Changes of Copepoda populations in Lake Kiret during 1969–1985

Long period analysis of Copepoda populations in Lake Kinneret indicated significant reductions in total biomass and production as well as densities of nauplii and adult stages, particularly females. The reduction of adult Mesocyclops sp. was statistically significant, but that of Thermocyclops sp. was not. Annual averages of eggs/female values and densities of copepodite stages did not show significant changes. Concentrations of adult Thermocyclops sp. became higher relative to the larger genus Mesocyclops sp. It is suggested that densities of large copepods were affected by intensification of predation pressure by particulate feeding fishes. The densities of nauplii were probably reduced by increased predation pressure of filter feeding fish.     

General water chemistry and quality in newly-created subtropical wetland lake

Sixty km2 of the southern Hula Valley (northern Israel) peat lands were flooded in 1994 as part of the Hula Valley Restoration Project. The small, shallow lake (110 ha, mean depth < 1 m) and network of ca. 90 km of canals created were designed to ameliorate problems (e.g., underground fires, soil subsidence, increased nutrient loading downstream to Lake Kinneret) resulting from drying the Lake Hula wetlands in the 1950s. This new wetland area now serves as the focus for developing eco-tourism in northern Israel. The initial development of this new ecosystem has been followed closely by a multi-disciplinary team of researchers, with an emphasis on water quality in the new lake and the potential impact of the project on Lake Kinneret. Here we report an overview of developments in general water chemistry of Lake Agmon during its first three years (1994–1996). Water quality in Agmon was within general expectations for a shallow lake situated on peat. The first year of Agmon was characterized by the heavy influence of stream and drainage inflows with high pH, alkalinity, turbidity and electrical conductivity and high concentrations of sulfate and total dissolved solids. By the third year, however, many in lake processes (e.g., nutrient cycling and algal and macrophytic production) were well-developed and thus strongly affected lake water quality. Excessive phosphorus and nitrogen concentrations in the lake have led to hypertrophy, characterized by low dissolved oxygen concentrations and prolific blooms of nuisance algae. The management of this new ecosystem in the near future will require persistent, and innovative measures.     

Metabolic activity of herbivorous zooplankton in Lake Kinneret (Israel) during 1972-1977

Metabolic activities of herbivores zooplankton in Lake Kinneretduring 1972–1977 were analysed, based on experimentaldata. Two multiannual cycles of activity were observed: I-1972-1974;II-1975-1977. Some changes of nutritional value of the grazedmaterial in the 1975–1977 cycle are suggested.     

Acoustic assessment of the number of pelagic fish in Lake Kinneret,Israel

The pelagic fish stock (mostly lavnun, Mirogrex terraesanctae) of Lake Kinneret was assessed using HADAS, a newly developed echo-counting analysis system. Total fish numbers (during Mar–Apr) were estimated to be 61 million in 1987, 218 million in 1988, 120 million in 1989, and 69 million in April 1990. The total fish number for March 1981 was recalculated from published data and estimated at 170 million. The decline in total numbers between 1988 and 1990 resulted primarily from a decrease in the relative abundance of the smallest size classes of fish. Since fish abundance apparently declined between 1981 and 1990, the decline in zooplankton standing stocks documented during these years cannot be attributed to higher fish predation caused by larger fish stocks. The HADAS estimates of lavnun population size are large enough to account for the known catch of lavnun.Contribution from Israel Oceanographic & Limnological Research, Ltd. This research was supported in part by the Israeli Water Commissioner, the Israeli Electric Company, and by a grant from the U.S. Agency for International Development.     

Effect of Physical Factors on Bacterial Distribution in the Jordan River Mouth Area in Israel

The Jordan River mouth is an area of complicated hydrography and variable and vulnerable ecology. It has changeable water and sediment regimes, for example, large spatial gradients of biological, chemical, physical, and hydrological characteristics. The Jordan River enters Lake Kinneret in the north and carries with it sewage from the Upper Galilee; thus the river is the major source of enteric bacteria in the lake. This research is based on theoretical analysis as well as on biological-hydrodynamical measurements along a discharging free, turbulent jet flow. Three components of velocities were recorded downstream from the exit cross-section for a distance of 700 m, and the mean streamwise velocity turbulence intensity and the turbulence scale were calculated. The measuring devices were an original three-dimensional velocity-fluctuation meter. Results of these measurements show that the Jordan River flows through the whole transect until it reaches the crest of a bar. After this, there is a separated exponential flow from the bottom and upper layers for ~100m after the bar. Fecal coliforms, Escherichia coli, and Klebsiella spp. were enumerated as colony-forming units from samples taken along the Jordan River path as it entered Lake Kinneret. Bacterial numbers were similar in surface and bottom waters in front of the bar. Behind the bar, however, there was a sharp decrease of bacterial numbers in the surface water, probably because of photooxidative stress. Despite the more protective environment of the bottom waters, the numbers here also decreased, indicating that the bar is a physical barrier for bacterial distribution. Furthermore, we found a significant effect of the flow velocity of Jordan River water on the bacterial distribution in Lake Kinneret. When the velocity is high, bacteria are distributed over a longer distance, regardless of their numbers in Jordan River mouth.     

