Phytoplankton distribution and survival in the thermocline

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.