Regional warming around West Antarctica, including the Antarctic Peninsula, is related to the retreat of glaciers that has resulted in significant ice mass loss in recent decades. We examined freedrifting icebergs in the Atlantic sector of the Southern Ocean in December 2005, aboard ARSV Laurence M. Gould, and in June 2008 and March/April 2009, aboard RVIB Nathaniel B. Palmer. Prior to these studies, little information was available about the effects of icebergs on the pelagic realm.Facultad de Ciencias Naturales y Muse

Cefarelli, Adrián Oscar

Helly, John J.

Kaufmann, Ronald S.

Lin, Hai

Long, David G.

Murray, Alison E.

Robison, Bruce H.

Ruhl, Henry A.

Shaw, Timothy J.

Sherman, Alana D.

Smith Jr., Kenneth L.

Sprintall, Janet

Stephenson Jr., Gordon R.

Stuart, Keith M.

Twining, Benjamin S.

Vernet, Maria

Oceanography

Servicio de Difusión de la Creación Intelectual

Oceanography |  Vol.  25, No. 338A N tA r c t i c  O c e A N O g r A p h y  i N  A  c h A N g i N g  W O r l d  >>  S i d e B A rregional warming around West Antarctica, including the Antarctic peninsula, is related to the retreat of glaciers that has resulted in significant ice mass loss in recent decades (de Angelis and Skvarca, 2003). large icebergs (> 18.5 km long) originating from ice shelves in the ross and Weddell Seas (Scambos et al., 2000) are attributed primarily to major loss events in these regions. Once free, icebergs become entrained in the counterclockwise Antarctic coastal current (Figure 1), eventu-ally entering a strong northward flow in the Northwest Weddell Sea. We examined free-drifting icebergs in the Atlantic sector of the Southern Ocean in december 2005, aboard ArSV Laurence M. Gould, and in June 2008 and March/April 2009, aboard rViB Nathaniel B. Palmer. prior to these studies, little information was available about the effects of icebergs on the pelagic realm. On these cruises, we investigated the “iceberg ecosystem” (Smith et al., 2007; Smith, 2011) to assess the degree to which icebergs are (1) hotspots of biological activity across multiple trophic levels, and (2) focal points for enhanced export of organic carbon to the deep sea. An important focus of this work was to examine the fundamental mechanisms by which icebergs affect the pelagic ecosystem, including physical disrup-tion and effects on the availability of critical nutrients (e.g., iron, nitrate). ecOSySteM Struc tureWe observed elevated densities of seabirds, seals, and whales in the open water regions near icebergs (Figure 2). These organisms may be feeding on Antarctic krill (Euphausia superba) and salps (Salpa thompsoni) that we collected at highest densities within ~ 2 km of an iceberg. turbulence created by iceberg mixing and meltwater input maintains an active diatom-enriched phytoplankton community with high photosynthetic efficiency. intense zooplankton grazing and meltwater dilution seem to explain relatively low phytoplankton densities measured close to icebergs and a “ring” of elevated phytoplankton ~ 2 km away. Other biotic effects of icebergs are apparent: diatom mats (Thalassiosira signyensis) attach to minerals encrusted in the ice on the iceberg flanks, providing food for krill. An iceberg-related bacterioplankton community occurs at 75–250 m depth.icebergs affect their physical and chemical surroundings in significant ways. Meltwater and turbulent advection alter water column structure, as reflected in the distribution of salinity, temperature, and carbon dioxide. At the surface, meltwater is detected as far away as 19 km and persists for at least 10 days. At depth, iceberg sidewall and basal melting con-tribute meltwater to the water column through diffusive and turbulent upwelling processes. upwelling of basal meltwater mixtures appears to be localized and intermittent; however, thermohaline “staircases” consistent with sidewall melting are evident up to 19 km from iceberg c18-A in data from 2009. horizontal spreading of meltwater provides a means by which the seasonal mixed layer can be enriched in nutrients from ice melt. concentrations of dissolved iron in iceberg ice are elevated 5- to 600-fold above seawater, a result of accumu-lated terrestrial material in the ice. highest levels of dissolved iron concentrations are associated with low-salinity waters, regardless of distance from the iceberg. Slight surface depletion of dissolved iron occurs within 1 km of icebergs, while surface enrichment is evident at most stations more than 10 km away. This observation suggests general iron enrichment in the lower-salinity surface mixed layer and enhanced iron removal, either via scavenging or biological uptake, at the iceberg face.ecOSySteM FuNc tiON The iceberg ecosystem is enriched at higher trophic levels with respect to lower levels, suggesting top-down control of phytoplankton by zooplankton. concentrations of photo-synthetic pigments in the guts of E. superba and S. thompsoni track patterns in surface chlorophyll a and are comparable to maximum gut pigment concentrations from highly pro-ductive marginal ice zones. iceberg systems also islands of ice: influence of Free-drifting Antarctic icebergs on pelagic Marine ecosystemsB y  M A r i A  V e r N e t ,  K e N N e t h  l .  