Picocyanobacteria and Bacteria in Lake Kinneret

In Lake Kinneret, we have been monitoring the numbers and depth distributions of bacteria and picocyanobacteria since early 1988. Picocyanobacteria were found throughout the year at all depths, at concentrations ranging from < 10³ to 10⁵ cells·ml⁻¹. Highest abundances were recorded during summer and fall in the epilimnion. The lowest areal standing stocks occurred from March to May, the period which corresponds to the peak of the annual dinoflagellate bloom. Bacterial numbers ranged from about 10⁵ to 10⁷ cells·ml⁻¹. The picocyanobacteria were evenly distributed in the water column during the mixed period. With the onset of thermal stratification and the end of the dinoflagellate bloom the numbers of picocyanobacteria increased rapidly in the epilimnion.     

Non-siliceous algae in a five meter core from Lake Kinneret (Israel)

The composition and succession of non-siliceous algae, studied in a five meter core from Lake Kinneret (Israel), are described. Only Chlorophyta species were recorded, probably due to the standard palynological sample processing which was used. In the lower part of the core, from the bottom to 300 cm (interval 5500–2500 years B.P.), Botryococcus braunii was the only common alga. Relevant changes in algal diversity and abundance occur at 300 cm. Many species of green algae were recorded for the first time (Pediastrum, Scenedesmus, Coelastrum, etc.). These changes may be related to an increase in nutrient concentration as a consequence of cultural disturbance. In the interval 300-0 cm, a succession of Pediastrum species is followed. The recovered green algae are extant in the present plankton of Lake Kinneret. They also constitute an important part of the algae found in the profundal sediments today.     

多甲藻对不同光照强度的反应:色素沉着及暗光适应的可能性

在ＨＰＬＣ色素分析的基础上,研究了多甲藻对不同光照强度（１６．６～２５０μ．Ｅｍ＾０２ｓ＾－１）的反应。类胡萝卡素中最重要的多甲藻素和硅甲藻黄素占叶黄素总量的９０％,随着光照强度的逐步增加,多甲藻细胞中叶绿素ａ含量呈现明显的下降趋势。结果表明硅甲藻黄素和β胡萝卜素具有光作用：１）当多甲藻暴露于较强光下时,硅甲藻黄素／叶绿素ａ之比呈现出明显增加的趋势,β胡萝卜素／叶绿素ａ之比也显现出明显的增加趋势;     

Appearance and establishment of diazotrophic cyanobacteria in Lake Kinneret,Israel

1. We propose that the appearance and establishment of Nostocales (cyanobacteria) species of the genera Aphanizomenon and Cylindrospermopsis in the warm subtropical Lake Kinneret (Sea of Galilee, Israel) from 1994 was linked to changes in climate conditions and summer nitrogen (N) availability. 2. From 1994 to 2009, an increase in frequency of events of elevated water temperature (>29 °C) in summer, and to some extent a greater frequency of lower summer wind speed events, affected water turbulence and water column stratification, thus providing better physical conditions for the establishment of these populations. 3. In recent years, N‐depleted conditions in Lake Kinneret in early summer have promoted the development and domination of Nostocales that could gain an ecological advantage owing to their N₂‐fixing capability. 4. Nitrogen fixation rates coincided both with heterocyst abundance and with Nostocales biomass. The N supplied to the lake via nitrogen fixation ranged from negligible quantities when Nostocales represented only a minor component of the phytoplankton community to 123 tonnes when Cylindrospermopsis bloomed in 2005. This high N₂ fixation rate equals the average summer dissolved inorganic nitrogen load to the lake via the Jordan River.     

