Abstract
Abstract. Variable physical conditions such as vertical turbulent exchange, internal wave, and mesoscale eddy action affect the availability of light and nutrients for phytoplankton (unicellular algae) growth. It is hypothesized that changes in ocean temperature may affect ocean vertical density stratification, which may hamper vertical exchange. In order to quantify variations in physical conditions in the northeast Atlantic Ocean, we sampled a latitudinal transect along 17 ± 5∘ W between 30 and 63∘ N in summer. A shipborne conductivity–temperature–depth (CTD) instrumented package was used with a custom-made modification of the pump inlet to minimize detrimental effects of ship motions on its data. Thorpe-scale analysis was used to establish turbulence values for the upper 500 m from three to six profiles obtained in a short CTD yo-yo, 3 to 5 h after local sunrise. From south to north, average temperature decreased together with stratification while turbulence values weakly increased or remained constant. Vertical turbulent nutrient fluxes did not vary significantly with stratification and latitude. This apparent lack of correspondence between turbulent mixing and temperature is likely due to internal waves breaking (increased stratification can support more internal waves), acting as a potential feedback mechanism. As this feedback mechanism mediates potential physical environment changes in temperature, global surface ocean warming may not affect the vertical nutrient fluxes to a large degree. We urge modellers to test this deduction as it could imply that the future summer phytoplankton productivity in stratified oligotrophic waters would experience little alterations in nutrient input from deeper waters.
Highlights
The physical environment is important for ocean life, including variations therein
Between −30 < z < 0 m, turbulence dissipation rate values between our minimum detectable level 10−11 and > 10−8 m2 s−3 are similar to those found by others, using microstructure profilers (e.g. Oakey, 1982; Gregg, 1989), lowered acoustic Doppler current profiler, or CTD Thorpe-scale analysis (e.g. Ferron et al, 1998; Walter et al, 2005; Kunze et al, 2006)
The indirect comparison with turbulence values determined from previous microstructure profiler observations along the same transect (Jurado et al, 2012) confirms the same trends, occasionally turbulence values were lower
Summary
The physical environment is important for ocean life, including variations therein. For example, the sun stores heat in the ocean with a stable vertical density stratification as result. Stratification supports internal waves, which (i) may move near-floating particles like phytoplankton (unicellular algae) up and down towards and away from the surface and (ii) may induce enhanced turbulence via vertical current differences (shear) resulting in internal waves breaking (Denman and Gargett, 1983). Such changes in the physical environment are expected to affect the availability of phytoplankton growth factors such as light and nutrients
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