D I S C U S S I O N
H.B. Gunner, USA
One could not anticipate that given its subtropical location, high light intensity and the high
concentrations of nitrogen in its watershed solid, Lake Kinneret could be oligotrophic and poor in
algae, the more so in that it is a lake rich in major elements from a variety of sources which are further
concentrated by intense evaporation. Thus, given the physical and chemical conditions of the Lake,
intensive studies by Serruya and her colleagues during the past three years suggest that the dinoflagellate population is less dangerous to the lake equilibrium than its natural competitor, the blue
green alga Microcystis. The latter with its high rate of nitrate uptake will tend to generate conditions
unfavourable to denitrification and to increase the amount of organically bound nitrogen. Conversely,
the development of a species such as Peridinium which required a more modest amount of nitrogen,
will be, for an equal biomass more favorable to denitrification. Nor will other links in the food chain
remain untouched: if the Microcystis constitutes an available source of food to the Zooplankton, the
herbivorous species of Zooplankton will increase their biomass with a subsequent further increase in
the total amount or organically bound nitrogen.
Thus from an ecological point of view good policy would appear to dictate the encouragement of
the natural population and in fact to try to maintain the Peridinium against Microcystis. It could,
however, be asked whether since both are dependent on relatively high levels of nitrogen they could
both not be eliminated by the drastic reduction of nitrogen input into the Lake.
Unfortunately, the probable result of such an undertaking would be the emergence of blue green
algae such as Nostoc or Anabaena which are capable of fixing molecular nitrogen. These algae now
present in Lake Kinneret in low numbers, relieved of the competitive pressure of Peridinium and able
to avail themselves of atmospheric nitrogen, could generate blooms virtually beyond control.
Unquestionably the decrease in organic matter accretion must be seen as one of the long term
treatments necessary to preserve the present equilibrium of the lake. However, the decrease of inorganic nutients and especially of nitrates must be progressive; otherwise the atmospheric nitrogen
fixing algae will displace the dissolved nitrogen users.
Therefore modifications of the present situation must be weighed very carefully for a variety of
potential consequences. Dr. Eren's paper carries the suggestion that the pumping schedule of lake
water might be changed according to the vertical migration of Peridinum. Certainly he is aware that
pumping of deep water will accentuate the warming of the lake which apart from the wide spectrum
of other changes this will invoke, will also encourage Anabaena bloom. This is not to deny the costly
nuisance generated by Peridinum in the Israel National Water Systems but to stimulate recognition of
its very critical role at another remove. Ideally, the insinuation of an additional link in the food chain
such as Tilapia whose harvest removes a portion of the biomass from the immediate environment,
remains the biological treatment of choice and it is perhaps in this direction that present efforts should
be channelled.
1. Serruya, C. Kinneret Limnological Laboratory. Progress Report No. 7. 1971.
M.C.B. Hotz, Canada
If there is bacterial degradation in the aquifer, is this aerobic or anaerobic? How deep are these
aquifers?
Reply
The organic matter accumulating in recharge wells is first decomposed aerobically, as the water
which is recharged contains dissolved oxygen.
When the recharge is terminated there is rapid depletion of oxygen and anaerobic conditions
appear. The sandy aquifer in which most of the recharge operations take place is few tens of meters
deep.
A.A. Rosen, USA.
In earlier work, Eren suggested that H2S evolved from anaerobic benthic deposits may directly
stimulate the growth of oscillatoria. Has further work supported that view?
Reply
We have evidence that our strain of Oscillatoria chalybea requires a reducing environment and low
light intensity. It is lysed rapidly in strong light (2000 lux or more) in presence of oxygen or Η 2 θ 2 ·
It seems that presence of H 2 S affects survival of the alga, as it protects it from photooxidative
lysis.
M. Halm an n, Israel
Experiments I have made with bottle tests of algal growth prove that in lake Kinneret the only
element limiting algal growth is phosphorus.
