ATOLL RESEARCH BULLETIN NO. 214
OBSERVATION ON REDOX POTENTIAL IN FRESHWATER POOLS ON ALDABRA by B. A. Whitton and M. Potts
Issued by
THE SMITHSONIAN INSTITUTION Washington, D. C., U.S.A.
May 1 9 7 7
OBSERVATION ON REDOX POTENTIAL IN FRESHWATER POOLS ON ALDABRA by B. A. Whitton and M. Potts
ABSTRACT
In comparison with most values reported in the literature for freshwaters, small freshwater pools on Aldabra were found always to have values for redox potential which were low, the great majority lying below
+
200 mV at a standard pH of 7.0, using a correction of 58 mV per unit pH differencefrom the standard pH. Values were especially low in pools in Casuarina forest, where readings below+
50 mV were recorded frequently. Typical freshwater pools on Aldabra combine a well oxygenated environment with low redox potential, an environmental combination which has received little investigation in the literature.INTRODUCTION
In a previous study of the chemistry of freshwater pools on Aldabra (Donaldson and Whitton, 1976), it was found that most of the inorganic nitrogen present was apparently in the form of ammonia rather than nitrite or nitrate. This occurred in spite of the fact that the pools were often highly super-saturated with oxygen by daytime.
The only other observations in the literature reported a similar (apparent) loack of nitrification are those made by Ganning and Wulff (1969) in brackish rockpools by the Baltic Sea. These latter authors suggested that some inhibitory substances might be present in the pools which hindered oxidation of ammonia.
Since the previous account of the pools on Aldabra, the present authors have had the opportunity to collect further data from the atoll which would help account for any lack of nitrification. A full report of these will be published elsewhere, but a summary of observations on redox potential in the pools is given here in order that it may be read together with the previous account. These observations were all made during the period December 1974
-
January 1975.l ~ e ~ a r t n e n t of Botany, University of Durham, Durham, ~ n ~ l a n b . (Manuscript received 8 July 1975 --Eds.)
METHODS
The system used f o r naming t h e p o o l s h a s been d e s c r i b e d i n t h e p r e v i o u s a c c o u n t (Donaldson and Whitton, 1 9 7 6 ) .
Measurements of redox p o t e n t i a l were made u s i n g a PYE UNICAM p o r t a b l e meter model 293 w i t h PYE UNICAM combined redox e l e c t r o d e s , t h e s e c o n s i s t i n g of a p l a t i n u m i n d i c a t o r e l e c t r o d e and a s i l v e r
/
s i l v e r c h l o r i d e r e f e r e n c e system. C l e a n i n g and b u f f e r i n g of t h e e l e c t r o d e was made a t f r e q u e n t i n t e r v a l s d u r i n g sampling. The P t i n d i c a t o r e l e c t r o d e was f i r s t c l e a n e d by r u b b i n g g e n t l y w i t h emery p a p e r and t h e n immersing i n c o n c e n t r a t e d , c h l o r i n e - f r e e HN03. I t was t h e n r i n s e d and c a l i b r a t e d a g a i n s t a redox b u f f e r . Measurements of pH were t a k e n a t t h e same t i m e a s redox p o t e n t i a l , u s i n g a s i m i l a r m e t e r , b u t w i t h a pH e l e c t r o d e . Data on redox p o t e n t i a l i s r e p o r t e d h e r e b o t h a s a d i r e c t r e a d i n g (Eh) and one " c o r r e c t e d " t o pH 7 . 0 ( E h 7 ) . Due t o wide d i u r n a l f l u c t u a t i o n s i n pH, i t i s e s s e n t i a l t o make some i n i t i a l comparison of r e s u l t s w i t h redox p o t e n t i a l v a l u e s c o r r e c t e d t o a s t a n d a r d pH v a l u e . The c o r r e c t i o n v a l u e of 58 mV p e r u n i t pH u s e d i n t h e e a r l i e r l i t e r a t u r e was used h e r e . Where s i m p l e t e s t s of adding a c i d o r a l k a l i t o samples of p o o l w a t e r werk c a r r i e d o u t , t h e s e d i d i n f a c t i n d i c a t e t h a t t h i s was a r e a s o n a b l e c h o i c e of c o r r e c t i o n v a l u e .
I t was u n f o r t u n a t e l y n o t p o s s i b l e t o t a k e measurements of d i s s o l v e d oxygen s i m u l t a n e o u s l y w i t h t h o s e of redox p o t e n t i a l due t o l o s s o f
equipment on r o u t e t o t h e a t o l l . However o b s e r v a t i o n s on ammonia, n i t r i t e , n i t r a t e , phosphate and a l g a l v e g e t a t i o n i n d i c a t e d t h a t t h e b e h a v i o u r of t h e p o o l s was i n g e n e r a l v e r y s i m i l a r d u r i n g t h e p r e s e n t p e r i o d of s t u d y t o t h a t d u r i n g t h e p r e v i o u s p e r i o d .
