RV Kilo Moana, Bering Sea 2003
RV Kilo Moana
Bering Sea 2003

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The Bering Sea, Alaska

Trace metal micronutrients and phytoplankton dynamics - a focus on the Bering Sea and the role of iron
Funded by the National Science Foundation

We investigated the role of micronutrient trace metals (with a focus on iron) in influencing phytoplankton communities. We examined the major high-nutrient, low-chlorophyll regions of the oceanic Bering Sea gyre - a region we predicted would be iron limited. We also studied the Bering Sea Shelf - a productive region that covers almost half of the Bering Sea. This is an extremely wide continental shelf, ranging from 500 to over 800 km in width.

There has been mention of an "iron curtain" occurring over the inner shelf of the Bering Sea; however there was no data available to confirm this idea. There is a "Green Belt" of high chlorophyll and primary production that occurs throughout the summer at the shelf break which must receive adequate iron along with macro-nutrients to sustain itself.

An emphasis of this project was to examine the distribution of iron (and other micronutrient trace metals) relative to the macronutrient distributions in order to gain insight into the relative supply and demand of micro- and macronutrient elements in the various regions of the Bering Sea.

Background

figure 1 showing  primary production in the Bering Sea and the Green Belt
Figure 1. Primary production in the Bering Sea and the "Green Belt"

The Bering Sea covers almost 3 million km2 and is unusual in having an extremely wide continental shelf, ranging from 500 to over 800 km in width. Approximately half of the geographic area of the Bering Sea is shelf, with an average water depth of 50 to 75 m. The remainder is an oceanic basin 3,000 to 4,000 m deep. The Bering Sea is separated from the western and eastern subarctic Pacific by the Aleutian Island arc. The shelf region is unusual in being extremely smooth and generally featureless, with the exception of a few islands. At the shelf break there is a very steep slope separating the shelf region from the deep basin. Hydrographic structure over the eastern Bering Sea shelf region consists of three domains (National Academy Press, 1996; Springer et al. 1996). The inner front coincides with approximately the 50 m isobath, the middle front with the 80 m to 100 m isobath and the shelf break front with the 170 m isobath. Alaska coastal water is found shoreward of the 50 m isobath and is the result of coastal freshwater river discharge combined with more saline water from the deep basin. It is generally well mixed vertically by the winds and tides. Offshore of the inner front, the waters tend to become thermally stratified due to seasonal heating and a two-layer system develops.

Figure 1 presents a generalized pattern of primary production in the Bering Sea (Springer et al. 1996). The open Bering Sea is an HNLC regime, presumably limited by lack of Fe. The inner shelf regime in the summer time is lower in chlorophyll and biomass because of lack of macronutrients that were depleted during the early spring bloom in the area (Sambretto et al. 1986). At the shelf break, there is a "Green Belt" of high chlorophyll and primary production that lasts through the summer. Coachman (1986) has argued that the interaction of strong tidal currents with the abrupt, steep shelf break promotes upwelling at the front and that this supplies nutrients to the euphotic zone. As a result, primary production apparently remains elevated throughout summer, long after the termination of the spring bloom over the inner shelf. Hansell et al. (1993) has estimated up to 110 g C m-2 y-1 of new production occurring near the shelf break. This sustained Green Belt of productivity helps fuel the various important fisheries in this region. Springer et al. (1996) hypothesize that the shelf edge Green Belt is a locus for carbon and energy transfer to higher trophic levels. The prolonged blooming period allows better phasing between periods of primary productivity and biomass production of key mesozooplankton species, primarily the large, oceanic copepods that are critical prey of numerous species at higher trophic levels. They hypothesize that this shelf-edge process is necessary to sustain the abundant stocks of marine fishes, birds and mammals in the Bering Sea.

figure 2, contour plot of dissolved manganese in the Bering Sea during Aug./Sept. 2003
Figure 2. Contour plot of dissolved manganese during Aug./Sept.2003

In this research project we are investigating the role of iron in allowing this green belt of diatom productivity to be maintained throughout the summer in this region. Our prediction is that the adjacent oceanic Bering Sea will be a typical Fe-limited HNLC regime. We predict that the inner shelf area in July will be mainly a nitrate-limited, Fe-replete regime. There has been mention of an "iron curtain" that occurs over the shelf of the Bering Sea that plays an important role in primary production in this region, however there are no iron data available in this region to confirm this idea. The mid-shelf regime may be co-limited by both macro- and micronutrients, particularly for diatoms. The Green Belt occurring at the shelf break must receive adequate Fe along with the macronutrients to sustain the high primary production and high chlorophyll concentrations. An emphasis of this project will be to examine the distribution of Fe (and other micronutrient trace metals) relative to the macronutrient distributions in order to gain insight into the relative supply and demand of these nutrient elements in the Bering Sea.

The distribution of dissolved manganese is of interest both in its own right and for its possible use as a tracer of the sources of the micronutrient iron. Dissolved manganese was measured on several surface transects in the southeastern Bering Sea during Aug/Sept., 2003 (Figure 2.). The measured dissolved Mn concentration ([Mn]) exhibits a strong east to west gradient and reflects the hydrographic regimes of the Bering Sea. An island enrichment effect was observed in samples taken near the Pribilof Islands, but not observed in samples taken off the Aleutians. Near the Pribilof Islands the surface mixed layer deepened to 65 m and entrained near-bottom mid-shelf waters with elevated [Mn], while off the Aleutians it appeared that high nutrient, low manganese subsurface waters were freshly upwelled leading to surface waters highly enriched in nutrients, but relatively low in [Mn]. Manganese, in combination with temperature, salinity, and nutrients, can be used as a hydrographic tracer, and a tracer for the origin of surface waters providing insight into the sources of dissolved Fe (Aguilar-Islas and Bruland, 2004).

BERING SEA 2003 PUBLICATIONS

Hurst, M.P., A.M. Aguilar-Islas, and K.W. Bruland. Iron in the southeastern Bering Sea: Elevated leachable particulate Fe in shelf bottom waters as an important source for surface waters. Submitted to Continental Shelf Research, June 2009.

Buck, K.N. and K.W. Bruland. The physico-chemical speciation of dissolved iron in the Bering Sea, Alaska. Limnology and Oceanography, 52(5): 1800-1808 (2007).

Aguilar-Islas, A.M., M.P. Hurst, K.N. Buck, B. Sohst, G.J. Smith, M.C. Lohan and K.W. Bruland. Micro- and macronutrients in the southeastern Bering Sea: Insight into iron-replete and iron-deplete regimes. Progress in Oceanography, 73(2): 99-126 (2007).

Leblanc, K., C.E. Hare, P.W. Boyd, K.W. Bruland, B. Sohst, S. Pickmere, M.C. Lohan, K.N. Buck, M. Ellwood and D.A. Hutchins. Fe and Zn effects on the Si cycle and diatom community structure in two contrasting high and low-silicate HNLC areas. Deep-Sea Research I, 52: 1842-1864 (2005).

References

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