Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 225

7.3.3 Phytoplankton motility and phytoplankton thin layers

In the previous section we described phytoplankton growth within the SCM that
involves the transfer of dissolved inorganic nutrients from the bottom layer into the
thermocline, with the nutrients then assumed to be available for uptake by the phyto-
plankton. The phytoplankton were treated as non-motile organisms, with the thermo-
cline providing a suitable niche for access to the required light and nutrient resources.
Survival of phytoplankton within the SCM is then a balance between nutrient supply
and growth on the one hand, and respiration, grazing and turbulent export on the
other. What happens if the phytoplankton can swim in response to their resource
requirements? For instance, dinoflagellates have been observed to swim downward
when nutrient-deplete, gathering nutrients from deeper water before ascending back
towards the light (Eppley et al., 1968 ; Cullen and Horrigan, 1981 ) .
Given the often strong tidal currents and associated turbulence in shelf seas, is
the swimming capability of single-celled plants significant? For instance, typical
swimming speeds of dinoflagellate cells range between v c ¼ 0.1 and 0.5 mm s 1

(Kamykowski and McCollum, 1986 ), which is two orders of magnitude lower than
the typical turbulent velocities in the tidally energetic bottom layers of the NW
European shelf. An assessment of the likely importance of motility in a turbulent
environment can be made by calculating the cell Pe ´ clet number, as described in
Chapter 5 .
For tidal conditions similar to those of the NW European shelf, the strength of the
turbulence in the bottom mixed layer overwhelms typical phytoplankton swimming
capabilities (Ross and Sharples, 2008 ) . However, swimming can have an impact close to
and within the thermocline. Results from a random walk Lagrangian model of cell
trajectories in a tidal, stratified water column, shown in Fig. 7.12 , suggest that tidal
turbulence periodically transports cells into the base of the thermocline, regardless of
whether a cell can swim or not (Ross and Sharples, 2008 ) . These cells will be nutrient-
replete, having been able to take up bottom layer nutrients. Once in the base of the
thermocline, swimming becomes advantageous as the weakly turbulent environment of
the thermocline allows a motile cell to swim upward, into higher light and away from
the risk of being entrained back down into the mixed layer by the next pulse of tidal
turbulence. Notice in Fig. 7.12a how the motile particle is able to reach deeper into the
thermocline, undertaking a number of migration cycles within the nitracline, compared
to the non-motile particle ( Fig. 7.12b ).The results also show that the motile cells are
able to migrate up and down within the base of the thermocline, swimming down to
extract nutrients from the nitracline and then upward to photosynthesise. Experiments
with this model suggested that a motile species can out-compete a non-motile species
even if the motile cell has some relative growth rate disadvantage 2 (Ross and Sharples,

2

It is interesting to note that a similar model experiment applied to the surface of the open ocean
suggests that motility confers no advantage (Broekhuizen, 1999 ). This may reflect the role of the tidal
turbulence, absent from the open ocean, in providing a rapid transport mechanism from deep water back
up to the thermocline.