Lower Trophic Levels, Climate change, and the Seward Line

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Lower Trophic Levels, Climate change, and the
Seward Line

• Russ Hopcroft, Ken Coyle, Tom Weingartner, Terry
  Whitledge

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Why care?
Changes catches in a small mesh bottom trawl in
Pavlof Bay, Alaska, through the regime shift of
the mid-1970s.
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The currents and water column properties respond
to changes in the Aleutian Low Pressure System
and convey physical and biological climate
signals from lower latitudes.
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COASTAL GULF OF ALASKA STUDY AREA

• Data over 3 decades
• GLOBEC 1997-2004 Why is this system so
  productive.. Climate Salmon
• NPRB 2005 2006
• Long-term?

200km
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Annual cycle

• Surface water warms seasonally
• PWS and Alaska Coastal Current freshen over
  summer fall

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April 1, 2003 May
16, 2003
Primary production starts on the inner shelf
earlier (0.5 1 month) than the mid- and outer
shelf where it peaks in May
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The success of the zooplankton that dominate the
spring is related to their unique adaptations to
the production cycles of the Gulf
Neocalanus
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Would climate change lead to a year-round
dominance of small species (poor fish food)????
Note that while Neocalanus may dominate biomass,
Pseudocalanus dominates annual production due to
year-round growth, especially in warm Summer/Fall
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How does 2005 compare?

• Mixed layer was warmer than all other GLOBEC Mays
• (2003 was later than all other years, therefore
  warmer)

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At Gak1 (where we have 10-20 years of data) mixed
layer was warmer than long-term means
MAY
SEPT
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Across the line, the average temperature (i.e.
total amount of heat) looks less unusual
MAY
SEPT
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1
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May 2005

• Nothing unusual about chlorophyll or zooplankton
• But higher number of pteropods (pink salmon
  food?)

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• Copepods have 6 stages, more late stages suggests
  faster growth

• 2005 appears to have faster growth than many
  years (for both species)

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Fall 2005
Several other southern species also observed
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Summary

• The Gulf of Alaska was warmer in 2005 than
  typical for a non-El Niño year, and this
  appears to have favored faster development of the
  spring zooplankton
• Much of the North American West coast experienced
  such anomalous conditions (with delayed pelagic
  production south of Alaska)
• These basin-wide conditions favored the transport
  of southern species northward into the Gulf of
  Alaska
• Implications for Fisheries?