1 Chemostasis and Homeostasis in Aquatic Ecosystems; Principles of Water Pollution Control WERNER STUMM
and E L I S A B E T H S T U M M - Z O L L I N G E R
1
2
Laboratories of A p p l i e d Chemistry and A p p l i e d Biology, respectively, H a r v a r d University, Cambridge, Mass.
In view of man's inability changes, balance
pollution
to adapt
is equated
to major
environmental
with disturbance
and loss of stability.
Increasing
of
ecological
the chemical
sity (number
of components
and phases) makes an
rium
more resistant
toward
system
posed
on the
interlocked thus adapted diversity enhances
system.
by
feedback
to coexistence
loops
influences
its members (homeostasis)
for mutual advantage;
its survival.
Because
of pollution
various
of change
kinds of
in aquatic
in a predictable
and
disturbance
ecosystems
way, general
mitigate
the conflict
between
and protection
of natural
waters.
are
increased
control beyond those of waste treatment
which
im and
makes the system less subject to perturbations
cause similar patterns
ploitation
In an ecosystem,
various
affect their stability outlined
external
diver equilib
and
measures can
resource
be ex
u n d e r s t a n d i n g of the c h e m i s t r y a n d b i o l o g y of n a t u r a l w a t e r s is a p r e r e q u i s i t e for a n u n d e r s t a n d i n g of t h e w a y s t h e e n v i r o n m e n t is affected b y man's p o l l u t i o n .
I n a b r o a d sense, p o l l u t i o n has b e e n c h a r
a c t e r i z e d as a n a l t e r a t i o n of m a n s s u r r o u n d i n g s i n s u c h a w a y t h a t t h e y b e c o m e u n f a v o r a b l e to h i m a n d to h i s life. T h i s c h a r a c t e r i z a t i o n i m p l i e s that p o l l u t i o n is not solely c a u s e d b y c o n t a m i n a n t s or p o l l u t a n t s a d d e d to the e n v i r o n m e n t b u t c a n also result f r o m other d i r e c t or i n d i r e c t c o n sequences of man's a c t i o n . Present address: Swiss Federal Institute of Technology, (ΕΤΗ), C H Zürich, Switzerland. Present address: C H 8700 Küsnacht, Switzerland. 1
2
1
NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS
2
M A N AGAINST N A T U R E .
M a n is a n i n t e g r a l p a r t of the
ecosystem;
d e s p i t e l o c a l i z e d large p o p u l a t i o n densities, m a n as the h u m a n a n i m a l p l a y s a r e l a t i v e l y m i n o r role i n the p h y s i o l o g y of the ecosphere.
Domestic
waste a n d g a r b a g e represent a v e r y s m a l l f r a c t i o n of the t o t a l detritus p r o d u c e d b y organisms. man's m e t a b o l i s m ( 2 χ
W i t h i n the b i o s p h e r e , the e n e r g y i n v o l v e d i n 10
1 5
K c a l per year) m a y be compared w i t h p r i
m a r y p r o d u c t i v i t y — i . e . , the energy fixed b y a l l the plants ( ^ - Ί Ο
1 8
Kcal
p e r y e a r ) . T h e s e estimates are b a s e d o n a d a i l y p e r c a p i t a c o n s u m p t i o n of 2 0 0 0 K c a l a n d a p r i m a r y p r o d u c t i v i t y of 1 0
1 6
moles y e a r " of c a r b o n ( 1 ). 1
I f e v e n l y d i s t r i b u t e d over the w o r l d , man's wastes h a v e a n e g l i g i b l e effect o n the energy transfer of the ecosphere. D o m e s t i c wastes cause l o c a l i z e d or t e m p o r a r y u n f a v o r a b l e e n v i r o n m e n t a l a l t e r a t i o n o n l y w h e r e t h e y are d i s c h a r g e d i n h i g h c o n c e n t r a t i o n . O n the other h a n d , m a n , as a n i n v e n t i v e i n t e l l e c t u a l b e i n g , w i t h his c a p a c i t y of m a n i p u l a t i o n a n d d o m i n a n c e dissipates 1 0 to 2 0 times ( i n the U S A , 5 0 to 1 0 0 times ) as m u c h energy as h e r e q u i r e s for his m e t a b o l i s m . T h e stress i m p o s e d u p o n the e n v i r o n m e n t as a d i r e c t or i n d i r e c t result of this energy d i s s i p a t i o n o u t w e i g h s b y f a r the d i s t u r b a n c e s c a u s e d b y the d i s p o s a l of d o m e s t i c wastes. I n w h a t w a y does e n e r g y d i s s i p a t i o n cause p o l l u t i o n ?
