Stammerjohn Sharon E.

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Sharon E.

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  • Article
    West Antarctic Peninsula : an ice-dependent coastal marine ecosystem in transition
    (The Oceanography Society, 2013-09) Ducklow, Hugh W. ; Fraser, William R. ; Meredith, Michael P. ; Stammerjohn, Sharon E. ; Doney, Scott C. ; Martinson, Douglas G. ; Sailley, Sevrine F. ; Schofield, Oscar M. E. ; Steinberg, Deborah K. ; Venables, Hugh J. ; Amsler, Charles D.
    The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality. The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state.
  • Preprint
    Particle export from the upper ocean over the continental shelf of the west Antarctic Peninsula: A long-term record, 1992–2007
    ( 2008-03) Ducklow, Hugh W. ; Erickson, Matthew ; Kelly, Joann ; Montes-Hugo, Martin ; Ribic, Christine A. ; Smith, Raymond C. ; Stammerjohn, Sharon E. ; Karl, David M.
    We report on results of a long-term (1993-2007) time series sediment trap moored at 170 m to the west of the Antarctic Peninsula in the mid-continental shelf region (350 m depth; 64º30’ S, 66º00’ W). This is a region characterized by late spring-summer diatom blooms, moderately high seasonal primary productivity (50-150 mmol C m-2 d-1 in December-February) and high phytoplankton and krill biomass in the seasonal sea ice zone. The mass flux ranged from near 0 to over 1 g m-2 d-1 and was near 0 to >30% organic carbon (mean 8%). Sedimentation from the upper ocean as estimated by the trap collections at 170 m exhibited strong seasonality with high fluxes (1-10 mmol C m-2 d-1) in November-March following ice retreat and very low fluxes (<0.001 mmol C m-2 d-1) during the Austral winter and under sea ice cover. An average of 85% of the annual export of 212 mmol C m-2 occurred during the seasonal peak flux episodes. Over the trap record, the annual peak flux episode has tended to occur later in the Austral summer, advancing by about 40 days since 1993. The time-integrated sedimentation during the peak flux episode was <1 – 50% of the SeaWiFS-estimated primary production (mean 4%) at the trap site over the period 1998-2006. The elemental composition of material captured in the traps had an average C:N:P of 212:28:1, greater than the canonical Redfield values. High C:P ratios (400- 600) corresponded with the annual flux peak, indicating preferential loss of P from the sinking particles in the summer, ice-free period. The composition of the exported material more closely approximated the Redfield composition during the low-flux, winter period.
  • Article
    Overview of the Arctic Sea state and boundary layer physics program
    (American Geophysical Union, 2018-04-16) Thomson, Jim ; Ackley, Stephen ; Girard-Ardhuin, Fanny ; Ardhuin, Fabrice ; Babanin, Alexander ; Boutin, Guillaume ; Brozena, John ; Cheng, Sukun ; Collins, Clarence ; Doble, Martin ; Fairall, Christopher W. ; Guest, Peter ; Gebhardt, Claus ; Gemmrich, Johannes ; Graber, Hans C. ; Holt, Benjamin ; Lehner, Susanne ; Lund, Björn ; Meylan, Michael ; Maksym, Ted ; Montiel, Fabien ; Perrie, Will ; Persson, Ola ; Rainville, Luc ; Rogers, W. Erick ; Shen, Hui ; Shen, Hayley ; Squire, Vernon ; Stammerjohn, Sharon E. ; Stopa, Justin ; Smith, Madison M. ; Sutherland, Peter ; Wadhams, Peter
    A large collaborative program has studied the coupled air‐ice‐ocean‐wave processes occurring in the Arctic during the autumn ice advance. The program included a field campaign in the western Arctic during the autumn of 2015, with in situ data collection and both aerial and satellite remote sensing. Many of the analyses have focused on using and improving forecast models. Summarizing and synthesizing the results from a series of separate papers, the overall view is of an Arctic shifting to a more seasonal system. The dramatic increase in open water extent and duration in the autumn means that large surface waves and significant surface heat fluxes are now common. When refreezing finally does occur, it is a highly variable process in space and time. Wind and wave events drive episodic advances and retreats of the ice edge, with associated variations in sea ice formation types (e.g., pancakes, nilas). This variability becomes imprinted on the winter ice cover, which in turn affects the melt season the following year.
