Routine metabolism of Antarctic krill (Euphausia superba) in South Georgia waters: Absence of metabolic compensation at its range edge

Routine respiration rates in the South Georgia stock of Antarctic krill (Euphausia superba) were measured to compare with previously published measurements on stocks from colder locations further south. Within the natural temperature range of this species (− 1.8° to 5.5 °C), respiration rate data from both the present and previous studies were adequately fitted by a single Arrhenius regression (Q10 of 2.8), although South Georgia krill showed an upward deviation from this regression between 0° and 2 °C (the lower temperature range at South Georgia). Metabolic compensation (i.e. the comparative lowering of respiration rate) at the high temperatures experienced at South Georgia was not apparent, although the higher than predicted metabolic rates at low temperatures suggests acclimation of South Georgia krill to a warm water lifestyle. Weight-specific respiration rate was significantly higher in sub-adults and adults compared to juveniles, highlighting the metabolic burden of reproduction. South Georgia krill showed no further increase in respiration rate when exposed to acute temperatures (5.5–12.2 °C), indicating that they were already at the limit of aerobic capacity by 5.5 °C. Overall, this study shows that even small degrees of additional warming to South Georgia waters are likely to make conditions there metabolically unsustainable for Antarctic krill

Marine ecology: a wonderland of marine activity in the Arctic night

Studies carried out on a wide variety of Arctic species during the polar night reveal continued feeding, growth and reproduction, changing our view of this period from one of biological stasis to a time of continued high activity level

Southern Ocean mesopelagic fish comply with Bergmann’s Rule

The applicability of macroecological rules to patterns in body size varies between taxa. One of the most examined is Bergmann’s rule, which states that body size increases with decreasing temperature and increasing latitude, although the rule is not universal and the proposed mechanisms underpinning it are multifarious and lack congruence. This study considers the degree to which Bergmann’s rule applies to the Southern Ocean mesopelagic fish community. We studied patterns in body size, temperature, and latitude across a 12° latitudinal gradient within the Scotia-Weddell sector. Intraspecific Bergmann’s rule was found to apply to 8 of the 11 biomass-dominant species in the family Myctophidae. The rule was also apparent at an interspecific level. Our study suggests that greater attainable body size in this community is a necessary attribute to reach colder regions further south. The adherence of these taxa to Bergmann’s rule enables such species to act as sentinels for identifying the drivers and consequences of ocean warming for the Southern Ocean ecosystem

Scaling of size, shape and surface roughness in Antarctic krill swarms

Antarctic krill are obligate swarmers and the size and shape of the swarms they form can have a major influence on trophic interactions and biogeochemical fluxes. Parameterizing variability in size and shape is therefore a useful step toward understanding the operation of the Southern Ocean ecosystem. We analyse the relationships between the length L L⁠, thickness T T⁠, perimeter P P⁠, and area A A of 4650 vertical cross-sections of open-ocean krill swarms obtained within the Atlantic sector of the Southern Ocean in summer 2003. Our data show that these parameters are tightly interrelated. The thickness T T increases on average as L 0.67 L0.67 and has a log-normal distribution within each length class. The perimeter and area scale with L L and T T as P∼L 0.77 T P∼L0.77T and A∼L 0.86 T 0.48 A∼L0.86T0.48⁠. The swarm aspect ratio, T/L T/L⁠, decreases approximately as L −0.32 L-0.32⁠. The surface roughness (defined as P/A P/A⁠) has a weak dependence on swarm length and decreases approximately as T −0.46 T-0.46⁠, which can be explained only by the appearance of indentations and cavities in the swarm shape. Overall, our study finds that there are distinct limits to the size and shape of swarms that Antarctic krill appear to be capable of forming and we explore the potential explanatory factors contributing to these limitations

Respiration rates and active carbon flux of mesopelagic fishes (Family Myctophidae) in the Scotia Sea, Southern Ocean

