Long term record (1933-2014) of Water Level (WL), nutrient concentrations, plankton densities, and temperatures in the epilimnion of Lake Kinneret was analyzed. The aim is to identify if water quality is deteriorated when the WL is low. It was found that water temperature increased and the composition and biomass of plankton communities were modified. Nitrogen and TDP decreased but TP slightly increased in the epilimnion during low WL conditions. The quality of epilimnetic water was not deteriorated and followed by a slight oligotrophism trend.
Lake Kinneret is the only natural freshwater lake in Israel located in the Syrian-African Rift Valley in northern Israel. The Israeli climate conditions are varied between desert in the south to subtropical in the north and mild Mediterranean in the center. Above 95% of the Israeli natural water resources are utilized. Rain distribution over Israel (total 7.9 bcm/y) varied between 1300 (north) < 100 (southern desert) mm/y: 70% evapo-transpiration, 5% runoffs, and 25% infiltration. Total national water supply is 2.11 bcm (109 m3) of which 0.55 bcm from the Kinneret-Jordan system and 0.7 bcm from desalination sources. Thirty percent (0.750 bcm/y) of supplied water is given as drinking qualities for housing consumption. The only open options to enhance water resources are desalination and recycling. Averaged water budget for Lake Kinneret is given in
Lake Kinneret was designed as major source of drinking water supply. In 1933 the south end of the lake was
Input | |
---|---|
River Jordan | 480 |
Golan Heights Rivers | 145 |
Direct rain | 75 |
Eastern Galilee | 75 |
Diversion from Jarmuch | 25 |
Total input | 800 |
Output | |
Evaporation | 280 |
National Water Carrier | 380 |
To South Jordan via Dam | 80 |
Local consumers | 60 |
Total output | 800 |
blocked by a dam. From that time the outflow is controlled by lake management policy which is limited by hydrological, national regulations, management design and environmental constrains. Recently, desalinated water replaces most of the Kinneret resources. The National Water Carrier (NWC) was constructed during the 1950s (operated 10.6.1964). During the last 49 years the NWC system withdrew approximately 15 bcm of water from the lake (ca 3.8 times the lake volume) for drinking, agriculture, industry and aquifer recharging. Nevertheless, as a result of water level restriction management, about 4 bcm of water were released through the south dam to the Dead Sea. Lake water salinity is highly fluctuated: 300 - 330 ppm chloride before 1960, 400 ppm during early 1960s and 200 - 210 ppm during early 1980s and then gradually increased to 280 - 300 ppm in 2002. Lake kineret became a significant supplier of salts to the Israeli soils and aquifers. About 9 million tons of dissolved salty ions (Cl, CO3, Na, Mg, K, Ca, SO4) deteriorate soils and aquifer qualities in the southern part of Israel.
Regional HydrologyThree major rivers (Hatzbani, Banyas and Dan) flow from the Hermon mountain region, located in the north part of the Kinneret drainage basin. These rivers joint into one river of Jordan which is crossing the Hula Valley through two major man-made canals: 80% and 20% of the Jordan water flow via the eastern and western canals respectively. In the south end of the valley the two canals joint into one water flow, the Jordan route flowing downstream into Lake Kinneret. Hula Valley altitude is between 170 - 61 m above sea level and Lake Kinneret WL is fluctuated between 208.80 - 214.87 m below sea level. The upper and lower WL legislation was highly discussed over the years. The upper limit (red line) was decided as −208.8 and was never changed. The background for that regulation was due to existed housing distance from shoreline and predicted compensations resulted by potential damage. The lower limit was mostly due to limnological trait and predicted impact on water quality. Because of the flexibility of this issue and water supply vs rain gauge (drought) constrains, the lower limit was changed several times. The constrains of water supply during drought forced to lower the bottom limit which was therefore changed several times: −212, −213, and −215. The cardinal dispute that is never clearly settled is therefore: How low can WL be? The Jordan river contributes about 63% of the Kinneret water budget and more than 50% of total external nutrient inputs of the total originate in the Hula Valley Region. The drainage basin area of Lake Kinneret is 2730 km2, located mostly northern to the lake of which “Hula Valley” is about 200 km2. From 1972 the hydrological management of the Kinneret-River Jordan system was controlled by both rain gauge and the “NWC management”. NCW management included maximum close dam that was aimed at maximum storage and national supply by pumping through the “carrier”. It is likely that close dam policy might be disadvantageous. Before Dam construction nutrient rich winter input floods crossed the lake in upper layers resulted by their higher temperature than that of the epilimnetic water and naturally left out through the open outlet.
