The situation was reversed in late winter: the contribution of phytoplankton larger dian 20 µm ranged from 50 to 8 0% of total Chl a.
Light availability is a good candidate to explain the winter bloom, but
the halostratification (which reduces the depth of the mixed layer, zm)
and the decrease of turbidity (expressed by the vertical light attenuation
coefficient K) seem to have a greater influence on light availability than
the incident irradiance (Eo) per se at this time of the year.
Each curve corresponds to one value of the minimum inddent irradiance (Eo) required
for phytoplankton bloom initiation (according to Riley’s critical value of Em) as a function
of zm and K. K and zm conditions for intermediate plume and oceanic waters are plotted
during Biomet II (respectively 12, 02) and Biomet III (respectively 13, 03).
llie depth-averaged available PAR received by phytoplankton in the mixed
layer is given by the relation:
Em = (Eo - Eo . e
- Kzm )/Kzm
The calculation of the minimum incident irradiance required for onset of
phytoplankton blooms according to the empirical critical mean irradiance (Em = 20.9 W.m ) of Riley (1957) and with actual K and zm (fig.)
shows that this minimum is significandy lower in intemiediate (I) and oceanic waters (( )) during Biomet III (respectively ss 50 and 175 W.nr
2 ) than
during Biomet II (respectively 200 and 525 W.m-) because of change of K
and zm conditions (K and zm decreased). This minimum during Biomet
III was actually reached since the incident irradiance averaged 128 W.m
2
in
February.
138