Sources, cycling and biological impacts of trace metals in the Subarctic Pacific Ocean

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Every year, marine phytoplankton (unicellular algae) convert ~45 billion kilograms of carbon dioxide (CO2) to organic carbon in the surface ocean, accounting for half of the total photosynthesis on Earth. This organic carbon supports the vast majority of marine ecosystems (including commercially important fisheries, such as herring, anchovy and salmon), and plays a key role in the oceanic sequestration of atmospheric CO2. Variations in phytoplankton productivity thus have a strong influence on fisheries and climate, and significant research has been directed towards understanding the environmental controls on this productivity.***Marine primary productivity is controlled by a combination of light and nutrients, and grazing by zooplankton. Traditionally macronutrients, such as nitrogen, were thought to determine phytoplankton growth. More recently, the trace element iron has been shown to limit phytoplankton productivity in ~30% of the ocean. The Subarctic NE Pacific is one of three major iron-limited areas in the world's oceans. Iron concentrations in this region decrease dramatically from the coast to the open ocean, and atmospheric inputs are a dominant iron source in offshore waters. The importance of these inputs was demonstrated by a volcanic ash deposition event in 2008, which triggered a massive phytoplankton bloom in the Gulf of Alaska. While atmospheric aerosols rich in dust may potentially stimulate phytoplankton growth via natural iron enrichment, aerosols may also be derived from anthropogenic sources, which are often rich in metal contaminants, such as lead and copper. Aerosols inputs to surface waters of the Subarctic Pacific can thus exert important beneficial or detrimental effects on phytoplankton productivity and community composition, with strong food web implications.***Metal pollutants can inhibit phytoplankton growth, and may also be transferred to higher trophic levels, including fish consumed by humans. Highly urbanized and industrialized areas of East Asia are increasing as a source of harmful pollutants that may be entrained with mineral dust from deserts during long-range aerosol transport across the Pacific. These anthropogenic aerosols are highly soluble in seawater, increasing their potential toxicity to organisms in the NE Pacific. The proposed research will elucidate the composition, flux and biological effects of trace elements, both essential and potentially toxic, in Asian-derived aerosols that are deposited in the Subarctic NE Pacific. Using complementary laboratory and field studies, the proposed research will also determine whether trace metals are bio-accumulating in the food web to levels that pose health risks to humans. Anticipated results will improve our understanding of trace metal cycling in the ocean, and the role of trace elements in controlling phytoplankton productivity, fisheries production and oceanic CO2 sequestration. **