Obviously,
smoke, s u l f u r d i o x i d e , excess heat a n d w a t e r loss b y e v a p o r a t i o n , s p i l l a g e of o i l , pesticides, a n d other p e t r o c h e m i c a l s i n t o fresh w a t e r a n d oceans, a n d the leakage of f e r t i l i z e r s f r o m l a n d i n t o the w a t e r are some of the b y p r o d u c t s of p o w e r c o n s u m p t i o n a n d c u l t u r a l d e v e l o p m e n t . A g r i c u l t u r e , forestry, g e o l o g i c a l e x p l o i t a t i o n , c o n s t r u c t i o n of d a m s , m a n i p u l a t i o n s of the l a n d s c a p e , u r b a n c o n s t r u c t i o n , a n d other means of c i v i l i z a t i o n c o u n t e r act the forces of n a t u r a l s e l e c t i o n ; t h e y affect the s o - c a l l e d b a l a n c e of n a t u r e a n d interfere w i t h b i o l o g i c a l r e l a t i o n s h i p s . M o s t of t h e
energy
u t i l i z e d b y o u r i n d u s t r i a l society for its o w n a d v a n t a g e u l t i m a t e l y causes a s i m p l i f i c a t i o n of the e c o s y s t e m — s p e c i f i c a l l y , a r e d u c t i o n of the f o o d w e b a n d a s h o r t e n i n g of the f o o d c h a i n ( 2 ) . T h e less c o m p l e x a n a t u r a l ecosystem, the less stable a n d the m o r e l i a b l e it is to p e r t u r b a t i o n s a n d to catastrophe. M o s t of o u r c o n c e r n , thus, s h o u l d be w i t h this s i m p l i f i c a t i o n of the ecosystem
a n d w i t h the c o n c o m i t a n t l a c k of b a l a n c e
and
stability. Instability as a Measure of Pollution. M a n ' s a b i l i t y to a d a p t to a c h a n g i n g e n v i r o n m e n t is v e r y l i m i t e d because the range of p h y s i o l o g i c a l a d a p t a t i o n is n a r r o w a n d e v o l u t i o n a r y a d a p t a t i o n is slow.
W h e n man
e v o l v e d , h e f o u n d a stable e n v i r o n m e n t c a p a b l e of r e s i s t i n g c h a n g e a n d p e r t u r b a t i o n . T h e c h e m i c a l c o m p o s i t i o n s of the v a r i o u s oceans are q u i t e similar and have probably been
essentially constant for the last 1 0 0
m i l l i o n years. S i m i l a r l y , t h e c o m p o s i t i o n of the a t m o s p h e r e has r e m a i n e d u n c h a n g e d , a n d c l i m a t i c v a r i a t i o n s h a v e b e e n e x t r e m e l y slow.
I n the
i n t e g r a t e d g l o b a l e c o l o g i c a l system, w e h a v e a r e m a r k a b l y w e l l - e s t a b -
1.
STUM M
AND STUMM-ZOLLINGER
Water Pollution
3
Control
l i s h e d b a l a n c e o f p r o d u c t i o n a n d d e s t r u c t i o n o f o r g a n i c m a t e r i a l as w e l l as o f p r o d u c t i o n a n d c o n s u m p t i o n o f 0 , p r o v i d i n g a constant s u r p l u s o f 2
0
2
i n the atmosphere.
I n v i e w o f man's i n a b i l i t y t o a d a p t t o major
e n v i r o n m e n t a l changes, p o l l u t i o n m a y b e i n t e r p r e t e d as a d i s t u r b a n c e i n the e c o l o g i c a l b a l a n c e c a u s i n g loss o f s t a b i l i t y o f the e n v i r o n m e n t . OBJECTIVES.
