The long-term Record of Zooplankton, as part of the scientific routine comprehensive limnological research of the
Figure 1. Sampling stations (A)-(G) in lake Kinneret.
reduction of sampling stations was essential. Daily Spatial and Bathymetrical distributions of zooplankton in Lake Kinneret were previously studied as well as zooplankton ecological trait and diurnal migrations were previously documented   -  . The present paper represents a long term (1969-2015) analysis of the related distributions of fish and zooplankton.
2. Material and Methods
The zooplankton sampling procedure is given in Gophen  and Gophen and Azoulay  . The data in tables 1 & 2 illustrate the sampling capacity and statistical evaluation (mean) of zooplankton (Table 1-annually; Table 2-monthly) in
Table 1. Annual means (all stations, all dates) of Zooplankton Biomass and annual means of C.V. (see text) and total number of sampled stations, in Lake Kinneret during 1969- 1985.
@Each sampled Station is a subsample removed from mixed 7 samples collected; total samples collected at fixed depth during 1969-1985 was app. 1765 × 7 = 12,355. *C.V. = Coefficient of Variation.
Table 2. Monthly means (all stations, all dates) of zooplankton biomass (g(ww)/m2) and C.V. values, during 1969-1985.
but only Epilimnetic data were filtered for the present study. Original zooplankton data presented here were converted into Biomass units (g(ww) per m2)  . The usage of biomass parameters of fish and its food consumption in an ecosystem with respect to long-term impacts is essential. Nevertheless, due to technical difficulties (calibration), fish data is not given by biomass but numerically. The biomass approach to the ecosystem long-term analysis of zooplankton-fish interrelation is essential because fish food (zooplankton) consumption rate is more biomass than numerically dependent. The study was restricted just to the Epilimnion because neither zooplankton nor fish densities in the Hypolimnion are negligible.
Such a retrospective analysis of data, including numerous items, require suitability of statistical methods. Raw date was taped and mean values per station, per periodical intervals (monthly, annual), were computed using indicative parameters of Coefficient of Variation (C.V). CV parameter expresses the Relative Standard Variation (RSD), i.e. the Ratio of the Standard Deviation (SD) to the Mean (X). C.V. shows the extent of variability in relation to the Mean of the population:
Comparative analysis between stations and periods was carried out by Test of significance known as “null hypothesis” which is assessing the strength of the evidence against it. The “null hypothesis” is a definition of “no effect” or “no difference”. The test of significance calculates the probability (p) of having an outcome at least as far from expected if “no difference” (null hypothesis; H0) was true, the computed value of p (probability) assuming H0 is true. Therefore, the smaller the p-value is, the stronger is the evidence against null hypothesis (H0) provided by the data. Practically, if p < 0.05, H0 is rejected, indicating that differences truly existed, and if p > 0.05, “no difference” is accepted.
A summary of acoustic surveys carried out in
Figure 2. Acoustic surveys in Lake Kinneret during 1987-2005: Polynomial regression (p and r2 are given) between All Annual recordings of Fish number and years.
Figure 3. Acoustic surveys in lake Kinneret during 1987-2015: Polynomial regression (Confidence interval is indicated) between annual maximun recorded fish number and year.
species fingerlings and sub-commercial body length. Larger size frequencies are due to sub-commercial and commercial fishes of about larger than 15 cm (Total Length) (TL). Zooplankton predation pressure is mostly operated by the small sizes including larvae, fingerlings of all species and adult Sardines  -  . Presently, some of the Tilapias partly consume zooplankton. The data about stock assessment of fish in
Figure 4. Annual Means of all stations (A)-(G) of zooplankton biomass (g(ww)/m2). In the epilimnion of lake Kinneret during 1970-1995.
lake (all stations) averages of zooplankton biomass during the two periods 1970-1982 and 1983-1995 were 35 (SD 6) and 21 (SD 2) g(ww)/m2, respectively. Consequently, there is an overlap between the time of the decline of zooplankton biomass and the time the fish stock increase is confirmed. As a result of these observations, a periodical and spatial (station locations) analysis of zooplankton biomass was carried out. Results shown in Figure 5 indicate higher biomass of zooplankton in the central zone than in the peripheral stations of the lake and higher biomass in the Northern region compared to the Southern zone during 1969-1985. The probability value (p) that resulted from a comparative t-test (p < 0.05) between North and South stations indicated clear dissimilarity between those two lake regions. Correlation coefficients (r2) between monthly means of zooplankton biomass in all stations (each station Vs. all others) and grand total average were varied within the range of 0.4 - 0.9. Those values of correlation and others shown in Figure 6 indicate mutual ecological parameters other than fish predation which has an impact on zooplankton development. Fractional Polynomial Regressions of station A vs. all other stations, as well as northern Vs southern stations, are significantly similar (left and lower panels Figure 6). Nevertheless, results shown in the upper-left panel (Figure 6) indicate the highest biomass in the central region and higher biomass in the north than in the southern parts of the lake. For the indication of temporal effect, a periodical comparative study of individual stations was carried out (Figures 7-9): Stations D and K (south) (Figure 7), Station A (center) and station G (north) (Figure 8), as well as Stations D, G, and K, (Figure 9 lower left panel) confirm higher biomass in 1969-1985 than in 1986-1995. Moreover, the zooplankton biomass as averaged for the whole lake (Figure 9 upper left panel) and separately (1970-1982 and 19873-1995) clearly indicates the decline of zooplankton biomass from mid-1980’s and onwards. Periodical (1:1972-1994; 2: 1995-2005) Linear regression was tested between monthly lake means of zooplankton biomass (g(ww)/ m2: LKDB  ) record and fish stock (106 fishes /lake;   ). The result indicates
Figure 5. Monthly mean of Epilimnetic zooplankton biomass (g(ww)/m2) during 1969-1985, in the Peripherial stations: Southern-L, K, D; Northern-C, G, F. And A (Central). Left: A-Blue (upper), Peripherial-Brown (lower); Wright: Southern-Blue (lower), Northern-Brown (upper).
