WETLANDS, Vol. 3, 1983, pp. 13~ 152
A CONCEPTUAL MODEL OF NUTRIENT CYCLING IN WETLANDS USED FOR WASTEWATER TREATMENT: A LITERATURE ANALYSIS Francis D. Heliotis and Calvin B. DeWitt Institute for Environmental Studies 70 Science University of Wisconsin-Madison Madison, Wisconsin 53706 Abstrmc£. A conceptual model of nutrient dynamics in wetlands used for wastewater treatment provided the basis for reviewing the literature on the subject. Papers were selected in order to describe the storages and transfers of the conce!0tual model. This approach serves to develop an understanding of wetland systems behavior and allows easy identification of gaps in the literature and of research needs. The consensus of the literature is that incorporation into peat is the major long-term mechanism for phosphorus removal, whereas denitrification is very significant for nitrogen removal. Long-term, mass balance studies are required in order to identify and quantify the nutrient removal mechanisms and assess the potential of different wetland types for treating waetewater. Areas particularly lacking mass balance studies include hydrology, decomposition processes, and physicochemical reactions between water and wetland soils. INTRODUCTION T h e p u r i f i c a t i o n p o t e n t i a l of w e t l a n d p l a n t s a n d s o i l s h a s b e e n recognized for centuries. The natives of the Sudanese villages along the Nile used indigenous plants and clay soils to purify water from the river during the flood season (Al Azharla Jahn 1976). As early as 1932, a study of the water quality in African lakes found that water passing through a wetland before entering the lake was of a higher quality than in lakes without wetlands (Beadle 1932). Scientific research began in 1953 with the w o r k of Kathe Seidet of the M a x Planck Institute in West Germany, on the ability of aquatic macrophytes to remove pollutants from water (Seidel 1976). With the requirements for high quality of U.S. surface waters resulting from the passage of the Water Pollution Control Act A m e n d m e n t s in 1972, interest increased in assessing wastewater treatment potential of wetland ecosystems. A variety of research projects initiated in different parts of the United States has resulted in a considerable amount of literature on the subject. A review of these publications discloses the following problems and related needs (Heliotis 1982): 13A
Heliotis and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING
135
i. Most research to date is site specific and few generalizations or extrapolations of results to other areas can be made. Treatment effects are determined through a "black box" approach rather than on mechanisms and rates involved. 2. S t u d i e s o f t h e i m p a c t of w a s t e w a t e r o n w e t l a n d s a r e s c a r c e a n d inconclusive because they require tong-term observations. Lack o f i n f o r m a t i o n o n t h e e c o l o g i c a l c o n s e q u e n c e s l e a d s to s k e p t i c i s m r e g a r d i n g the advisability of this practice (Stearns and Guntenspergen 1981). 3. F ew a t t e m p t s h a v e b e e n m a d e t o i n t e g r a t e t h e e x i s t i n g i n f o r m a t i o n on t h e u s e o f w e t l a n d s f o r w a s t e w a t e r t r e a t m e n t ( S l o e y e t al. 1978; S t o w e l l e t al. 1980). N e e d e d i n f o r m a t i o n i n c l u d e s t h e t r e a t m e n t potential of d i f f e r e n t wetland types~ e s p e c i a l l y identification and quantification of nutrient removal mechanisms, the environmental impact of this practice, and the available m a n a g e m e n t options. A CONCEPTUAL MODEL OF WIg~LAND NUTRIENT PRO CESSES A n understanding of wetland processes, especially those related to nutrient dynamics, is essential for evaluation of their potential for n u t r i e n t r e m o v a l . I n a q u a l i t a t i v e s e n s e t h e s e p r o c e s s e s a r e well k n o w n ( K a d l e c a n d K a d l e c 1978; S l o e y e t al. 1978; v a n d e r Valk e t al. 1978 } b u t o n l y in a f e w c a s e s h a v e t h e s i z e s o f n u t r i e n t p o o l s a n d t h e m a g n i t u d e o f t r a n s f e r r a t e s b e e n m e a s u r e d {Davis a n d v a n d e r Valk 1978c; K l o p a t e k 1975; P r e n t k i e t aL 1978; R i c h a r d s o n e t al. 1978). T w o approaches have been used in studying nutrient relationships in wetlands: the qualitative approach and the mass balance approach. T h e y both follow the methodological frameworks s h o w n in Figure I. T h e "black box framework" describes nutrient relationships only in terms of system inputs and outputs. The "compartmentalized framework" describes nutrient relationships as wetland compartments; inputs