-flagellates and small diatoms dominated. It is suggested that the more opportunistic feeding mode of R. gigas gave stability to its growth rate, whereas C. acutus, which is predominantly herbivorous, was affected by the fluctuations in phytoplankton concentrations and species composition. Moulting occurred within a narrow range of carbon and dry mass for both species, although this range varied between stations

The spring mesozooplankton community at South Georgia: a comparison of shelf and oceanic sites

Mesozooplankton (predominantly 200–2000 μm) were sampled at a shelf and an oceanic station close to South Georgia, South Atlantic, during austral spring (October/November) 1997. Onshelf zooplankton biomass was extremely high at 10–16 g dry mass m−2 (0–150 m), 70% comprising the small neritic clausocalaniid copepod Drepanopus forcipatus. Large calanoid species, principally Calanoides acutus and Rhincalanus gigas, contributed only 8–10%. At the oceanic station, biomass in the sampled water column (0–1000 m) was ∼6.5 g dry mass m−2 and 4–6 g dry mass m−2 in the top 200 m. Here, large calanoids composed 40–50% of the standing stock. Antarctic krill (Euphausia superba) occurred in low abundances at both stations. Vertical profiles obtained with a Longhurst Hardy Plankton Recorder indicated that populations of C. acutus and R. gigas, which overwinter at depth, had completed their spring ascent and were resident in surface waters. Dry mass, carbon and lipid values were lower than found in summer but were consistent with overwintered populations. Phytoplankton concentrations were considerably higher at the oceanic station (2–3 mg chlorophyll a m−3) and increased over the time on station. In response to this, egg production of both large calanoid species and growth rates of R. gigas approached those measured in summer. Onshelf phytoplankton concentrations were lower (<1 mg m−3), and low egg production rates suggested food limitation. Here phytoplankton rations equivalent to 6% zooplankton body C would have been sufficient to clear primary production whereas at the oceanic station daily carbon fixation was broadly equivalent to zooplankton carbon biomass

The deep-sea copepod fauna of the Southern Ocean: patterns and processes

In recent years, much attention has been paid to the Antarctic epipelagic fauna, as a result of the desire to increase our knowledge of ecosystem function and resource management. Unfortunately, our understanding of the polar pelagic deep-sea has not progressed as fast, and in common with many other parts of the world's deep ocean, knowledge is still fragmentary. As yet, we have an incomplete but evolving knowledge of species presence and distribution, but very little idea of how the extreme seasonality seen in the Southern Ocean might influence the deep-water fauna. An examination is made of species distribution and diversity, in relation to the latitudinal cline seen in many benthic groups, and the historical perspective offered by changing circulation patterns and sea temperature through geological time. Although a number of important frontal systems are found within the circumpolar Southern Ocean, the boundary is marked by the Sub-Tropical Convergence, which appears to be the major biogeographic boundary between it and surrounding provinces. Evidence for seasonality in various families is reviewed in light of what we know and can infer about their biology and particularly in respect of their bathymetric distribution, which in some groups appears to change with latitude

Response of the copepod community to a spring bloom in the Bellingshausen Sea

During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m−3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northern regions of the Southern Ocean. However, the estimated grazing impact of the copepod community at all the stations was negligible, rising to a maximum of only 8.4% of daily primary production at the most northerly bloom station. These low values result from the very low numbers of copepods in the epipelagic, particularly under the ice. Of the copepods measured, grazing was mainly by the adult female population of Oithona spp. before the bloom, and appeared to be mainly by Oithona spp. and C. acutus during the bloom