  • Article
    Emerging trends in the sea state of the Beaufort and Chukchi seas
    (Elsevier, 2016-07-06) Thomson, James M. ; Fan, Yalin ; Stammerjohn, Sharon E. ; Stopa, Justin ; Rogers, W. Erick ; Girard-Ardhuin, Fanny ; Ardhuin, Fabrice ; Shen, Hayley ; Perrie, Will ; Shen, Hui ; Ackley, Stephen ; Babanin, Alexander ; Liu, Qingxiang ; Guest, Peter ; Maksym, Ted ; Wadhams, Peter ; Fairall, Christopher W. ; Persson, Ola ; Doble, Martin J. ; Graber, Hans C. ; Lund, Bjoern ; Squire, Vernon ; Gemmrich, Johannes ; Lehner, Susanne ; Holt, Benjamin ; Meylan, Michael ; Brozena, John ; Bidlot, Jean-Raymond
    The sea state of the Beaufort and Chukchi seas is controlled by the wind forcing and the amount of ice-free water available to generate surface waves. Clear trends in the annual duration of the open water season and in the extent of the seasonal sea ice minimum suggest that the sea state should be increasing, independent of changes in the wind forcing. Wave model hindcasts from four selected years spanning recent conditions are consistent with this expectation. In particular, larger waves are more common in years with less summer sea ice and/or a longer open water season, and peak wave periods are generally longer. The increase in wave energy may affect both the coastal zones and the remaining summer ice pack, as well as delay the autumn ice-edge advance. However, trends in the amount of wave energy impinging on the ice-edge are inconclusive, and the associated processes, especially in the autumn period of new ice formation, have yet to be well-described by in situ observations. There is an implicit trend and evidence for increasing wave energy along the coast of northern Alaska, and this coastal signal is corroborated by satellite altimeter estimates of wave energy.
  • Preprint
    Multiscale control of bacterial production by phytoplankton dynamics and sea ice along the western Antarctic Peninsula : a regional and decadal investigation
    ( 2012-03-07) Ducklow, Hugh W. ; Schofield, Oscar M. E. ; Vernet, Maria ; Stammerjohn, Sharon E. ; Erickson, Matthew
    We present results on phytoplankton and bacterial production and related hydrographic properties collected on nine annual summer cruises along the western Antarctic Peninsula. This region is strongly influenced by interannual variations in the duration and extent of sea ice cover, necessitating a decade-scale study. Our study area transitions from a nearshore region influenced by summer runoff from glaciers to an offshore, slope region dominated by the Antarctic Circumpolar Current. The summer bacterial assemblage is the product of seasonal warming and freshening following spring sea ice retreat and the plankton succession occurring in that evolving water mass. Bacterial production rates averaged 20 mgC m-2 d-1 and were a low (5%) fraction of the primary production (PP). There was significant variation in BP between regions and years, reflecting the variability in sea ice, Chlorophyll and PP. Leucine incorporation was significantly correlated (r2 ranging 0.2-0.7, p<0.001) with both chlorophyll and PP across depths, regions and years indicating strong phytoplankton-bacteria coupling. Relationships with temperature were variable, including positive, negative and insignificant relationships (r2 <0.2 for regressions with p<0.05). Bacterial production is regulated indirectly by variations in sea ice cover within regions and over years, setting the levels of phytoplankton biomass accumulation and PP rates; these in turn fuel BP, to which PP is coupled via direct release from phytoplankton or other less direct pathways.
  • Article
    Penguin biogeography along the West Antarctic Peninsula : testing the canyon hypothesis with Palmer LTER observations
    (The Oceanography Society, 2013-09) Schofield, Oscar M. E. ; Ducklow, Hugh W. ; Bernard, Kim S. ; Doney, Scott C. ; Patterson-Fraser, Donna ; Gorman, Kristen ; Martinson, Douglas G. ; Meredith, Michael P. ; Saba, Grace ; Stammerjohn, Sharon E. ; Steinberg, Deborah K. ; Fraser, William R.