or outputs of the wetland m a y or m a y not be considered depending u p o n the boundary of the system under study. T h e q u a l i t a t i v e a p p r o a c h m e a s u r e s t h e c o n c e n t r a t i o n of n u t r i e n t s whereas the mass balance approach measures the nutrient fluxes and attempts to account for their total mass. Clearly the mass balance approach can give more meaningful information on the importance of d i f f e r e n t w e t l a n d p r o e e s s e ~ ( K a d le c a n d K a d l e c 1978; S l o e y e t al. 1978; Valiela a n d T e a l 1978; W h i g h a m a n d B a y l e y 1978). In m a n a g i n g a w e t l a n d f o r n u t r i e n t a s s i m i l a t i o n , it is e q u a l l y i m p o r t a n t t o c h o o s e t h e appropriate methodological framework. A mass balance study using the b l a c k b o x a p p r o a c h c a n a s s e s s t h e o v e r a l l f u n c t i o n of a w e t l a n d a s nutrient sink or source, but does not provide information about the relative importance of different nutrient pools and cycling rates. This information is necessary for identification of key components and appropriate management strategies. A c o n c e p t u a l model o f n u t r i e n t p r e s e n t e d in F i g u r e 2. Each of t h e d i s c u s s e d in a q u a l i t a t i v e w a y b a s e d B e c a u s e of t h e d i v e r s i t y of w e t l a n d m e t h o d o l o g i e s , a n d s c a r c i t y of d a t a ,
c y c l i n g in a h y p o t h e t i c a l w e t l a n d is p o o l s a n d t r a n s f e r s s h o w n is upon findings from the literature. t y p e s , d i f f e r e n c e s in r e s e a r c h t h e r e v i e w is l i m i t e d to l i t e r a t u r e
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n e c e s s a r y for b r i n g i n g t o g e t h e r r e s u l t s from as many wetland t y p e s and c o n c e p t u a l m o d el c o m p o n e n t s a s p o s s i b l e , a n d f o r i n d i c a t i n g t h e r e l a t i v e i m p o r t a n c e of t h e s e t y p e s a n d c o m p o n e n t s in w a s t e w a t e r n u t r i e n t assimilation. RXTMRNAL
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T h e h y d r o l o g i c a l c y c l e of w e t l a n d s is i n f l u e n c e d b y s u r f a c e a n d groundwater, direct atmospheric precipitation, and evapotranspiration. Their relative contributions, as determined by the flux volume of water a n d i t s m i n e r a l c o n t e n t , a l s o d e t e r m i n e t h e m i n e r a l s u p p l y of w e t l a n d s . Both t h e f l u x v o l u m e of t h e w a t e r a n d its c h e m i c a l c o m p o s i t i o n a r e , in n a t u r a l c o n d i t i o n s , r e s u l t s of t h e h y d r o g e o l o g l c a l s e t t i n g of t h e system. T h u s , it is r e a s o n a b l e t h a t m o s t c l a s s i f i c a t i o n s c h e m e s r e c o g n i z e t h e i m p o r t a n c e of h y d r o l o g y a n d i n c o r p o r a t e h y d r o l o g i c a l c h a r a c t e r i s t i c s ( C o w a r d i n , e t al. 1979; G o s s e l i n k a n d T u r n e r 1978; Moore a n d B e l l a m y 1974; N o v i t z k i 1978b}. K n o w l e d g e of t h e h y d r o l o g i c a l c o n d i t i o n s is n e c e s s a r y in w a t e r q u a l i t y m a n a g e m e n t b e c a u s e w e t l a n d h y d r o g e o l o g y i s t h e f a c t o r m o s t a m e n a b l e t o m a n i p u l a t i o n ( S l o e y e t al. 1978). F o r w a s t e w a t e r a d d i t i o n a n u n d e r s t a n d i n g a n d q u a n t i f i c a t i o n of t h e w a t e r b u d g e t is e s s e n t i a l f o r e f f e c t i v e p l a n n i n g . Groundwater. T h e i n t e r a c t i o n of g r o u n d w a t e r w i t h w e t l a n d s is d e t e r m i n e d b y t h e c o n f i g u r a t i o n of t h e a d j a c e n t w a t e r t a b l e a n d t h e h y d r a u l i c c o n d u c t i v i t y of t h e u n d e r l y i n g s u b s t r a t e . T h e c h e m i c a l c o m p o s i t i o n of g r o u n d w a t e r d e p e n d s u p o n t h e n a t u r e of t h e g e o l o g i c a l s t r a t a t h r o u g h w h i c h it m o v e s . T h e s e d i m e n t s i m m e d i a t e l y a d j a c e n t to t h e w e t l a n d b a s i n a r e of e x t r e m e i m p o r t a n c e b e c a u s e t h e y a r e u s u a l l y r i c h e r in n u t r i e n t s t h a n t h e underlying bedrock. V e r y few s t u d i e s h a v e c o n s i d e r e d t h e c o n t r i b u t i o n of g r o u n d w a t e r to the nutrient budget of wetlands. N o v i t z k i (1978a) e s t i m a t e d t h a t s p r i n g w a t e r c o n t r i b u t e s 63% o f t h e t o t a l n i t r o g e n a n d 34~. of t h e t o t a l phosphorus entering a southern Wisconsin wetland. Surface Water.