Close dam management enhanced nutrient retaining in the hypolimnion and the sediments. Water withdrawal was done by daily pumping from upper layers. In comparison with “no dam” condition, “close dam” management might enhance nutrients accumulation. Moreover, winter demands for supply are lower than in summer, therefore additional nutrients retaining in the lake sediments is predicted because in winter nutrient concentrations are higher. Nevertheless, when lakes are comparatively analyzed it has to be considered that in Lake Konstanz for example only 1% of the input is pumped and no Dam construction whilst in lake Kinneret app. 60% of the total inputs were pumped prior to the desalination period. Moreover, water retention time is very significant parameter. In Lake Kinneret this value is 5.6 years when maximum permitted WL (208.8 m below sea level) is maintained.
The Kinneret ecosystem has undergone significant changes during the last 70 years. Some of the changes are natural like droughts and floods, and others are anthropogenic like land-use in the drainage basin or fishery and salts diversion in the lake. Increasing population up to above 200,000 inhabitants in the drainage basin, sewage removal and fishpond restriction in the catchment, operating new agricultural technologies (crop types, irrigation) and development of eco-tourism [
The lake is exploited for its fishing by ca 200 licensed fishermen which remove commercially an average of 1600 ton of fish (94 kg/ha) per annum. The zooplanktivorous Lavnun (bleak, Acanthobrama spp.) comprised 55% by weight of total catches and >50% of the stock biomass. Among 8 commercial species out of 24 recorded the native Tilapias (averaged 326 t/year) and the exotic Mugil (averaged 160 t/year) are the most important in the commercial landings [
Data of temperatures, nutrient concentrations and plankton densities in the epilimnion, were taken from the Lake Kinneret Data Base ( [
Statistical analyses used in this study were taken from STATA 9.1, Statistics-Data Analysis and Stat View 5.1, SAS Institute Inc. The analyses used were: ANOVA (p < 0.05), polynomial and linear regressions, fractional polynomial prediction, LOWESS (0.8). These statistical evaluations were used in two ways: 1) Regressions between each parameter and its monitored orderly decline respected WL values. Results are presented in figures; 2) Analysis between periodical (years) groups by comparative evaluation of the parameters (plankton, nutrients, etc.) in that time frame. The WL values were also included in the analysis to enable consideration of WL impact. Results are presented in tables.
Number | Period | Average WL (mbsl) | Relative scale (high > low) | Probability (p) |
---|---|---|---|---|
1 | 1933-1953 | -210.55 | 1 < 2 | <0.0001 S |
2 | 1954-1973 | -209.77 | 1 < 3 | 0.1069 NS |
3 | 1974-1993 | -210.39 | 1 > 4 | <0.0001 S |
4 | 1994-2013 | -211.84 | 2 > 3 | <0.0001 S |
2 > 4 | <0.0001 S | |||
3 > 4 | <0.0001 S |
The period after the construction of the south Dam, (1933-2014) or the period later than the operation of the NWC (1965-2014) were considered and divided differently: the first divided into 4 groups of 20 years each. The second period was divided into two groups, 24 years each. The two methods indicate clearly low WL later than 199O. The paper is focused on comparative analysis of the epilimnetic water quality and temperature aimed at an attempt to identify long term impact of low WL. The formal daily monitoring of WL in Lake Kinneret WL started in 1926. The precision of WL information prior to 1931 is doubtful. Therefore these data will not be considered. Measurements of the Jordan outlet threshold bottom indicated an altitude of −212.35 mbsl and WL data consequently indicate water depths at the Jordan outlet of 4 m and 2 m in winter and summer respectively (Fig- ure 1). Reports and photos from late 1920s and early 1930s evidently show people, and horses pulling wagons crossing this river section by foot. Consequently the period of 1926-1933 is neglected in this paper. The WL of Lake Kinneret was prominently lower during 1994-2013 (
discharge was reduced on the concentrations of TN, TIN, TDP, zooplankton (all groups, herbivorous and predator), pyrrhophyta biomass in the epilimnion of Lake Kinneret (Figures 4-11).