S M i t h  J r . ,  A d r i á N  O .  c e F A r e l l i ,  J O h N  J .  h e l ly , r O N A l d  S .  K A u F M A N N ,  h A i  l i N ,  d AV i d  g .  l O N g ,  A l i S O N  e .  M u r r Ay,  B r u c e  h .  r O B i S O N , h e N r y  A .  r u h l ,  t i M O t h y  J .  S h A W ,  A l A N A  d .  S h e r M A N ,  J A N e t  S p r i N tA l l , g O r d O N  r .  S t e p h e N S O N  J r . ,  K e i t h  M .  S t u A r t,  A N d  B e N J A M i N  S .  t W i N i N gFigure 1. tracks of icebergs around Antarctica detected by SeaWinds satellite (1999–2009) show-ing concentration through iceberg Alley in the Northwest Weddell Sea (Stuart and long, 2011). Oceanography  |  September 2012 39show efficient trophic transfer to heterotrophic microbial production, with tight coupling of bacteria-phytoplankton production. higher iron-binding ligand concentrations are associ-ated with active summer plankton, indicating an important role for biologically produced ligands in enhancing the solubility and reten-tion of dissolved iron in the waters around icebergs. Significantly, export fluxes of total mass and carbon are two to three times higher near icebergs than in surrounding iceberg-free waters. These enhanced processes, combined with the altered biological, physical, and chemi-cal properties, create and define the iceberg ecosystem (Figure 2). AcKNOWledgeMeNtSThanks to the captains and crews of ArSV Laurence M. Gould and rViB Nathaniel B. Palmer, to raytheon polar Services for technical and logistics support, and to d. chakos and t. gunther for figure design. This work was funded by the National Science Foundation Office of polar programs grants Ant-0636730, Ant-0636319, Ant-0636809, Ant-0636723, and Ant-0636543. AuthOrSMaria Vernet (mvernet@ucsd.edu) is Research Biologist, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA. Kenneth L. Smith Jr. is Senior Scientist, Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA. Adrián O. Cefarelli is PhD Candidate, Universidad de La Plata, La Plata, Argentina. John J. Helly is Director, Laboratory for Earth and Environmental Sciences, San Diego Supercomputer Center, University of California, San Diego, La Jolla, CA, USA. Ronald S. Kaufmann is Professor, University of San Diego, San Diego, CA, USA. Hai Lin earned his PhD at University of South Carolina, Columbia, SC, USA. David G. Long is Professor, Brigham Young University, Provo, UT, USA. Alison E. Murray is Associate Research Professor, Desert Research Institute, Reno, NV, USA. Bruce H. Robison is Senior Scientist, Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA. Henry A. Ruhl is on the Ocean Biochemistry and Ecosystems staff at the National Oceanography Centre, Southampton, UK. Timothy J. Shaw is Professor, University of South Carolina, Columbia, SC, USA. Alana D. Sherman is Electrical Engineer, Monterey Bay Aquarium Research Institute, Moss Landing, CA, USA. Janet Sprintall is Research Oceanographer, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA. Gordon R. Stephenson Jr. is PhD Candidate, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA. Keith M. Stuart is PhD Candidate, Brigham Young University, Provo, UT, USA. Figure 2. A free-drifting iceberg acts as a float-ing island of ice, surrounded by meltwater, turbulent mixing, terrigenous input, and dis-solved iron. Abundant krill and salps sustain localized populations of seabirds and marine mammals; all contribute to enhanced partic-ulate carbon sedimentation to the seafloor. Benjamin S. Twining is Senior Research Scientist, Bigelow Laboratory for Ocean Sciences, Booth Bay Harbor, ME, USA.reFereNceS de Angelis, h.d., and p. Skvarca. 2003. glacier surge after ice shelf collapse. Science 299:1,560–1,562, http://dx.doi.org/10.1126/science.1077987.Scambos, t.A., c. hulbe, M. Fahnestock, and J. Bohlander. 2000. The link between climate warming and break-up of ice shelves in the Antarctic peninsula. Journal of Glaciolology 46:516–530, http://dx.doi.org/10.3189/172756500781833043.Smith, K.l. Jr., B.h. robison, J.J. helly, r.S. Kaufmann, h.A. ruhl, t.J. Shaw, B.S. twining, and M. Vernet. 2007. Free-drifting icebergs: hot spots of chemi-cal and biological enrichment in the Weddell Sea. Science 317:478 –482, http://dx.doi.org/10.1126/science.1142834.Smith, K.l. Jr. 2011. Free-drifting icebergs in the Southern Ocean: An overview. Deep Sea Research Part II 58:1,277–1,284, http://dx.doi.org/10.1016/ j.dsr2.2010.11.003.Stuart, K.M., and d.g. long. 2011. tracking large tabular icebergs using the SeaWinds Ku-band microwave scatterometer. Deep-Sea Research Part II 58:1,285–1,300, http://dx.doi.org/10.1016/ j.dsr2.2010.11.004.