S u r f a c e w a t e r may e n t e r t h e w e t l a n d a s s t r e a m f l o w a n d a s o v e r l a n d flow. S u r f a c e flow is a f u n c t i o n of t h e a r e a o f t h e w a t e r s h e d , s l o p e a n d h y d r a u l i c p e r m e a b i l i t y of t h e u p l a n d s o i l s . W at er q u a l i t y v a r i e s c o n s i d e r a b l y b e t w e e n h i g h a n d low w a t e r c o n d i t i o n s in w e t l a n d s t h a t d e p e n d p r i m a r i l y on s u r f a c e w a t e r f o r t h e i r e x i s t e n c e ( N o v i t z k i 1981). Land u s e a n d soil t y p e s of t h e w a t e r s h e d a r e c r i t i c a l f a c t o r s d e t e r m i n i n g the q u a l i t y of the s u r f a c e waters. W a s t e w a t e r a p p l i e d to a w e t l a n d is a c o m p o n e n t of s u r f a c e i n f l o w that can greatly alter the water and the nutrient budget, depending upon i t s flow a n d c o m p o s i t i o n . I t s c o n t r i b u t i o n to t h e w a t e r a n d n u t r i e n t b u d g e t of t h e w e t l a n d c a n be e s t i m a t e d e a s i l y b e c a u s e t h e r e a r e u s u a l l y r e c o r d s of v o l u m e t r i c f l o w s a n d c h e m i c a l c o m p o s i t i o n (Heliotis t 9 8 t ) . It is n e c e s s a r y to a s s e s s t h e r e l a t i o n s h i p b e t w e e n s u r f a c e w a t e r a n d w e t l a n d s r e c e i v i n g w a s t e w a t e r b e c a u s e r u n o f f from u n u s u a l s t o r m s c o u l d fl o o d t h e w e t l a n d , f h J s h i n g t h e e f f l u e n t s i n t o n e i g h b o r i n g w a t e r bodies. T h i s c o u l d u p s e t t h e t r e a t m e n t o p e r a t i o n as well a s p o l l u t e
Heliotis and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING surface waters. A atormwater analysis of adjacent areas is therefore n e c e s s a r y to d e t e r m i n e t h e e x p e c t e d f r e q u e n c y a n d p o s s i b l e d a m a g e f r o m storm runoff (Fritz a n d Helle 1978). Precipitation and Evapotranspiration. Precipitation is the most important input of water for certain t y p e s o f w e t l a n d s , e s p e c i a l l y s o m e n o r t h e r n b o g s ( K a d l e c 1976, Mo o r e a n d Betlamy 1974). G o r h a m {1961) e a r l y r e c o g n i z e d t h e i m p o r t a n c e o f precipitation as a mineral source for the wetlands. A literature review s h o w e d t h a t p r e c i p i t a t i o n c o n t r i b u t e s 0.5-30.16 k g / h a / y r a n d 0.07-4.75 k g / h a / y r of t o t a l n i t r o g e n a n d p h o s p h o r u s , r e s p e c t i v e l y { C h a p i n a n d U t t o r m a r k 1973). In t e r m s o f n u t r i e n t b u d g e t , p r e c i p i t a t i o n a c c o u n t e d f o r 3% o f t h e t o t a l P a n d 2% o f t h e t o t a l N i n p u t to N e v i n w e t l a n d ( N o v i t z k i 1978a), w h e r e a s in o m b r o t r o p h i c ( r a i n - f e d ) p e a t l a n d s it c a n a c c o u n t f o r a l m o s t 100Z. Evapotranspiration removes water from the land or water surface, t h e u n s a t u r a t e d zone or the s a t u r a t e d zone. It can a c c o u n t for a substantial loss of water from wetlands especially during the growing season, Its obvious effect on water quality is the increase in concentration of nutrients because of the reduced a m o u n t of water {Heliotia 1982). INTERNAL INTERACTIONS
Nutrient Uptake by Wetland Plants. Mechanisms o f n u t r i e n t u p t a k e . A g e n e r a l h i g h a f f i n i t y a c t i v e absorption m e c h a n i s m is responsible for ionic uptake by ptants at low environmental concentrations (14utchinson 1975). This mechanism involves various phases that closely agree with Michaelis-Menten kinetics and t h a t h a v e been d e m o n s t r a t e d o v e r a wide taxonomic r a n g e of p l a n t s i n c l u d i n g some a l g a e (Ni~sen 1973). T h e e q u a t i o n d e s c r i b i n g t h e n u t r i e n t t r a n s f e r i n i t s l i n e a r i z e d f o r m is: 1 / v : (Km/Vm) (1/Co) + 1/Vm w h e r e v is t h e o b s e r v e d r a t e o f n u t r i e n t u p t a k e a t e x t e r n a l n u t r i e n t c o n c e n t r a t i o n Co; Vm is t h e maximum r a t e of u p t a k e ; a n d Km is t h e M i c h a e l i s c o n s t a n t e q u a l to t h e n u t r i e n t c o n c e n t r a t i o n a t v = 1 / 2 V s . At h i g h e x t e r n a l c o n c e n t r a t i o n s a l o w - a f f i n i t y , d i f f u s i o n - - l i k e p r o c e s s predominates. T h e e f f e c t o f f l o w r a t e on n u t r i e n t u p t a k e h a s r a r e l y b e e n investigated. T h i s is a n i m p o r t a n t c o n s i d e r a t i o n in w e t l a n d s r e c e i v i n g wastewater because different loading rates, methods of application, and c o n f i g u r a t i o n o f t h e w e t l a n d r e s u l t in d i f f e r e n t r a t e s o f w a t e r m o v e m e n t . S t a n f o r t h (1976) r e p o r t e d t h a t t h e u p t a k e of P b y Myriophyllum i n c r e a s e s w i t h i n c r e a s i n g flow r a t e . At r e l a t i v e l y low r a t e s ( l e s s t h a n 4 m l / s e c ) , t h e r e l a t i o n s h i p c a n be d e s c r i b e d b y a Michaeiis-Menten curve where flow rate is used instead of concentration. T h i s d e p e n d e n c e is d u e to t h e d e c r e a s e in t h i c k n e s s o f t h e n u t r i e n t d e p l e t i o n z o n e b e t w e e n t h e p l a n t a n d t h e w a t e r w i t h i n c r e a s i n g flow rates. At t h e e c o s y s t e m l e v e l , flow r a t e s a r e u s u a l l y h i g h e n o u g h s o t h a t t u r b u l e n t f l o w p r e d o m i n a t e s a n d t h e t h i c k n e s s of t h e d e p l e t i o n z o n e i s a s s u m e d to be minimal ( S t a n f o r t h 1976; Wetzel 1975). In t h a t case,
139
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WETLANDS, Vol. 3, 1983
n u t r i e n t r e m o v a l f r o m w a t e r is r e l a t e d to t h e time t h e w a t e r s p e n d s c o n t a c t with the plant; a n d n u t r i e n t r e d u c t i o n in t h e flowing w a t e r d e c r e a s e s w i t h i n c r e a s i n g flow. A n o t h e r f a c t o r s h o w n to h a v e d r a m a t i c e f f e c t s o n P u p t a k e
in
by
Myriophyllum spicatum is the presence of trace metal contaminants such
a s c o p p e r a n d z i n c , w h i c h " p o i s o n " t h e u p t a k e p r o c e s s ( S t a n f o r t h 1976). T h i s may be i m p o r t a n t in t h e c a s e of w a s t e w a t e r a p p l i c a t i o n to w e t l a n d s because wastewater often contains trace metals. T h e r e a l s o s e e m to be c o n s i d e r a b l e d i f f e r e n c e s in t h e s e l e c t i v e absorption capacity of different aquatic maerophytes. D y k y j o v a (1978) i n d i c a t e d t h a t s o m e s p e c i e s s u c h a s S p a r g a n i u m e r e c t u m , A c o r u s calamus, a n d e s p e c i a l l y Gtjzcvria maxima a r e h i g h l y p l a s t i c in t h e i r n u t r i e n t u p t a k e a s r e l a t e d to a v a i l a b i l i t y , a b s o r b i n g h i g h a m o u n t s o f n u t r i e n t s from eutrophic environments. O t h e r s p e c i e s s u c h as P h r a g m i t e s australia and Schoenoplec~us (Scirpus) lacustris possess a smaller plasticity in highly eutrophic habitats.