    The West Antarctic Peninsula (WAP) is home to large breeding colonies of the ice-dependent Antarctic Adélie penguin (Pygoscelis adeliae). Although the entire inner continental shelf is highly productive, with abundant phytoplankton and krill populations, penguin colonies are distributed heterogeneously along the WAP. This ecological conundrum targets a long-standing question of interest: what environmental factors structure the locations of Adélie penguin "hot spots" throughout the WAP?
  • Article
    The disappearing cryosphere : impacts and ecosystem responses to rapid cryosphere loss
    (American Institute of Biological Sciences, 2012-04) Fountain, Andrew G. ; Campbell, John L. ; Schuur, Edward A. G. ; Stammerjohn, Sharon E. ; Williams, Mark W. ; Ducklow, Hugh W.
    The cryosphere—the portion of the Earth's surface where water is in solid form for at least one month of the year—has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change.
  • Article
    Stable Isotope clues to the formation and evolution of refrozen melt ponds on Arctic Sea ice.
    (American Geophysical Union, 2018-11-15) Tian, Lijun ; Gao, Yongli ; Ackley, Stephen ; Stammerjohn, Sharon E. ; Maksym, Ted ; Weissling, Blake
    Sea ice is one of the determining parameters of the climate system. The presence of melt ponds on the surface of Arctic sea ice plays a critical role in the mass balance of sea ice. A total of nine cores was collected from multiyear ice refrozen melt ponds and adjacent hummocks during the 2015 Arctic Sea State research cruise. The depth profiles of water isotopes, salinity, and ice texture for these sea ice cores were examined to provide information about the development of refrozen melt ponds and water balance generation processes, which are otherwise difficult to acquire. The presence of meteoric water with low oxygen isotope values as relatively thin layers indicates melt pond water stability and little mixing during formation and refreezing. The hydrochemical characteristics of refrozen melt pond and seawater depth profiles indicate little snowmelt enters the upper ocean during melt pond refreezing. Due to the seasonal characters of deuterium excess for Arctic precipitation, water balance calculations utilizing two isotopic tracers (oxygen isotope and deuterium excess) suggest that besides the melt of snow cover, the precipitation input in the melt season may also play a role in the evolution of melt ponds. The dual‐isotope mixing model developed here may become more valuable in a future scenario of increasing Arctic precipitation. The layers of meteoric origin were found at different depths in the refrozen melt pond ice cores. Surface topography information collected at several core sites was examined for possible explanations of different structures of refrozen melt ponds.
  • Article
    Antarctic sea ice—A polar opposite?
    (The Oceanography Society, 2012-09) Maksym, Ted ; Stammerjohn, Sharon E. ; Ackley, Stephen ; Massom, Robert A.
    As the world's ice diminishes in the face of climate change—from the dramatic decline in Arctic sea ice, to thinning at the margins of both the Greenland and Antarctic ice sheets, to retreating mountain glaciers the world over—Antarctic sea ice presents something of a paradox. The trend in total sea ice extent in the Antarctic has remained steady, or even increased slightly, over the past three decades, confounding climate model predictions showing moderate to strong declines. This apparent intransigence masks dramatic regional trends; declines in sea ice in the Bellingshausen Sea region that rival the high-profile decline in the Arctic have been matched by opposing increases in the Ross Sea. Much of the explanation lies in the unique nature of the Antarctic sea ice zone. Its position surrounding the continent and exposure to the high-energy wind and wave fields of the open Southern Ocean shape both its properties and its connection to the atmosphere and ocean in ways very different from the Arctic. Sea ice extent and variability are strongly driven by large-scale climate variability patterns such as the El Niño-Southern Oscillation and the Southern Annular Mode. Because many of these patterns have opposing effects in different regions around the continent, decreases in one region are often accompanied by similar, opposing increases in another. Yet, the failure of climate models to capture either the overall or regional behavior also reflects, in part, a poor understanding of sea ice processes. Considerable insight has been gained into the nature of these processes over the past several decades through field expeditions aboard icebreakers. However, much remains to be discovered about the nature of Antarctic sea ice; its connections with the ocean, atmosphere, and ecosystem; and its complex response to present and future climate change.
