Open Journal of Modern Hydrology, 2012, 2, 1-6 Published Online January 2012 (
The Ecology of Keratella cochlearis in Lake Kinneret
Moshe Gophen1,2
1Migal-Galilee Technology Center, Kiryat Shmona, Israel; 2Tel Hai College, Kiryat Shmona, Israel.
Received July 27th, 2011; revised October 5th, 2011; accepted November 12th, 2011.
A long term record of rotifers in Lake Kinneret (Israel) indicated that Keratella cochlearis is the most common zoo-
plankter in it. Population dynamics parameters (“egg/female ratio” procedure) were combined with limnological data to
analyze temporal fluctuations of K. cochlearis in the Kinneret ecosystem. Lake Kinneret is characterized by meso-
oligotrophic conditions. During the winter-spring period, when the lake is completely mixed and at low temperatures,
water and nutrient inputs are maximal. The lake is stratified in the summer, with an oligotrophic epilimnion; at high
temperatures, nutrient and water inputs are minimal. Since the early 1990’s, a decline in rainfall and water inputs and a
reduction in Peridinium, coinciding with an ascending level of blue greens, were documented. K. cochle aris is affected
by the water input regime, temperature, nutrient concentrations and the phytoplankton composition. The densities of
individuals and egg bearing females are low in the summer and fall, and high during the late winter-spring, with E/F
values being the highest during January-April. Population dynamics parameters were respectively similar. Regressions
of densities of individuals and egg bearing females vs time (years) revealed a prominent long term decline.
Keywords: Keratella cochlearis; Lake Kinneret; Long Term; Seasonal Dynamics
1. Introduction
Keratella cochlearis (Brachionidae) is distributed worl-
dwide. The species is common and is a constant con-
stituent of the Lake Kinneret (Israel) zooplankton com-
munities, although it only contributes a low biomass to
the entire assemblage. The common spatial and temporal
occurrence of K. cochlearis, coinciding with limnologi-
cal changes, initiated the present study, which deals with
the impact of ecological conditions on the distribution of
this species. The long term data record, which was estab-
lished by the Kinneret Limnological Laboratory, was the
basis of the study [1,2].
Lake Kinneret, Israel, supplies 16% - 30% of the na-
tional water demands and >55% of the drinking water.
The drainage basin area of the lake is 2730 km–2, of
which, about 200 km–2 is the “Hula Valley” that contrib-
utes >50% of the nutrient input. The lake is stratified
from May to mid-December (with an anoxic hypoli-
mnion) and is totally mixed from mid-December through
April. The trophic status of Lake Kinneret can be classi-
fied as mesotrophic in the winter-spring season and olig-
otrophoic in the summer-fall period [3]. Over the last 60
years, the Kinneret ecosystem has undergone several
man-made modifications: construction of a dam at the
south end; diversion of salty springs and salinity fluctua-
tions (200 - 300 ppm Cl); construction of the National
Water Carrier (NWC) through which about 1 million m–3
(mcm) is pumped from the lake; exotic and native fish
stocking and subsidized bleak fishing [4]. High range
fluctuations of natural conditions were recorded, with
seasonal ups and downs, as well as a long term decline in
the water level, water inputs, plankton biomass and spe-
cies composition [5]. The subtropical climate conditions
of the Kinneret region are characterized by high and low
levels of water and nutrient inputs in winter and summer
months, respectively. The epilimnetic loads and plankton
biomass present similar trends [6,7].
The lake is exploited for its fishing by ca 200 licensed
fishermen, who commercially remove an average of
1832 tons of fish (108 kg/ha) per annum (1970-2005). In
recent years, fishery landings have declined extremely.
A decrease in total phosphorus (TP) concentrations
during the 1980’s and 1990’s, a decrease in organic ni-
trogen throughout 1970-2001 a decline in TSS and ni-
trates during the 1990’s and a slight increase in (SRP) in
the Jordan River waters have been recorded.