NuLrient u p t a k e b y periphyT, on. Q u a n t i f i c a t i o n of n u t r i e n t u p t a k e a n d p r o c e s s i n g h a s b e e n e s p e c i a l l y l i m i t e d in t h e c a s e o f e p i p h y t i c organisms, which generally have been lumped with macrophytes as a single a g g r e g a t e d c o m p o n e n t ( v a n d e r Valk e t al. 1978}. T h e r o l e o f p e r i p h y t o n t h u s is n o t e x p l i c i t l y c o n s i d e r e d e v e n t h o u g h a c c o r d i n g to some e v i d e n c e , t h i s r o l e is q u i t e a p p r e c i a b l e , a c c o u n t i n g f o r m o s t of t h e p h o s p h o r u s r e m o v a l in a T y p h a - d o m i n a t e d m a r s h ( C o r r e l e t al. 1975). N u t r i e n t L e v e l s in Wetland Plants.
Nutritional r e q u i r e m e n t s and limitations. T h e c o n c e n t r a t i o n o f a g i v e n n u t r i e n t i n p l a n t t i s s u e ~ p r o v i d e s a g o o d i n d i c a t i o n of i t s nutritional and ecological status. When p l a n t g r o w t h c e a s e s to b e l i m i t e d b y n u t r i e n t a v a i l a b i l i t y , t h e c o n c e n t r a t i o n of t h e g i v e n n u t r i e n t in t i . s u e is it s " c r i t i c a l c o n c e n t r a t i o n . " Thus plants whose t i s s u e n u t r i e n t s are f o u n d at or below the this critical c o n c e n t r a t i o n a r e n u t r i e n t limited. Plants whose t i s s u e n u t r i e n t s a r e a b o v e this c r i t i c a l c o n c e n t r a t i o n a r e e n g a g e d in " l u x u r y c o n s u m p t i o n . " It u s u a l l y is t h e i n o r g a n i c n u t r i e n t s N a n d P t h a t i m p o r t a n t as l i m i t i n g f a c t o r s f o r a q u a t i c p l a n t g r o w t h . o r t r a c e e l e m e n t s m i g h t a l s o be l i m i t i n g , P o t a s s i u m , f o r s h o w n to be l i m i t i n g b y G e r l o f f 11975) f o r Myriophyllum eutrophic Lake Wingra in Wisconsin.
are critically Other nutrients example, was spicatum in
The t e c h n i q u e s fnr t i s s u e a n a l y s i s and c r i t i c a l n u t r i e n t concentration determination for assessing nutrient supplie~ in aquatic environments were developed by Gerloff and Krombholz (1966) and applied to a s s e s s i n g n u t r i e n t s u p p l i e s in a q u a t i c e n v i r o n m e n t s . However, critical concentrations have not been established for emergent wetland macrophytes, Prentki et al. (1978) found for some wetland environments that N, P, K, Ca, and Mg concentrations frequently fall within the ranges of growth limitation, and sometimes are well below critical concentrations that may indicate severe nutrient deficiencies in some wetland environments,
Heliotis and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING
t h e m / c a / composition o f w e t l a n d p / a n t e . Data o n t h e c h e m i c a l c o m p o s i t i o n of w e t l a n d p l a n t s a p p e a r e x t e n s i v e l y in t h e l i t e r a t u r e f o r p u r p o s e s of n u t r i e n t b u d g e t and c y c l i n g estimations, for p h y s i o l o g i c a l ecology s t u d i e s , a n d r e c e n t l y , f o r a s s e s s m e n t o f p o t e n t i a l f o r a n i m a l f e e d s a n d nutrient removal from polluted waters. H u t c h i n s o n (1975) c o m p i l e d d a t a f r o m t h e l i t e r a t u r e on t h e c h e m i c a l c o m p o s i t i o n o f a q u a t i c p l a n t s , w h e r e a s B o y d ' s r e v i e w (1978) w a s b a s e d o n data from hie own r e s e a r c h . B o y d ' e p r i m a r y c o n c l u s i o n was t h a t t h e r e a r e no m e a n i n g f u l a v e r a g e c o n c e n t r a t i o n s of n u t r i e n t s in w e t l a n d p l a n t s a n d t h a t m e a s u r e m e n t s would h a v e to be p e r f o r m e d f o r each p a r t i c u l a r case. He s t a t e s t h a t v a r i a t i o n in c o m p o s i t i o n i n c r e a s e s in t h e following order: within-site intraepecific variation, between-site intraapecffic variation, and interspecific variation. C h e m i c a l c o m p o s i t i o n c h a n g e s s e a s o n a l l y a s well a s b e t w e e n d i f f e r e n t p a r t s o f t h e p l a n t (Boyd 1978; D y k y j o v a 1978; K v e t 