Evaluation of the seasonal formation of subsurface negative preformed nitrate anomalies in the subtropical North Pacific and North Atlantic

Academic Article

Abstract

  • <p><strong>Abstract.</strong> Summertime mixed-layer drawdown of dissolved inorganic carbon in the absence of measurable nutrients in the ocean's subtropical gyres and non-Redfieldian oxygen<span class="thinspace"></span>:<span class="thinspace"></span>nitrate relationships in the underlying subsurface waters are two biogeochemical phenomena that have thus far eluded complete description. Many processes are thought to contribute to one or both, including lateral nutrient transport, carbon overconsumption or non-Redfield <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">C</mi><mo>:</mo><mi mathvariant="normal">N</mi><mo>:</mo><mi mathvariant="normal">P</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="41pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="89baeb1bef20dd832e5430c0fb7046a4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-6461-2018-ie00001.svg" width="41pt" height="10pt" src="bg-15-6461-2018-ie00001.png"/></svg:svg></span></span> organic matter cycling, heterotrophic nutrient uptake, and the actions of vertically migrating phytoplankton. To obtain insight into the likely magnitude of potential contributing mechanisms that can remove nitrate from the nutricline while supporting dissolved inorganic carbon (DIC) drawdown tens of meters higher in the water column, we investigated the seasonal formation rates for negative preformed nitrate (<span class="inline-formula">preNO<sub>3</sub></span>) anomalies (oxygen consumption without stoichiometric nitrate release) in the subsurface and positive preformed nitrate anomalies (oxygen production without stoichiometric nitrate drawdown) in the euphotic zone at the subtropical ocean time series stations ALOHA (A Long-Term Oligotrophic Habitat Assessment) in the North Pacific and BATS (Bermuda Atlantic Time-series Study) in the North Atlantic. Non-Redfield <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mo>-</mo><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>:</mo><mi mathvariant="normal">N</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="39pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="8685a2c40f29f412e6d48873a8503c89"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-6461-2018-ie00002.svg" width="39pt" height="13pt" src="bg-15-6461-2018-ie00002.png"/></svg:svg></span></span> stoichiometry for dissolved organic matter (DOM) remineralization accounts for up to <span class="inline-formula">∼15</span><span class="thinspace"></span>mmol<span class="thinspace"></span>N<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span><span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span> of negative <span class="inline-formula">preNO<sub>3</sub></span> anomaly formation at both stations. We present a new formulation for calculating <span class="inline-formula">preNO<sub>3</sub></span> (residual <span class="inline-formula">preNO<sub>3</sub></span>) that includes components resulting from non-Redfield DOM cycling. Residual negative <span class="inline-formula">preNO<sub>3</sub></span> anomalies in excess of that which can be accounted for by non-Redfield DOM cycling are found to accumulate at a rate of <span class="inline-formula">∼32</span>–46<span class="thinspace"></span>mmol<span class="thinspace"></span>N<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span><span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span> at Station ALOHA and <span class="inline-formula">∼46</span>–87<span class="thinspace"></span>mmol<span class="thinspace"></span>N<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span><span class="thinspace"></span>yr<span class="inline-formula"><sup>−1</sup></span> at the BATS station. These negative anomaly formation rates are in approximate balance with residual positive <span class="inline-formula">preNO<sub>3</sub></span> anomaly formation rates from the euphotic zone located immediately above the nutricline in the water column. We evaluate three mechanisms to explain these anomalies, calculating that transparent exopolymer particle (TEP) cycling and heterotrophic nitrate uptake can contribute to the formation of both residual <span class="inline-formula">preNO<sub>3</sub></span> anomalies. However, a significant fraction, estimated at <span class="inline-formula">∼50</span><span class="thinspace"></span>%–95<span class="thinspace"></span>%, is unexplained by the sum of these processes. Vertically migrating phytoplankton possess the necessary distribution, nutrient acquisition strategy, and biogeochemical signature to simultaneously remove nitrate at depth and transport it above the nutricline. Reported transport rates by known migrators equal or exceed the residual <span class="inline-formula">preNO<sub>3</sub></span> anomaly formation rates and potentially explain both the negative and positive residual <span class="inline-formula">preNO<sub>3</sub></span> anomalies as well as the mixed-layer DIC drawdown at the stations ALOHA and BATS within the limits of scarce detailed abundance profiles. However, the three processes examined are not independent and mutually exclusive. The model <i>Rhizosolenia</i> mat system (and perhaps other migrators) produces TEPs, suggesting that migration could provide accelerated vertical transport of TEPs and provide labile carbon for heterotrophic nitrate uptake. These results based on geochemical distributions suggest that, in the absence of additional mechanisms and rates, phytoplankton vertical migrators, although rare and easily overlooked, play a larger role in subtropical ocean nutrient cycling and the biological pump than generally recognized.</p>
  • Authors

  • Letscher, Robert
  • Villareal, Tracy A
  • Status

    Publication Date

  • November 2, 2018
  • Published In

  • Biogeosciences  Journal
  • Digital Object Identifier (doi)

    Start Page

  • 6461
  • End Page

  • 6480
  • Volume

  • 15
  • Issue

  • 21