I t i s the objective o f this p r e s e n t a t i o n t o r e v i e w some
of the c h e m i c a l a n d b i o l o g i c a l factors that regulate the c o m p o s i t i o n o f n a t u r a l w a t e r s , t o illustrate t h e v a r i a b l e s a n d m o d e s b y w h i c h s t a b i l i t y is i m p a r t e d t o n a t u r a l systems, t o i n t e r p r e t p o l l u t i o n i n terms o f d i s t u r b ance o f e c o l o g i c a l balances a n d m i t i g a t i o n o f ecosystem s t a b i l i t y , a n d t o discuss s o m e means o f w a t e r p o l l u t i o n c o n t r o l b e y o n d those o f w a s t e treatment. Chemical Factors Regulating
the Composition of Natural
Waters
T e r r e s t r i a l waters v a r y i n c h e m i c a l c o m p o s i t i o n ; these v a r i a t i o n s c a n be u n d e r s t o o d , at least p a r t i a l l y , i n terms o f the different histories o f the waters. A p p r e c i a t i o n o f some o f the p e r t i n e n t reactions b y w h i c h n a t u r a l waters a c q u i r e t h e i r characteristics c a n be o b t a i n e d b y c a r r y i n g out some s i m p l e i m a g i n a r y experiments
(Figure l a ) .
M i n e r a l s are m i x e d w i t h
d i s t i l l e d w a t e r a n d exposed to a n a t m o s p h e r e c o n t a i n i n g C O * .
Congruent
Atmosphere IONS, MOLECULES (DISPERSED SOLO PHASES)
COMPOUNDS, IONS (INCLUSION WATERS)
MATERIAL „ . OUTFLOWS . Ot FROM SYSTEM 0.
CHEMICAL REACTIONS
m
PÇOg
pco
Controlling System
Controlled System
(b)
2
pco
2
OUttCT CHCUKAL _ WOWCCT CXWCM. «FlUCNCf S
Figure 1.
Generalized
models for description
of natural water systems
(a) Mixing rocks, water, and atmosphere (b) Equilibrium models establish boundary conditions toward which aquatic environments must proceed, however slowly (c) Steady state model permits the description of the time-invariant conditions of dynamic and open systems (d) Living systems are controlled by negative feedback (homeostasis)
4
NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS
a n d i n c o n g r u e n t d i s s o l u t i o n reactions ( w e a t h e r i n g r e a c t i o n s ) t a k e p l a c e because m a n y constituents of the earth's crust are t h e r m o d y n a m i c a l l y u n s t a b l e i n the presence of w a t e r a n d the a t m o s p h e r e ; for e x a m p l e : CaC0 (s) Calcite
+
3
CaCOs(s) +
H 0 = Ca + +
HC0 - + OH~
H C0 * = Ca
+
2
2
2
3
NaAlSi 0 (s) + 8
2 +
8
H 0 = Na+ +
8
2 HC0 " 3
2
OH- +
2 H Si0 4
+ A
Al Si 0 (OH)4(s) Kaolinite
l
4
2
Albite NaAlSi 0 (s) + 3
H C0 * + = Na+ +
8
2
H 0 HCOr +
3
CaAl Si 0 (s) +
2 H C0 * + H 0 = Ca + 2 HC0 ~ +
2
2
8
2
8
2
2 +
3
+
2 H4S1O4 +
3 H 0 = Ca
2
2 OH" +
4
6 7
7
3
4
3
Al Si 0 (OH) (s) 2
2
5
4
Al Si 0 (OH) (s) Kaolinite 2
2
5
4
Al Si 0 (OH) (s) 2
2
3 Ca . A] . Si . 3(OH) (s) + 2 H C 0 * = Ca + 2 H C 0 " + 8 H Si0 Montmorillonite 3 3
A
l
2
2 +
0
5
2
CaAl Si 0 (s) + Anorthite 2
2
2
2 +
5
4
3
3
4
4
+
7 Al Si 0 (OH) (s) Kaolinite 2
2
5
4
R e c o g n i t i o n of the c h e m i c a l processes i n v o l v e d p e r m i t s i d e n t i f i c a t i o n of the v a r i a b l e s a n d m e c h a n i s m s that regulate a n d c o n t r o l t h e m i n e r a l c o m p o s i t i o n of n a t u r a l waters.
W i t h the h e l p of e q u i l i b r i u m constants
for the p e r t i n e n t reactions, b o u n d a r y c o n d i t i o n s t o w a r d s w h i c h a q u a t i c e n v i r o n m e n t s m u s t p r o c e e d c a n be established. W e c a n also c a r r y out o u r i m a g i n a r y e x p e r i m e n t b y m i x i n g rocks w i t h w a t e r i n a closed bottle, w h e r e w e leave a little space at the top for the atmosphere.