  • Article
    ASPIRE : the Amundsen Sea Polynya International Research Expedition
    (The Oceanography Society, 2012-09) Yager, Patricia L. ; Sherrell, Robert M. ; Stammerjohn, Sharon E. ; Alderkamp, Anne-Carlijn ; Schofield, Oscar M. E. ; Abrahamsen, E. Povl ; Arrigo, Kevin R. ; Bertilsson, Stefan ; Garay, D. Lollie ; Guerrero, Raul ; Lowry, Kate E. ; Moksnes, Per-Olav ; Ndungu, Kuria ; Post, Anton F. ; Randall-Goodwin, Evan ; Riemann, Lasse ; Severmann, Silke ; Thatje, Sven ; van Dijken, Gert L. ; Wilson, Stephanie
    In search of an explanation for some of the greenest waters ever seen in coastal Antarctica and their possible link to some of the fastest melting glaciers and declining summer sea ice, the Amundsen Sea Polynya International Research Expedition (ASPIRE) challenged the capabilities of the US Antarctic Program and RVIB Nathaniel B. Palmer during Austral summer 2010–2011. We were well rewarded by both an extraordinary research platform and a truly remarkable oceanic setting. Here we provide further insights into the key questions that motivated our sampling approach during ASPIRE and present some preliminary findings, while highlighting the value of the Palmer for accomplishing complex, multifaceted oceanographic research in such a challenging environment.
  • Article
    Seasonal forcing of summer dissolved inorganic carbon and chlorophyll a on the western shelf of the Antarctic Peninsula
    (American Geophysical Union, 2010-03-30) Montes-Hugo, Martin ; Sweeney, Colm ; Doney, Scott C. ; Ducklow, Hugh W. ; Frouin, Robert ; Martinson, Douglas G. ; Stammerjohn, Sharon E. ; Schofield, Oscar M. E.
    The Southern Ocean is a climatically sensitive region that plays an important role in the regional and global modulation of atmospheric CO2. Based on satellite-derived sea ice data, wind and cloudiness estimates from numerical models (National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis), and in situ measurements of surface (0–20 m depth) chlorophyll a (ChlSurf) and dissolved inorganic carbon (DICSurf) concentration, we show sea ice concentration from June to November and spring wind patterns between 1979 and 2006 had a significant influence on midsummer (January) primary productivity and carbonate chemistry for the Western Shelf of the Antarctic Peninsula (WAP, 64°–68°S, 63.4°–73.3°W). In general, strong (>3.5 m s−1) and persistent (>2 months) northerly winds during the previous spring were associated with relatively high (monthly mean > 2 mg m−3) ChlSurf and low (monthly mean < 2 mmol kg−1) salinity-corrected DIC (DICSurf*) during midsummer. The greater ChlSurf accumulation and DICSurf* depletion was attributed to an earlier growing season characterized by decreased spring sea ice cover or nearshore accumulation of phytoplankton in association with sea ice. The impact of these wind-driven mechanisms on ChlSurf and DICSurf* depended on the extent of sea ice area (SIA) during winter. Winter SIA affected phytoplankton blooms by changing the upper mixed layer depth (UMLD) during the subsequent spring and summer (December–January–February). Midsummer DICSurf* was not related to DICSurf* concentration during the previous summer, suggesting an annual replenishment of surface DIC during fall/winter and a relatively stable pool of deep (>200 m depth) “winter-like” DIC on the WAP.
  • Article
    Physical and biological properties of early winter Antarctic sea ice in the Ross Sea.
