Abstract:
The temporal and spatial variations in the hiomass and photosynthetic production of
phytoplankton in relation to some physico-chemical environmental factors were
investigated at two (offshore and near-shore) stations in Lake Chamo from August,
2003 to May, 2004. Inorganic nutrients varied temporally, with their low and high
concentration levels generally corresponding to the dry and rainy periods
respectively. Lake's transparency was always low (Zsd < 40 cm). Because
phytoplankton hiomass was high throughout the study period, its contrihution to the
extinction of underwater light is expected to he considerahle. Phytoplankton hiomass
measured as chlorophyll a varied 43 to 95 (mean = 77.2) and 47 to 108 (mean=69.4)
mg Chi a m'^ at the near-shore and central stations, respectively. The vertical
distrihutlon of photosynthetic activity was of a typical pattern for phytoplankton with
light-inhihition on all sampling dates. The light-saturated rate of photosynthesis (Amax)
ranged from 721 to 1789 mg O2 (~224.95 to 558.17 mg C) m'^ h'^ at the central
station andtfrom 543 to 1803 mg 02( »169. 42 mg C) m'^ h'^ at the near-shore
station. Lack of correspondence hetween phytoplankton hiomass and
Amax was
evident. Bipmass-specific rate of photosynthetic production at light-saturation,
photosynthetic capacity (<Pmax) ranged from 9 to 34 mg O2 (mg Chi a)''' h'^ at the nearshore
station
and
from
10
to
34
mg
O2
(mg
Chi
a)'''
h'''
at
the
central
station.
An
inverse
relationship
hetween
photosynthetic
capacity
and
phytoplankton
hiomass
was
also
ohserved.
Hourly
integral
photosynthesis
ranged
from
0.36
to
1.72
g
O2
("
0.11
to
0.54
g
C)
m'^
h'^
at
the
central
station
and
from
0.47
to
1.8
g
O2
(^
0.15
to
0.56
g
C)
m'^h'^
at
the
near-shore
station.
The
production
rate
per
unit
area
seems
to
have
heen
influenced
hy
the
light-saturated
rate
of
photosynthesis
as
the
strong
and
positive
correlation
hetween
the
two
(r
=
0.77)
suggests.
The
temporal
variations
in
phytoplankton
hiomass
and
primary
production
in
relation
to
environmental
factors
are
discussed
with
a
view
to
identify
those
environmental
variahies
that
have
regulatory
role.
