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

TY - JOUR

T1 - Turbulence measurements suggest high rates of new production over the shelf edge in the northeastern North Sea during summer

AU - Bendtsen, Jørgen

AU - Richardson, Katherine

PY - 2018/12/10

Y1 - 2018/12/10

N2 - 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 Nm-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 Nm-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 10-8W 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.

AB - 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 Nm-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 Nm-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 10-8W 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.

U2 - 10.5194/bg-15-7315-2018

DO - 10.5194/bg-15-7315-2018

M3 - Journal article

SN - 1726-4170

VL - 15

SP - 7315

EP - 7332

JO - Biogeosciences

JF - Biogeosciences

IS - 23

ER -