The following trends in the limnological processes
within the Kinneret epilimnion (0 - 15 m) were recorded
during 1969-2004: a positive relationship between the
total nitrogen (TN) concentration and the biomass of
Copyright © 2012 SciRes. OJMH
The Ecology of Keratella cochlearis in Lake Kinneret (Israel)
Pyrrhophyta, with a consequent Peridinium biomass sup-
pression; a slightly increased TP concentration; a decline
in TN [8]; a reduction in large cell phytoplankter (Perid-
inium) biomass, an enhancement in that of nanophyto-
plankton (Chlorophyta, diatoms and mostly Cyanobacte-
ria, such as Microcistis and Aphnizomenon); a decline in
the N/P mass ratio enhanced biomass of N2-fixing cya-
nobacteria [9]; modification of Lake Kinneret from a P to
an N limited ecosystem; a slight decline in the total bio-
mass of phytoplankton (nano-phytoplankton dominance)
under a lower water level (WL); and a decrease until
1990, with a subsequent increase, in the zooplankton bio-
mass (92% due to Copepoda, and Cladocera). The lower
water level that was followed by a decline in N and a
slight increase in P accelerated the shift from a domi-
nance of large cell algae (Peridinium) to that of the sma-
ller cell size algae (nano-phytoplankton). The decline in
N together with an increase in P (lower TN/TP mass ra-
tio), with an enhanced N deficiency and a P sufficiency,
resulted in cyanophyte blooming [9]. It is likely that the
high biomass of small cells (particles) caused a decline in
light penetration (higher reflection), as indicated by the
shallower depth recorded by the Secchi measurements
[8]. A decline in water temperatures during 1979-mid-
1980’s, with a subsequent increase, was documented [8].
The number of zooplankton species in the Lake Kin-
neret assemblages included: Copepoda-4, Cladocera-7
and Rotifera-35 [5]; There are 24 fish species in Lake
Kinneret, of which19 are native, 5 are exotic and eight
are commercially harvested. Most fishes in Lake Kin-
neret are planktivorous and the most common are the
zooplanktivorous bleaks (Acanthobrama sp., Mirogrex
sp.), contributing to ca. 80% of the total number of fish
and more than 50% of fish biomass in the lake (Walline
et al. 2000). In this paper, I present an analysis of the
response of K. cochlearis populations to the temporal and
seasonal fluctuations in the limnological conditions dur-
ing 1972-2001.
2. Methods
Hydrological data on rain and water inputs were taken
from the annual reports of the Hydrological Service Wa-
ter Authority of Israel (1972-2001) (Figure 1). The re-
tention time of the water in Lake Kinneret was calculated
according to the relation between the inflow rates [6],
and the lake volume. Water level data were provided by
the Hydrological Service/Israeli Water Authority [6].
Zooplankton monitoring in Lake Kinneret was carried
out during 1969-2001 with weekly and biweekly sam-
pling at seven stations at 12 distinct depths. In the sample
analyses, the total number of Keratella cochlearis spe-
cimens and egg bearing females were counted, and mon-
thly averages of the number per liter were evaluated.
Physical, hydrological, chemical and plankton data were
collected from the Lake Kinneret Data Base [7], and ex-
pressed as monthly means (1969-2001) (for sampling
procedures see [9] and [3]); professional responsibilities
for the data are: chemistry-A. Nishri, phytoplankton-U.
Pollingher and T. Zohary, zooplankton-M. Gophen. Her-
zig (1983) compiled information from several studies on
temperature impact on the duration of embryonic devel-
opment in brachionid rotifers. Data given in Figure 1(b)
in Herzig [10], which are respectively related to the Lake
Kinneret epilimnion (15˚C - 28˚C) indicate that its em-
bryonic development value is shorter than a day. An av-
erage of 19 hours, i.e., 0.8 day was used in the present
study. The parameters below were employed for the
analysis of population dynamics [2,11,12].
B = E/D (1)
where B = Finite hatching rate; E = no. of eggs per fe-
male; D = Egg development time
Keratella (no./L)
Keratella W. Egg (no./L)
RT (years)
E/F (no./L)
Water Level (mbsl)
Volume (m cm)
Total inflow (mcm/m)
Figure 1. Monthly averaged (1972-2001) values of Keratella
cochlearis (individuals·L–1, Females carrying eggs·L–1 and
Eggs·Female–1) (left panels) in the epilimnion of Lake Kin-
neret and Hydrological data (Lake volume in mcm, Water
Level in mbsl, Retention time in years, and inflows in mcm·
month–1) (right and mid panels).
Copyright © 2012 SciRes. OJMH
The Ecology of Keratella cochlearis in Lake Kinneret (Israel) 3
1/B = Population Turnover Time (2)
Instantaneous birth rate,
b:b = ln (E+1)/D (3)
Instantaneous rate of population change,
r:r = (lnNt lnNo)/t (4)
where: t = sampling time interval, 14 days, Nt = popula-
tion at the end of 14 days, No = population at the begin-
ning of 14 days; Instantaneous death rate,
d:d = b r (5)
These data were collected bi-weekly and monthly aver-
ages were calculated. Annual cycles were grouped as two
periodical classes: Winter: December and January-May;
and Summer: June-November. Similarly, temporal clas-
ses were grouped in two classes: 1972-1985 and 1986-
2001, and epilimnetic (0 - 15 m) monthly averaged tem-
peratures were grouped as two classes: 1) 15.0˚C -
20.5˚C: and 2) 20.6˚C - 28.1˚C. ANOVA tests (p < 0.05)
were carried out for the population dynamics parameters
separately against the periodical and thermal groups.
