The distribution of trace metals and their relationship to net community production during two marine heatwave events in the subarctic northeast Pacific Ocean




Taves, Robyn

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The marine biological carbon pump (BCP) leads to the transfer of carbon from the atmosphere to the ocean interior through the growth of photosynthetic plankton in the euphotic zone and subsequent sinking of particulate matter to depth. In the subarctic northeast Pacific Ocean, the BCP is limited by the availability of the micro-nutrient iron (Fe) in the offshore, high nutrient, low chlorophyll (HNLC) region and by the macronutrient nitrate in the coastal region. In the last decade, two atmospheric events led to anomalously high sea surface temperatures, or marine heatwaves, in 2014-2015 and 2019 that were related to changes in the phytoplankton community composition and the BCP. Given that the bioavailability of trace metals can control microbial community composition, and important rate processes related to the BCP, we studied the spatial and temporal variation in the distribution of dissolved trace metals manganese, iron, cobalt, nickel, copper, and cadmium and macronutrients nitrate, phosphate, and silicic acid from 5 stations along the Line P Time Series transect in the subarctic northeast Pacific Ocean from winter (February) and summer (August) 2012-2019. In 2014 the mixed layer shallowed by diminished winter storm mixing, resulted in lower trace metal and macronutrient concentrations, but sustained dissolved iron and net community production (NCP) in the offshore, while the coastal region biogeochemistry remained similar to previous years (2012-2013). In 2015, further mixed layer shallowing allowed for similarly diminished trace metal (most notably iron), macronutrient concentrations, and NCP in the offshore and coastal regions. The phytoplankton community in the offshore shifted towards a higher relative abundance of pico-nano size species in both 2014 and 2015, with biomass sustained in 2014 and reduced in 2015. During the second marine heatwave event in 2019, the offshore was subject to similar winter mixed layer shallowing, warming, and reduced macronutrient concentrations. In the summer, surface trace metal and macronutrient concentrations were greatly diminished, resulting in nitrate limitation in the offshore. As the oceans continue to warm in response to anthropogenic CO2, a likely consequence may be more frequent, and severe, marine heatwaves that can have internal and external effects on micro-macronutrient distributions, phytoplankton production, and the efficiency of the biological carbon pump.



Trace metals, Marine heatwave