Samples for phytoplankton biomass were preserved and fixed with Lugol’s iodine. Species were identified and the
biomass was determined using an inverted plankton micro- scope Olympus IMT-2, magnification of 400 × Uter-
mo¨hl, 1958.
Algal pigments
chlorophyll a,
pheopigments were extracted with 90 vv acetone and analysed spectrophotometrically Strickland and Parsons,
1972. Zooplankton samples were preserved in the 4 vv formaldehyde solution and studied by conventional quanti-
tative analysis Kiselev, 1956. Chemical analyses were performed in depth integrated water samples, using the
methods described by Grasshoff et al. 1983.
2.4. Statistics For comparisons of independent groups, Wilcoxon signed
rank non-parametric tests were used. Comparing differences between single samples in one time series, known distribu-
tion Poisson distribution for counting measurements, normal distribution for others and independence of single
samples was assumed. Absolute difference between samples was compared using confidence intervals of each single
measurement.
3. Results
3.1. Bacterioplankton abundance and heterotrophic activity The total number of bacteria TNB decreased from the
beginning sampling week 1, 7 :
02 × 10
6
cells ml
21
to the end sampling week 5, 1
: 91 × 10
6
cells ml
21
of the experi-
ment. In sampling week 4, a small peak occurred
3 :
65 × 10
6
cells ml
21
; different from the weeks 3 and 5 values at
p 0 :
021 and p 0 :
0007 ;
respectively; Fig. 1a. Bacterial production BP decreased rapidly within 3 weeks from the
initial value of 10.9 to 1.4–2.0 pmol l
21
h
21
at sampling weeks 4 and 5. The number of colony forming units
CFU was in the range of 750–950 cells ml
21
, with the exception of 2146 cells ml
21
at sampling week 4 Fig. 1a. 3.2. Extracellular enzyme activities and monomer uptake
rates V
max
of glucose uptake GI and b-glucosidase activity b-Gluc had approximately the same range of values GI
V
max
7–150 nmol l
21
h
21
, median 38.6 nmol l
21
h
21
; b- Gluc V
max
11–20 nmol l
21
h
21
, median 16.8 nmol l
21
h
21
; Fig. 2a. In contrast, V
max
values for leucine uptake LI
V. Kisand, H. Tammert Soil Biology Biochemistry 32 2000 1965–1972 1967
Fig. 1. Dynamics of basic limnological parameters: a bacterioplankton, TNB — total number of bacteria, BP — bacterial production as leucine uptake into proteins in pmol l
21
h
21
; CFU — number of colony forming units; b phytoplankton biomass and activity, Chl a — concentration of chlorophyll a and PP — primary production; c physicochemical parameters, concentration of some inorganic ions and temperature; d biomass of dominating algae, CY —
cyanobacteria, BAC — diatoms in g WW m
23
and macrozooplankton ZOOPL in g WW m
23
SE represents standard error.
were approximately 1000 times lower V
max
17.5– 441 pmol l
21
h
21
, median 132 pmol l
21
h
21
compared to aminopeptidase hydrolysis LAP values which were 100–
285 nmol l
21
h
21
, median 275.6 nmol l
21
h
21
Fig. 2a. The uptake K
M
had lower values than the K
M
of hydrolysis Wilcoxon sign rank test, p
, 0 :
05; Fig. 2b. In addition the K
M
of GIb-Gluc were lower GI 2.4–161 mM, median 13.7 mM, b-Gluc 409–883 mM, median 435 mM than the
K
M
of LILAP LI 1.6–43 nM, median 15.8 nM, LAP 5.7– 41 mM, median 15.3 mM.
For comparison of K
M
and V
max
with the other parameters the specific activity i.e. K
M
or V
max
per cell values were calculated. Glucose uptake affinity K
M
and activity V
max
coincided with the release of the low molecular weight products of primary production PP
diss
in sampling week 1 September. On the other hand, b-glucosidase affinity
was better correlated with polymeric substrates released measured as PP
diss
after the end of the cyanobacterial bloom Fig. 3a and c. While enzyme specific activity per
cell and affinity increased, the uptake of glucose declined from sampling week 3.
LAP switched from a less active but higher affinity lower K
M
system in sampling weeks 1–2 September to a more active higher V
max
system. The leucine uptake kinetics changed in a similar manner. The only difference being
that in September, leucine uptake was more active and had higher affinity Fig. 3b and d. In October, leucine
uptake was less active with slightly lower affinity. The specific affinity and activity of leucine uptake had the lowest
values in sampling week 3, its K
M
increased almost 10 times by sampling week 5 Fig. 3d.
V
max
and K
M
coefficients of uptake and hydrolysis, which may also be used to compare efficiency, had a reciprocal
relationship with each other Fig. 4: uptake efficiency was higher when enzyme efficiency was lower. Glucose uptake
was much greater than the amount of glucose generated by hydrolysis of b-glucosidase. On the other hand, the rate at
which leucine was taken up and the aminopeptidase effi- ciency had nearly the same values, which were also close
to glucose uptake values.
3.3. Physicochemical, phytoplankton and zooplankton variables
Water temperature decreased from 17 8C sampling week
1 to 1.1 8C sampling week 5, Fig. 1c, NO
2
-N concentration was very low, throughout the sampling period and near to
the detection limit 1 mg N m
23
. NO
3
-N was constant for most of the sampling period approx. 0.05 mg N m
23
, but twice as much as the mean level at sampling week 4
0.1 mg N m
23
. The concentration of PO
4
-P was the high- est in sampling week 2 0.02 mg N m
23
and the concentra- tion
of NH
4
-N was
highest in
sampling week
3 0.021 mg N m
23
, Fig. 1c. The concentrations of total nitrogen 0.92–1.9 mg N m
23
and phosphorus 0.036– 0.059 mg P m
23
followed the changes in chlorophyll a concentration Chl a, which had a maximum value in
sampling week 2 Fig. 1b. Although within the phytoplank- ton community cyanobacteria dominated, abundance of
cyanobacteria decreased continuously
throughout the
sampling period from 33 to 13.8 g WW m
23
. In contrast, the abundance of diatoms increased almost five times with
the maximum value 4.6 g WW m
23
occurring in sampling week 5 Fig. 1d. Some species of Chrysophycea e.g. mixo-
trophic Dinobryon sp., Hyalobryon sp., Pseudokephyrion sp. became more abundant in sampling week 3 biomass
0.4 g WW m
23
. PP
part
was at
a high
level 81.5
mg C m
22
h
21
in sampling week 1, but declined suddenly in sampling week 2. This coincided with the highest values
of released PP
diss
and Chl a Fig. 1b. PP
part
increased in weeks 2 and 3, but declined again in week 5 the lowest
recorded values, 6.7 mg C m
22
h
21
. Macrozooplankton biomass decreased to about 10 of
V. Kisand, H. Tammert Soil Biology Biochemistry 32 2000 1965–1972 1968
Fig. 2. Extracellular hydrolysis and uptake kinetic parameters. Points are the mean of five measurements: GIb-Gluc — glucose and b-glucosidase
uptake; LILAP — leucine uptake and aminopeptidase.
the original level during the investigation period 1.5– 0.15 g WW m
23
, Fig. 1d. Numerically, rotator Polyarthra luminosa dominated with the exception of week 1–2 when
copepods, cladocerans and rotators had almost equal abun- dance. Biomass of copepods dropped in week 2. At the same
time biomass of rotators mainly Polyarthra luminosa increased and reached the same level as that of cladocerans
dominant species Chydorus sphaericus.
4. Discussion