RESULTS
Of t h e 20 p o o l s whose c h e m i s t r y was d e s c r i b e d by Donaldson and Whitton ( 1 9 7 6 ) , s i x were chosen f o r d e t a i l e d s t u d y of redox p o t e n t i a l changes w i t h d e p t h and t i m e on 17 J a n u a r y 1975. P a r t of t h e d a t a o b t a i n e d i s summarized i n T a b l e 1. A s t h e sediment s u r f a c e t e n d s t o be b o t h i l l - d e f i n e d and a r e g i o n o f r a p i d change, n o t t o o much a t t e n t i o n should be p a i d t o s m a l l d i f f e r e n c e s between r e a d i n g s t a k e n immediately above t h e sediment s u r f a c e .
I n a d d i t i o n t o t h e s e r e a d i n g s , measurements were t a k e n i n o t h e r s m a l l p o o l s on 17 J a n u a r y , and a l s o i n a r a n g e of p o o l s on o t h e r d a t e s . These somewhat extended t h e upper r a n g e of redox p o t e n t i a l v a l u e s , t h e h i g h e s t b e i n g a s m a l l p o o l f i l l e d w i t h Oedogonium:
A l l f r e s h w a t e r p o o l s found w i t h an Eh7 v a l u e i n t h e w a t e r n e a r t h e s u r f a c e of t h e p o o l lower t h a n
+
50 mV ( i n c l u d i n g W7, W8, W9) were a s s o c i a t e d w i t h t h e C a s u a r i n a F o r e s t , and had w a t e r s c o l o u r e d a p a l ebrown.
DISCUSSION
These data are obviously fragmentary, and any conclusions should be treated with caution until a more intensive study has been carried out on them. Nevertheless they do indicate the probability of several features of interest.
1. All the pools showed marked variations in redox potential during the day. As repeated readings at any one time were consistent, and as the pattern of changes was quite different in different pools, it seems unlikely that these variations were associated with any sort of
instrument error. It may be pointed out that the pool which showed an increase between successive readings during the day, W2, is also the pool for which Donaldson and Whitton (1976) reported a late afternoon peak in nitrite and nitrate values.
2. In comparison with data for soils and lake sediments, the literature on values for redox potential in freshwaters is rather sparse. It would however seem clear from the literature available
(Baas-Becking e t a l . , 1955; Hutchinson, 1957) that in comparison with most oxygenated waters, the values for pools on Aldabra are low, and those for the pools in the C a s u a r i n a Forest remarkably low.
3. If freshwaters behave in a manner similar to soils (Pearsall, 1938;
Reddy and Patrick 1975), then the redox potential measurements recorded for the majority of Aldabra pools correspond with environments which do not favour nitrification. If the values quoted by Pearsall (1938) are corrected to Eh7 values, then he found that with only three exceptions, all soils studied where nitrate predominated over ammonia had an Eh7 greater than + 234 mV, and soils lacking nitrates had an Eh7 less than 204 mV. The three exceptions found by Pearsall were all soils
receiving drainage from stream waters containing nitrate.
These observations would provide an explanation for the rarity of detectable nitrate in Aldabra pools. In most instances the redox potential values correspond to an environment which is sufficiently reducing that nitrification would not be expected to occur. Further, pool sediments were always found to be markedly reducing, and as the sediments are in many cases disturbed frequently by the activity of crabs, any chemical changes taking place in the sediments may be expected to have a marked effect on the chemistry of the water above the sediment.
It is less clear what are the probable agents responsible for bringing about these low redox potential measurements. The fact that all the pools in the C a s u a r i n a Forest had low values suggests that,the brown materials leaching from the debris of "needle" and other fallen parts of the tree may here be partly responsible. However the
variations taking place during a single day within one pool suggest that some other quite different factors must play a role. Disturbance
of t h e s e d i m e n t s and r e l e a s e of e x c r e t a by c r a b s would seem l i k e l y such f a c t o r s .
ACKNOWLEDGEMENTS
We a r e most g r a t e f u l t o t h e Royal S o c i e t y and t h e N a t u r a l Environment Research C o u n c i l f o r f i n a n c i a l s u p p o r t .
REFERENCES
Baas Becking, L. G . M . , Kaplan, I . R. and Moore, D . 1960. L i m i t s of t h e n a t u r a l environment i n t e r m s o f pH and o x i d a t i o n -
r e d u c t i o n p o t e n t i a l . J. G e o l . 68: 243-284.
