37 New England Coastal Waters—An Infinite Estuary
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CHARLES S. YENTSCH Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Me. 04575
I f o c e a n i c r e g i o n s c o u l d be graded u s i n g t h e c o n v e n t i o n a l motives o f our s o c i e t y , the c o a s t a l w a t e r s would g e t h i g h marks. They h o s t most o f t h e p r o d u c t i v e f i s h e r i e s and p l a y a prime r o l e i n n e a r shore r e c r e a t i o n a l a c t i v i t i e s . Oddly, t h e s t u d y o f t h e p r o c e s s e s which cause c o a s t a l w a t e r s t o be p r o d u c t i v e and a e s t h e t i c a l l y p l e a s i n g , has n o t r e c e i v e d t h e u n i f i e d a t t e n t i o n o f t h e océanographie community. The e a r l y 1930 s t u d i e s o f H. B. B i g e l o w r e p r e s e n t e d a g i a n t s t e p towards s u c h a g o a l ; however, what f o l l o w e d was a p e r i o d where t h e s t u d y o f c o a s t a l ocean p r o c e s s e s was d i s p l a c e d by i n t e r e s t s i n t h e deep o c e a n s . S h o r t l y a f t e r World War I I , i n t e r e s t s changed; some o c e a n o g r a p h e r s began t o f o c u s i n t e r e s t on t h e i n t e r a c t i o n o f f r e s h w a t e r and seawater v i a t h e e s t u a r y . T h i s i n t e r e s t g a i n e d momentum l a r g e l y t h r o u g h t h e r e s e a r c h i n t e r e s t s Β . H. Ketchum. Armed w i t h t e c h n i q u e s l a r g e l y d e r i v e d from e s t u a r i n e s t u d y , Ketchum (1) examined t h e i n t e r a c t i o n o f f r e s h w a t e r and s e a water i n c o a s t a l w a t e r s . I n e s s e n c e , he t r e a t e d t h e s e as a g i g a n t i c e s t u a r y . In the l i g h t o f our present day c o n c e r n o f t h e e c o l o g i c a l i m p l i c a t i o n s o f p o l l u t i o n o f n e a r - s h o r e w a t e r s , t h i s approach must be c o n s i d e r e d one o f t h e major advancements i n t h e s t u d y o f t h e c o a s t a l ocean environment. The a p p r o a c h used by Ketchum c o n s i d e r e d c o a s t a l w a t e r s as a m i x t u r e o f open-ocean water and r i v e r inflow. A t any d i s t a n c e from e i t h e r s o u r c e , t h e d i l u t i o n o f seawater c a n be d e f i n e d by a r a t i o between t h e s a l i n i t y o f t h e open-ocean and t h e s a l i n i t y o f t h e c o a s t a l water. The a p p r o a c h c a n be u s e d d i r e c t l y t o estimate the d i l u t i o n of a conservative p o l l u t a n t . Less d i r e c t i s the use w i t h a non-conservative p o l l u tant. I t i s t h e i n t e n t o f t h i s paper t o demonstrate a 608
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
37.
