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. Summertime mixed-layer drawdown of dissolved inorganic carbon inthe absence of measurable nutrients in the ocean's subtropical gyres andnon-Redfieldian oxygen : nitrate relationships in the underlying subsurfacewaters are two biogeochemical phenomena that have thus far eluded completedescription. Many processes are thought to contribute to one or both,including lateral nutrient transport, carbon overconsumption or non-Redfield C : N : P organic matter cycling, heterotrophic nutrient uptake, and theactions of vertically migrating phytoplankton. To obtain insight into thelikely magnitude of potential contributing mechanisms that can remove nitratefrom the nutricline while supporting dissolved inorganic carbon (DIC)drawdown tens of meters higher in the water column, we investigated theseasonal formation rates for negative preformed nitrate ( preNO3 )anomalies (oxygen consumption without stoichiometric nitrate release) in thesubsurface and positive preformed nitrate anomalies (oxygen productionwithout stoichiometric nitrate drawdown) in the euphotic zone at thesubtropical ocean time series stations ALOHA (A Long-Term Oligotrophic HabitatAssessment) in the North Pacific and BATS (Bermuda Atlantic Time-series Study) inthe North Atlantic. Non-Redfield - O 2 : N stoichiometry for dissolvedorganic matter (DOM) remineralization accounts for up to ∼15  mmol N m −2  yr −1 of negative preNO3 anomalyformation at both stations. We present a new formulation for calculating preNO3 (residual preNO3 ) that includes components resultingfrom non-Redfield DOM cycling. Residual negative preNO3 anomalies inexcess of that which can be accounted for by non-Redfield DOM cycling arefound to accumulate at a rate of ∼32 –46 mmol N m −2  yr −1 at Station ALOHA and ∼46 –87 mmol N m −2  yr −1 at the BATSstation. These negative anomaly formation rates are in approximate balancewith residual positive preNO3 anomaly formation rates from theeuphotic zone located immediately above the nutricline in the water column.We evaluate three mechanisms to explain these anomalies, calculating thattransparent exopolymer particle (TEP) cycling and heterotrophic nitrateuptake can contribute to the formation of both residual preNO3 anomalies. However, a significant fraction, estimated at ∼50  %–95 %,is unexplained by the sum of these processes. Vertically migratingphytoplankton possess the necessary distribution, nutrient acquisitionstrategy, and biogeochemical signature to simultaneously remove nitrate atdepth and transport it above the nutricline. Reported transport rates byknown migrators equal or exceed the residual preNO3 anomaly formationrates and potentially explain both the negative and positive residual preNO3 anomalies as well as the mixed-layer DIC drawdown at the stationsALOHA and BATS within the limits of scarce detailed abundance profiles.However, the three processes examined are not independent and mutuallyexclusive. The model Rhizosolenia mat system (and perhaps othermigrators) produces TEPs, suggesting that migration could provide acceleratedvertical transport of TEPs and provide labile carbon for heterotrophicnitrate uptake. These results based on geochemical distributions suggestthat, in the absence of additional mechanisms and rates, phytoplankton verticalmigrators, although rare and easily overlooked, play a larger role insubtropical ocean nutrient cycling and the biological pump than generallyrecognized.

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