Ken Dunton

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Advection of Carbon on the Western Arctic
Shelf Implications for Benthic-Pelagic Coupling
Ken Dunton Susan Schonberg University of Texas
Marine Science Institute
With contributions from Jackie Grebmeier, Ari
Balsom, and Craig Aumack A SBI-ARCSS Project
funded by NSF
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Benthic Biomass (g m-2) Bering, Chukchi
Beaufort 1970-1996
Patterns in benthic biomass reflects the
advective processes which transport carbon to
adjacent shelves.
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Chukchi Sea Productivity
The Chukchi Sea is characterized by a very high
benthic biomass owing to its relatively shallow
depth and high water column productivity. Little
of the produced carbon is grazed by zooplankton
before reaching the seabed.
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M. Webber-USFWS
Walrus herd June 2002
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Seabed at 18m in Central Alaskan Beaufort
gouging from icebergs
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Major Circulation Patterns Superimposed on the
Distribution of Benthic Biomass
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Gradients in the Stable Carbon Isotope Ratios
d13C signatures of fauna show extremely clear
depletion in 13C from the Bering to the eastern
Beaufort Sea
Schell et al. (1998)
Dunton, unpub. data
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13C isotopic variations in Bowhead Whale Baleen
BEAUFORT SEA
Cyclical variations reflect summer feeding of 13C
depleted zooplankton in the eastern Beaufort.
CHUKCHISEA
RUSSIA
Barrow
Pt. Hope
Prudhoe Bay
NORTHWESTTERRITORIES
ALASKA
YUKON
Summering Area Wintering Area
BERINGSEA
Fraker (1984)
Schell et al. (1989)
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Some questions.

• What is the fate of allochthonous carbon from the
  north Pacific and where is it detected on the
  western Arctic shelf?
• What is the relative importance of known carbon
  sources to pelagic and benthic consumers?
• Do differences in the isotopic composition of
  primary producers and/or consumers reflect
  overall ice or water mass movements over the
  shelf and slope?
• How do trophic linkages between pelagic and
  benthic components differ on adjacent western
  arctic shelves?

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SBI Spring and Summer Cruises 2002 (two six-week
cruises, four shelf-basin transects)
Arctic Basin (gt3000 m)
Hanna
West Beaufort
Barrow Canyon
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d13C and d15N - Pelagic Organisms

• d13C values are more negative in west Hanna
  Canyon and north of shelf edge this could
  possibly reflect influence of 13C enriched ice
  algal carbon over the shelf.
• d15N data reflect clear and consistent trophic
  enrichments between producers and primary and
  secondary consumers.

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d13C values of POM are 2-5 lower (more negative)
in late summer compared to spring, especially
over the shelf break and basin.
Rolf Gradinger, June 2003, Chukchi Shelf
Based on the isotopic values of ice algae (-15,
Bluhm and Gradinger), average late spring and
summer POM (-24 and -27, respectively), ice
algal carbon potentially contributes up to 25 of
the POC pool over the Chukchi Shelf during the
spring bloom.
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d13C Values of Pelagic and Benthic Organisms

• Benthic organisms are clearly more 13C enriched
• d15C values of the bivalve Nucula are greater
  than Calanus, which has a comparable d15N
  signature

d13C signatures become increasingly depleted
eastward along the Alaskan arctic coast also
note increased separation in 13C values in E.
Beaufort
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Comparison of a Bivalve Suspension Feeder and a
Polychaete Deposit Feeder
Polychaetes and bivalves in the Chukchi Sea
occupy widely different trophic levels yet have
identical 13C ratios. Is this explained by
utilization of benthic microalgae?
Carbon Source POC d13C -26
Carbon Source POC d13C -22 and Benthic
Microalgae d13C -15
Alaska
Grebmeier and Cooper (2002)
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C and N Sources for Consumers in a Georgia Salt
Marsh
Carbon and nitrogen isotope ratios of food
sources and consumers for a salt marsh. Numbers
represent consumers. Note the corresponding
enrichment in 15N and 13C with trophic level for
consumers dependent solely on plankton.
Peterson Howarth (1987)
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Carbon and Nitrogen Isotope Ratios of Benthic
Organisms
No correlation is apparent between 13C and 15N
with increasing trophic level on the Chukchi
Shelf. This would be expected if the organisms
were assimilating more than one carbon source.
However, in the western Beaufort a correlation
begins to develop.
r2 0.04
r2 0.11
r2 0.41
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Summary

• 13C signatures of fauna in the western Beaufort
  reflect the advection of C that originated on the
  Bering/Chukchi shelf and physical transport
  processes on the western arctic shelf.
• The relatively large and consistent 13C depletion
  in POM is attributed to loss of 13C enriched
  ice algae following summer ice retreat. Ice algae
  may comprise a large fraction of the POC pool
  assimilated by zooplankton.
• A measurable amount of the carbon assimilated by
  fauna living on and within the sediments on the
  Chukchi Shelf is derived from benthic microalgae.
• The high benthic biomass and tight coupling
  between producers and consumers is unique to the
  Chukchi shelf.

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Implications for Early Retreat of Ice Cover from
Shelves
Loss of ice algal subsidy to shelf fauna,
including fraction that contributes to benthic
algal component, will compromise efficient
coupling of carbon between producers and benthic
consumers. Benthic biomass will decrease with
effects that cascade directly to the highest
trophic levels.
Cross Island Beaufort Sea, AK early July