1978; Wetzel 1975). I n g e n e r a l , t i s s u e n u t r i e n t l e v e l s a r e maximum a t t h e b e g i n n i n g of t h e g r o w i n g s e a s o n and d e c l i n e t h e r e a f t e r (Davis a n d Harris 1978; K l o p a t e c 1978). F o r P h r a g m i t e s australia s h o o t s , t h e N a n d P c o n c e n t r a t i o n s (% d r y w e i g h t ) w a r e 2.77 a n d 0.48 in May a n d 1.00 a n d 0,17 in O c t o b e r , w h e r e a s f o r T y p h a anguatifolia t h e y w e r e 2,9 a n d 0.49 in May a n d 0.8 a n d 0.15 in S e p t e m b e r ( D y k y j o v a i978). During decomposition, h o w e v e r , it has b e e n o b s e r v e d t h a t n i t r o g e n c o n t e n t of p l a n t s i n c r e a s e s a s a r e s u l t o f i n v a s i o n b y b a c t e r i a a n d f u n g i (Hill 1979; K l o p a t e k 1978). For nutrient allocation within the plant, it has been documented that the aboveground parts and especially the young and p h o t o s y n t h e t i c a l l y a c t i v e t i s s u e s c o n t a i n h i g h e r p e r c e n t a g e s of n u t r i e n t s t h a n t h e u n d e r g r o u n d r o o t s a n d r h i z o m e s (Heliotis 1982). N u t r i e n t s t a n d i n g s t o c k s . T h e s t a n d i n g s t o c k o f n u t r i e n t s in v e g e t a t i o n a t a g i v e n time is t h e p r o d u c t of t h e b i o m a s s a n d t i s s u e n u t r i e n t c o n c e n t r a t i o n : N u t r i e n t S t a n d i n g S t o c k (g/m21 : B i o m a s s (g/m2} x T i s s u e N u t r i e n t C o n c e n t r a t i o n (% d r y w e i g h t } . Typical tissue nutrient levels and standing stocks for several c o m m o n w e t l a n d m a c r o p h y t e e a r e g i v e n in T a b l e 1. It s e e m s t h a t t h e r e is a r a n g e of v a l u e s o f a n o r d e r o f m a g n i t u d e b e t w e e n t h e d i f f e r e n t species. H o w e v e r , minimum a n d maximum t i s s u e n u t r i e n t c o n c e n t r a t i o n s vary much less than standing stocks because tissue levels are determined b y t h e n u t r i t i o n a l e c o l o g y of t h e p l a n t ( u p t a k e r a t e s a n d l u x u r y consumption) and b e c a u s e s t a n d i n g s t o c k s a r e controlled by biomass and r e f l e c t t h e t r o p h i c s t a t u s of t h e h a b i t a t . E s t i m a t i o n of t h e a b o v e g r o u n d n u t r i e n t pool is of p a r t i c u l a r i m p o r t a n c e if t h e w e t l a n d is to be m a n a g e d f o r n u t r i e n t a s s i m i l a t i o n , a n d if h a r v e s t i n g is c o n s i d e r e d a m a n a g e m e n t p r a c t i c e . However, the l a c k o f p a r a l l e l b i o m a s s a n d t i s s u e n u t r i e n t c o n c e n t r a t i o n d a t a in t h e l i t e r a t u r e m a k e s it d i f f i c u l t to e s t i m a t e t h i s pool. B i o m a s s a l o n e is u s u a l l y a m u c h b e t t e r p r e d i c t o r o f nutriet~t s t a n d i n g s t o c k s than t i s s u e l e v e l (Hoyd 1978). I n f o r m a t i o n on l e a c h i n g a n d d e c o m p o s i t i o n p r o ( : e s s e s is a l s o n e c e s s a r y in o r d e r to a s s e s s t h e time of maximum n u t r i e n t standing stock.
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Heliotis and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING
F l o w o f e n e r g y a n d m a t e r / a / s to h i g h e r t r o p h i c l e v e l s . T h e r o l e of a n i m a l s in n u t r i e n t c y c l i n g in w e t l a n d e c o s y s t e m s h a s b e e n l a r g e l y n e g l e c t e d d e s p i t e t h e p o s s i b i l i t y t h a t t h e y may b e p r o d u c i n g p a r t i c u l a t e matter, maintaining the stability of the sediments, influencing n u t r i e n t s in w a t e r b y e x c r e t i n g ammonium i o n s a n d h a r v e s t i n g n u t r i e n t s b y g r a z i n g {Valiela a n d T e a l 1978}. In g e n e r a l it c a n b e s t a t e d t h a t a l t h o u g h a n i m a l s d o n o t s e e m t o b e i m p o r t a n t in t r a n s p o r t i n g n u t r i e n t s o u t s i d e t h e b o u n d a r i e s o f w e t l a n d s , t h e y a r e i m p o r t a n t n u t r i e n t p r o c e s s o r s a f f e c t i n g t h e r a t e of m i n e r a l cycling.
