Chemostasis and Homeostasis in Aquatic Ecosystems; Principles of

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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