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To follow parcels of nutrient rich waters (HNLC) from the open Gulf of Alaska
to coastal waters, at surface and at depth, the model must represent both ecosystems.
Iron limitation employs a Michaelis-Menton function (as in Fennel et al.
2003) as a multiplicative factor affecting growth of both small and large
phytoplankton. It affects large phytoplankton more strongly.
- Iron field initialized with onshore/offshore values based on data
- Iron depleted, but not followed through ecosystem
- Nudged back to initial profile with 30 day timescale
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Section of iron along the Seward Line.
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Onshore, the main nitrogen pathway is from nitrate through large
phytoplankton to copepods.
Offshore without iron limitation, the main nitrogen pathway is also
through large phytoplankton to copepods, which is not what we see
in the open ocean.
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If we add iron limitation, which mostly affects the oceanic areas due to
lower levels of iron in those areas, the largest part of the fluxes now goes
through small phytoplankton to small microzooplankton.
We are better able, therefore, to reproduce the main characteristics of the
HNLC areas.
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Flowcharts of model with and without iron limitation
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- With no iron limitation term, during late May/ early June
(after spring blooms), nitrate is depleted in the surface in all areas
(left). Offshore areas should retain high surface nutrients at all times.
- With the iron limitation scheme implemented (right), the oceanic areas no
longer show iron depletion in the surface waters because lower iron
concentrations there inhibit the bloom of phytoplankton.
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Surface nitrate in NEP grid without iron limitation in late May
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Surface nitrate in NEP grid with iron limitation in late May
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In the movie comparing measured chlorophyll from SEAWIFS and model output:
- March: very little activity.
- April: some activity beginning on the shelf, particularly in the Seward
area and Southeast Alaska.
- May: somewhat more activity in the data around Kodiak Island, whereas
the model shows more activity in the southeast.
- June: the model shows nutrient depletion beginning in the nearshore areas.
Both show activity around Kodiak Island and Prince William Sound.
- July: more depletion on the coast in the model. Interesting
shelf-break front of high production.
Lots of mesoscale activity (eddies) in May and June. These are often
seen in snapshots of chlorophyll
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Animation of chlorophyll from SEAWIFS and model.
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a)
b)
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Computed the average over the top 100 m of model output in three areas:
the inner shelf (represented by GAK stations 1 and 2), the middle shelf
(GAK stations 6 and 7) and the outer shelf (stations 11, 12, and 13).
- Nitrate (upper left): the model does a pretty good job
- Chlorophyll (lower left): magnitudes OK but miss timing of bloom
- Copepods (upper right): magnitudes reasonable but miss timing of bloom
- Neocalanus (middle right): underestimated by model
- Euphausiids (lower right): look good
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c)
d)
e)
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Depth averaged model output (black line) and data (red stars) for 3 regions along the Seward Line,
a) nutrients, b) chlorophyll.
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Depth averaged model output and data for 3 regions along the Seward Line,
c) Copepods, d) Neocalanus, and e) Euphausiids.
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To the left is a picture of the 2 mg/m3 chlorophyll isosurface in
quasi-3D from the region just to the northeast and east of Kodiak Island on
DOY 209 (July 28th). Bathymetry is gray, and the chlorophyll isosurface is
green. The solid black line is the location of the transect in the right
figure.
Note how Portlock and Albatross Banks have chlorophyll "holes".
To the right, is a side view of the same chlorophyll isosurface
with a transect of nitrate shaded from orange (high nitrate) deep in the water
column to blue (depleted) at the surface. Topography is white, shaded with
gray. Note:
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Chlorophyll isosurface.
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- The column of water over Portlock Bank in the center, marked by an
intermediate value of nitrate, probably due to enhanced mixing over the
bank
- A possible, localized front around the bank marked with slanted arrows,
where perhaps nitrate is mixing up from the deeper depths
- The chlorophyll ³hole² in the surface waters, perhaps because
- Vigorous mixing over the bank itself mixes the chlorophyll too deep
- Higher chlorophyll around edges responding to enhanced nutrient
levels from the deeper waters at the front
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Nutrient cross-section and Chlorophyll isosurface over Portlock Bank.
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This is a picture of the nitrogen flux at 50 m - the velocity times the
nitrogen concentration at each location.
- "Rivers" of nutrients
- The cross-shelf transport produced by a 200 km scale eddy southeast of
Kayak Island pulls deep water onto the shelf.
- We see this at other times of year as well.
- Some evidence of influx at Amatuli Canyon.
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Nutrient flux.
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