Decomposition P r o c e s s e s . The standing l i t t e r compartment. Standing litter in emergent macrophytes differs from fallen litter in being more widely spaced, leas fragmented, and in less contact with water. Microbial colonization and decomposition are of minor importance, and los~es of dry weight and nutrients are due to leaching and fragmentation. These are controlled by autumnal and winter rains and winds, the structural nature of the s t a n d s , a n d m e c h a n i c a l d r a i n a g e by a n i m a l s (Davis a n d v a n d e r Valk 1978b}. T h e s t a n d i n g l i t t e r c o m p a r t m e n t is i m p o r t a n t in c a s e s of w a s t e w a t e r a d d i t i o n s t o w e t l a n d s s i n c e it r e p r e s e n t s a " c o n f i n e d " n u t r i e n t p o o l amenable to management practices such as harvesting.
The fallen l i t t e r c o m p a r t m e n t a n d t h e d~composigion ~ q u e n o e . Upon the entrance of dead plant tissues into the fallen litter compartment, a complex series of events begins whose rates and specific pathways are determined by environmental conditions (see Figure 3). In general, oxygen concentrations in the wetland substrate determine the types of microbial populations that colonize the litter. Waterlogged soils, especially those rich in organic matter, develop a thin oxidized layer a few millimeters thick in the surface that undergoes aerobic respiration while the rest of the substrate undergoes anaerobic respiration and fermentation (Chamie and Richardson 1978). Initial reduction of dead tissues to small particles is generally attributed to invertebrate "shredders" such ag amphipod~, or to mechanical causes such as tidal action (Axelrad et el. 1976}. This r e d u c t i o n i n c r e a s e s the s u r f a c e / v o l u m e ratio r e n d e r i n g the p a r t i c l e s more a v a i l a b l e f o r m i c r o b i a l c o l o n i z a t i o n . T h e s u b s e q u e n t d e g r a d a t i o n p r o c e s s e s a r e d e p e n d e n t o n t h e a v a i l a b i l i t y of s p e c i f i c s u b s t r a t e s w i t h i n t h e p a r t i c l e (i.e., i t s r e f r a c t i b i l i t y ) , t h e r a t e o f m i c r o b i a l m e t a b o l i s m t h a t is t e m p e r a t u r e d e p e n d e n t , a v a i l a b i l i t y of e l e c t r o n a c c e p t e r s { o x y g e n a n d o t h e r s ) , told t h e a v a i l a b i l i t y o f n u t r i e n t s .
N u t r i e n t f l u x e s durirlg d e c o m l ~ i t i o n . 1Jeaching d e p e n d s u p o n t y p e a n d p h y s i c a l c o n d i t i o n of v e g e t a t i o n , r e s p i r a t o r y r a t e s , a n d p r e s e n c e o f a e r o b i c o r a n a e r o b i c c o n d i t i o n s (Chamie a n d R i c h a r d s o n 1978). For instance, Davis and van der Valk (197~b), found that Typha seems more susceptible to leaching than Scirpus during the same time interval. They also found that site characteristics can influence leaching rates and an increasing degree o f submergence usually r e s u l t s in greater leaching (Davis and van der Valk 1978c).
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i PLANTS
11' ~1 : ~oo,, : Processes
: Conditions
LITTER
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FERMENTATIO~
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Nutrient Pathways durin~ Decomposition (Heliotis 1981).
Heliotis and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING
Although nutrients are rapidly leached from the fresh litter, nutrient concentration of the litter compartment as a whole might not decrease. In f a c t , it u s u a l l y i n c r e a s e s a s a r e s u l t of m i c r o b i a l uptake. This microbial uptake can be very substantial, reaching in one case 85~ of the total macrophyte P uptake {Davis and Harris 1978). Morphology also plays an important role in nutrient uptake patterns. Davis and van der Valk (1978c) attributed similar uptake patterns of the litter of Scirpus f]uviatilis a n d Carvx at_berodes to the morphological similarities of the two species in terms of structure and surface area available for microbial colonization. At the ecosystem level, all the previously mentioned factors ptus environmental conditions such as temperature, redox potential, and flow patterns, determine the role of litter as a nutrient sink or source. In g e n e r a l , o l d e r l i t t e r t e n d s to a c c u m u l a t e n u t r i e n t s a c t i n g a s a n u t r i e n t sink for the wetland {Heliotis 1981; Brinson 1977}.
