Nitrate uptake and diffusive nitrate supply in the Central Atlantic

Planas, Dolors; Agustı́, Susana; Duarte, Carlos M.; Granata, Timothy C.; Merino, Martı́n

doi:10.4319/lo.1999.44.1.0116

Cited by 68 publications

(45 citation statements)

References 46 publications

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“…The DCM in shelf seas contrasts with, for example, the DCM in oligotrophic gyres where relatively weak vertical mixing results in upward nitrate fluxes more than one order of magnitude lower than those measured in the Celtic Sea (Planas et al 1999), an optically deep DCM, and low carbon export (Painter et al 2007). Aside from these differences however, the DCM in the seasonal thermocline of shelf seas is located in a region of marked vertical gradients of light and nutrient availability (Holligan et al 1984a,b;Sharples et al 2001), as is common for the DCM in other hydrographic systems (Cullen 1982).…”

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confidence: 75%

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Hickman

Holligan

Moore

et al. 2009

Limnology & Oceanography

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Observations of vertical gradients in phytoplankton community structure were made through the water column of the seasonally stratified Celtic Sea, including within the thermocline. A deep chlorophyll maximum (DCM) was located within the thermocline at all stations, coupled to the nitracline. Vertical gradients in phytoplankton community composition were routinely observed within the thermocline. The cell abundance maxima for Synechococcus occurred in the upper part of the DCM coincident with a picoeukaryote abundance minima. Picoeukaryote abundance typically increased at or just above the peak of the DCM. Diatoms were observed occasionally at the DCM peak. Pigment compositions and phytoplankton absorption spectra indicated that the different phytoplankton communities were chromatically well adapted to the spectral composition of irradiance at the depths where they occurred in the water column. Profiles of vertical eddy diffusivity revealed that timescales for mixing between the phytoplankton layers within the thermocline were in excess of typical phytoplankton growth rates. The observed vertical gradients in community structure could therefore result from selection and niche partitioning of phytoplankton types on the light and nutrient gradient within the thermocline. The data further indicate that the pigments, light absorption characteristics, and cell size contribute to the phytoplankton selection process.The Celtic Sea, part of the temperate Northwest European shelf, is a tidally dynamic environment where water column structure is strongly influenced by the balance of solar heating and tidally generated mixing (Simpson and Hunter 1974). Much of the region becomes thermally stratified in April, initiating the spring phytoplankton bloom (Pingree et al. 1976). The water column remains stratified throughout the summer, with the thermocline forming a boundary between nutrient-depleted surface mixed layer (SML) above and the nutrient-rich bottom mixed layer (BML) below (Pingree et al. 1977). During this seasonal stratification, a deep chlorophyll maximum (DCM) is present within the thermocline (Pingree et al. 1977). The DCM is typically located toward the base of the density gradient, lies within the euphotic zone, and is strongly coupled to the nitracline (Holligan et al. 1984a,b;Sharples et al. 2001).Energy from tidally induced turbulence is dissipated at the base of the thermocline, causing upward mixing of nitrate into the thermocline from the BML and downward mixing of phytoplankton from the base of the DCM (Sharples et al. 2001). Previous observations in the Celtic Sea have revealed a vertical flux of nitrate into the thermocline of around 2 mmol N m 22 d 21 with the DCM maintained at a depth corresponding to ,5% of surface irradiance (Sharples et al. 2001). The DCM is often observed to be a biomass and photosynthesis maximum as well as a pigment maximum (Holligan et al. 1984a,b;Pemberton et al. 2004;Moore et al. 2006). Monospecific blooms with chlorophyll a (Chl a) concentrations .10 mg Chl a m 23...

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confidence: 75%

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Hickman

Holligan

Moore

et al. 2009

Limnology & Oceanography

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“…1 (Lewis et al, 1986;Planas et al, 1999). This mixing drives a vertical turbulent flux of nitrate into the base of the thermocline, where it fuels new production.…”

Section: Horizontal Gradients In Vertical Turbulent Mixing and Nutriementioning

confidence: 99%

Internal tidal mixing as a control on continental margin ecosystems

Sharples

Moore

Hickman

et al. 2009

Geophysical Research Letters

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Abstract.We show that a breaking internal tide at a shelf edge is a fundamental control on the structural and functional properties of ecosystems. Contrasts in vertical mixing of nitrate between the shelf and the open ocean correspond with horizontal and vertical changes in phytoplankton communities, with largest cells found in surface waters at the shelf edge. Intense fishing activity is commonly seen at continental shelf edges, targeting spawning fish stocks. We suggest that the internal tide, a globally ubiquitous physical process at steep shelf edge bathymetry, supports shelf edge fisheries by providing large-celled phytoplankton for first-feeding fish larvae. The repeatability of the internal tide removes fish from the need to time spawning with a spring bloom. Also, with large phytoplankton cells dominating particulate organic carbon export, the internal tides could be an important influence on spatial and temporal variability in patterns of global carbon sequestration in deep water and sediments.

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“…These numbers represent the first estimate of the contribution of both processes in a coastal upwelling system. Few studies have simultaneously quantified BNF and nitrate diffusion (Mouriño-Carballido et al, 2011;Painter et al, 2013;Fernández-Castro et al, 2015), mainly due to the methodological difficulties to estimate vertical mixing (Kz) in the field, which has frequently motivated the use of constant values of Kz , and empirical parameterizations (Planas et al, 1999;Fernández-Castro et al, 2014). Capone et al (2005) estimated in the tropical North Atlantic that BNF could equal or even exceed nitrate vertical diffusion into the euphotic zone.…”

Section: Magnitude and Biogeochemical Relevance Of Bnfmentioning

confidence: 99%

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

et al. 2017

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The classical paradigm about marine N 2 fixation establishes that this process is mainly constrained to nitrogen-poor tropical and subtropical regions, and sustained by the colonial cyanobacterium Trichodesmium spp. and diatom-diazotroph symbiosis. However, the application of molecular techniques allowed determining a high phylogenic diversity and wide distribution of marine diazotrophs, which extends the range of ocean environments where biological N 2 fixation may be relevant. Between February 2014 and December 2015, we carried out 10 one-day samplings in the upwelling system off NW Iberia in order to: (1) investigate the seasonal variability in the magnitude of N 2 fixation, (2) determine its biogeochemical role as a mechanism of new nitrogen supply, and (3) quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N 2 fixation in this region was relatively low (0.001 ± 0.002 -0.095 ± 0.024 µmol N m −3 d −1 ), comparable to the lower-end of rates described for the subtropical NE Atlantic. Maximum rates were observed at the surface during both upwelling and relaxation conditions. The comparison with nitrate diffusive fluxes revealed the minor role of N 2 fixation (<2%) as a mechanism of new nitrogen supply into the euphotic layer. Small diazotrophs (<10 µm) were responsible for all N 2 fixation activity detected in the region. Quantitative PCR targeting the nifH gene revealed the highest abundances of two sublineages of Candidatus Atelocyanobacterium thalassa or UCYN-A (UCYN-A1 and UCYN-A2), mainly at surface waters during upwelling and relaxation conditions, and of Gammaproteobacteria γ-24774A11 at deep waters during downwelling. Maximum abundance for the three groups were up to 6.7 × 10 2 , 1.5 × 10 3 , and 2.4 × 10 4 nifH copies L −1 , respectively. Our findings demonstrate measurable N 2 fixation activity and presence of diazotrophs throughout the year in a nitrogen-rich temperate region.

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Nitrate uptake and diffusive nitrate supply in the Central Atlantic

Cited by 68 publications

References 46 publications

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Internal tidal mixing as a control on continental margin ecosystems

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

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