Islands of ice: Influence of free-drifting Antarctic icebergs on pelagic marine ecosystems

SEDICI - Repositorio de la UNLP

Oceanography |  Vol.  25, No. 338
A N tA r c t i c  O c e A N O g r A p h y  i N  A  c h A N g i N g  W O r l d  >>  S i d e B A r
regional warming around West Antarctica, 
including the Antarctic peninsula, is related 
to the retreat of glaciers that has resulted in 
significant ice mass loss in recent decades 
(de Angelis and Skvarca, 2003). large icebergs 
(> 18.5 km long) originating from ice shelves 
in the ross and Weddell Seas (Scambos et al., 
2000) are attributed primarily to major loss 
events in these regions. Once free, icebergs 
become entrained in the counterclockwise 
Antarctic coastal current (Figure 1), eventu-
ally entering a strong northward flow in the 
Northwest Weddell Sea. We examined free-
drifting icebergs in the Atlantic sector of the 
Southern Ocean in december 2005, aboard 
ArSV Laurence M. Gould, and in June 2008 and 
March/April 2009, aboard rViB Nathaniel B. 
Palmer. prior to these studies, little information 
was available about the effects of icebergs 
on the pelagic realm. On these cruises, we 
investigated the “iceberg ecosystem” (Smith 
et al., 2007; Smith, 2011) to assess the degree 
to which icebergs are (1) hotspots of biological 
activity across multiple trophic levels, and 
(2) focal points for enhanced export of organic 
carbon to the deep sea. An important focus 
of this work was to examine the fundamental 
mechanisms by which icebergs affect the 
pelagic ecosystem, including physical disrup-
tion and effects on the availability of critical 
nutrients (e.g., iron, nitrate). 
ecOSySteM Struc ture
We observed elevated densities of seabirds, 
seals, and whales in the open water regions 
near icebergs (Figure 2). These organisms 
may be feeding on Antarctic krill (Euphausia 
superba) and salps (Salpa thompsoni) that we 
collected at highest densities within ~ 2 km 
of an iceberg. turbulence created by iceberg 
mixing and meltwater input maintains an active 
diatom-enriched phytoplankton community 
with high photosynthetic efficiency. intense 
zooplankton grazing and meltwater dilution 
seem to explain relatively low phytoplankton 
densities measured close to icebergs and a 
“ring” of elevated phytoplankton ~ 2 km away. 
Other biotic effects of icebergs are apparent: 
diatom mats (Thalassiosira signyensis) attach 
to minerals encrusted in the ice on the iceberg 
flanks, providing food for krill. An iceberg-
related bacterioplankton community occurs at 
75–250 m depth.
icebergs affect their physical and chemical 
surroundings in significant ways. Meltwater 
and turbulent advection alter water column 
structure, as reflected in the distribution of 
salinity, temperature, and carbon dioxide. At 
the surface, meltwater is detected as far away 
as 19 km and persists for at least 10 days. At 
depth, iceberg sidewall and basal melting con-
tribute meltwater to the water column through 
diffusive and turbulent upwelling processes. 
upwelling of basal meltwater mixtures appears 
to be localized and intermittent; however, 
thermohaline “staircases” consistent with 
sidewall melting are evident up to 19 km from 
