Alkaline phosphatase activity (APA) is traditionally a proxy for phosphate (DIP)-limitation because it is induced by DIP-limited microbes to access the labile ester fraction of the organic phosphorus ...(OP) pool. Here, we present multi-year summertime depth distributions of APA and enzyme kinetics in the DIP-replete Celtic Sea. Our findings support the cumulating body of evidence that APA has a potentially widespread role in OP remineralization through the water column. APA and V
max were positively correlated with depth and DIP, with total APA being threefold higher below (0.93 ± 0.32 nM P h−1) compared to above the thermocline (0.30 ± 0.24 nM P h−1, p<0.001). Separation of particles by sinking speed demonstrated that APA was eightfold higher on fast sinking (F
fast) particles compared to slow sinking particles (F
slow; p<0.05). When normalized to particulate organic carbon (POC) and bacterial production (BP), APAPOC and APABP associated with F
fast (0.76 ± 0.10 nmol P μmol C−1 h−1, 21.13 ± 2.2 nmol P nmol C−1, respectively) were fourfold and 25-fold higher compared to the combined APA associated with dissolved plus suspended (F
susp) and F
slow fractions (0.19 ± 0.06 nmol P μmol C−1 h−1 and 0.84 ± 0.23 nmol P nmol C−1, respectively). We postulate that this may reflect enhanced ectoenzyme activity associated with bacteria colonizing particle surfaces and/or release by zooplankton via faecal pellet excretion. Knowledge of the disparity between APA and BP associated with particle and dissolved phases is required to accurately define the O₂ : P ratio of regenerated P derived from sinking particles as a result of AP-facilitated remineralization.
The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating ...a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ13C) signatures in dissolved inorganic carbon (δ13C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ13C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ13C values in consumers can change as a result of environmentally driven variation in the δ13C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ13C baseline, with spatial gradients in the δ13C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ13C‐DIC values (−0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO2) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ13C‐POC values and δ13C in arctic marine mammals reflects the anthropogenic CO2 signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ13C‐POC values in the Arctic Ocean and partially explain the δ13C values in marine mammals with consideration of time‐varying integration of δ13C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.
Warming of the Arctic has led to changes in multiple environmental factors that control the carbon isotope ratio at the base of the food web. Here, we present the first comprehensive spatial and temporal assessment of the carbon isotopic baseline for the Arctic Ocean, which is essential for understanding and predicting changes in Arctic food web structure. The response of the Arctic ecosystem to ongoing environmental change presented here is stronger than we would predict theoretically.
The atmospheric and deep sea reservoirs of carbon dioxide are linked via physical, chemical, and biological processes. The last of these include photosynthesis, particle settling, and organic matter ...remineralization, and are collectively termed the "biological carbon pump." Herein, we present results from a 13-y (1992-2004) sediment trap experiment conducted in the permanently oligotrophic North Pacific Subtropical Gyre that document a large, rapid, and predictable summertime (July 15-August 15) pulse in particulate matter export to the deep sea (4,000 m). Peak daily fluxes of particulate matter during the summer export pulse (SEP) average 408, 283, 24.1, 1.1, and 67.5 μmol.m⁻².d⁻¹ for total carbon, organic carbon, nitrogen, phosphorus (PP), and biogenic silica, respectively. The SEP is approximately threefold greater than mean wintertime particle fluxes and fuels more efficient carbon sequestration because of low remineralization during downward transit that leads to elevated total carbon/PP and organic carbon/PP particle stoichiometry (371:1 and 250:1, respectively). Our long-term observations suggest that seasonal changes in the microbial assemblage, namely, summertime increases in the biomass and productivity of symbiotic nitrogen-fixing cyanobacteria in association with diatoms, are the main cause of the prominent SEP. The recurrent SEP is enigmatic because it is focused in time despite the absence of any obvious predictable stimulus or habitat condition. We hypothesize that changes in day length (photoperiodism) may be an important environmental cue to initiate aggregation and subsequent export of organic matter to the deep sea.