Wl decline in Lake Kinneret is mostly resulted by rain reduction and partly by water supply (pumping) regime. The major outcomes of the Kinneret WL decline were primarily changes of nutrients dynamic and consequently
Nutrient | r2 | p | Significance |
---|---|---|---|
Lake WL | |||
PON | 0.126 | <0.0001 | S |
TIN | 0.086 | <0.0001 | S |
Kieldhal total | 0.085 | <0.0001 | S |
TDN | 0.130 | <0.0001 | S |
TDP | 0.090 | <0.0001 | S |
TP | 0.010 | 0.1359 | NS |
Pyrrhophyta | 0.127 | <0.0001 | S |
TN | 0.187 | <0.0001 | S |
Jordan disch. (mcm/m) | |||
PON | 0.098 | <0.0001 | S |
TIN | 0.558 | <0.0001 | S |
Kieldhal total | 0.099 | <0.0001 | S |
TDN | 0.368 | <0.0001 | S |
TDP | 0.079 | <0.0001 | S |
TP | 0.208 | <0.0001 | S |
Pyrrhophyta | 0.135 | <0.0001 | S |
TN | 0.339 | <0.0001 | S |
Total zooplankton | |||
---|---|---|---|
Periodical mean WL (mbsl) | Periodical average (g/m2) | Relative scale | Significance (p) |
2 (−209.77) | 41.4 | 2 > 3 | <0.0001 S |
3 (−210.39) | 25.7 | 2 > 4 | <0.0001 S |
4 (−211.84) | 29.9 | 3 < 4 | 0.0079 S |
Cladocera | |||
2 (−209.77) | 23.4 | 2 > 3 | <0.0001 S |
3 (−210.39) | 15.2 | 2 > 4 | 0.0032 S |
4 (−211.84) | 18.3 | 3 < 4 | 0.0102 S |
Copepoda | |||
2 (−209.77) | 14.9 | 2 > 3 | <0.0001 S |
3 (−210.39) | 8.7 | 2 > 4 | <0.0001 S |
4 (−211.84) | 9.5 | 3 < 4 | 0.2255 NS |
Rotifera | |||
2 (−209.77) | 3.1 | 2 > 3 | 0.0037 S |
3 (−210.39) | 1.9 | 2 > 4 | 0.0365 S |
4 (−211.84) | 2.1 | 4 > 3 | 0.4445 NS |
Herbivorous zooplankton | |||
---|---|---|---|
Periodical mean WL (mbsl) | Periodical average (g/m2) | Relative scale | Significance (p) |
2 (−209.77) | 32.0 | 2 > 3 | <0.0001 S |
3 (−210.39) | 20.3 | 2 > 4 | <0.0001 S |
4 (−211.84) | 23.9 | 3 < 4 | p = 0.0084 S |
Herbivorous copepoda | |||
2 (−209.77) | 5.5 | 2 > 3 | <0.0001 S |
3 (−210.39) | 3.2 | 2 > 4 | <0.0001 S |
4 (−211.84) | 3.5 | 3 < 4 | p = 0.2230 NS |
Predator copepoda | |||
2 (−209.77) | 9.4 | 2 > 3 | <0.0001 S |
3 (−210.39) | 5.5 | 2 > 4 | <0.0001 S |
4 (−211.84) | 6.0 | 3 < 4 | p = 0.2230 NS |
TN (ppm) | |||
---|---|---|---|
Periodical mean WL (mbsl) | Mean concentration (ppm) | Relative scale | Significance (p) |
2 (−209.77) | 0.568 | ||
3 (−210.39) | 0.582 | 4 < 3 | <0.0001 S |
4 (−211.84) | 0.439 | 4 < 2 | p = 0.0005 S |
TP (ppm) | |||
2 (−209.77) | 0.013 | ||
3 (−210.39) | 0.017 | 2 < 3 | <0.0001 S |
4 (−211.84) | 0.017 | 2 < 4 | <0.0001 S |
TIN (ppm) | |||
2 (−209.77) | 0.160 | ||
3 (−210.39) | 0.150 | 4 < 2 | p = 0.0294 S |
4 (−211.84) | 0/115 | 4 < 3 | p = 0.0220 S |
TDN (ppm) | |||
2 (−209.77) | 0.368 | ||
3 (−210.39) | 0.431 | ||
4 (−211.84) | 0.321 | 4 < 3 | <0.0001 S |
TDP | |||
2 (−209.77) | No data | ||
3 (−210.39) | 0.008 | ||
4 (−211.84) | 0.006 | 4 < 6 | <0.0001 S |
Zooplankters | Temper. (˚C) | Production (mgC/mgC/day) | Respiration (mgC/mgC/day) | Consumption (mgC/mgC/day) | Efficiency (%) |
---|---|---|---|---|---|
Hebivorous copepods | 15 | 0.050 | 0.288 | 0.750 | 45 |
Hebivorous copepods | 20 | 0.113 | 0.550 | 2.750 | 24 |
Hebivorous copepods | 27 | 0.167 | 1.125 | 3.250 | 39 |
Predator copepods | 15 | 0.048 | 0.163 | 0.465 | 45 |