Mesopelagic fish have recently been highlighted as an important, but poorly studied component of marine ecosystems, particularly regarding their role in the marine pelagic food webs and biogeochemical cycles. Myctophids (Family Myctophidae) are one of the most biomass-dominant groups of mesopelagic fishes, and their large vertical migrations provide means of rapid transfer of carbon to the deep ocean where it can be sequestered for centuries or more. In this study, we develop a simple regression for the respiration rate of myctophid fish using literature-based wet mass and habitat temperature data. We apply this regression to net haul data collected across the Scotia-Weddell sector of the Southern Ocean to estimate respiration rates of the biomass-dominant myctophid species. Electrona carlsbergi, Electrona antarctica, and Gymnoscopelus braueri made a high contribution (up to 85%) to total myctophid respiration. Despite the lower temperatures of the southern Scotia Sea (-1.46 to 0.95°C), total respiration here was as high (reaching 1.1 mg C m-2 d-1) as in the warmer waters of the mid and northern Scotia Sea. The maximum respiratory carbon flux of the vertically migrating community was 0.05 to 0.28 mg C m-2 d-1, equivalent to up to 47% of the gravitational particulate organic carbon flux in some parts of the Scotia-Weddell region. Our study provides the first baseline estimates of respiration rates and carbon flux of myctophids in the Southern Ocean. However, direct measurements of myctophid respiration, and of mesopelagic fish generally, are needed to constrain these estimates further and incorporate these fluxes into carbon budgets

Pteropod eggs released at high pCO2 lack resilience to ocean acidification

The effects of ocean acidification (OA) on the early recruitment of pteropods in the Scotia Sea, was investigated considering the process of spawning, quality of the spawned eggs and their capacity to develop. Maternal OA stress was induced on female pteropods (Limacina helicina antarctica) through exposure to present day pCO2 conditions and two potential future OA states (750 μatm and 1200 μatm). The eggs spawned from these females, both before and during their exposure to OA, were incubated themselves in this same range of conditions (embryonic OA stress). Maternal OA stress resulted in eggs with lower carbon content, while embryonic OA stress retarded development. The combination of maternal and embryonic OA stress reduced the percentage of eggs successfully reaching organogenesis by 80%. We propose that OA stress not only affects the somatic tissue of pteropods but also the functioning of their gonads. Corresponding in-situ sampling found that post-larval L. helicina antarctica concentrated around 600 m depth, which is deeper than previously assumed. A deeper distribution makes their exposure to waters undersaturated for aragonite more likely in the near future given that these waters are predicted to shoal from depth over the coming decades

Hydrostatic Pressure and Temperature Effects on the Membranes of a Seasonally Migrating Marine Copepod

Marine planktonic copepods of the order Calanoida are central to the ecology and productivity of high latitude ecosystems, representing the interface between primary producers and fish. These animals typically undertake a seasonal vertical migration into the deep sea, where they remain dormant for periods of between three and nine months. Descending copepods are subject to low temperatures and increased hydrostatic pressures. Nothing is known about how these organisms adapt their membranes to these environmental stressors. We collected copepods (Calanoides acutus) from the Southern Ocean at depth horizons ranging from surface waters down to 1000 m. Temperature and/or pressure both had significant, additive effects on the overall composition of the membrane phospholipid fatty acids (PLFAs) in C. acutus. The most prominent constituent of the PLFAs, the polyunsaturated fatty acid docosahexanoic acid [DHA – 22:6(n-3)], was affected by a significant interaction between temperature and pressure. This moiety increased with pressure, with the rate of increase being greater at colder temperatures. We suggest that DHA is key to the physiological adaptations of vertically migrating zooplankton, most likely because the biophysical properties of this compound are suited to maintaining membrane order in the cold, high pressure conditions that persist in the deep sea. As copepods cannot synthesise DHA and do not feed during dormancy, sufficient DHA must be accumulated through ingestion before migration is initiated. Climate-driven changes in the timing and abundance of the flagellated microplankton that supply DHA to copepods have major implications for the capacity of these animals to undertake their seasonal life cycle successfully

Assessing key influences on the distribution and life-history of Arctic and boreal Calanus: are online databases up to the challenge?

