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
Subtropical ocean gyre ecosystems exhibit low rates of primary productivity
caused by thermal stratification of the water column that acts as an
impediment to sustained nutrient supply to the surface ocean. Yet these
regions exhibit significant annual net community production (ANCP),
estimated at
The nutrient sources supporting this seasonal DIC drawdown at the time series
sites have eluded oceanographers since the phenomenon was first documented
(Michaels et al., 1994; Toggweiler, 1994; Gruber et al., 1998; Keeling et
al., 2004), and identifying the nutrient sources is essential to understanding
how the drawdown occurs. Numerous nutrient input mechanisms have been
investigated, including vertical mixing,
The oxygen
To date, no work has attempted to partition the contribution of the multiple
mechanisms to formation of the negative
In this work, we expand the examination of
Time series biogeochemical data including concentrations of total dissolved
nitrogen, nitrate
The preformed
The residual
The seasonal formation rate of rNPN anomalies at each time series site was
estimated by a linear regression of the residual
A residual preformed phosphate (
Empirically derived, annual values of
Empirically derived values of
The climatology of the residual
The monthly averaged climatology of residual
Volumetric rates of rNPN anomaly formation at Station ALOHA are estimated at
Residual negative
The volumetric rate of rPPN anomaly formation within the euphotic zone at
Station ALOHA is estimated at
The climatology of the residual
The monthly averaged climatology of residual
The volumetric rate of rNPN anomaly formation at the BATS station is
estimated at
The climatology of the residual
The climatology of the residual
The monthly averaged climatology of residual
Our analysis found DOM remineralization within the upper mesopelagic layer to
follow non-Redfield
We now undertake a quantitative examination of the potential contributing mechanisms to explain the subsurface rNPN anomalies and euphotic-zone rPPN anomalies.
Both the ALOHA and BATS station climatologies of the residual
Subsurface rNPN anomalies begin to appear at a depth of
What is the role of lateral physical mixing in creating the observed rPPN and
rNPN anomalies? To address this question, we turn to the World Ocean Atlas
(WOA) annual
On the
Examination of the residual
On the
Lateral mixing may explain the observed rPPN anomaly below
Contribution of N-poor DOM remineralization to NPN and PPN anomaly
formation rates as well as contributions to rNPN and rPPN anomaly formation
rates by presumed processes at the time series sites. NPN and rNPN is for
features
on
Transparent exopolymer particles are the polysaccharide-rich exudate of
phytoplankton that accumulate in the size range
We use field data of TEP concentrations near the stations BATS and ALOHA and
a few simplifying assumptions to test for the importance of this process as a
contributor to the dual rNPN and rPPN anomalies. Our estimates of the
potential for TEP cycling to contribute to the observed euphotic-zone rPPN
and subsurface rNPN anomaly formation rates assume (1) that TEPs are pure
carbohydrate (
Cisternas-Novoa et al. (2015) measured TEP concentration depth profiles in
the Sargasso Sea northeast of Bermuda on five separate occasions from
February 2012 to June 2013. The profiles show a
The potential contribution of TEP cycling to rPPN and rNPN anomaly
formation near the BATS station can be also be estimated using published
sinking rates for negatively buoyant TEPs. Again, using the TEP profiles from
the Sargasso Sea from Cisternas-Novoa et al. (2015), we estimate a
concentration difference of
Seasonal depth profiles of TEP concentration are unavailable for the
subtropical North Pacific near Station ALOHA. Wurl et al. (2011) measured TEP
concentration profiles south of the island of Hawaii during
August/September 2009. We use the upper-ocean concentration excess present in
these profiles with the TEP sinking rate of 0.04 d
Multiple studies have shown the uptake of nitrate and/or phosphate by
heterotrophic bacteria during the remineralization of organic matter or that
inorganic nutrients can limit bacterial OM consumption (e.g., Zweifel et al.,
1993; Kirchman, 1994; Cotner et al., 1997; Rivkin and Anderson, 1997; Caron
et al., 2000; Letscher et al., 2015). Bacterial nitrate uptake to
remineralize OM has the effect of creating an NPN anomaly (
To assess the potential contribution of bacterial nitrate uptake to our
observed rNPN anomaly formation rates at the stations ALOHA and BATS, we
examined the literature for estimates of bacterial C production rates and
bacterial biomass
We can make a similar calculation for Station ALOHA using published
3H-leucine bacterial production rate measurements (Church et al., 2004). The
mean bacterial C production rate at 100 m is
From our analyses of the available published data, remineralization of N-poor DOM, TEP cycling, or heterotrophic bacterial nitrate uptake cannot quantitatively explain both the seasonal rPPN anomaly formation within the euphotic zone and subsurface rNPN anomaly formation observed at the stations ALOHA and BATS, even with the generous assumptions made. We now examine the vertical migration of phytoplankton down to the nutricline as a potential biological mechanism to explain the dual rPPN and rNPN anomalies.