Predator copepods | 20 | 0.103 | 0.287 | 2.142 | 18 |
Predator copepods | 27 | 0.160 | 0.499 | 2.517 | 26 |
Cladocera | 15 | 0.140 | 0.153 | 1.200 | 24 |
Cladocera | 20 | 0.160 | 0.190 | 2.750 | 13 |
Cladocera | 27 | 0.210 | 0.413 | 4.650 | 13 |
Rotifera | 15 | 0.047 | 0.153 | 0.750 | 27 |
Rotifera | 20 | 0.080 | 0.190 | 2.375 | 11 |
modification of phytoplankton community structure and biomass. Other effects were modifications of zooplankton population composition and biomass densities. Fish stocks changes as reflected by annual landings were insignificantly correlated with WL Studies of the impact of WL decline on the limnological conditions of lakes commonly include examples of extreme cases like Lake Chad, Aral Sea and Lake Sivan. These kind of comparative consideration as well as those with man-made reservoirs should be carefully evaluated. Moreover, even if WL fluctuations are moderate in lakes under different anthropogenic operational management the system response might be different. WL decline in Lake Kinneret varied mostly within the maximum amplitude of 6 meters and the Kinneret is a deep lake with Max. and mean depths of 45 m and 26 m respectively. The lowest WL permitted in Lake Kinneret is limited to the depth of the intake of the NWC (215 m mbsl) and the upper limit is legislated to 208.8 mbsl. About 55% of water inputs in Lake Kinneret are pumped for human consumption. The comparison of WL decline conditions between two lake ecosystems which have different type of Hypsometric curve are misleading. The shoreline length of Lake Kinneret is 53 km and the Value of the Development of
Shoreline is 1.16 and residence time is 5.6 years. Therefore comparative study of WL decline in Lake Kinneret with shallow lakes or very deep lakes, with different residence time, thermal structure (amictic, monomictic or polymictic), the amplitude range of the WL change, and others, are mostly irrelevant due to significant differ of limnological conditions between those ecosystems. Human population (and sewage production) and pollutant inputs are significant parameters involved. In Lake Konstanz (Germany-Austria-Swiss) population density in the vicinity of the lake is app. 4170 whilst in Lake Kinneret 950 residents per 1 km of shoreline, respectively, or, 19,500 residents and 12,500 capita per 1 km3 of lake water in Konstanz and Kinneret respectively. In Lake Sivan in Armenia, WL was lowered by 19 meters, whilst in Lake Kinneret only maximum of 6 m but usually about 1.5 - 2.0 m. The shallow lakes of Aral Sea and Lake Chad were dried as a result of extreme WL decline whilst in Lake Kinneret only 10.3 km2 are exposed when WL decline from maximum to minimum permitted (208.8 - 214 mbsl). WL decline in lake Chad caused water surface reduction of 12,000 km2 (48%) due to climate change and increased water consumption. In the Aral Sea 70% of the water inflows were diverted for agriculture and WL declined by 15 meters. Neither Chad, and Aral nor Sivan lake are dissimilar ecosystems to Lake Kinneret and comparison is misleading.