    (Cambridge University Press, 2020-06-24) Tison, Jean-Louis ; Maksym, Ted ; Fraser, Alexander D. ; Corkill, Matthew ; Kimura, Noriaki ; Nosaka, Yuichi ; Nomura, Daiki ; Vancoppenolle, Martin ; Ackley, Stephen ; Stammerjohn, Sharon E. ; Wauthy, Sarah ; Van der Linden, Fanny ; Carnat, Gauthier ; Sapart, Célia ; de Jong, Jeroen ; Fripiat, Francois ; Delille, Bruno
    This work presents the results of physical and biological investigations at 27 biogeochemical stations of early winter sea ice in the Ross Sea during the 2017 PIPERS cruise. Only two similar cruises occurred in the past, in 1995 and 1998. The year 2017 was a specific year, in that ice growth in the Central Ross Sea was considerably delayed, compared to previous years. These conditions resulted in lower ice thicknesses and Chl-a burdens, as compared to those observed during the previous cruises. It also resulted in a different structure of the sympagic algal community, unusually dominated by Phaeocystis rather than diatoms. Compared to autumn-winter sea ice in the Weddell Sea (AWECS cruise), the 2017 Ross Sea pack ice displayed similar thickness distribution, but much lower snow cover and therefore nearly no flooding conditions. It is shown that contrasted dynamics of autumnal-winter sea-ice growth between the Weddell Sea and the Ross Sea impacted the development of the sympagic community. Mean/median ice Chl-a concentrations were 3–5 times lower at PIPERS, and the community status there appeared to be more mature (decaying?), based on Phaeopigments/Chl-a ratios. These contrasts are discussed in the light of temporal and spatial differences between the two cruises.
  • Article
    Effect of continental shelf canyons on phytoplankton biomass and community composition along the western Antarctic Peninsula
    (Inter-Research, 2015-03-30) Kavanaugh, Maria T. ; Abdala, F. N. ; Ducklow, Hugh W. ; Glover, David M. ; Fraser, William R. ; Martinson, Douglas G. ; Stammerjohn, Sharon E. ; Schofield, Oscar M. E. ; Doney, Scott C.
    The western Antarctic Peninsula is experiencing dramatic climate change as warm, wet conditions expand poleward and interact with local physics and topography, causing differential regional effects on the marine ecosystem. At local scales, deep troughs (or canyons) bisect the continental shelf and act as conduits for warm Upper Circumpolar Deep Water, with reduced seasonal sea ice coverage, and provide a reservoir of macro- and micronutrients. Shoreward of many canyon heads are Adélie penguin breeding colonies; it is hypothesized that these locations reflect improved or more predictable access to higher biological productivity overlying the canyons. To synoptically assess the potential impacts of regional bathymetry on the marine ecosystem, 4 major canyons were identified along a latitudinal gradient west of the Antarctic Peninsula using a high-resolution bathymetric database. Biological-physical dynamics above and adjacent to canyons were compared using in situ pigments and satellite-derived sea surface temperature, sea ice and ocean color variables, including chlorophyll a (chl a) and fucoxanthin derived semi-empirically from remote sensing reflectance. Canyons exhibited higher sea surface temperature and reduced sea ice coverage relative to adjacent shelf areas. In situ and satellite-derived pigment patterns indicated increased total phytoplankton and diatom biomass over the canyons (by up to 22 and 35%, respectively), as well as increases in diatom relative abundance (fucoxanthin:chl a). While regional heterogeneity is apparent, canyons appear to support a phytoplankton community that is conducive to both grazing by krill and enhanced vertical export, although it cannot compensate for decreased biomass and diatom relative abundance during low sea ice conditions.
  • Article
    Modeling of the influence of sea ice cycle and Langmuir circulation on the upper ocean mixed layer depth and freshwater distribution at the West Antarctic Peninsula
    (American Geophysical Union, 2020-08-03) Schultz, Cristina ; Doney, Scott C. ; Zhang, Weifeng G. ; Regan, Heather ; Holland, Paul R. ; Meredith, Michael P. ; Stammerjohn, Sharon E.
    The Southern Ocean is chronically undersampled due to its remoteness, harsh environment, and sea ice cover. Ocean circulation models yield significant insight into key processes and to some extent obviate the dearth of data; however, they often underestimate surface mixed layer depth (MLD), with consequences for surface water‐column temperature, salinity, and nutrient concentration. In this study, a coupled circulation and sea ice model was implemented for the region adjacent to the West Antarctic Peninsula, a climatically sensitive region which has exhibited decadal trends towards higher ocean temperature, shorter sea ice season, and increasing glacial freshwater input, overlain by strong interannual variability. Hindcast simulations were conducted with different air‐ice drag coefficients and Langmuir circulation parameterizations to determine the impact of these factors on MLD. Including Langmuir circulation deepened the surface mixed layer, with the deepening being more pronounced in the shelf and slope regions. Optimal selection of an air‐ice drag coefficient also increased modeled MLD by similar amounts and had a larger impact in improving the reliability of the simulated MLD interannual variability. This study highlights the importance of sea ice volume and redistribution to correctly reproduce the physics of the underlying ocean, and the potential of appropriately parameterizing Langmuir circulation to help correct for biases towards shallow MLD in the Southern Ocean. The model also reproduces observed freshwater patterns in the West Antarctic Peninsula during late summer and suggests that areas of intense summertime sea ice melt can still show net annual freezing due to high sea ice formation during the winter.