Surface waters in upwelling regions are thought to be nutrient rich and hence inhibit nitrogen fixation (diazotrophy) because diazotrophs can preferentially assimilate nitrate and ammonia instead of ...expending energy to fix dinitrogen. We found average nitrogen fixation rates to be two to seven times higher in the surface waters of the upwelling region of the eastern equatorial Atlantic than typically measured here during non‐upwelling periods. We posit that in this region, low nitrate‐phosphate ratio waters are upwelled, and an initial bloom of non‐diazotrophic phytoplankton removes recently upwelled nitrate. Thereby, diazotrophy is fuelled by residual phosphate and by a combination of aeolian and upwelled sources of iron. Annually, we estimate that approximately 47 Gmol of new nitrogen is introduced by diazotrophy in upwelled waters alone and 195 Gmol N is fixed in the equatorial Atlantic region. Our findings challenge the paradigm that the highest nitrogen fixation rates occur in oligotrophic gyres and instead provide evidence of its importance in upwelling regimes where phosphate‐ and iron‐rich waters rich are upwelled.
Key Points
Nitrogen fixation rates are higher during upwelling than non‐upwelling times
Diazotrophy is fueled by low N:P, iron rich water that is upwelled
Annually, 47 Gmol of new nitrogen is fixed in upwelled waters
We present nearly 9 yrs (June 2005–December 2013) of measurements of upper-ocean (0 m to 125 m) dinitrogen (N₂) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ...ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N₂ fixation rates were measured based on adding the 15N₂ tracer as a gas bubble. Beginning in August 2012, 15N₂ was first dissolved into filtered seawater and the 15N₂-enriched water was subsequently added to N₂ fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N₂-enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a 15N₂ gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N₂ fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N₂ fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N₂ fixation rates ranged between 502 and 919 μmol N m−2 d−1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m−2 yr−1, consistent with previous independent estimates of NCP at this site.
The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change. How the nitrogen cycle responds will determine long-term trends in net ...primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evolve. Here we show that intensifying nitrogen limitation of phytoplankton, associated with near-term reductions in NPP, causes detectable declines in nitrogen isotopes (δ
N) and constitutes the primary perturbation of the 21
century nitrogen cycle. Model experiments show that ~75% of the low latitude twilight zone develops anomalously low δ
N by 2060, predominantly due to the effects of climate change that alter ocean circulation, with implications for the nitrogen source-sink balance. Our results highlight that δ
N changes in the low latitude twilight zone may provide a useful constraint on emerging changes to nitrogen limitation and NPP over the 21
century.
The concentration of phosphate and dissolved organic phosphorus (DOP) is chronically low and limits phytoplankton growth in the subtropical North Atlantic relative to other ocean basins. Transport of ...phosphate and DOP from the productive flanks of the gyre to its interior has been hypothesized as an important phosphorus supply pathway. During a cruise in the eastern Atlantic in spring 2011, the rates of phosphate uptake, alkaline phosphatase activity (APA), and DOP production were measured in the northwest African shelf region, subtropics, and tropics. Rates of DOP production were sixfold higher in the shelf region (43 ± 41 nM d−1) relative to the subtropics (6.9 ± 4.4 nM d−1). In contrast, APA was threefold higher in the subtropics (8.0 ± 7.3 nM d−1), indicative of enhanced DOP utilization, relative to the shelf region (2.6 ± 2.1 nM d−1). Hence, observations suggest net production of DOP in the shelf region and either net consumption of DOP or a near balance in DOP production and consumption in the gyre interior. Eddy‐permitting model experiments demonstrate that (i) DOP accounts for over half the total phosphorus in surface waters, (ii) DOP is transported westward from the shelf region by a combination of gyre and eddy circulations, and (iii) advected DOP supports up to 70% of the particle export over much of the subtropical gyre. Our combined observational and modeling study supports the view that the horizontal transport of DOP from the shelf region is an important mechanism supplying phosphorus to the surface subtropical North Atlantic.
