New production across the shelf-edge in the northeastern North Sea during the stratified summer period

Turbulence measurements suggest high rates of new production over the shelf edge in the north-eastern North Sea during summer

10.5194/bg-2018-385 ◽

2018 ◽

Author(s):

Jørgen Bendtsen ◽

Katherine Richardson

Keyword(s):

North Sea ◽

Vertical Mixing ◽

Euphotic Zone ◽

Shelf Edge ◽

Nutrient Inputs ◽

New Production ◽

The North ◽

North Eastern ◽

The North Sea ◽

Using Data

Abstract. New production, i.e., that driven by allochthonous nutrient inputs, is the only form of primary production that can lead to net increases in organic material and is, therefore, important for understanding energy flow in marine ecosystems. The spatial distribution of new production is generally, however, not well known. Here, using data collected in July 2016, we analyse the potential for vertical mixing to support new production in the upper layers of the north eastern portion of the North Sea. Estimated nitrate fluxes due to turbulent vertical mixing into the euphotic zone were up to 0.5–1 mmol N m−2 d−1 over the shelf-edge (f-ratios > 0.1) while values of

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Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer

Biogeosciences ◽

10.5194/bg-15-7315-2018 ◽

2018 ◽

Vol 15 (23) ◽

pp. 7315-7332 ◽

Cited By ~ 1

Author(s):

Jørgen Bendtsen ◽

Katherine Richardson

Keyword(s):

North Sea ◽

Vertical Mixing ◽

Euphotic Zone ◽

Bottom Layer ◽

Depth Range ◽

Shelf Edge ◽

Nutrient Inputs ◽

New Production ◽

Edge Zone ◽

The North

Abstract. New production, i.e. that driven by allochthonous nutrient inputs, is the only form of primary production that can lead to net increases in organic material and is, therefore, important for understanding energy flow in marine ecosystems. The spatial distribution of new production is generally, however, not well known. Using data collected in July 2016, we analyse the potential for vertical mixing to support new production in the upper layers of the northeastern portion of the North Sea. Relatively large (up to >0.5 mmol N m−2 d−1) nitrate fluxes due to turbulent vertical mixing into the euphotic zone were found at some stations over the shelf edge, while low values (< 0.1 mmol N m−2 d−1) were found in the deeper open area north of the shelf edge. The low vertical mixing rates (dissipation rates of turbulent kinetic energy below 10−8 W kg−1, corresponding to vertical turbulent diffusion coefficients of 10−6–10−5 m2 s−1) implied f ratios of <0.02 in the open waters north of the shelf edge. In the shallow (<50 m) southern and central part of the study area, inorganic nutrients were low and nitrate undetectable, suggesting negligible new production here, despite relatively high concentrations of chlorophyll a being found in the bottom layer. Thus, high rates of new production seem to be concentrated around the shelf-edge zone and in association with localized features exhibiting enhanced vertical mixing. We find that the nutricline depth is significantly deeper at the shelf edge and interference with increased mixing in this deeper depth range can explain the increased diapycnal nitrate fluxes. Overall, this suggests that the shelf-edge zone may be the major nutrient supplier to the euphotic zone in this area during the period of summer stratification.

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REVIEW of “Turbulence measurements suggest high rates of new production over the shelf edge in the north-eastern North Sea during summer” by Bendtsen and Richardson.

10.5194/bg-2018-385-rc2 ◽

2018 ◽

Author(s):

Anonymous

Keyword(s):

North Sea ◽

Shelf Edge ◽

New Production ◽

Turbulence Measurements ◽

The North ◽

North Eastern

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Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer

10.5194/egusphere-egu2020-11427 ◽

2020 ◽

Author(s):

Jørgen Bendtsen ◽

Katherine Richardson

Keyword(s):

North Sea ◽

Vertical Mixing ◽

Euphotic Zone ◽

Bottom Layer ◽

Ecosystem Structure ◽

Depth Range ◽

Shelf Edge ◽

New Production ◽

Edge Zone ◽

Turbulence Measurements

The potential for vertical mixing to support new production in the upper layers of the northeastern portion of the North Sea was analysed from observations obtained during the stratified period in July 2016. Five transects across the shelf edge between the relatively shallow central North Sea and the deep Norwegian trench showed a clear frontal structure in hydrography, turbulent mixing, nutrients and chlorophyll a across the shelf edge. Relatively large (up to >0.5&#8201;mmol&#8201;N&#8201;m&#8722;2&#8201;d&#8722;1) nitrate fluxes due to turbulent vertical mixing into the euphotic zone were found at some stations over the shelf edge, while low values (<&#8201;0.1&#8201;mmol&#8201;N&#8201;m&#8722;2&#8201;d&#8722;1) were found in the deeper open area north of the shelf edge. The low vertical mixing rates implied f ratios less than 0.02 in the open waters north of the shelf edge. In the shallow (<50&#8201;m) southern and central part of the study area, inorganic nutrients were low and nitrate undetectable, suggesting negligible new production here, despite relatively high concentrations of chlorophyll&#160;a being found in the bottom layer. Thus, high rates of new production seem to be concentrated around the shelf-edge zone and in association with localized features exhibiting enhanced vertical mixing. We find that the nutricline depth is significantly deeper at the shelf edge and interference with increased mixing in this deeper depth range can explain the increased diapycnal nitrate fluxes. Overall, this suggests that the shelf-edge zone may be the major nutrient supplier to the euphotic zone in this area during the period of summer stratification. Potential impacts on plankton ecosystem structure are discussed.Reference:Bendtsen, J. and Richardson, K.: Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer, Biogeosciences, 15, 7315&#8211;7332, https://doi.org/10.5194/bg-15-7315-2018, 2018.

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