  • Article
    Sea-ice production and air/ice/ocean/biogeochemistry interactions in the Ross Sea during the PIPERS 2017 autumn field campaign
    (Cambridge University Press, 2020-06-11) Ackley, Stephen ; Stammerjohn, Sharon E. ; Maksym, Ted ; Smith, Madison M. ; Cassano, John ; Guest, Peter ; Tison, Jean-Louis ; Delille, Bruno ; Loose, Brice ; Sedwick, Peter N. ; De Pace, Lisa ; Roach, Lettie ; Parno, Julie
    The Ross Sea is known for showing the greatest sea-ice increase, as observed globally, particularly from 1979 to 2015. However, corresponding changes in sea-ice thickness and production in the Ross Sea are not known, nor how these changes have impacted water masses, carbon fluxes, biogeochemical processes and availability of micronutrients. The PIPERS project sought to address these questions during an autumn ship campaign in 2017 and two spring airborne campaigns in 2016 and 2017. PIPERS used a multidisciplinary approach of manned and autonomous platforms to study the coupled air/ice/ocean/biogeochemical interactions during autumn and related those to spring conditions. Unexpectedly, the Ross Sea experienced record low sea ice in spring 2016 and autumn 2017. The delayed ice advance in 2017 contributed to (1) increased ice production and export in coastal polynyas, (2) thinner snow and ice cover in the central pack, (3) lower sea-ice Chl-a burdens and differences in sympagic communities, (4) sustained ocean heat flux delaying ice thickening and (5) a melting, anomalously southward ice edge persisting into winter. Despite these impacts, airborne observations in spring 2017 suggest that winter ice production over the continental shelf was likely not anomalous.
  • Dataset
    Numerical model simulating the sea ice and ocean conditions in the Amundsen Sea over the period Jan. 1, 2006 to Dec. 31, 2013
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact:, 2017-08-03) St-Laurent, Pierre ; Hofmann, Eileen E. ; Sherrell, Robert M. ; Stammerjohn, Sharon E. ; Yager, Patricia L. ; Biddle, Mathew ; York, Amber D.
    Numerous coastal polynyas fringe the Antarctic continent and strongly influence the productivity of Antarctic shelf systems. Of the 46 Antarctic coastal polynyas documented in a recent study, the Amundsen Sea Polynya (ASP) stands out as having the highest net primary production per unit area. Incubation experiments suggest that this productivity is partly controlled by the availability of dissolved iron (dFe). As a first step toward understanding the iron supply of the ASP, we introduce four plausible sources of dFe and simulate their steady spatial distribution using conservative numerical tracers. The modeled distributions replicate important features from observations including dFe maxima at the bottom of deep troughs and enhanced concentrations near the ice shelf fronts. A perturbation experiment with an idealized drawdown mimicking summertime biological uptake and subsequent resupply suggests that glacial meltwater and sediment-derived dFe are the main contributors to the prebloom dFe inventory in the top 100 m of the ASP. The sediment-derived dFe depends strongly on the buoyancy-driven overturning circulation associated with the melting ice shelves (the “meltwater pump”) to add dFe to the upper 300 m of the water column. The results support the view that ice shelf melting plays an important direct and indirect role in the dFe supply and delivery to polynyas such as the ASP. The data are from a numerical model simulating the sea ice and ocean conditions in the Amundsen Sea over the period Jan. 1, 2006 to Dec. 31, 2013. The data files provide the daily averaged model fields during this period. The numerical model and experiment are thoroughly described in St-Laurent et al., J. Geophys. Res. Oceans, doi:10.1002/2017jc013162.
