Discussion by N.E. Armstrong, USA
The solar radiation bridget in Eqn 1 is incomplete since it ignores absorption and reflection of light
bay water. The dimensions of each term are important for assessing the applicability of the model;
therefore what units of radiation are being used? The term t' is defined later (Eqn 4)as the reciprocal
of Secchi Disc measurements. Such measurements include non-photosynthetic and photosynthetic
turbidity elements, thus t' should be redefined or the analysis restricted to low phytoplankton
population periods. The growth rate μ for phytoplankton is defined as a linear function of light.
Actually a Monod-type relationship exists for algal growth and light intensity. Again it is doubtful
whether the simplification for average light (Eqn 3) has added any flexibility to the model since other
exponentials are included. Any of these factors may be responsible for the "two distinctly different
populations" described in Fig 1. Although the term "Heterotrophic activity" is not defined in the
paper, its use in the paper is tied with Zooplankton populations. This activity is hypothesized to be
responsible for the turbidity levels observed in the summer or the "non-photosynthetic" component
mentioned in Eqn 1. In actuality the Zooplankton biomass during the summer was only % to
l
A of the
total suspended solids in the water column and therefore its contribution is only part of the turbidity
levels. Another more plausible explanation for the turbidity levels is the presence of "Gelbstoff" or
"yellow substance" which is derived from decay of organic material. The presence of this material
would correlate with temperature since the bacterial action producing it is temperature dependent.
In the discussion section of the paper, temperature is specified as the causative factor for
phytoplankton population decrease. This statement ignores the large body of evidence from other
estuaries and oceanic waters that shows Zooplankton as a predator and phytoplankton population
controller along with nutrients (although nutrients are not limiting here). In Jamaica Bay the
predation of Zooplankton on the phytoplankton coincides with temperature change, and this
simultaneous action apparently is responsible for the statement discussed here.
Finally the conclusion that summertime phytoplankton populations may be further reduced by
increased transparency (Conclusion 4) is contradictory to the author's basic equations for
phytoplankton growth rate and average light intensity, Eqns 2 and 3 respectively. If Eqn 3 is
substituted into Eqn 2, then phytoplankton growth rate is shown to be directly proportional to
increases in temperature and Secchi disc transparency. Thus by this equation, with increases in
Secchi-Disc values, (i.e. more transparency) the growth rate M, should increase, not decrease, and
phytoplankton populations should likewise increase.
Although the use of models to describe biological population changes and causative factors should
be encouraged, models such as this one need to be carefully tested and analysed for applicability.
H. Jones, Austrialia
What processes are to be used to reduce nutrient concentrations in effluents from the new plants
discharging into Jamaica Bay?
S.W. Loewensom, Rhodesia
What is the ratio of wet weather flow to dry weather flow? Are the treatment plants designed to
treat the total wet weather flow? Is it feasible to recycle the treated effluent for re-use in the urban
system? How important is the recreational use of the Bay?
Norman E. Cooke, Canada
What would be the relative cost of pumping the effluents now going into Jamaica Bay across the
bar out into the open sea?
M.C.B. Hotz, Canada
Streams carrying effluents usually have a high colloidal content. Experience in delta areas and the
Etang de Berre (France) indicate that coagulation of colloids on mixing with saline seawater would
have a major effect on turbidity.
241
The solar radiation bridget in Eqn 1 is incomplete since it ignores absorption and reflection of light
bay water. The dimensions of each term are important for assessing the applicability of the model;
therefore what units of radiation are being used? The term t' is defined later (Eqn 4)as the reciprocal
of Secchi Disc measurements. Such measurements include non-photosynthetic and photosynthetic
turbidity elements, thus t' should be redefined or the analysis restricted to low phytoplankton
population periods. The growth rate μ for phytoplankton is defined as a linear function of light.
Actually a Monod-type relationship exists for algal growth and light intensity. Again it is doubtful
whether the simplification for average light (Eqn 3) has added any flexibility to the model since other
exponentials are included. Any of these factors may be responsible for the "two distinctly different
populations" described in Fig 1. Although the term "Heterotrophic activity" is not defined in the
paper, its use in the paper is tied with Zooplankton populations. This activity is hypothesized to be
responsible for the turbidity levels observed in the summer or the "non-photosynthetic" component
mentioned in Eqn 1. In actuality the Zooplankton biomass during the summer was only % to
l
A of the
total suspended solids in the water column and therefore its contribution is only part of the turbidity
levels. Another more plausible explanation for the turbidity levels is the presence of "Gelbstoff" or
"yellow substance" which is derived from decay of organic material. The presence of this material
would correlate with temperature since the bacterial action producing it is temperature dependent.
In the discussion section of the paper, temperature is specified as the causative factor for
phytoplankton population decrease. This statement ignores the large body of evidence from other
estuaries and oceanic waters that shows Zooplankton as a predator and phytoplankton population
controller along with nutrients (although nutrients are not limiting here). In Jamaica Bay the
predation of Zooplankton on the phytoplankton coincides with temperature change, and this
simultaneous action apparently is responsible for the statement discussed here.
Finally the conclusion that summertime phytoplankton populations may be further reduced by
increased transparency (Conclusion 4) is contradictory to the author's basic equations for
phytoplankton growth rate and average light intensity, Eqns 2 and 3 respectively. If Eqn 3 is
substituted into Eqn 2, then phytoplankton growth rate is shown to be directly proportional to
increases in temperature and Secchi disc transparency. Thus by this equation, with increases in
Secchi-Disc values, (i.e. more transparency) the growth rate M, should increase, not decrease, and
phytoplankton populations should likewise increase.
Although the use of models to describe biological population changes and causative factors should
be encouraged, models such as this one need to be carefully tested and analysed for applicability.
H. Jones, Austrialia
What processes are to be used to reduce nutrient concentrations in effluents from the new plants
discharging into Jamaica Bay?
S.W. Loewensom, Rhodesia
What is the ratio of wet weather flow to dry weather flow? Are the treatment plants designed to
treat the total wet weather flow? Is it feasible to recycle the treated effluent for re-use in the urban
system? How important is the recreational use of the Bay?
Norman E. Cooke, Canada
What would be the relative cost of pumping the effluents now going into Jamaica Bay across the
bar out into the open sea?
M.C.B. Hotz, Canada
Streams carrying effluents usually have a high colloidal content. Experience in delta areas and the
Etang de Berre (France) indicate that coagulation of colloids on mixing with saline seawater would
have a major effect on turbidity.
241