Coastal fluxes of dissolved iron to NE Pacific surface waters: A driver of the marine ecosystem and carbon cycle

Throughout most of the ocean the productivity of phytoplankton is limited by the availability of one or more essential nutrients. In recent years we have come to realize that dissolved iron is one such limiting nutrient. Indeed, it is now thought that the availability of dissolved iron limits productivity in 30-40% of the ocean surface (Moore et al., 2002), including the vast majority of the northern North Pacific. Because of its impact on phytoplankton, the supply of iron to North Pacific surface waters is a driver of processes of enormous regional and global significance. Iron addition in the northern North Pacific has been shown to stimulate the growth of phytoplankton (mostly large diatoms; Boyd et al., 2004). In this manner iron supply to surface waters may lead to a transfer of carbon from surface-waters to the deep sea, and has the potential to contribute to a reduction in atmospheric CO2 (Martin, 1990; Buesseler and Boyd, 2003). Iron supply may also impact the fish yields in iron-limited regions where they feed, if bottom-up control of fisheries predominates (Ware and Thomson, 2005). All of these topics are active areas of exciting, interdisciplinary research. Iron must be delivered to surface waters in order to be used by phytoplankton. However, this delivery mechanism is poorly understood. Aeolian transport has long been thought to be most important, therefore it is the best studied mechanism. However, it is not well known how much of this aeolian iron dissolves in the ocean, nor is it known what the relative importance of local versus distant sources are. Furthermore, recent research, suggests that other sources of dissolved iron may also be important, including inputs from rivers, diagenetic remobilization from sediments (Elrod et al., 2004), sediment resuspension (Chase et al., 2002; C. Measures, personal communication, 2005), and groundwater (Charette and Sholkovitz., 2002; Windom, Moore and Jahnke, unpublished data, 2005). All of these processes vary over time and space due to climatic, geologic and other influences. Given the important influences of iron on the marine ecosystem and carbon cycle, the USGS has a tremendous opportunity to develop an interdisciplinary research program aimed at understanding and quantifying these sources of iron, and their biogeochemical impacts, in collaboration with other institutions (developed further in "strategy and approach" section.)

This work is envisioned as a USGS contribution to a long-term, multi-institution, multi-disciplinary study, and the long-term objectives outlined within this project summary reflect this. These long-term overarching goals will take many years to achieve, but they are highlighted upfront so that the motivation behind the shorter-term goals (including the FY07 statement of work) will be clear. The developing collaborations and synergies that will allow this collaboration to come to fruition will be discussed within the ¿strategy and approach¿ section. The long-term objectives of this work include: 1) identifying and quantifying various mechanisms of iron delivery to the iron-limited waters of the North Pacific, with an emphasis (by USGS) on coastal processes. 2) Examining the degree to which iron and other nutrients limit biological productivity, with an emphasis on the high-productivity coastal regions 3) Examining whether the populations of resident fish in productive, nearshore areas are limited from the ¿bottom up¿ by the availability of nutrients, including nitrate and iron. Objectives for the next year or two will focus on study of some of mechanisms of iron delivery to the Copper River region (see FY07 Statement of Work) to participate in an NSF-funded cruise led by Prof. Ken Bruland of UC Santa Cruz. The emphasis is on the Copper River region for several reasons: 1) it is the single largest source of fresh water to the Gulf of Alaska; 2) winds have been observed to transport glacier-derived dust down the river valley and hundreds of km offshore; 3) there are little or no iron data available for this region of the Gulf of Alaska. The immediate objective of this USGS-funded portion is to quantify offshore transport of iron and of the Copper River plume using short-lived Ra isotopes (Moore, 2000 a,b). The offshore iron measurements will be carried out by Bruland. Possible additional objectives to be achieved within the next two years, depending on resources, are to: 1) examine the magnitude, solubility and source of the aerosol-derived Fe flux; and 2) examine Fe concentrations in various tributaries of the Copper River (both glacierized and unglacierized); and 3) examine the Fe flux from submarine groundwater discharge (SGD), including tidally driven recirculation of seawater through sediments.