Epilimnetic temperatures data indicates cooling trend of Kinneret water during 1970-mid 1980s when WL was more or less consistently high. Epilimnetic lake water warming (by 1.8˚C) documented afterwards during WL decline. During WL decline the temperature of the thermocline increased and its depth was reduced (shallower) with consequent reduction of the volume of the epilimnion. Warmer epilimnion, shallower and warmer thermocline indicates elevation of epilimnetic specific Heat capacity (cal/m3). It is in agreement with intensification of light absorbance expressed as shallower Secchi depth, due to enhancement of the density of small particles (nano-phytoplankton). The shift of phytoplankton composition from large cells Peridinium to small sized algae with higher particle density probably enhanced heat capacity in the epilimnion. The implication of the warming process might have an impact on the lake metabolism: enhancement of biological, microbiological, chemical and obviously physical rate of processes. A parameter which contributed to the warming process of the epilimnion under regime of WL decline is the Albedo factor. Temporal changes of the epilimnetic and 3 m above water surface (
pattern of air temperature changes in Hula Valley, located northern to the Kinneret. It was suggested that in the Hula Valley changes are due to fluctuations of Albedo values resulted in by Land Use modifications in the Hula Valley. Increasing of epilimnetic heat load in Lake Kinneret occur because of consecutive events: WL decline, reduction of the surface water area, and reduction of the epilimnion volume. The ratio between radiation and reflectance of solar energy (Albedo) from water surface (app. 5%) is the same in both high and low WL. Nevertheless, change of heat capacities emerged from dimensions (surface area and epilimnetic volume) reduction. The decline of Kinneret water level was followed by diminishing of water surface area which lowered total capacity of evaporation induced cooling impact, even so, heat capacity was enhanced. WL decline accompanied by surface reduction has created two factors of heat enhancement: 1) reduction of cooling effect by smaller capacity of evaporation; and 2) lower Albedo total heat addition. Those two factors probably caused an elevation of epilimnetic temperatures. If low WL is a continual case, elevated heat capacity is accumulated. To understand how WL decline contribute to the increase of epilimnetic temperature the following computation is presented:
Berman (1976) Stanhill and Neuman (1978), and Serruya (1978), documented annual averaged incident radiation (solar plus sky) for the Kinneret region as 15.1 × 106 J∙m−2d−1.
Albedo value for water covered surface is consider as 5%.
Lake water surface at altitude of 209 mbsl and at 214 mbsl is 169.5 km2 and 159.2 km2 respectively (10.3 km2 = 6.1% reduction).
Epilimnion volume at 209 mbsl is 3184 × 106 m3 (thermocline depth 22 m) and at 214 mbsl −2258 × 106 m3 (thermocline depth 16 m) [
Annual total solar and sky radiation on the entire surface of lake water is 949 × 1015 J and 892 × 1015 J for 209 and 214 mbsl respectively.
If reflectance of 5% (Albedo) is subtracted from incident radiation, annual absorbance is 9020 × 1014 J and 8470 × 1014 J for 209 and 214 WL altitude respectively.
Each m3 of the epilimnion absorb annually an additional 2.833 and 3.751 × 1014 J per m3 at 209 and 214 WL altitude respectively.
The increase of heat absorbance from 2.833 to 3.751 × 1014 J per m3 represent additional heat capacity of 32.4% per year. Consequently, WL decline by itself enhance additional heat load to the epilimnion of Lake Kinneret.
Undoubtedly the volume of the hypolimnion is significantly (r2 = 0.462) reduced but volume of the epilimnion is just slightly become smaller when WL is decline from 209 to 214 mbsl. Even if the rate of biological activities in the epilimnion does not modified, their wastes (plankton mortality, fish and zooplankton excretion, etc.) are settled into a smaller volume of the hypolimnion. Consequently, the concentration of nutrients (H2S, NH4, CO2, etc.) together with un-aerobic microbial products significantly increased (H2S: r2 = 0.582, NH4: r2 = 0.401; TP: r2 = 0.409; data is not given here). Moreover, biological activities are probably enhanced as a result of the epilimnetic temperature increase resulted by WL decline. Recent report of The Kinneret Limnological Laboratory [
Serruya & Pollingher [
The zooplankton biomass density (g/m2) data presented in
Parameter | Low WL characterization |
---|---|
Temperature | Increase |
Nitrogen | Low |
Phosphorus | High |
%Peridinium | Low |
%Cyanobacteria | High |
%Chlorophyta | High |
%Diatoms | High |
Total phytoplankton | Low |
Zooplankton | Low |
Water quality: W toxic Cyanobacteria | Low |
Water quality: WO toxic Cyanobacteria | High |
(W = with; WO = without).
impact of zooplnktivorous fish, (bleaks) on zooplankton was previously discussed [
The impact of WL decline on the epilimnion of Lake Kinneret is summarized in
This paper should not be considered as a recommendation to decline WL in Lake Kinneret and high WL is also not ultimately preferred. The paper is aimed at an attempt to consider provisional WL decline resulted by external constrains as an acceptable temporary option.