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Carol Ladd
Carol LaddPacific Marine
Environmental LabNOAA
Outline
Gulf of AlaskaCross‐shelf exchange mechanisms
Bering SeaSea IceTemperatureStratification
BeringSea
Gulf of Alaska
BasinWinter mixing replenishes macronutrients to surfaceIron limited (HNLC)
Shelf‐breakAlaska Current / Alaskan StreamCross‐shelf exchange provides nitrate to shelf and iron to basin
ShelfCoastal Current Highly productive Macronutrient limited (after spring bloom)
Cross‐shelf Exchange Mechanisms
upwelling
downwelling
Movie – sea surface height
= Formation regions (Yakutat, Sitka, Haida)
Y
S
H
Kodiak
I
Yakutat
Sitka
Advection of coastal chlorophyll into basin
Vertical processes within the eddy supplying macronutrients and/or iron to euphotic zone.
Apr 2003
May 2003
Sep 2003
Eddies and Chlorophyll
April2003
May2003
Sept2003
Coastal Windstress Curl
• Conditions favorable to barrier jets occur 1/3 of time during cool season
• avg Ekman pumping velocity of 10 m/day over cool season
SAR satellite wind data (warm colors are high wind speeds)
Bathymetric Steering
15 May – 29 May 2002
Drifter Data
SeaWiFS chlorophyll data
Downwelling winds ⇒ onshore flow at surface (inhibited by boundary currentsEpisodic upwelling winds⇒ onshore flow at depthEddies carry coastal water off‐shelf both in core of eddy and advected around edges; may also result in on‐shelf flow by reducing the strength of shelf‐break frontCoastal wind jets ⇒ variations in upwelling strength Bathymetric steering ⇒ on/off‐shelf flow and mixing in canyons
Bering Sea
⇒
inner
middle
outerSeasonalSea Ice
• Wide shelf (>500 km)
• 3 shelf domains (coastal, middle shelf, and outer shelf)
• Marginal Ice zone
• Sea ice, temperature, stratification important to ecosystem
Influences onEcosystem
Copepods more abundant in SE in years of most southerly ice extent (Baier and Napp, 2003)
SST positively related to primary productivity (Mueter, et al. 2009)
Stratification associated with changes in zooplankton community (Coyle, et al. 2008)
Weak stratification (1999): large zooplankton
strong stratification (2004): small zooplankton
Summer productivity negatively influenced by high stratification (Sambrotto, et al. 2008; Strom and Fredrickson, 2008)
Stronger stratification (2004) associated with nutrient limitation, reduced microzooplankton grazing, and weak trophic coupling (Strom and Fredrickson, 2008)
Bottom Temperatures (Cold Pool)
Stratification
Strength of maximum annual stratification
Timing of initiation and break‐down of stratification
phytoplankton
stratification
Phytoplankton response to stratificationInterannual variability
of stratification
Sea Ice:•Very extensive in early 1970s•Very little ice in early 2000s•Back to cold/ice covered conditions in last couple of years•Influences ice associated marine mammals and birds, timing of spring bloom, water column vs benthic dominated ecosystem
Temperature:•Variability is associated with sea ice distributions•Influences habitat available to some species (northward shift indistribution of some species has been observed with warming)
Stratification:•Weak stratification in late 1990s, early 2000s•Stronger stratification since 2002•Trend toward later stratification break‐down in fall may result in reduction or absence of fall bloom