Incorporation o f N u t r i e n t s i n t o t h e S u b s t r a t a . The constant state of inundation of wetland soils makes them share c e r t a i n c o m m o n c h a r a c t e r i s t i c s d e s p i t e t h e i r d i f f e r e n c e s in a m o u n t o f organic matter present and other physical and chemical properties. The a b s e n c e of m o l e c u l a r o x y g e n is d u e t o t h e e x t r e m e l y slow r a t e s of g a s e x c h a n g e s b e t w e e n soil a n d a i r w h e n t h e soil is s a t u r a t e d w i t h w a t e r { P o n n a m e r u m a 1972L F u r t h e r m o r e , t h e m o l e c u l a r o x y g e n p e n e t r a t i n g t h e soil is r a p i d l y u t i l i z e d b y a e r o b i c a n d f a c u l t a t i v e a n a e r o b i c m i c r o o r g a n i s m s ( E n g l e r e t al. 1976). O n l y a v e r y t h i n l a y e r i n t h e s o i l - w a t e r i n t e r f a c e r e m a i n s in a n o x i d i z e d s t a t e . The chemical and b i o l o g i c a l c o n d i t i o n s o f t h e i n t e r f a c e r e s e m b l e t h o s e in a e r o b i c s o i l s and are responsible for its unique ecological importance { P o n n a m e r u m a 1972). Studies of the factors controlling the oxygenated layer s h o w e d that both microbial respiration and chemical oxidation of iron (II} control its thickness {Hutchinson 1957; P o n n a m e r u m a 1972}. The importance of the layer in nutrient cycling is related to (a} its capacity to sorb and retain c o m p o u n d s such as phosphate, silica, manganese, cobalt, nickel, and zinc that are present in the supernatant water or diffused .from the reduced zone below {Engler et al. 1976; P o n n a m e r u m a 1972}, and (b) its ability to cause nutrient transformations such as nitrification~ which releases nitrate to the water column (Keeney 1972). Below t h e o x i d i z e d z o n e t h e soil is in a r e d u c e d c o n d i t i o n c h a r a c t e r i z e d by the p r e s e n c e of ammonium, h y d r o g e n sulfide, m a n g a n e s e (I I) , i r o n (II), a n d m e t h a n e . The r e d u c e d soil a c t s as sink or s o u r c e f o r c e r t a i n c o m p o u n d s w h i l e it t r a n s f o r m s o t h e r s . F o r i n s t a n c e , it r e l e a s e s m o r e p h o s p h a t e t o s o l u t i o n s low i n s o l u b l e p h o s p h a t e a n d s o r b s m o r e p h o s p h a t e f r o m s o l u t i o n s h i g h in t h e s p e c i e s t h a n do a e r o b i c s o i l s b e c a u s e of t h e g r e a t e r s u r f a c e a r e a of t h e g e l - l i k e c o l l o i d a l r e d u c e d f e r r o u s a n d m a n g a n e s e c o m p o u n d s (Li e t al. 1972; P a t r i c k a n d K h a | i d 1974). It a l s o p r o v i d e d t h e n e c e s s a r y c o n d i t i o n s f o r t h e p r o c e s s o f d e n i t r i f i c a t i o n t h a t t r a n s f o r m s n i t r a t e to m o l e c u l a r n i t r o g e n s u b s e q u e n t l y released to the a t m o s p h e r e , t h e r e f o r e a c t i n g as a v e r y s i g n i f i c a n t N - s i n k in a q u a t i c s y s t e m s ( K e e n e y 1972). C h a n g e s in t h e r e d o x s t a t e o f t h e w e t l a n d s o i l s h a p p e n w i t h t h e d i f f u s i o n of o x y g e n f r o m t h e a b o v e g r o u n d D a r t s o f t h e p l a n t to t h e r o o t s
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as well a s w i t h c h a n g e s in t h e w a t e r l e v e l , m e c h a n i c a l d i s t u r b a n c e of t h e soil, a n d h a r v e s t i n g o f r o o t e d m a c r o p h y t e s ( B o y l e 1979; Moore a n d Bellamy 1974}. I n g e n e r a l , low pH a n d r e d o x p o t e n t i a l t e n d to f a v o r t h e f o r m a t i o n of s o l u b l e s p e c i e s o f m a n y m e t a l s , b u t in n a t u r a l s y s t e m s t h e y a l s o r e g u l a t e o t h e r p r o c e s s e s t h a t may c o u n t e r t h i s f o r m a t i o n .
The cation exchange capacity of wetland soils, typically ranging between 80-200 meq/100 g of oven-dried soil, is important because it determines the a m o u n t of nutrients adsorbed from the water (Sjors 1961; Tilton 1976). As a general trend, this capacity increases with increase in the mineral content of the soil (Sjors 1961). Experiments with peat cores s h o w e d that nutrient removal capacity rapidly declines with time, as a result of the saturation of the most active adsorption sites, and reaches a steady level that reflects a filtration process (Mechenich 1980). I n n a t u r a l c o n d i t i o n s , h o w e v e r , t h e r e m o v a l c a p a c i t y is r e n e w e d b e c a u s e the a d s o r p t i o n sites of the p e a t a r e l i b e r a t e d as p l a n t s pump n u t r i e n t s f r o m t h e soil ( D a v i s e t el. 1981). Humic a n d f u l v i c a c i d s , c o m m o n in organic soils, also remove P by forming stable organometallic phosphates (Sinha 1971). S e d i m e n t a t i o n is t h e p r i m a r y m e c h a n i s m f o r r e m o v a l o f s e t t l e a b l e s o l i d s a n d c o n t r i b u t e s to r e m o v a l o f c o l l o i d s , n i t r o g e n , p h o s p h o r u s , h e a v y m e t a l s , a n d r e f r a c t o r y o r g a n i c s ( T c h o b a n o g i o u s e t el. 1979), In t h e i r l i t e r a t u r e r e v i e w of the role of w e t l a n d s for s e d i m e n t control, B e t s a n d P a t r i c k (1978) i n d i c a t e d t h a t t h e r e m o v a l o f s u s p e n d e d sediments is due to the decreased velocity of flowing water because of the presence of vegetation, and to the f]occulation of clay particles in salt marsh and estuarine systems because of mixing. P r e c i p i t a t i o n r e a c t i o n s c o n t r i b u t e to the t r a n s f e r of c o m p o u n d s f r o m t h e w a t e r c o l u m n to t h e s u b s t r a t e . For p h o s p h o r u s and h e a v y m e t a l s , p r e c i p i t a t i o n is a m a j o r r e m o v a l m e c h a n i s m . As a g e n e r a l t r e n d , a t pH g r e a t e r t h a n 8, p h o s p h a t e s p r e c i p i t a t e f r o m s o l u t i o n s . F o r t h e p u r p o s e o f m a n a g e m e n t of w e t l a n d s f o r n u t r i e n t a s s i m i l a t i o n it is d e s i r a b l e to a c h i e v e p e r m a n e n t i n c o r p o r a t i o n o f n u t r i e n t s i n t o t h e d e p o s i t , o r t r a n s f e r o f n u t r i e n t s to t h e a b o v e g r o u n d p a r t s o f t h e vegetation for harvesting. T h e s e t w o p r o c e s s e s r e s u l t in t h e l o n g - t e r m r e m o v a l of n u t r i e n t s f r o m t h e s y s t e m a n d t h e r e f o r e in t h e i m p r o v e m e n t o f t h e q u a l i t y of w a t e r l e a v i n g t h e w e t l a n d . T h e i m p o r t a n c e o f p e a t d e p o s i t s a s p e r m a n e n t s i n k s f o r n u t r i e n t s is e m p h a s i z e d in a s t u d y b y F r i e d m a n a n d DeWitt (1978), w h o c a l c u l a t e d t h a t Waubesa Wetlands, Wisconsin, stored each year for thousands of years approximately I0~ of the critical annual loading of phosphorus leading to eutrophication of the adjacent Lake Waubesa.