A s e p i t o m i z e d b y Sillén ( 3 ) a n d m a n y other researchers
( 4 ), s u c h a c l o s e d r o c k - w a t e r - a t m o s p h e r e system constitutes a s i m p l i f i e d b u t representative m o d e l of w h a t has
fittingly
been
called
"spaceship
e a r t h . " I n a n e q u i l i b r i u m system, the c o n c e n t r a t i o n of i n o r g a n i c solutes ( o c e a n ) a n d the C 0
2
pressure i n the gas phase ( a t m o s p h e r e ) are p r i -
m a r i l y r e g u l a t e d b y the heterogeneous reactions i n v o l v i n g carbonates a n d v a r i o u s a l u m i n u m silicates, thus i l l u s t r a t i n g p l a u s i b l y that the C O 2 c o n tent of the atmosphere is r e g u l a t e d at the s e a - s e d i m e n t interface.
The
v o l u m e p r o p o r t i o n s i n this m o d e l ( F i g u r e l a ) a p p e a r u n r e l a t e d to the r e a l system, b u t m e t a p h o r i c a l l y the i d e a of a "gas b u b b l e " is reflected i n the mass p r o p o r t i o n of C 0
2
i n the geosphere; for every C a t o m i n the
atmospace, there are about 60 C atoms ( m o s t l y as H C O 3 " ) i n the h y d r o space a n d a b o u t 40,000 C atoms ( l a r g e l y as C 0
8
2
" ) i n the sediments
(4).
Buffering. H e t e r o g e n e o u s d i s s o l u t i o n a n d p r e c i p i t a t i o n reactions are the p r i n c i p a l p H buffer m e c h a n i s m s i n n a t u r a l waters. It has b e e n s h o w n
1.
Water Pollution
S T U M M AND STUMM-ZOLLINGER
that t h e buffer i n t e n s i t y of heterogeneous
5
Control
systems is m u c h l a r g e r t h a n
that of a h o m o g e n e o u s s o l u t i o n ; for e x a m p l e , t h e buffer intensities at pH =
8 of a n o r t h i t e - k a o l i n i t e a n d of C a C O - C 0 a
2
(10~
35
a t m ) suspen
sions at h y p o t h e t i c a l e q u i l i b r i u m a r e , r e s p e c t i v e l y , 3000 a n d 3 0 times larger t h a n that of a 1 0 " M H C C V s o l u t i o n ( 4 , 5 ) . I n a s i m i l a r w a y , as 3
[ H ] i s k e p t constant b y heterogeneous +
e q u i l i b r i u m , t h e concentrations
of other cations a n d anions i n n a t u r a l waters are buffered b y heteroge neous reactions.
A w a t e r that is i n e q u i l i b r i u m w i t h s o l i d C a C O ^ w i l l
t e n d to m a i n t a i n a r a t h e r constant p C a even i f C a
2 +
is i n t r o d u c e d t o t h e
w a t e r f r o m e x t e r n a l sources. At
equilibrium,
the G i b b s
independent variables. e q u i l i b r i u m models.
phase
r u l e restricts t h e n u m b e r
of
It is t h e basis f o r o r g a n i z i n g a n d i n t e r p r e t i n g
A f e w s i m p l e e q u i l i b r i u m systems
(Figure l b )
are c o n s i d e r e d i n T a b l e I . T h e y are c o n s t r u c t e d b y i n c o r p o r a t i n g t h e specific components
into closed systems a n d b y s p e c i f y i n g t h e n u m b e r
of phases to b e i n c l u d e d . T h e phase r u l e restricts the n u m b e r of i n d e p e n d e n t v a r i a b l e s ( degrees of f r e e d o m ) , F , to w h i c h one c a n assign values o n the basis of the n u m b e r of components, C , a n d of phases, P: F — C
+
2 -
(2)
P.
I n T a b l e l b , m o d e l s c o n t a i n i n g i d e n t i c a l components
b u t differing
w i t h respect to t h e n u m b e r of phases are c o m p a r e d w i t h e a c h other. A n increase i n F must b e a c c o m p a n i e d b y a decrease i n F . T h e activities i n the system, such as n u m b e r 3 o r 5, r e m a i n constant a n d i n d e p e n d e n t of the c o n c e n t r a t i o n of the c o m p o n e n t s as l o n g as the phases coexist i n e q u i librium.
