Gulf of Alaska (GoA) 2007
Funded by the National Science Foundation
Figure 1. Horizontal Transects (solid) and Vertical Stations (circles),
Gulf Of Alaska 2006
We examined the high-chlorophyll regions that develop at the boundaries of high-nitrate, iron-deplete waters of the open Gulf of Alaska (GoA) and iron-rich, nitrate-poor coastal waters. The region receiving the bulk of our attention was the productive waters of the northern and northwestern Gulf of Alaska where the HNLC waters of the subarctic eastern Alaska Gyre mix with the iron-rich waters of the Alaskan Coastal Current (ACC). Exchange between coastal waters, which were predicted to be rich in iron, and central GoA waters, rich in macro-nutrients (but deplete in iron), is evident in the form of eddies, resulting in filaments of high chlorophyll extending far into the central GoA (Figure 2).
Figure 2. Sea surface chlorophyll image of the Gulf of Alaska
showing the large eddies in the northwestern portion
that have higher biomass than the surrounding waters.
This image is a composite from a 6-day period centered
around July 30, 2004. This image was generated from the
CCAR Global Near Real-Time Ocean Color Data Viewer
using Level 3 Aqua-MODIS data.
We tested the hypothesis that the high biomass observed in satellite imagery in the northwestern GoA in mid-summer is the result of the high river runoff during this time of year into the ACC enriching this region with both dissolved and leachable particulate iron (Figure 3.), and the resultant mixing of this high iron coastal water with the HNLC waters of the adjacent GoA via mesoscale eddies (Figure 4.). Anticyclonic, mesoscale eddies are of critical importance in mixing of these water types in this region. The iron-rich coastal waters mixing with the macronutrient-rich HNLC waters of the GoA leads to the development of high productivity bands within this eddy-rich region during mid-summer months (Figure 2). The source and the role of both dissolved and leachable particulate iron concentrations resulting in the observed high phytoplankton biomass in these productive waters was examined.
Figure 3. Satellite images of the Copper River and eddies delineated
with the suspended particulate matter from the river plumes.
These are true color images from SeaWiFs.
We expected that there would be an "Fe-limitation mosaic" in the GoA/ACC region, and proposed an exploratory effort studying dissolved and particulate iron, along with macronutrients and supporting hydrographic data in this region. Alaskan coastal rivers likely introduce a variable range of macro- and micronutrients, while the mesoscale eddies in this region mix the Fe-deplete HNLC waters of the open GoA with these more productive, but often macronutrient-depleted, shelf waters. This study will provide the data and the impetus to justify including the micronutrient iron in future CoOP- or GLOBEC-type studies in this region (or perhaps justify the exclusion of iron), and provide the background data to plan future, more sophisticated, multi-disciplinary studies with physical and biological oceanographers within this productive and economically important region of the GoA.
Figure 4. Gulf of Alaska currents (After Stabeno et al. 2004)
In the course of addressing these questions, we provided a large data set on both dissolved and leachable particulate iron and other trace metals concentrations, along with macronutrient (nitrate, silicic acid and phosphate) and hydrographic data, for oceanic modelers. Existing oceanic models do not include the source of dissolved or particulate iron from coastal regions as there is little information on iron inputs from rivers and continental margins (Fung et al. 2000; Moore et al. 2002). This study, carried out in a highly productive regime of the northwestern GoA during mid-summer, provides valuable information on the role of iron in this type of system.
GoA 2007 PUBLICATIONS
Silver, M.S., S. Bargu, S.L. Coale, C.R. Benitez-Nelson, A.C. Garcia, K.J. Roberts, E.S. Wood, K.W. Bruland and K.H. Coale. Natural and iron-fertilized oceanic communities contain toxic algae. Submitted July 2009.
Rovegno, P.S., C.A. Edwards and K.W. Bruland. Observations of a Kenai Eddy and a Sitka Eddy in the Northern Gulf of Alaska. Journal of Geophysical Research: Oceans, accepted July 2009, doi:10.1029/2009JC005451 (2009).>
Fiechter, J., A.M. Moore, C.A. Edwards, K.W. Bruland, E. DiLorenzo, C.V.W. Lewis, T.M. Powell, E.N. Curchitser, and K. Hedstrom. Modeling iron limitation of primary production in the coastal Gulf of Alaska. Deep Sea Research II, doi:10.1016/j.dsr2.2009.02.010 (2009).
Hurst, M.P. and K.W. Bruland. The effects of the San Francisco Bay plume on trace metal and nutrient distributions in the Gulf of the Farallones. Geochimica et Cosmochimica Acta, 72: 395-411 (2008).
Hurst, M.P. and K.W. Bruland. An investigation into the exchange of iron and zinc between soluble, colloidal, and particulate size-fractions in shelf waters using low-abundance isotopes as tracers in shipboard incubation experiments. Marine Chemistry, 103: 211-226 (2007).