Donaldson, A . and Whitton, B. A . 1976. Chemistry of f r e s h w a t e r p o o l s i n A l d a b r a A t o l l . A t o l l R e s e a r c h B u l l e t i n , t h i s i s s u e .
Ganning, B. and Wulff, F . 1969. The e f f e c t o f b i r d d r o p p i n g s on c h e m i c a l and b i o l o g i c a l dynamics i n b r a c k i s h w a t e r r o c k p o o l s . O i k o s 20: 274-286.
Hutchinson, G . E. 1957. A T r e a t i s e on L i r n n o l o g y . 1. G e o g r a p h y , P h y s i c s a n d C h e m i s t r y . 1015 pp. John Wiley & Sons I n c . , N.Y., U.S.A.
P e a r s a l l , W. H . 1938. The s o i l complex i n r e l a t i o n t o p l a n t communities 1. O x i d a t i o n - r e d u c t i o n p o t e n t i a l s i n s o i l s . J. E c o l . 26: 180-193.
Reddy, K. R. and P a t r i c k , W . H . , Jr 1975. E f f e c t o f a l t e r n a t e a e r o b i c and a n a e r o b i c c o n d i t i o n s on redox p o t e n t i a l , o r g a n i c m a t t e r decomposition and n i t r o g e n l o s s i n a f l o o d e d s o i l , . S o i l B i o l . B i o c h e m . 7 : 87-94.
T a b l e 1 T e m p e r a t u r e ( O C ) , p ~ a n d r e d o x p o t e n t i a l (mv) v a l u e s f o r s i x p o o l s ( s e e t e x t )
p~ p~~~
P o s i t i o n i n p o o l and t i m e o f d a y W2 W3 W4 W7 W8 W9
t pH E h E h 7 t pH E h E h 7 t pH Eh E h 7 t pH E h E h 7 t pH Eh E h 7 t pH E h E h 7
2 0 mm b e l o w s u r f a c e o f p o o l
...
0 8 2 0 - 1 0 3 0 2 5 . 8 6 . 2 5 + l o 0 + 5 7 2 8 . 8 7 . 3 5 + 1 2 0 + 1 4 0 2 9 . 8 8 . 3 9 + 1 7 2 + 2 5 3 2 9 . 9 6 . 2 0 + 2 2 5 + 1 7 9 2 8 . 4 6 . 3 1 + 7 8 + 3 8 2 8 . 0 6 . 5 8 + 8 0 + 5 6 1 4 2 5 - 1 4 5 5 3 3 . 4 7 . 9 5 + l o 0 + 1 5 5 4 0 . 2 9 . 3 5 - 8 5 + 5 1 3 7 . 2 1 0 . 1 8 + 1 5 + 1 9 9 4 1 . 5 7 . 8 0 - 6 0 - 1 4 3 1 . 7 6 . 2 2 - 4 8 - 9 3 3 1 . 7 6 . 7 6 + 5 8 + 4 4 1 8 1 0 - 1 8 5 0 3 2 . 8 7 . 8 + 1 8 0 + 2 2 6 3 2 . 2 8 . 6 0 + 4 5 + 1 3 8 3 4 . 2 10.0 -10 + 1 6 4 3 5 . 1 7 . 6 0 + 1 0 + 4 5 2 9 . 2 8 . 0 - 1 3 0 -72 2 9 . 6 7 . 6 5 - 8 0 - 4 2
o v e r s u r f a c e o f s e d i m e n t
...
0 8 2 0 - 1 0 3 0 2 5 . 2 5 . 5 8 + 4 5 -37 2 8 . 5 7 . 4 0 - 8 0 -57 2 9 . 2 7 . 9 0 + I 6 5 + 2 1 7 2 8 . 2 6 . 2 2 + 7 5 + 3 0 2 8 . 3 5 . 9 0 + 2 5 - 3 9 2 8 . 2 6 . 2 5 + l o 5 +62 1 4 2 5 - 1 4 5 5 3 2 . 2 7 . 1 0 - 9 5 - 8 9 3 2 . 3 7 . 1 0 -95 - 8 9 4 0 . 2 7 . 2 5 - 1 8 2 - 1 6 7 4 0 . 8 7 . 5 3 - 5 8 - 2 7 3 1 . 3 6 . 0 9 - 9 2 - 1 4 5 3 0 . 8 6 . 7 0 - 1 5 - 3 2 1 8 1 0 - 1 8 5 0 3 0 . 8 7 . 4 - 1 4 0 - 1 1 7 3 2 . 4 8 . 7 0 + 1 0 + l o 9 3 3 . 8 9 . 6 - 4 0 -111 3 5 . 8 7 . 8 0 - 4 0 +6 2 9 . 2 8 . 0 - 1 8 0 - 1 2 2 2 9 . 4 7 . 8 0 - 1 6 0 - 1 1 4