I n M o d e l 6, o n l y one degree of f r e e d o m remains f o r t h e g i v e n
n u m b e r of components
a n d phases; t h e n Ρ
Γ θ 2
i n the gas phase of t h e
m o d e l w i l l b e d e t e r m i n e d b y t h e e q u i l i b r i u m a n d cannot b e v a r i e d (manostat).
T h e models given i n T a b l e I c a n b e enlarged; the addition
of each a d d i t i o n a l c o m p o n e n t
to a n e q u i l i b r i u m system m u s t result i n
either a n e w phase o r a n a d d i t i o n a l degree of f r e e d o m .
Sillén ( 3 ) , w h o
d e m o n s t r a t e d t h e relevance of e q u i l i b r i u m m o d e l s , has p r o p o s e d
equi-
l i b r i u m systems o f different c o m p l e x i t y as m o d e l s f o r t h e c o m p o s i t i o n o f the ocean a n d t h e atmosphere. MINIMIZING E X T E R N A L DISTURBANCE.
T h e d i s p l a c e m e n t of a c h e m -
i c a l e q u i l i b r i u m b y a change of t h e parameters ( a c t i v i t y , pressure, t e m p e r a t u r e ) o n w h i c h e q u i l i b r i u m d e p e n d s is i n d e p e n d e n t of t h e p a t h o f the change, b u t t h e r m o d y n a m i c a l l y one c a n p r e d i c t t h e sign of t h e d i s p l a c e m e n t . T h e p r i n c i p l e of L e C h a t e l i e r has b e e n expressed q u a l i t a t i v e l y as f o l l o w s : " A system tends to change so as to m i n i m i z e t h e external stress." A s w e h a v e seen, f o r a g i v e n n u m b e r of c o m p o n e n t s t h e n u m b e r of i n d e p e n d e n t v a r i a b l e s is s m a l l e r , t h e larger t h e n u m b e r of coexisting
NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS
Table I. a: C0
and CaCOz Solubility
2
1
2
2 H 0, C0
Ρ
Components
2
2
Variables'
3
Aqueous Solution Calcite(s) d
2 H 0 , C 0 , CaO
2 2 t = 25°C •log pco = 3.5
C F
Models 2
Aqueous Solution C0 (g)
Phases
Equilibrium Models;
2
2
2
Aqueous Solution C0 (g) Calate(s) 2
3 H 0 , C 0 , CaO 2
2
3 2 t = 25°C -log Pco = 3.5
3 3 t = 25°C — log ρ = 0 [Ca +] = C V
b
2
2
Composition pH pHC0 pCa pH Si0 4
9.9 4.1 3.9
5.7 5.7
3
8.3 3.0 3.3
e
4
α 6
From Stumm and Morgan (1). H2CO3* is treated as a nonvolatile acid. The system is under a total pressure of 1
atm.
B y specifying pco*, the total pressure ρ is determined (P = pco2 + pH o). For the calculation, constants valid at Ρ = 1 atm were used. c
2
phases.
T h e s i m p l e e q u i l i b r i u m system C a C 0 , H 0 , C 0 3
2
2
w i t h three
phases ( N o . 3, T a b l e I ) has a n infinite buffer i n t e n s i t y w i t h r e g a r d to dilution ( H 0 )
a n d to the a d d i t i o n of the base C a ( O H )
2
2
o r the a c i d
C 0 ; i.e., the system ( as l o n g as t h e three phases coexist i n e q u i l i b r i u m ) 2
resists attempts to p e r t u r b a t i o n c a u s e d b y the a d d i t i o n ( o r w i t h d r a w a l ) of components
of the system.
H e n c e , i n c r e a s i n g the n u m b e r of c o m p o
nents a n d phases—i.e., i n c r e a s i n g the c h e m i c a l d i v e r s i t y — m a k e s the sys t e m m o r e resistant t o w a r d a larger n u m b e r of e x t e r n a l influences i m p o s e d o n the system a n d h e n c e less subject
to p e r t u r b a t i o n s r e s u l t i n g f r o m
e x t e r n a l stresses. Steady State. I n contrast to the models discussed a b o v e , n a t u r a l w a ters are systems o p e n to t h e i r e n v i r o n m e n t , a n d m u c h of t h e i r c h e m i s t r y d e p e n d s o n the k i n e t i c s of p h y s i c a l a n d c h e m i c a l processes. If, i n s u c h a system, i n p u t is b a l a n c e d b y o u t p u t , a steady state c o n d i t i o n is a t t a i n e d , a n d the system r e m a i n s u n c h a n g e d i n t i m e . S u c h a t i m e - i n v a r i a n t c o n -
1.