Nutrient R e m o v a l Mechanisms. The r e l a t i v e i m p o r t a n c e of wetland n u t r i e n t c y c l i n g mechanisms v a r i e s g r e a t l y f r o m s i t e to s i t e , a n d d a t a on t h e i r q u a n t i f i c a t i o n a r e u s u a l l y n o t a v a i l a b l e . ] t is, t h e r e f o r e , n o t s u r p r i s i n g t h a t a t t e m p t s to c o r r e l a t e n u t r i e n t a b s o r p t i v e c a p a c i t i e s w i t h v a r i o u s w e t l a n d features have not been successful. In one such study, Whigham and Bayley (1978) were unable to demonstrate any correlation of the amounts of N and P accumulated by the aboveground vegetation with latitude, length of the growing season, hydroperiod, turnover rate of water in the
Helio~is and DeWitt, WETLAND WASTEWATER NUTRIENT CYCLING
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w e t l a n d , n u t r i e n t s t a t u s of w a t e r , o r d i v e r s i t y of p l a n t s p e c i e s i n d i f f e r e n t w e t l a n d s . T h e o n l y t r e n d was t h a t w e t l a n d s w i t h p r e d o m i n a n t l y organic substrates accumulated l e s s N and P in the aboveground v e g e t a t i o n b u t w e r e a b l e to s t o r e n u t r i e n t s i n t h e p e a t . S e v e r a l a u t h o r s , h o w e v e r , h a v e s u g g e s t e d t h e " k e y c o m p o n e n t s " of t h e i r s t u d y sites regulating nutrient cycling. F r o m this information, the following g e n e r a l t r e n d s a r e o b s e r v e d (Heliotis, 1982}: Conditions favoring phosphorus
removal:
-Organic soil in poor nutrient regimes -Vegetation limited by phosphorus supply or able to take up e x c e s s phosphorus (luxury consumption) - P r e s e n c e of Pe, AI c o m p o u n d s Conditions favoring nitrogen removal: - R e d u c e d soil-water i n t e r f a c e (denitrification) - V e g e t a t i o n l i m i t e d b y N o r a b l e to t a k e u p e x c e s s n i t r o g e n (luxury consumption) Conditions favoring nitrogen and phosphorus
removal:
- L o w or no energy subsidy to the wetland by tides, w a v e action, streamflow, etc. SU ~ Y A s y s t e m a t i c r e v i e w of n u t r i e n t c y c l i n g s t u d i e s i n w e t l a n d s resulted in the construction of a conceptual model of nutrient dynamics. The following needs were identified: (1) quantification of hydrological relationships emphasizing contribution of different water sources to the nutrient budget of the wetland; and (2) mass-balance studies of wetland processes especially decomposition, adsorption reactions, and peat ace umulation. LITERATURE CITED Al A z h a r i a J a h n , Samia. 1976. S u d a n e s e n a t i v e m e t h o d s f o r t h e p u r i f i c a t i o n of Nile w a t e r d u r i n g t h e f l o o d s e a s o n . I n : Biological control of water pollution. J. Tourbier and R. W. Pierson, Jr. ( e d s . ) . University of Pennsylvania Press, pp. 95-106. Axelrad, D, M.; Moore, K. A.; and Bender, M. E. 1976. Nitrogen, p h o s p h o r u s a n d c a r b o n flux i n C h e s a p e a k e Bay m a r s h e s . OWRT-B-O27-A. Bulletin 79, Virginia Water Resources Center, Virginia Polytechnic Institute and State University, Blackburg, Virginia 24061. Beadle, L. C. 1932. S c i e n t i f i c r e s u l t s of t h e C a m b r i d g e e x p e d i t i o n to E a s t A f r i c a n lakes~ 1930-1. IV. T h e w a t e r s of some E a s t A f r i c a n lakes in relation to their fauna & flora. J. Linn. Soc. (zool.}, 38: 157-21]. Boto, K. G.p a n d P a t r i c k , W. H., J r . 1978. Role of w e t l a n d s i n t h e r e m o v a l of s u s p e n d e d s e d i m e n t s . In: W e t l a n d f u n c t i o n s a n d v a l u e s : t h e s t a t e of o u r u n d e r s t a n d i n g . G r e e s o n , P. E.; C l a r k , J. R.; a n d C l a r k , J. E. ( e d s . L P r o c e e d i n g s o n t h e National S y m p o s i u m o n
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