Abstract. 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 (preNO3)
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 -O2:N stoichiometry for dissolved
organic matter (DOM) remineralization accounts for up to ∼15 mmol N m−2 yr−1 of negative preNO3 anomaly
formation at both stations. We present a new formulation for calculating
preNO3 (residual preNO3) that includes components resulting
from non-Redfield DOM cycling. Residual negative preNO3 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 preNO3 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 preNO3
anomalies. However, a significant fraction, estimated at ∼50 %–95 %,
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 preNO3 anomaly formation
rates and potentially explain both the negative and positive residual
preNO3 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 Rhizosolenia 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.