Water-column iron (and other metal) measurements during the Gulf of Alaska cruise will be carried out by Bruland. While these measurements will indicate the lateral extent of the Copper River plume they do not convey any information about rates of metal flux from the plume. These rates will be determined, for the period of the cruise, by measuring short-lived radium isotopes (onboard the ship) using the method of Moore and Arnold (1996). Radium behaves conservatively in seawater and it has two short lived isotopes, Ra-223 (11.4 d half life) and Ra-224 (3.6 d half life). The source of radium to the water column is nearshore sediments. Thus, assuming the sediments have a unique ratio of these two isotopes, this ratio between the two isotopes will change as nearshore waters are advected or mixed offshore, reflecting the aging of this coastally derived signal. By this means these radium isotopes have been used to quantify coastal mixing rates and the age of waters on the continental shelf (Moore, 2000 a, b). Whether Fe fluxes are driven by submarine groundwater discharge (SGD) will be assessed primarily by making time-series (~hourly) measurements of both radon and iron in nearshore waters over two or more tidal cycles. The flux of SGD wil be evaluated using the radon-based approach of Burnett and Dulaiova (2003). Together with the iron measurements, this will allow an estimate of the iron flux by this mechanism. It is worth noting that construction and deployment of time-series water sampling equipment that can be deployed in this high-energy nearshore environment would facilitate collection of a longer timeseries, and a better estimate of the flux. Development of such sampling equipment will be a goal for the future, possibly in collaboration with Dr. Thomas Chapin (USGS Denver). Dissolved and colloidal iron samples will be collected from Copper River tributaries by pumping with a peristaltic pump through acid-washed polyethylene tubing, and filtering through acid-washed filters (0.45 um for dissolved and 0.1 um for colloidal). Samples of glacial flour will be collected if possible in the late summer/early fall from glaciers in the vicinity of the Copper River because glacially-derived dust plumes have been observed to be transported hundreds of kilometers offshore of the Copper River (see http://www.nasa.gov/multimedia/imagegallery/image_feature_472.html). Dissolution experiments of this glacial flour and collected dust will be carried out under the supervision of Dr. Ed Sholkovitz (WHOI). It is important to point out that participation in this cruise and in similar efforts to examine the processes delivering iron to the coastal North Pacific positions the USGS for participation, with a coastal ocean emphasis, in the upcoming international program of Geotraces (www.geotraces.org). The mission of Geotraces is ¿to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions.¿ Furthermore, there are other upcoming international programs with related objectives with which there are opportunities for synergy/collaboration. These include IMBER (the Integrated Marine Biogeochemistry and Ecosystem Research; www.imber.info/) which has the broad goal ¿to understand how interactions between marine biogeochemical cycles and ecosystems respond to and force global change¿. Another program is BEST (the Bering Sea Ecosystem STudy Program http://www.arcus.org/Bering/index.html) which has the goal ¿to develop a fundamental understanding of how climate change will affect the marine ecosystem of the Eastern Bering Sea, the continued use of its resources, and the economic, social and cultural sustainability of the people who depend on it.¿ Finally, the OCCC (Ocean Carbon and Climate Change) program will focus research on the marine carbon cycle.

This project will improve our understanding of what controls biological productivity and possibly resident fish populations in this region of extremely important fisheries. Thus, the information gleaned from a long-term study of these processes can provide fisheries managers with a science-based means by which to manage this resource. Furthermore, if we can interact with the carbon cycle community (see OCCC discussion later), there is the potential to improve our understanding of the connections between iron (and nitrate) flux to surface waters and CO2 drawdown caused by nutrient-stimulated algal uptake of CO2 . From a USGS management perspective, this project has the potential to contribute to a major USGS science goal of establishing the geologic framework for ecosystem structure and function. Furthermore, if we are able to examine temporal variability in these processes we hope to achieve another science goal of anticipating the environmental impacts of climate variability.