3. Results
The population turnover time (T) and instantaneous rate
of population change (r) parameters at low temperatures
were found to be significantly lower than those in the high
temperature class (p = 0.0426, and 0.0454, respectively):
2.8 (SD 2.5) and 4.2 (SD 5.2) months for low and high
temperatures, respectively. Similar results were obtained
for the periodical factors (winter vs summer): p = 0.0126
and 0.0166 for T and r, respectively; 0.012 (SD 0.104)
and –0.029 (SD 0.098) for r values and 2.9 (SD 2.4) and
4.6 (SD 5.8) for T values, respectively. Instantaneous
birth rates were significantly higher (p: 0.0080 - 0.0285)
at temperatures above 24˚C in comparison to those at
20˚C - 24˚C. Moreover, according to the periodical dis-
tribution, there were high densities of K. cochlearis (Fig-
ure. 1) and egg bearing females between March and July.
The summer and winter populations behaved differently,
and the thermal range of 18˚C - 22˚C is probably optimal.
Analysis of the relations between K. cochlearis popula-
tion parameters (individual abundance, density of egg
carrying females and eggs /female) and hydrological pa-
rameters (Jordan discharge [m3·s–1], water inputs into the
lake [m3·month–1], lake water retention time [years], lake
water level [m] below sea level and lake water volume
[million·m3]) was carried out by simple regression (Fig-
ures 2(a) and 2(b)). Significant inverse relations were
indicated between egg bearing females and lake retention
time. Jordan inflows vs number of individuals and E/F
were significantly positive. Regressions between lake
volume and the respective WL, and Keratella, E/F and
egg bearing female densities were all significantly posi-
tive (Figures 2(a) and 2(b)). These significant levels are
3920 4000 4080
R ^2=0.75
Keratella (no/ l)
Lake Volume (m cm )
3920 40004080
R^2= 0.644
Lake Volume ( m c m )
Ker atella + Eg g
3920 40004080
R^2= 0.481
–211.4 –210.6 –209.8
WL (mbsl)
Keratella (no/ l)
–211.4 –210.6 –209.8
R ^2=0.735
WL (mbsl)
Ker at e lla+ E gg
–211.4 –210.6 –209.8
E/ F (ev e r)
WL (mbsl)
Lake Volume ( m c m )
E/ F (ev e r)
26 10 14
R^2 = 0.423
p =0.0221
RT (years )
E/F (aver )
20 5080 110
R^2 = 0.356
p = 0.0404
Inflow (mcm/m)
20 5080 110
R^2 = 0.57
p =0.0045
E/F ( ave r )
Inflow (mcm/m)
Figure 2. Simple regressions (R2 and p are given) between po-
pulation densities (total number, females with eggs, Female–1)
of of Keratella cochlearis and hydrological parameters (lake
volume, water level, Inflows and retention time).
Copyright © 2012 SciRes. *****
The Ecology of Keratella cochlearis in Lake Kinneret (Israel)
due to the rainy and cold winter season. Comparative
analysis (ANOVA; p < 0.05) of the hydrological values
vs winter (December and January-May) and summer
(June-No- vember) months clearly show the higher levels
of Jordan discharge and total water inputs when tem-
peratures are colder, as well as a shorter lake water reten-
tion time in winter. The entire period (1972-2000) was
divided into two terms: 1972-1985 and 1986-2000. The
ANOVA test was carried out where population dynamics
parameters were related to the two periods. Results
demonstrated that there was a significantly higher con-
centration of egg bearing females and death and birth
rates during 1986-2000, whilst population turnover val-
ues and the number of individuals were lower after 1985.
The seasonal pattern of rate of change, instantaneous
birth rate and instantaneous death rate fluctuations are
presented in Figure 4. During the six winter months (Feb.
- Apr. and Oct. - Dec.) the r values were positive and in
summer (May through September) negative. The instan-
taneous birth and death rate values were similar, with an
exceptional decline in October.
4. Discussion
It is proposed that the dry, warm summer months provide
suboptimal conditions for the growth of K. cochlearis in
Lake Kinneret. Due to the worldwide zoogeographical
distribution, K. cochlearis has drawn the attention of
zooplankton ecologists. Its feeding habits have been
documented by Bogdan and Gilbert [13]; its feeding in-
terference with Daphnia has been studied by Burns and
Gilbert [14,15], and Gilbert and Stemberger [16]; the
association of K. cochlearis with a predator cyclopoid
and the consequent feeding interference were investi-
gated by Gilbert and Williamson [17]; Stemberger [18];
studied the impact of spine development induction by the
predator Asplanchna; and Walz [19-21], has documented
population development of K. cochlearis in cultures. Not
much attention has been given to the impact of diversi-
fied limnological parameters on the spatial and temporal
distribution of K. cochlearis.
