Patterns of diatom diversity correlate with dissolved trace metal concentrations and longitudinal position in the northeast Pacific coastal−offshore transition zone

Abstract

Diatoms are important primary producers in the northeast Pacific Ocean, with their productivity closely linked to pulses of trace elements in the western high nitrate, low chlorophyll (HNLC) region of the oceanographic time series transect ‘Line P.’ Recently, the coastal−HNLC transition zone of the Line P transect was identified as a hotspot of phytoplankton productivity, potentially controlled by a combination of trace element and macronutrient concentrations. Here we describe diatom community composition in the eastern Line P transect, including the coastal− HNLC transition zone, with a method using high-throughput sequencing of diatom 18S gene amplicons. We identified significant correlations between shifting diatom community composition and longitude combined with concentrations of dissolved copper and 2 other dissolved trace metals (dissolved Fe [dFe] and/or dissolved zinc) and/or a physical factor (salinity or density). None of these variables on its own was significantly correlated with shifts in community composition, and 3 of the factors (dFe, salinity, and density) correlated with one another. Longitude could incorporate multiple factors that may influence diatom communities, including distance from shore, proximity of sampling stations, and an integration of previous pulses of macro- and micro-nutrients. We also evaluated in situ Fe limitation of the diatom Thalassiosira oceanica using a quantitative reverse-transcription polymerase chain reaction method, and found biological evidence of Fe stress in samples from the coastal−HNLC transition zone. Combined, our results support a prior hypothesis that dissolved trace metals as well as longitudinal distance may be important to diatom diversity in the coastal−HNLC transition zone of the Line P transect.