iceberg c18-A in data from 2009. horizontal 
spreading of meltwater provides a means by 
which the seasonal mixed layer can be enriched 
in nutrients from ice melt. concentrations of 
dissolved iron in iceberg ice are elevated 5- to 
600-fold above seawater, a result of accumu-
lated terrestrial material in the ice. highest 
levels of dissolved iron concentrations are 
associated with low-salinity waters, regardless 
of distance from the iceberg. Slight surface 
depletion of dissolved iron occurs within 1 km 
of icebergs, while surface enrichment is evident 
at most stations more than 10 km away. This 
observation suggests general iron enrichment 
in the lower-salinity surface mixed layer and 
enhanced iron removal, either via scavenging or 
biological uptake, at the iceberg face.
ecOSySteM FuNc tiON 
The iceberg ecosystem is enriched at higher 
trophic levels with respect to lower levels, 
suggesting top-down control of phytoplankton 
by zooplankton. concentrations of photo-
synthetic pigments in the guts of E. superba 
and S. thompsoni track patterns in surface 
chlorophyll a and are comparable to maximum 
gut pigment concentrations from highly pro-
ductive marginal ice zones. iceberg systems also 
islands of ice: influence of Free-drifting Antarctic 
icebergs on pelagic Marine ecosystems
B y  M A r i A  V e r N e t ,  K e N N e t h  l .  S M i t h  J r . ,  A d r i á N  O .  c e F A r e l l i ,  J O h N  J .  h e l ly , 
r O N A l d  S .  K A u F M A N N ,  h A i  l i N ,  d AV i d  g .  l O N g ,  A l i S O N  e .  M u r r Ay,  B r u c e  h .  r O B i S O N , 
h e N r y  A .  r u h l ,  t i M O t h y  J .  S h A W ,  A l A N A  d .  S h e r M A N ,  J A N e t  S p r i N tA l l , 
g O r d O N  r .  S t e p h e N S O N  J r . ,  K e i t h  M .  S t u A r t,  A N d  B e N J A M i N  S .  t W i N i N g
Figure 1. tracks of icebergs around Antarctica 
detected by SeaWinds satellite (1999–2009) show-
ing concentration through iceberg Alley in the 
Northwest Weddell Sea (Stuart and long, 2011). 
Oceanography  |  September 2012 39
show efficient trophic transfer to heterotrophic 
microbial production, with tight coupling of 
bacteria-phytoplankton production. higher 
iron-binding ligand concentrations are associ-
ated with active summer plankton, indicating 
an important role for biologically produced 
ligands in enhancing the solubility and reten-
tion of dissolved iron in the waters around 
icebergs. Significantly, export fluxes of total 
mass and carbon are two to three times higher 
near icebergs than in surrounding iceberg-free 
waters. These enhanced processes, combined 
with the altered biological, physical, and chemi-
cal properties, create and define the iceberg 
ecosystem (Figure 2). 
AcKNOWledgeMeNtS
Thanks to the captains and crews of ArSV 
Laurence M. Gould and rViB Nathaniel B. 
Palmer, to raytheon polar Services for technical 
and logistics support, and to d. chakos and 
t. gunther for figure design. This work was 
funded by the National Science Foundation 
Office of polar programs grants Ant-0636730, 
Ant-0636319, Ant-0636809, Ant-0636723, 
and Ant-0636543. 
AuthOrS
Maria Vernet (mvernet@ucsd.edu) is Research 
Biologist, Scripps Institution of Oceanography, 
