Observations of the vertical structure of density, concentrations of chloro
phyll a and nitrate, and turbulent dissipation rates were made over a perio
d 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 o
f the thermocline occurred almost 5 h after maximum tidal currents. This tu
rbulence aids phytoplankton growth by supplying bottom-layer nutrients into
the subsurface chlorophyll maximum but reduces phytoplankton concentration
s in the thermocline by mixing cells from the base of the subsurface maximu
m 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 m(-2) d(-1), which is estimated to have b
een capable of supporting new phytoplankton growth at a rate of 160 (64-256
) mg C m(-2) d(-1). Turbulent entrainment of carbon from the base of the su
bsurface biomass maximum into the bottom mixed layer was observed to be 290
(120-480) mg C m(-2) d(-1). This apparent excess export from the chlorophy
ll maximum is suggested to be a feature of the spring-neap cycle, with expo
rt dominating as the tidal turbulence increases toward spring tides and ero
des the base of the thermocline. The observed rate of carbon export into th
e bottom mixed layer could account for as much as 25% of the gross annual p
rimary production in stratifying shelf seas. Such turbulent losses, combine
d with grazing losses and low light levels, suggest that phytoplankton need
to he highly adapted to environmental conditions within the thermocline in
order to survive.