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Temporal Variation and Cycling of Trace Elements in the Humboldt Bay Estuary

By Justin Martin & Matthew Hurst

Abstract – The distribution of metals (manganese, iron, cobalt, nickel, copper, zinc, cadmium, lead) and nutrients (nitrate, phosphate, silicate) was investigated during 2007-2008 in the waters of Humboldt Bay. Samples were collected at the Humboldt Bay entrance during the incoming tide, Indian Island at mid-tide, and Mad River Slough during the outgoing tide. The study established background levels, identified contributing end members, and resolved the temporal variation of these trace elements in Humboldt Bay. The physical speciation of the metals was determined by separating the water samples into particulate, colloidal and soluble fractions. The metal data suggest that both runoff and resuspended sediments contribute to elevated levels of trace metals (except cadmium) in the water column, with runoff during the winter months contributing to the highest dissolved metal concentrations. The contaminant metals (cobalt, nickel, copper, zinc, lead) delivered to Humboldt Bay through freshwater inputs accumulate in the sediments and are resuspended during spring wind events and subsequently removed from the estuary through tidal action. Lead is primarily associated with the particulate phase and can be used as a tracer of suspended sediments. Upwelled water entering Humboldt Bay is the primary source of cadmium with the highest concentrations found at the Bay entrance during late spring. The nutrient data show that the California coastal waters are the primary source of nutrients to Humboldt Bay. Although anthropogenic sources of metals and nutrients exist, levels of these trace elements are well below other well-studied estuaries in the continental United States.

Concentrations of metals

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