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1. Winter Female Krill Oocyte Size

   https://doi.org/10.15784/601919  

   Bernard, Kim (January 2025, U.S. Antarctic Program (USAP) Data Center)

   This dataset contains measurements of the diameter and area of Antarctic krill (Euphausia superba) oocytes collected from the northern Antarctic Peninsula during the austral winter from 2012 to 2016 (August each year). Female oocytes were examined using the "squash technique" developed by Cuzin-Roudy and Amsler (1991) for assessing ovarian development and sexual maturity. The squash method provides a detailed view of oocyte size, which serves as an indicator of reproductive status and energy allocation during the winter months. Data were collected and processed according to the methods described by Steinke et al. (2024). The dataset includes .csv files containing oocyte diameter and area measurements for individual female krill. These data are valuable for understanding krill reproductive cycles and their ecological responses to environmental variability. Data users should acknowledge the project and grant number, as well as credit Dr. Kim Bernard, Dr. Kirsten Steinke, and Dr. Christian Reiss. The research was conducted in collaboration with the US National Oceanic and Atmospheric Administration Antarctic Marine Living Resources group. 

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2. Winter condition, physiology, and growth potential of juvenile Antarctic krill

   https://doi.org/10.3389/fmars.2022.990853  

   Bernard, Kim S.; Steinke, Kirsten B.; Fontana, Julia M. (September 2022, Frontiers in Marine Science)

   In recent years, substantial efforts have been made to understand the implications of climate change on Antarctic krill, Euphausia superba , because of their pivotal role in the Southern Ocean food web and in biogeochemical cycling. Winter is one of the least studied seasons in Antarctica and we have limited understanding about the strategies Antarctic krill use to survive the winter. In particular, data on the winter physiology and condition of juvenile Antarctic krill are severely lacking. From May to September (the austral autumn-winter) of 2019, we maintained juvenile Antarctic krill in large (1,330 L) aquarium tanks at Palmer Station, Antarctica and, at monthly time intervals, measured their physiology and condition. Each tank served as a “food environment scenario”, representing possible food environments the krill may encounter during winter along the Western Antarctic Peninsula. We found that, unlike adults, juvenile krill maintain relatively high respiration rates through the winter and respond positively to increased food concentrations by increasing their ingestion rates. Unlike larval krill, juveniles use lipid stores accumulated during the summer and autumn to sustain themselves through periods of starvation in the winter. We used our empirically derived measurements of physiology and condition to estimate the energy budget and growth potential of juvenile krill during the winter. We found that, given their comparatively high respiration rates, small juvenile krill (20 mg dry weight) would need to encounter food at concentrations of ~ 0.15 mg C L -1 daily to avoid loss of body carbon. Without sufficient lipid reserves, this value increases to ~ 0.54 mg C L -1 , daily. The health of juvenile krill in the wintertime is dependent on their ability to accumulate lipid stores in the summer and autumn and to find sufficient food during the winter. Changes in food availability to Antarctic krill throughout the year may become problematic to juvenile krill in the future. Understanding the variability in the winter energy budget of juvenile Antarctic krill will allow us to improve population models that make assumptions on seasonal growth patterns. 

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3. Omnivorous summer feeding by juvenile Antarctic krill in coastal waters

   https://doi.org/10.1002/lno.12533  

   Conroy, John A; Steinberg, Deborah K; Nardelli, Schuyler C; Schofield, Oscar (April 2024, Limnology and Oceanography)

   Abstract The Antarctic krillEuphausia superbais often considered an herbivore but is notable for its trophic flexibility, which includes feeding on protistan and metazoan zooplankton. Characterizing krill trophic position (TP) is important for understanding carbon and energy flow from phytoplankton to vertebrate predators and to the deep ocean, especially as plankton composition is sensitive to changing climate. We used repeated field sampling and experiments to study feeding by juvenile krill during three austral summers in waters near Palmer Station, Antarctica. Our approach was to combine seasonal carbon budgets, gut fluorescence measurements, imaging flow cytometry, and compound‐specific isotope analysis of amino acids. Field measurements coupled to experimentally derived grazing functional response curves suggest that phytoplankton grazing alone was insufficient to support the growth and basal metabolism of juvenile krill. Phytoplankton consumption by juvenile krill was limited due to inefficient feeding on nanoplankton (2–20 μm), which constituted the majority of autotrophic prey. Mean krill TP and the metazoan dietary fraction increased in years with higher mesozooplankton biomass, which was not coupled to phytoplankton biomass. Comparing TP estimates using δ¹⁵N of different amino acids indicated a substantial and consistent food‐web contribution from heterotrophic protists. Phytoplankton, metazoans, and heterotrophic protists all were important contributors to a diverse krill diet that changed substantially among years. Juvenile krill fed mostly on heterotrophic prey during summer near Palmer Station, and this food web complexity should be considered more broadly throughout the changing Southern Ocean. 

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4. Krill Hotspot Formation and Phenology in the California Current Ecosystem

   https://doi.org/10.1029/2020GL088039  

   Fiechter, Jerome; Santora, Jarrod A.; Chavez, Francisco; Northcott, Devon; Messié, Monique (June 2020, Geophysical Research Letters)

   Abstract In the California Current Ecosystem, krill represent a key link between primary production and higher trophic level species owing to their central position in the food web and tendency to form dense aggregations. However, the strongly advective circulation associated with coastal upwelling may decouple the timing, occurrence, and persistence of krill hotspots from phytoplankton biomass and nutrient sources. Results from a coupled physical‐biological model provide insights into fundamental mechanisms controlling the phenology of krill hotspots in the California Current Ecosystem, and their sensitivity to alongshore changes in coastal upwelling intensity. The simulation indicates that dynamics controlling krill hotspot formation, intensity, and persistence on seasonal and interannual timescales are strongly heterogeneous and related to alongshore variations in upwelling‐favorable winds, primary production, and ocean currents. Furthermore, regions promoting persistent krill hotspot formation coincide with increased observed abundance of top predators, indicating that the model resolves important ecosystem complexity and function. 

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5. Linking Antarctic krill larval supply and recruitment along the Antarctic Peninsula

   https://doi.org/10.1093/icb/icaa111  

   Conroy, John A; Reiss, Christian S; Gleiber, Miram R; Steinberg, Deborah K (July 2020, Integrative and Comparative Biology)

   Synopsis Antarctic krill (Euphausia superba) larval production and overwinter survival drive recruitment variability, which in turn determines abundance trends. The Antarctic Peninsula has been described as a recruitment hot spot and as a potentially important source region for larval and juvenile krill dispersal. However, there has been no analysis to spatially resolve regional-scale krill population dynamics across life stages. We assessed spatiotemporal patterns in krill demography using two decades of austral summer data collected along the North and West Antarctic Peninsula since 1993. We identified persistent spatial segregation in the summer distribution of euphausiid larvae (E. superba plus other species), which were concentrated in oceanic waters along the continental slope, and E. superba recruits, which were concentrated in shelf and coastal waters. Mature females of E. superba were more abundant over the continental shelf than the slope or coast. Euphausiid larval abundance was relatively localized and weakly correlated between the North and West Antarctic Peninsula, while E. superba recruitment was generally synchronized throughout the entire region. Euphausiid larval abundance along the West Antarctic Peninsula slope explained E. superba recruitment in shelf and coastal waters the next year. Given the localized nature of krill productivity, it is critical to evaluate the connectivity between upstream and downstream areas of the Antarctic Peninsula and beyond. Krill fishery catch distributions and population projections in the context of a changing climate should account for ontogenetic habitat partitioning, regional population connectivity, and highly variable recruitment. 

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