S T U M M AND STUMM-ZOLLINGER
Water Pollution
Control
Application of Phase R u l e " b: Aluminum
Silicates
and
Aqueous Solution C0 (g) Kaolinite Ca-montmorillonite Calotte
Aqueous Solution C0 (g) Kaolinite Ca-montmorillonite
2
2
CaCOz
Aqueous Solution C0 (g) Kaolinite C a-montmorillonite Calcite Ca-feldspar 2
6 H 0, C 0 , CaO A1 0 , Si0 2
2
2
3
2
5 3 t = 25°C - l o g pco2 = 3.5 8[Ca +] = [ H S i 0 ] 2
4
4
5 2 t = 25°C •log pcoo = 3.5
5 1 t = 25°C
d
7.4 3.9 4.2 3.2
8.3 3.0 3.3 3.6
9.0 3.4 3.7 3.7 - l o g Pco =
4.5
2
This additional constraint is necessary for defining the system; other conditions could be specified. e pCOz ~ = 4.4. d
2
d i t i o n of a c h e m i c a l r e a c t i o n system represents a c o n v e n i e n t i d e a l i z e d m o d e l of a n a t u r a l w a t e r system. W h i l e a n e q u l i l i b r i u m system at c o n stant t e m p e r a t u r e a n d pressure is c h a r a c t e r i z e d b y a m i n i m u m i n the G i b b s free energy, energy is r e q u i r e d for the m a i n t e n a n c e of the steady state ( 6 , 7 ) . B e c a u s e the sea has r e m a i n e d constant i n its c o m p o s i t i o n for the recent geologic past, it has b e e n d e s c r i b e d p l a u s i b l y i n terms of a steady state m o d e l . F o r a steady state ocean, for each element, E, the e q u a t i o n (d[£]/dt)input =
( d [ E ] / d t ) sedimentation c a n be w r i t t e n . B e c a u s e the rate
of s e d i m e n t a t i o n is c o n t r o l l e d l a r g e l y b y the rate at w h i c h a n element is converted
( p r e c i p i t a t i o n , i o n exchange, b i o l o g i c a l a c t i v i t y ) into a n i n -
s o l u b l e a n d settleable f o r m , the residence t i m e is affected b y the readiness of the elements to react. H e n c e , elements that are h i g h l y o v e r s a t u r a t e d (e.g., A l , F e ) h a v e d e t e n t i o n times that c o r r e s p o n d to the t i m e necessary for ocean m i x i n g ( ~ 1 0
3
years).
O n the other h a n d , elements w i t h l o w
r e a c t i v i t y s u c h as N a or L i have v e r y l o n g residence times
—10
8
years)
NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS
8
that are p e r h a p s w i t h i n one or t w o orders of m a g n i t u d e of the age o f the ocean. S t e a d y state m o d e l s c a n also a i d i n u n d e r s t a n d i n g fresh w a t e r systems (Figure l e ) .
A p r e s u p p o s i t i o n of steady state f r e q u e n t l y p e r m i t s the
q u a n t i t a t i v e e v a l u a t i o n of processes s u c h as exchange reactions b e t w e e n a t m o s p h e r e a n d g r o u n d w a t e r s , m i x i n g relations, l i m n o l o g i c a l t r a n s f o r m a tions of constituents (see
F i g u r e 4 ) , a n d l o c a l h y d r o l o g i c a l cycles.
E v e n h i g h l y d y n a m i c n a t u r a l w a t e r systems m a y b e at e q u i l i b r i u m w i t h respect to c e r t a i n processes; this depends o n the t i m e scale of the process.
H e n c e , there a l w a y s exist i n n a t u r a l waters regions or e n v i r o n
ments that are l o c a l l y at e q u i l i b r i u m , even t h o u g h gradients exist t h r o u g h out the system as a w h o l e . Regulation
in Ecosystems
R e t u r n i n g to our i m a g i n a r y closed-bottle e x p e r i m e n t , w e m a y expose the bottle i n w h i c h w e m i x e d rocks w i t h w a t e r to some light. T h e r e w i l l n o w be a flow of energy t h r o u g h the system. If the bottle contains o r ganisms, our m o d e l ( F i g u r e l a ) becomes a m i c r o c o s m o s ; a s m a l l p o r t i o n of the l i g h t energy is u s e d i n a l g a l photosynthesis a n d becomes stored i n the f o r m of o r g a n i c m a t e r i a l .