In early studies on the Kinneret zooplankton, the au-
thor partly neglected rotifer ecology because of its low
contribution to the total biomass. Nevertheless, the con-
tinuous presence of K. cochlearis in the zooplankton re-
cords of Lake Kinneret emphasized the need to deter-
mine the impact of seasonal and long term ecological
modifications within the Kinneret ecosystem. As a con-
sequence of the geographical location of the Kinneret
ecosystem within a sub-tropical region, characterized by
short, cold and wet winters and long, dry and warm
summers, the river runoffs are intensive in winter. As a
result, the lake WL is high and obviously the lake vol-
ume is accompanied by a shorter retention time (Table 1;
Figures 2(a) and 2(b)). This probably initiates optimal
conditions for Keratella food resources. Their availabil-
ity is high and the abiotic conditions are suitable. Thus,
winter populations of Keratella are larger and more pro-
ductive (higher number of egg bearing females and E/F)
(Table 2; Figure 2). The lower rates of change in winter
(Table 2) are indicated by the higher level of stability
and lower death rate values (Table 2). The long term
analysis (Table 3, Figure 3) implies that the populations,
during 1972-1985, when droughts were less frequent [6],
were notably more stable, healthy and productive. The
lower death and birth rate values, as well as the longer
turnover time and higher densities, enhanced these fea-
tures. During 1986-2000, droughts became more frequent,
runoff discharges declined, and water temperature in-
creased [6,12]. Consequently, it was suggested that food
source renewal and the shift of the Kinneret ecosystem
from a P to N limitation [8], with a lower productivity (a
lower N content in suspended particles), caused suppres-
sion of Keratella communities (Figure 1). It is proposed
that the change in phytoplankton composition, from
Peridinium to nano-phtoplankton dominancy [8,9,12],
affected the nutritional value of Keratella food sources.
The seasonal fluctuations of the population dynamics
parameters, presented in Figure 4, imply that the sup-
pression of summer (May - Sep.) Keratella populations
(Nt < No, see equation 4) is probably a result of food
availability and temperature constraints. Productive and
healthy assemblages were present in winter (Oct. - Dec.
and Feb. - Apr.). The reason for exceptionally low values
of instantaneous birth and death rates in October (Figure
4) is unclear.
Table 1. Comparative analysis (ANOVA) between Hydro-
logical values (see text) and two seasons (see also Figure 4):
1) winter (12, 1 - 5) and 2) summer (6 - 11). S = significant,
NS = not significant.
Hydrological parameter Difference Probablity
Jordan Discharge (m3·s-1) 1 > 2 0.0013
Retention Time 1 < 2 0.0021
Inflow (m3·month-1) 1 > 2 0.0100
Water Level (mbsl) NS
Water Temperature (˚C) 1 < 2 <0.0001
Lake Volume (mcm) NS
Table 2. Comparative analysis (ANOVA) between popula-
tion dynamics values (see text) and two seasons: 1) winter
(12, 1 - 5) and 2) summer (6 - 11). S = significant, NS = not
Parameter Difference probability (p)
Number of Ind. 1 > 2 <0.0001
Female with eggs 1 > 2 <0.0001
Eggs/Female 1 > 2 0.0001
Rate of change 1 > 2 0.0296
Birth rate NS
Death rate 1 > 2 0.0174
Turnover NS
Copyright © 2012 SciRes. OJMH
The Ecology of Keratella cochlearis in Lake Kinneret (Israel) 5
Table 3. Comparative analysis (ANOVA) between popula-
tion dynamics values (see text) and two periods: 1) 1972-
1985, and 2) 1986-2000. S = significant, NS = not significant.
Parameter Difference probability (p)
Death rate 1 < 2 <0.0001
Birth rate 1 < 2 <0.0001
Rate of change NS
Turnover time 1 > 2 0.0041
Eggs/Female 1 < 2 <0.0001
Females w. Egg NS
Individuals/L 1 > 2 0.0139
Figure 3. Annual averages (1972-2001), (No. L–1) of Ker-
atella cochlearis in the epilimnion of Lake Kinneret.
Mont h
–0 .1
–0 .0 5
0. 1
0. 2
0. 3
d=(death rate)
r=(rateof change)
b=(birth rate)
Figure 4. Monthly (January - December from left to right)
fluctuations of population dynamic variables of Keratella
cochlearis averaged for the period of 1969-2001 in Lake
5. Conclusion
An attempt to explain the long term and seasonal distri-
bution of Keratella cochlearis in Lake Kinneret was
made by analyzing the impact of biotic and abiotic con-
ditions. The conclusion drawn was that the winter condi-
tions are suitable for the maintenance of healthy popula-
tions, whilst droughts and/or summer conditions suppress
Keratella assemblages.
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