Despite the importance of calanoid copepods to healthy ecosystem functioning of the Arctic Ocean and Subarctic Seas, many aspects of their biogeography, particularly in winter months, remain unresolved. At the same time, online databases that digitize species distribution records are growing in popularity as a tool to investigate ecological patterns at macro scales. The value of such databases for Calanus research requires investigation - the long history of Calanus sampling holds promise for such databases, while conditions at high latitudes may impose limits through spatial and temporal biases. We collated records of three Calanus species (C. finmarchicus, C. glacialis, and C. hyperboreus) from the Ocean Biodiversity Information System (OBIS) and the Global Biodiversity Information Facility (GBIF) providing over 230,000 unique records spanning 150 years and over 100 individual datasets. After quality control and cleaning, the latitudinal and vertical distribution of occurrences were explored, as well as the completeness of informative metadata fields. Calanus sampling was found to be temporally and spatially biased towards surfacemost layers (<10m) in spring and summer. Only 3.5% of records had an average collection depth ≥400m, approximately half of these in months important for diapause. Just over 40% of records lacked associated information on sampling protocol while 11% of records lacked life-stage information. OBIS data contained fields for maximum and minimum collection depth and so were subset into discrete “shallow summer” and “deep winter” life cycle phases and matched to sea-ice and temperature conditions. 23% of OBIS records north of 66° latitude were located in regions of seasonal sea-ice presence and occurrences show species-specific thermal optima during the shallow summer period. The collection depth of C. finmarchicus was significantly different to C. hyperboreus during the deep winter. Overall, online databases contain a vast number of Calanus records but sampling biases should be acknowledged when they are used to investigate patterns of biogeography. We advocate efforts to integrate additional data sources within online portals. Particular gaps to be filled by existing or future collections are (i) widening the spatial extent of sampling during spring/summer months, (ii) increasing the frequency of sampling during winter, particularly at depths below 400m, and (iii) improving the quality, quantity and consistency of metadata reporting

Oceanic swarms of Antarctic krill perform satiation sinking

Antarctic krill form some of the highest concentrations of animal biomass observed in the world's oceans potentially due to their prolific ability to swarm. Determining the movement of Antarctic krill within swarms is important to identify drivers of their behaviour and their biogeochemical impact on their environment. We examined vertical velocity within approximately 2000 krill swarms through the combined use of a shipborne echosounder and an acoustic Doppler current profiler. We revealed a pronounced downward anomaly in vertical velocity within swarms of −0.6 cm s−1 compared with vertical motion outside the swarm. The anomaly changed over the diel cycle, with smaller downward anomalies occurring at night. Swarms in regions of high phytoplankton concentrations (a proxy for food availability) also exhibited significantly smaller downward anomalies. We propose that the anomaly is the result of downward velocities generated by the action of krill beating their swimming appendages. During the night and in high phytoplankton availability, when krill are more likely to feed to the point of satiation, swimming activity is lowered and the anomaly is reduced. Our findings are consistent with laboratory work where krill ceased swimming and adopted a parachute posture when sated. Satiation sinking behaviour can substantially increase the efficiency of carbon transport to depth through depositing faecal pellets at the bottom of swarms, avoiding the reingestion and break-up of pellets by other swarm members

The contribution of zooplankton faecal pellets to deep carbon transport in the Scotia Sea (Southern Ocean)

The northern Scotia Sea contains the largest seasonal uptake of atmospheric carbon dioxide yet measured in the Southern Ocean. This study examines one of the main routes by which this carbon fluxes to the deep ocean, through the production of faecal pellets (FPs) by the zooplankton community. Deep sediment traps were deployed in two sites with contrasting ocean productivity regimes (P3, naturally iron-fertilized and P2, iron-limited), within the same water mass. The magnitude and seasonal pattern of particulate organic carbon (POC) and FPs in the traps was markedly different between the two sites. Maximum fluxes at P3 (22.91 mg C m−2 d−1; 2534 × 10 FP m−2 d−1) were an order of magnitude higher than at P2 (4.01 mg C m−2 d−1; 915 × 10 FP m−2 d−1), with flux at P3 exhibiting a double seasonal peak, compared to a single flatter peak at P2. The maximum contribution of FP carbon to the total amount of POC was twice as high at P3 (91%) compared to P2 (40%). The dominant FP category at P3 varied between round, ovoidal, cylindrical and tabular over the course of the year while, at P2, ovoidal FPs were consistently dominant, always making up more than 60% of the FP assemblage. There was also a difference in the FP state between the two sites, with FPs being relatively intact at P3, while FPs were often fragmented with broken peritrophic membranes at P2. The exception was ovoidal FPs, which were relatively intact at both sites. Our observations suggest that there was community shift from an herbivorous to an omnivorous diet from spring through to autumn at P3 while detritivores had a higher relative importance over the year at P2. Furthermore, the flux was mainly a product of the vertically migrating zooplankton community at P3 while the FP flux was more likely to be generated by deeper-dwelling zooplankton feeding on recycled material at P2. The results demonstrate that the feeding behavior and vertical distribution of the zooplankton community plays a critical role in controlling the magnitude of carbon export to the deep ocean in this region