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

• Published in: Biogeosciences
• Main Authors: R. T. Letscher, T. A. Villareal
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/bg-15-6461-2018; https://doaj.org/article/16d1fb0acdcd4c5588ac584d3c946ffb

Summertime mixed-layer drawdown of dissolved inorganic carbon in the absence of measurable nutrients in the ocean's subtropical gyres and non-Redfieldian oxygen : 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 C : N : P 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 (preNO 3 ) 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 −O 2 : N stoichiometry for dissolved organic matter (DOM) remineralization accounts for up to ∼ 15 mmol N m −2 yr −1 of negative preNO 3 anomaly formation at both stations. We present a new formulation for calculating preNO 3 (residual preNO 3 ) that includes components resulting from non-Redfield DOM cycling. Residual negative preNO 3 anomalies in excess of that which can be accounted for by non-Redfield DOM cycling are found 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 BATS station. These negative anomaly formation rates are in approximate balance with residual positive preNO 3 anomaly formation rates from the euphotic zone located immediately above the nutricline in the water column. We ...