The impact of zooplankton status on the management of Lake Kinneret (Israel)

Monthly averages of standing stock wet biomass of zooplankton in Lake Kinneret (Israel) varied between 11 and 76 g m^–2 during 1969–1981, with the exception of two months. Averaged contributions of different groups were: Cladocera 58%, Copepoda 35% and Rotifera 7%. Total standing crop wet biomass is highest during January–June, averages varied between 35 and 50 g m^–2, and decreases during summer–fall (23–36 g m^–2). The winter biomass of Cladocera fluctuated between 22 and 35 g m^–2 and dropped to a range of 9–23 g m^–2 in summer, whereas copepod biomass varied very little around an average of 18 g (ww) m^–2 with the exception of low values from April to June. The stock biomass of Rotifera is relatively high during winter floods season (December-March) whilst in summer it is very low.Young stages of fish in Lake Kinneret feed mostly on zooplankton and zoobenthic forms. The most abundant fish in the Kinneret ecosystem, Mirogrex terraesanctae terraesanctae, also feed on zooplankton at the adult stage throughout the year, and herbivorous fish consume zooplankton during the summer when lake plankton resources are limited.The summer ecosystem of Lake Kinneret is characterised as a steady state type, in which the impact of the zooplankton-chain is of great importance. Increase of predation pressure on zooplankton by fish can disequilibrate the balanced trophic relations existing between nannoplankton production and zooplankton grazing capacity. Such a situation can lead to organics accumulation as nannoplankton blooms, resulting in water quality deterioration. Management options aimed at preventing collapse of zooplankton populations are discussed.     

Seasonal Rotifer Dynamics in the Long-term (1969–2002) Record from Lake Kinneret (Israel)

Long-term (1969–2002) data record of biomass distribution of rotifers in Lake Kinneret is combined with previously published information on their metabolic activity and newly calculated population dynamics parameters to synthesize a model of their seasonal dynamics in Lake Kinneret. Nineteen rotifer species were recorded in routine samples collected in Lake Kinneret (Israel) in 7 offshore (deeper than 5 m), stations, at 12 discrete depths during 1969–2002. Organisms were sorted and counted (including external egg carrying females), biomass was measured and calculated for the entire lake stock (g_w.w m⁻²; mg l⁻¹). Rates of grazing, respiration and production were measured experimentally at three different temperature ranges. Results were extrapolated to the lake community for months with similar temperatures. Rotifera comprised 7% of total zooplankton biomass in Lake Kinneret whilst Cladocera and Copepoda 58 and 35% respectively. Rotifers were found to be more abundant during December–June and decline in summer months. Monthly (1969–2001) means indicated total grazing capacity of rotifers as 11%, respiration as 9% and production as 3.7% of the total zooplankton metabolic activity. Positive relations were indicated between rotifer and small bodied cladoceran numerical concentrations. Population growth models suggest that rotifers are not food limited in Lake Kinneret but that fish predation plays an important role in regulating abundance in spring-summer and fall.     

Zooplankton community changes in Lake Kinneret (Israel) during 1969–1985

Long-term records (1969–1985) of zooplankton density in Lake Kinneret indicated significant reduction of biomass (Copepods, Cladocera, Rotifera) and production (Copepods, Cladocera). Nauplius and adult copepod densities decreased but those of copepodites did not change. Mesocyclops was suppressed more than the smaller Thermocyclops and males of both genera became more abundant relative to the larger females. Ratios of small/large Cladocera densities became higher. Numbers of total cladocerans were stable, and therefore reduction of Cladocera grazing capacity is assumed. The abundant Keratella spp. were reduced. It is likely that intensification of fish visual-attack-predation pressure shifted the size-class structure towards smaller adult copepods and cladocerans. Reduction of Keratella spp. and copepod nauplii was probably affected by increasing pressure of fish filter-feeders. Data on fish food consumption, feeding behaviour and fisheries management suggested their direct impact on long-term changes of zooplankton in Lake Kinneret.     

湖水含盐量和Cu2+浓度变化对Kinneret湖浮游植物的影响

通过人为改变湖水中的Cu^2 浓度和含盐量的方法,Cu^2 浓度和含盐量变化对Kinneret湖水中浮游植物可能造成的影响进行了分析。结果表明,湖水Cu^2 浓度增加会抑制Kinneret湖水中藻类等浮游植物的生长。这对改善湖水水质来7说是非常有利的,但高的Cu^2 浓度对农作物生长和人类健康是有害的,在另一方面,对含盐量较低的约旦河水来说,适当增加Cu^2 浓度则有利于藻类的生长。Kinneret湖中浮游植物的年平均生物量随湖水含盐量的下降而有增加的趋势,特别是当含盐量低于200μ左右时,浮游植物中蓝藻占的比例会升高,这对作为饮用水资源的Kinneret湖水来说是不利的,由此可见,适当控制和维持Kinneret湖水Cu^2 浓度和含盐量对湖水水质保护具有重要的意义。