Key Points
There is net production of DOP in the northwest African shelf regionDOP is transported westward in the subtropical gyre from the eastern boundaryCycling of semilabile DOP supports productivity in the subtropical gyre
The spatial and diurnal tidal variability of dissolved organic carbon (DOC) concentrations and the composition of dissolved organic matter (DOM), as evaluated by high-temperature catalytic oxidation ...and excitation–emission matrix combined with parallel factor analysis (EEM–PARAFAC), respectively, were determined in Liverpool Bay. EEM–PARAFAC modeling resulted in six fluorescent components characterized as terrestrial humic-like (two), microbial humic-like (two), and protein-like (two). The spatial distributions of DOC and the four humic-like components were negatively correlated with salinity in the high-salinity waters observed in this study (30.41–33.75), suggesting that terrestrial DOM was conservatively distributed. The spatial patterns of protein-like components were largely different from those of DOC, humic-like components, and chlorophyll
a
, suggesting that these distributions were the combined result of production and degradation in the bay in addition to river inputs. These findings suggest that the DOM dynamics in Liverpool Bay are strongly controlled by river-dominated allochthonous DOM inputs with some less significant contributions of autochthonous DOM within the bay. In addition, the temporal variations of DOM associated with the diurnal tidal cycles were determined at one inshore (31.34–32.24 salinity) and one offshore (33.64–33.75 salinity) station in the bay. Negative linear relationships between salinity and DOM characteristics, i.e., DOC, humic-like, and protein-like components, were observed at the inshore station. In contrast, no relationship was observed at the offshore station, suggesting that the export of DOM through rivers and possibly tidal flats have a noticeable influence on DOM concentration and composition up to a relatively elevated salinity of around 33 in Liverpool Bay.
Abstract
The carbon cycle is a key regulator of Earth’s climate. On geological time-scales, our understanding of particulate organic matter (POM), an important upper ocean carbon pool that fuels ...ecosystems and an integrated part of the carbon cycle, is limited. Here we investigate the relationship of planktonic foraminifera-bound organic carbon isotopes (δ
13
C
org-pforam
) with δ
13
C
org
of POM (δ
13
C
org-POM
). We compare δ
13
C
org-pforam
of several planktonic foraminifera species from plankton nets and recent sediment cores with δ
13
C
org-POM
on a N-S Atlantic Ocean transect. Our results indicate that δ
13
C
org-pforam
of planktonic foraminifera are remarkably similar to δ
13
C
org-POM
. Application of our method on a glacial sample furthermore provided a δ
13
C
org-pforam
value similar to glacial δ
13
C
org-POM
predictions. We thus show that δ
13
C
org-pforam
is a promising proxy to reconstruct environmental conditions in the upper ocean, providing a route to isolate past variations in δ
13
C
org-POM
and better understanding of the evolution of the carbon cycle over geological time-scales.
The broad distribution and often high densities of the cyanobacterium Trichodesmium spp. in oligotrophic waters imply a substantial role for this one taxon in the oceanic N cycle of the marine ...tropics and subtropics. New results from 154 stations on six research cruises in the North Atlantic Ocean show depth‐integrated N2 fixation by Trichodesmium spp. at many stations that equalled or exceeded the estimated vertical flux of NO3− into the euphotic zone by diapycnal mixing. Areal rates are consistent with those derived from several indirect geochemical analyses. Direct measurements of N2 fixation rates by Trichodesmium are also congruent with upper water column N budgets derived from parallel determinations of stable isotope distributions, clearly showing that N2 fixation by Trichodesmium is a major source of new nitrogen in the tropical North Atlantic. We project a conservative estimate of the annual input of new N into the tropical North Atlantic of at least 1.6 × 1012 mol N by Trichodesmium N2 fixation alone. This input can account for a substantial fraction of the N2 fixation in the North Atlantic inferred by several of the geochemical approaches.
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