S o m e of the o r g a n i c m a t t e r becomes
o x i d i z e d , l i b e r a t i n g e n e r g y i n o r d e r to s u p p o r t the l i f e processes ( a s s i m •H6-.
Kcal Ε
Figure 2.
Photosynthesis and biochemical
cycle
1.
STUM M AND STUMM-ZOLLINGER
Water Pollution
9
Control
i l a t i o n ) o f h e t e r o t r o p h i c organisms ( c o n s u m e r s a n d d e c o m p o s e r s ) ure 2 ) .
(Fig
Organisms a n d their abiotic environment are interrelated a n d
interact u p o n e a c h other. T h e m a i n t e n a n c e o f l i f e r e s u l t i n g f r o m solar e n e r g y ( photosynthesis ) is the m a i n cause f o r n o n e q u i l i b r i u m c o n d i t i o n s ( F i g u r e 2 ) .
Photosyn
thesis m a y b e c o n c e i v e d as a process p r o d u c i n g l o c a l i z e d centers o f h i g h l y n e g a t i v e pc (p« = d u c e d components
—log electron activity) a n d oxygen ( h i g h pc). T h e re (organic compounds)
and the equivalent oxidation
p r o d u c t s ( 0 ) b e c o m e p a r t i a l l y stored—e.g., i n the sediments a n d i n the 2
a t m o s p h e r e - h y d r o s p h e r e , r e s p e c t i v e l y . T h e n o n p h o t o s y n t h e t i c organisms t e n d t o restore e q u i l i b r i u m b y c a t a l y t i c a l l y d e c o m p o s i n g products
o f photosynthesis
through
the unstable
energy-yielding redox
reactions,
t h e r e b y o b t a i n i n g a source o f energy f o r t h e i r m e t a b o l i c needs. T h e sequence o f r e d o x reactions o b s e r v e d i n a n a q u e o u s system as a f u n c t i o n of pc values ( p H =
7 ) i s also i n d i c a t e d i n this figure.
Energy
0
WASTE
Photosynthesis Ρ Production of Organic Materiol
Nutrients^
2
Respiration R Destruction of Organic Materiol CO2
Energy
Distance
Figure
3.
Balance
between photosynthesis and respiration
A disturbance of the P-R (photosynthesis-respiration) balance results from vertical (lakes) or longitudinal (rivers) separation of Ρ and R organisms. An unbal ance between Ρ and R functions leads to pollutional effects of one kind or another: depletion of 0> if Ρ < R or mass development of algae if production rates become larger than the rates of algal destruction by consumer and decomposer organisms (R < P).
B e c a u s e o f the energy flow t h r o u g h t h e b o t t l e , its contents
cannot
b e i n e q u i l i b r i u m , b u t u p o n c o n t i n u e d exposure to solar energy, e v e n t u a l l y a steady state b a l a n c e b e t w e e n p r o d u c t i o n a n d d e s t r u c t i o n o f o r g a n i c m a t e r i a l as w e l l as p r o d u c t i o n a n d c o n s u m p t i o n of 0 ( F i g u r e 3 ) . A t steady state, a constant s u r p l u s o f 0
2
2
w i l l be attained
(equivalent to the
10
NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS
r e d u c e d o r g a n i c m a t t e r p r e s e n t ) p r e v a i l s i n the gas a n d s o l u t i o n phase. W e r e c o g n i z e f r o m o u r e x p e r i m e n t that a system c o n t a i n i n g l i v i n g t h i n g s extracts e n e r g y f r o m the s t r e a m of r a d i a t i o n a n d uses this energy to or g a n i z e the system—i.e., the i n p u t of solar energy is necessary to m a i n t a i n l i f e — a n d that the flux of energy t h r o u g h the system is a c c o m p a n i e d cycles
of w a t e r , n u t r i e n t s , a n d of
other
elements
by
( hydrogeochemical
c y c l e s ) a n d b y cycles of l i f e t h r o u g h different t r o p h i c levels. T h u s , a n e c o l o g i c a l system m a y be d e f i n e d (8) contains a b i o l o g i c a l c o m m u n i t y
as a u n i t of the e n v i r o n m e n t that
( p r i m a r y producers,
various
trophic
levels of consumers a n d d e c o m p o s e r s ) i n w h i c h the flow of energy is reflected i n the t r o p h i c s t r u c t u r e a n d i n m a t e r i a l cycles. I n a n ecosystem, the e n e r g y flow f r o m a source to a s i n k m a y l e a d to a n e n t r o p y decrease i n the i n t e r m e d i a t e system. I n a s t a t i s t i c a l sense, e n t r o p y is a measure of d i s o r d e r .