  • Article
    Long‐term patterns in ecosystem phenology near Palmer Station, Antarctica, from the perspective of the Adélie penguin
    (Ecological Society of America, 2023-02-10) Cimino, Megan A. ; Conroy, John A. ; Connors, Elizabeth ; Bowman, Jeff ; Corso, Andrew ; Ducklow, Hugh ; Fraser, William ; Friedlaender, Ari ; Kim, Heather Hyewon ; Larsen, Gregory D. ; Moffat, Carlos ; Nichols, Ross ; Pallin, Logan ; Patterson‐Fraser, Donna ; Roberts, Darren ; Roberts, Megan ; Steinberg, Deborah K. ; Thibodeau, Patricia ; Trinh, Rebecca ; Schofield, Oscar ; Stammerjohn, Sharon
    Climate change is leading to phenological shifts across a wide range of species globally. Polar oceans are hotspots of rapid climate change where sea ice dynamics structure ecosystems and organismal life cycles are attuned to ice seasonality. To anticipate climate change impacts on populations and ecosystem services, it is critical to understand ecosystem phenology to determine species activity patterns, optimal environmental windows for processes like reproduction, and the ramifications of ecological mismatches. Since 1991, the Palmer Antarctica Long‐Term Ecological Research (LTER) program has monitored seasonal dynamics near Palmer Station. Here, we review the species that occupy this region as year‐round residents, seasonal breeders, or periodic visitors. We show that sea ice retreat and increasing photoperiod in the spring trigger a sequence of events from mid‐November to mid‐February, including Adélie penguin clutch initiation, snow melt, calm conditions (low winds and warm air/sea temperature), phytoplankton blooms, shallow mixed layer depths, particulate organic carbon flux, peak humpback whale abundances, nutrient drawdown, and bacterial accumulation. Subsequently, from May to June, snow accumulates, zooplankton indicator species appear, and sea ice advances. The standard deviation in the timing of most events ranged from ~20 to 45 days, which was striking compared with Adélie penguin clutch initiation that varied <1 week. In general, during late sea ice retreat years, events happened later (~5 to >30 days) than mean dates and the variability in timing was low (<20%) compared with early ice retreat years. Statistical models showed the timing of some events were informative predictors (but not sole drivers) of other events. From an Adélie penguin perspective, earlier sea ice retreat and shifts in the timing of suitable conditions or prey characteristics could lead to mismatches, or asynchronies, that ultimately influence chick survival via their mass at fledging. However, more work is needed to understand how phenological shifts affect chick thermoregulatory costs and the abundance, availability, and energy content of key prey species, which support chick growth and survival. While we did not detect many long‐term phenological trends, we expect that when sea ice trends become significant within our LTER time series, phenological trends and negative effects from ecological mismatches will follow.
  • Article
    Seasonal habitat preference and foraging behaviour of post-moult Weddell seals in the western Ross Sea
    (The Royal Society, 2023-01-25) Goetz, Kimberly T. ; Dinniman, Michael S. ; Hückstädt, Luis A. ; Robinson, Patrick W. ; Shero, Michelle R. ; Burns, Jennifer M. ; Hofmann, Eileen E. ; Stammerjohn, Sharon E. ; Hazen, Elliott L. ; Ainley, David G. ; Costa, Daniel P.
    Weddell seals (Leptonychotes weddellii) are important predators in the Southern Ocean and are among the best-studied pinnipeds on Earth, yet much still needs to be learned about their year-round movements and foraging behaviour. Using biologgers, we tagged 62 post-moult Weddell seals in McMurdo Sound and vicinity between 2010 and 2012. Generalized additive mixed models were used to (i) explain and predict the probability of seal presence and foraging behaviour from eight environmental variables, and (ii) examine foraging behaviour in relation to dive metrics. Foraging probability was highest in winter and lowest in summer, and foraging occurred mostly in the water column or just above the bottom; across all seasons, seals preferentially exploited the shallow banks and deeper troughs of the Ross Sea, the latter providing a pathway for Circumpolar Deep Water to flow onto the shelf. In addition, the probability of Weddell seal occurrence and foraging increased with increasing bathymetric slope and where water depth was typically less than 600 m. Although the probability of occurrence was higher closer to the shelf break, foraging was higher in areas closer to shore and over banks. This study highlights the importance of overwinter foraging for recouping body mass lost during the previous summer.