Figure 6. Monthly mean of Epilimnetic zooplankton biomass (g(ww)/m2) during 1969-1985; Left: in the Peripherial stations: Southern-L, K, D; Northern-C, G, F. And in A (Central); and, Wright: Fractional Polynomial Regression of Station A Vs. all Peripherial stations. Lower: Northern (C, G, F) Vs. Southern (L, K, D).
Figure 7. Monthly mean of epilimnetic zooplankton biomass (g(ww)/m2) during 2 periods: 1969-1985 (Blue) and 1986-1995 (Brown) in two southern peripherial stations: left: station D and Wright station K.
non-significant (r2 = 0.0004) and significant (r2 = 0.5325) relations for the earlier and later periods, respectively. Diet composition for Bleak fishes of all ages in
Figure 8. Monthly mean of epilimnetic zooplankton biomass (g(ww)/m2) during 2 periods: 1969-1985 (Blue) and 1986-1995 (Brown): Left: Station A (Central); Wright: Station G (Peripheral North).
Figure 9. Zooplankton biomass (g(ww)/m2). Wright: monthly means (all stations) of Epilimnetic zooplankton during two periods: Blue-1970-1982, Brown-1983-1995; Left: Monthly means in Peripherial Stations: Blue―1969-1985; Brown―1986-1995.
the 1995-2005 period, zooplankton biomass significantly declined and fish stock (mostly Bleaks) increased. It is likely that food source became limited and competition forcefully affected, making prey-predator relations significant in the second period.
The inverse relation between Zooplankton Biomass and fish (mostly Bleaks) densities was widely documented in previous studies. An increase of fish densities in the lake started from 1998 (Figure 2) when zooplankton (mostly prey favoured Cladocerans) started a decline  , Figure 3). Moreover, during 1970- 1993 a significant high harvest (app. 1000 tons per annum) of bleaks was recorded. Probably reflecting a productive stock biomass producing intensive pressure on zooplankton which continuously declined. Nevertheless optimal conditions for zooplankton growth dynamics was indicated except fish predation. Three major factors affecting Zooplankton ecophysiology in
The first step in attempting the evaluation of the relation of zooplankton distribution to fish stock data was done by the multiannual fluctuations of both fish and zooplankton densities (Figures 2-4). The data support the suggestion that zooplankton decline was due to Fish stock enhancement. The relevance of the measured fish stock size comprises from >95% of the recorded targets to small and sub-commercial body sizes which are known as zooplanktivores. Figures 2-4 confirm the inverse relation between Zooplankton density and the documented fish stock.
The second step of the study was an attempt aimed at spatial allocating of fish shoals and zooplankton population. The acoustic surveys clearly confirmed assembling of fish flocks in the peripheral parts of the Kinneret Pelagial. It is, therefore, suggested that zooplanktivory pressure is more intensive in the peripheral stations and lower in the central part of the lake. Consequently, zooplankton biomass is higher in the central parts of the Kinneret pelagial and lower in the peripheral zones. In all stations zooplkankton biomass in winter is higher than in summer. It is the result of the lower energy investment in winter by zooplankton and higher reproduction efficiency    together with the lower feeding rate of the fish.
The next step forward in the investigation was due to the dissimilarity of zooplankton density found between the northern and southern parts. Water Current and water-mass moving directions in Lake Kinneret were widely studied and documented as well as the distribution of the Jordan River input waters in the lake   . The direction of the flow of the
5. Summary and Conclusions
The grand total average of zooplankton biomass in all sampling stations (A, C, F, D, G, K, L) indicates its low level in the peripheral stations and its even lower levels in the southern region of the lake. Conclusively, the design of sampling program aimed at representing the entire Kinneret Pelagial Epilimnion should include at least one station in the southern part (stations D, K, L). If not, the lake mean value will be higher than reality. Furthermore, if sampling program eliminates the central zone (station A) the resulting value will be lower than real.
A future suggestion aimed at improvement of zooplankton sampling design is leaving 3 northern and 3 southern peripheral stations and an additional sampling station in the central region south of station A where the depth is about 30 meters.
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