T h e second l a w of
thermodynamics
d e m a n d s that a n y spontaneous process be a c c o m p a n i e d b y a n increase i n entropy—i.e.,
d S ( source,sink ) - f
d S ( ecosystem ) ^
dS(source,sink)
>
of
—dS(ecosystem) ^
0, the e n t r o p y dS(source,sink).
the
ecosystem
0.
B u t because
may
decrease:
T h i s decrease is reflected i n the
o r d e r i n g of the ecosystem a n d the presence of s u c h h i g h l y i m p r o b a b l e aggregations of energy as l i v i n g beings ( 9 ) . T h e i r o r g a n i z a t i o n has b e e n a c q u i r e d at the expense of a n increase i n e n t r o p y of the e n v i r o n m e n t . T h u s , the ecosystem m a y b e r e g a r d e d as a n " e n t r o p y p u m p " w h i c h e m p l o y s h i g h - g r a d e solar e n e r g y to dissipate excess e n t r o p y , t h e r e b y m a i n t a i n i n g its p h y s i c a l i n t e g r i t y ( 1 0 ) . Interaction Between Organisms and Abiotic Environment.
Steady
state m o d e l s m a y b e a p p l i e d to r e c o g n i z e a n d e v a l u a t e factors that r e g u late the i n t e r a c t i o n b e t w e e n b i o t i c a n d a b i o t i c v a r i a b l e s . F o r e x a m p l e , t h e g r o w t h rate of organisms ( e.g., b a c t e r i a d B / d t =
μΒ, w h e r e μ is t h e
net g r o w t h rate constant ( t i m e ) , is d e t e r m i n e d i n a c o m p l e t e l y m i x e d - 1
system b y the h y d r a u l i c d e t e n t i o n t i m e , Γ
Η 2
ο — μ" , because at steady 1
state, the g r o w t h of the o r g a n i s m s , μΒ, is e q u a l to the outflow of organisms Β/ΓΉ20. A n o t h e r e x a m p l e is i l l u s t r a t e d i n F i g u r e 4, w h e r e some of the i m p o r tant steps i n the l i m n o l o g i c a l t r a n s f o r m a t i o n of p h o s p h o r u s i n a l a k e are c h a r a c t e r i z e d i n terms of a steady state m o d e l (4).
T h e m o d e l simulates
a r e a l system b y g i v i n g a h y p o t h e t i c a l b a l a n c e of the a b u n d a n c e of Ρ i n v a r i o u s forms a n d of the exchange rates. T h e c y c l e of p h o s p h o r u s is d e t e r m i n e d l a r g e l y b y r e g e n e r a t i o n of Ρ f r o m b i o t a .
Primary production
d e p e n d s to a large extent o n the s u p p l y of Ρ to the t r o p h o g e n i c layer. F o r d e e p e r lakes, the rate of s u p p l y f r o m sediments is s m a l l i n c o m p a r i s o n w i t h t h e s u p p l y b y the h y p o l i m n i o n a n d b y the i n t r o d u c t i o n of Ρ f r o m waste a n d d r a i n a g e . A significant f r a c t i o n of Ρ i n t r o d u c e d i n t o the l a k e is i r r e t r i e v a b l y lost to the sediments.
1.
S T U M M A N D S T U M M-ZOLLINGER
Water Pollution
11
Control
-e»OI
Soluble 2
„ Phytoplonkton 60
-»» 14 Epilimnion
I Bocterio 0.1
Detrital Biomass 3? Herbivores
Detrital Biomass Benthol orgonisms 700
FeP0 ,AIP0 , Ç a » (P0 )e (9220 NriNiwi)
1 •1 1 1
\
'Energy
Production
(rtlotfvt unit*)
y
I 1 L. ,
•
P
