Sharples, Jonanthan, Moore, C. Mark, Rippeth, Tom P., Holligan, Patrick M., Hydes, David J., Fisher, Neil R. and Simpson, John H.
Phytoplankton distribution and survival in the thermocline
Limnology and Oceanography, 46, (3), .
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Observations of the vertical structure of density, concentrations of chlorophyll a and nitrate, and turbulent dissipation
rates were made over a period of 25 h in a well-stratified shelf region in the Western English Channel,
between neap and spring tides. Maximum turbulent dissipation at the base of the thermocline occurred almost 5 h
after maximum tidal currents. This turbulence aids phytoplankton growth by supplying bottom-layer nutrients into
the subsurface chlorophyll maximum but reduces phytoplankton concentrations in the thermocline by mixing cells
from the base of the subsurface maximum into the bottom mixed layer. The turbulent dissipation observations were
used to estimate an average nitrate flux into the thermocline of 2.0 (0.8–3.2, 95% confidence interval) mmol m22
d21, which is estimated to have been capable of supporting new phytoplankton growth at a rate of 160 (64–256)
mg C m22 d21. Turbulent entrainment of carbon from the base of the subsurface biomass maximum into the bottom
mixed layer was observed to be 290 (120–480) mg C m22 d21. This apparent excess export from the chlorophyll
maximum is suggested to be a feature of the spring-neap cycle, with export dominating as the tidal turbulence
increases toward spring tides and erodes the base of the thermocline. The observed rate of carbon export into the
bottom mixed layer could account for as much as 25% of the gross annual primary production in stratifying shelf
seas. Such turbulent losses, combined with grazing losses and low light levels, suggest that phytoplankton need to
be highly adapted to environmental conditions within the thermocline in order to survive.
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