In the absence of other mechanisms, the potential contribution of vertically
migrating phytoplankton to the rNPN and rPPN anomaly features at the two
stations can be determined by subtracting the contributions of TEP cycling
and bacterial nitrate uptake to rNPN and/or rPPN formation from the total
observed rNPN and rPPN anomaly formation rates (presented in Table 2). By
contrast, the phytoplankton vertical migration category must provide
16–21 mmol N m
As a rare, giant component of the flora, net collections, in situ
observations by divers, and towed optical systems constitute the bulk of
observations; hence vertical migrators are somewhat out of the mainstream of
current phytoplankton observations. The unique attributes and evidence for
vertical migration are briefly discussed here for background, as well as
documentation of their presence in both oceans at or near the time series
stations. Nitrate transport rates are then summarized for comparison to the
required contribution from the residual
Flagellate motility and cyanobacteria buoyancy control has been recognized
for decades as a strategy to exploit spatially disjunct light and nutrient
fields (Cullen, 1985; Eppley et al., 1968; Ganf and Oliver, 1982; Hasle,
1950; Steemann Nielsen, 1939) and was suggested for non-flagellated marine
species of the genus
Giant phytoplankton are found throughout the warmer oceans of the world and
are specifically noted at both Station ALOHA and the BATS station (Figs. S11–S13). The
required characteristics of buoyancy reversals, high-internal-nitrate pools,
and rapid ascent/descent have been found in multiple taxa from the Atlantic
and Pacific oceans (Villareal et al., 2014). The taxa
In the Pacific, the range of
For rNPN and rPPN development, the unique characteristics of vertical
migration are that cells acquire and store nitrate at depth, transport it
into the euphotic zone, and then reduce it internally to biomass concomitant
with oxygenic photosynthesis. Nitrate transport calculations by vertical
migration have a number of assumptions and caveats, including variability in
abundance, particularly for
Fraga (2001) developed an independent assessment of the impact of vertical
migration on the NO tracer (functionally related to our residual
Vertically migrating phytoplankton can also help explain the observed
summertime DIC drawdown (Gruber et al., 1998; Keeling et al., 2004) in the
absence of measurable nitrate from the mixed layer at both the stations
ALOHA and BATS. The concurrently operating migration cycles of different
individuals would continually bring intracellular nitrate-rich migrators into
the surface mixed layer, where their oxygenic photosynthesis would draw down
DIC and release dissolved
We can compare our estimated rates of rPPN formation within the euphotic zone
with the summertime mixed-layer DIC drawdown at each time series site using
assumptions on the appropriate
At Station ALOHA, net community production during the 6-month summer-to-fall period represents
The estimated rate of euphotic-zone rPPN formation during summer to fall at
the BATS station is 50–96 mmol N m
We also investigated for the presence of subsurface residual negative
Seasonal climatology of the residual
We hypothesize that P-limited or P-stressed vertically migrating
phytoplankton also take up phosphate at the nutricline in combination with
nitrate to contribute to both the observed rNPN and rPPN and the subsurface
residual negative
Subsurface residual
The data have been deposited with PANGEA and can be accessed at
The supplement related to this article is available online at:
RTL and TAV jointly conceived the study, performed the formal analysis, created visualizations, and wrote and edited the article. RTL created the methodology with input from TAV. RTL curated the datasets of residual preformed nitrate concentrations.
The authors declare that they have no conflict of interest.
Robert T. Letscher thanks the efforts of the scientists, crew, and staff that have supported the Hawaiian Ocean Time Series and Bermuda Atlantic Time-series Study for nearly 30 years, including the continuing financial support of the National Science Foundation Biological and Chemical Oceanography programs. Tracy A. Villareal acknowledges support from NSF OCE 1537546 and wishes to thank all the students, graduate and undergraduate, that provided the diving support essential to this work as well as the officers and crew of the research vessels involved in this work. Edited by: Manmohan Sarin Reviewed by: four anonymous referees