University of California, San Diego, La Jolla, CA, 
USA. Kenneth L. Smith Jr. is Senior Scientist, 
Monterey Bay Aquarium Research Institute, 
Moss Landing, CA, USA. Adrián O. Cefarelli 
is PhD Candidate, Universidad de La Plata, 
La Plata, Argentina. John J. Helly is Director, 
Laboratory for Earth and Environmental 
Sciences, San Diego Supercomputer Center, 
University of California, San Diego, La Jolla, 
CA, USA. Ronald S. Kaufmann is Professor, 
University of San Diego, San Diego, CA, USA. 
Hai Lin earned his PhD at University of South 
Carolina, Columbia, SC, USA. David G. Long is 
Professor, Brigham Young University, Provo, UT, 
USA. Alison E. Murray is Associate Research 
Professor, Desert Research Institute, Reno, NV, 
USA. Bruce H. Robison is Senior Scientist, 
Monterey Bay Aquarium Research Institute, 
Moss Landing, CA, USA. Henry A. Ruhl is 
on the Ocean Biochemistry and Ecosystems 
staff at the National Oceanography Centre, 
Southampton, UK. Timothy J. Shaw is 
Professor, University of South Carolina, 
Columbia, SC, USA. Alana D. Sherman is 
Electrical Engineer, Monterey Bay Aquarium 
Research Institute, Moss Landing, CA, USA. 
Janet Sprintall is Research Oceanographer, 
Scripps Institution of Oceanography, University 
of California, San Diego, La Jolla, CA, USA. 
Gordon R. Stephenson Jr. is PhD Candidate, 
Scripps Institution of Oceanography, 
University of California, San Diego, La Jolla, 
CA, USA. Keith M. Stuart is PhD Candidate, 
Brigham Young University, Provo, UT, USA. 
Figure 2. A free-drifting iceberg acts as a float-
ing island of ice, surrounded by meltwater, 
turbulent mixing, terrigenous input, and dis-
solved iron. Abundant krill and salps sustain 
localized populations of seabirds and marine 
mammals; all contribute to enhanced partic-
ulate carbon sedimentation to the seafloor. 
Benjamin S. Twining is Senior Research 
Scientist, Bigelow Laboratory for Ocean 
Sciences, Booth Bay Harbor, ME, USA.
reFereNceS 
de Angelis, h.d., and p. Skvarca. 2003. glacier surge after 
ice shelf collapse. Science 299:1,560–1,562, http://
dx.doi.org/10.1126/science.1077987.
Scambos, t.A., c. hulbe, M. Fahnestock, and J. Bohlander. 
2000. The link between climate warming and 
break-up of ice shelves in the Antarctic peninsula. 
Journal of Glaciolology 46:516–530, http://
dx.doi.org/10.3189/172756500781833043.
Smith, K.l. Jr., B.h. robison, J.J. helly, r.S. Kaufmann, 
h.A. ruhl, t.J. Shaw, B.S. twining, and M. Vernet. 
2007. Free-drifting icebergs: hot spots of chemi-
cal and biological enrichment in the Weddell Sea. 
Science 317:478 –482, http://dx.doi.org/10.1126/
science.1142834.
Smith, K.l. Jr. 2011. Free-drifting icebergs in the 
Southern Ocean: An overview. Deep Sea Research 
Part II 58:1,277–1,284, http://dx.doi.org/10.1016/ 
j.dsr2.2010.11.003.
Stuart, K.M., and d.g. long. 2011. tracking large 
tabular icebergs using the SeaWinds Ku-band 
microwave scatterometer. Deep-Sea Research 
Part II 58:1,285–1,300, http://dx.doi.org/10.1016/ 
j.dsr2.2010.11.004.


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Islands of ice: Influence of free-drifting Antarctic icebergs on pelagic marine ecosystems

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