201
H.B. Gunner, USA
One could not anticipate that given its subtropical location, high light intensity and the high
concentrations of nitrogen in its watershed solid, Lake Kinneret could be oligotrophic and poor in
algae, the more so in that it is a lake rich in major elements from a variety of sources which are further
concentrated by intense evaporation. Thus, given the physical and chemical conditions of the Lake,
intensive studies by Serruya and her colleagues during the past three years suggest that the dinoflagellate population is less dangerous to the lake equilibrium than its natural competitor, the blue
green alga Microcystis. The latter with its high rate of nitrate uptake will tend to generate conditions
unfavourable to denitrification and to increase the amount of organically bound nitrogen. Conversely,
the development of a species such as Peridinium which required a more modest amount of nitrogen,
will be, for an equal biomass more favorable to denitrification. Nor will other links in the food chain
remain untouched: if the Microcystis constitutes an available source of food to the Zooplankton, the
herbivorous species of Zooplankton will increase their biomass with a subsequent further increase in
the total amount or organically bound nitrogen.
Thus from an ecological point of view good policy would appear to dictate the encouragement of
the natural population and in fact to try to maintain the Peridinium against Microcystis. It could,
however, be asked whether since both are dependent on relatively high levels of nitrogen they could
both not be eliminated by the drastic reduction of nitrogen input into the Lake.
Unfortunately, the probable result of such an undertaking would be the emergence of blue green
algae such as Nostoc or Anabaena which are capable of fixing molecular nitrogen. These algae now
present in Lake Kinneret in low numbers, relieved of the competitive pressure of Peridinium and able
to avail themselves of atmospheric nitrogen, could generate blooms virtually beyond control.
Unquestionably the decrease in organic matter accretion must be seen as one of the long term
treatments necessary to preserve the present equilibrium of the lake. However, the decrease of inorganic nutients and especially of nitrates must be progressive; otherwise the atmospheric nitrogen
fixing algae will displace the dissolved nitrogen users.
Therefore modifications of the present situation must be weighed very carefully for a variety of
potential consequences. Dr. Eren's paper carries the suggestion that the pumping schedule of lake
water might be changed according to the vertical migration of Peridinum. Certainly he is aware that
pumping of deep water will accentuate the warming of the lake which apart from the wide spectrum
of other changes this will invoke, will also encourage Anabaena bloom. This is not to deny the costly
nuisance generated by Peridinum in the Israel National Water Systems but to stimulate recognition of
its very critical role at another remove. Ideally, the insinuation of an additional link in the food chain
such as Tilapia whose harvest removes a portion of the biomass from the immediate environment,
remains the biological treatment of choice and it is perhaps in this direction that present efforts should
be channelled.
1. Serruya, C. Kinneret Limnological Laboratory. Progress Report No. 7. 1971.
M.C.B. Hotz, Canada
If there is bacterial degradation in the aquifer, is this aerobic or anaerobic? How deep are these
aquifers?
Reply
The organic matter accumulating in recharge wells is first decomposed aerobically, as the water
which is recharged contains dissolved oxygen.
When the recharge is terminated there is rapid depletion of oxygen and anaerobic conditions
appear. The sandy aquifer in which most of the recharge operations take place is few tens of meters
deep.
A.A. Rosen, USA.
In earlier work, Eren suggested that H2S evolved from anaerobic benthic deposits may directly
stimulate the growth of oscillatoria. Has further work supported that view?
Reply
We have evidence that our strain of Oscillatoria chalybea requires a reducing environment and low
light intensity. It is lysed rapidly in strong light (2000 lux or more) in presence of oxygen or Η 2 θ 2 ·
It seems that presence of H 2 S affects survival of the alga, as it protects it from photooxidative
lysis.
M. Halm an n, Israel
Experiments I have made with bottle tests of algal growth prove that in lake Kinneret the only
element limiting algal growth is phosphorus.
201