Nearshore iron limitation in the Humboldt Current and potential impacts on toxicity and grazers

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Abstract
We tested if near-shore iron (Fe)-limitation of phytoplankton occurs near the strong upwelling center associated with Lengua de Vaca Point at 30°S in the Humboldt Current System. Fe-amended and control microcosms showed differences in variable fluorescence parameters (dark-adapted PSII maximum quantum efficiency, functional absorption cross-section of PSII, and the time constant for quinone QA oxidation) that were consistent with Fe-limitation. Evidence for Fe-limitation was strongest at a site within 4 km from shore of Lengua de Vaca Point, where warmer, low chlorophyll offshore waters came in due to an upwelling relaxation event, and weak at a site to the north of Lengua de Vaca Point with higher initial chlorophyll. Alleviation of Fe-limitation in waters increased chlorophyll and particulate organic nitrogen (PON) but did not significantly affect particulate organic carbon (POC) or opal. Fe-addition at another nearshore site caused only an increase in opal, but not changes in chlorophyll, PON, or POC. The diatom Pseudo-nitzschia sp., potential producers of the neurotoxin domoic acid, increased upon Fe addition. Increases in domoic acid correlated with alleviation of Fe-limitation indicated by variable fluorescence. Scallop larvae consumed Fe-amended phytoplankton. This study confirms Fe-limitation of primary production can occur in very nearshore waters in the Humboldt Current System where the continental shelf is exceptionally narrow. Besides increasing phytoplankton productivity, inputs of Fe might increase phytoplankton toxicity, affecting other trophic levels. Coastal and even neritic ecosystems in this region may be especially sensitive to alteration of Fe input whether due to global change or local contamination.

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