YENTSCH
New
England
Coastal
Waters
609
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means o f p r e d i c t i n g t h e d i s t r i b u t i o n o f n o n - c o n s e r v a tive properties. F i r s t , I s h a l l d e s c r i b e t h e compon e n t s f o r t h e development o f t h e p r e d i c t i v e model, and f o l l o w by t e s t i n g t h e model on d a t a c o l l e c t e d i n t h e c o a s t a l w a t e r s o f f New E n g l a n d . My p r i n c i p a l aim i s t o e s t i m a t e t h e v a l u e o f an " e s t u a r i n e t r e a t m e n t " o f c o a s t a l waters w i t h r e f e r e n c e t o one b i o l o g i c a l l y s i g n i f i c a n t compound — phosphate. Models f o r c o n s e r v a t i v e and n o n - c o n s e r v a t i v e ties
proper-
When f r e s h w a t e r f l o w s i n t o t h e ocean, a m i x t u r e d e v e l o p s t h a t r e p r e s e n t s a r a t i o between t h e two. The amount o f f r e s h w a t e r i n t h e m i x t u r e can be e s t i m a t e d by measuring t h e s a l i n i t y o f t h e m i x t u r e ( S ) and t h e s a l i n i t y o f t h e ocean (SR) n e i g h b o r i n g t h e e s t u a r y . The p e r c e n t a g e o f f r e s h w a t e r (F) i s e q u a l t o , 0
S
R
S
" Q R
(100)
(1)
S
or
the percentage ^o ° S
o f seawater
(S) i n the mixture
(100)
as (2)
SR
U n i t s a r e p a r t s p e r thousand. By making a s e r i e s o f s a l i n i t y measurements from t h e f r e s h w a t e r s o u r c e t o t h e open ocean, a r e l a t i o n s h i p can be d e v e l o p e d ( f i g u r e 1), which d e s c r i b e s the d i s t r i b u t i o n of f r e s h and oceanwater i n t h e system. I f a conservative p o l l u t a n t (a s u b s t a n c e n o t a l t e r e d by b i o c h e m i c a l a c t i v i t y ) e n t e r s w i t h f r e s h w a t e r a t a u n i f o r m r a t e , t h e n t h e down stream d i s t r i b u t i o n w i l l f o l l o w t h e c u r v e d e s c r i b i n g the fresh-oceanwater d i s t r i b u t i o n . I f however, t h e p o l l u t a n t i s n o n - c o n s e r v a t i v e (a p r o p e r t y whose c o n c e n t r a t i o n i s a l t e r e d by b i o l o g i c a l a c t i v i t y , f o r example phosphate) t h e n t h e d i s t r i b u t i o n would n o t f o l l o w t h e f r e s h - o c e a n w a t e r d i s t r i b u t i o n s i n c e t h e c o n c e n t r a t i o n s would be a l t e r e d by b i o l o g i c a l a c t i v i t y en r o u t e downstream. The o b s e r v e d c o n c e n t r a t i o n o f t h e n o n - c o n s e r v a t i v e p r o p e r t y a t any l o c a t i o n en r o u t e would be: (3)
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
610
MARINE
CHEMISTRY
where N f would be the c o n c e n t r a t i o n a t some p o i n t down stream from t h e f r e s h water s o u r c e , Nf would be the c o n c e n t r a t i o n o f the n o n - c o n s e r v a t i v e In t h e f r e s h water s o u r c e and N would be t h e amount removed by b i o l o g i c a l uptake. F~~is o b t a i n e d from e q u a t i o n 1. The d i m e n s i o n s o f Ν a r e mass p e r u n i t volume. Thus e q u a t i o n 3 i s o n l y v a l i d i f t h e r e i s no c o n t r i b u t i o n by ocean w a t e r . The c l o s e s t example would be a h e a v i l y p o l l u t e d r i v e r f l o w i n g i n t o phosphate-^poor ocean. However t h e u s u a l c a s e would be f o r f r e s h w a t e r t o c a r r y phosphate t o c o a s t a l o c e a n i c r e g i o n s which a l r e a d y have s i z a b l e q u a n t i t i e s o f phos phate present. To c o u n t e r t h i s s i t u a t i o n e q u a t i o n (3) i s r e w r i t t e n f o r the oceanic c o n t r i b u t i o n ( N ^ ) as; r
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u
s
N
s
T5Ô
=
(
N
s
'
r
"
>
N
r
u
(
4
)
where S% i s o b t a i n e d from e q u a t i o n ( 2 ) . Thus i n a r e a l i s t i c s i t u a t i o n any c o n c e n t r a t i o n o f a n o n - c o n s e r v a t i v e p r o p e r t y (N ) w i l l be d e t e r m i n e d by f r e s h and o c e a n i c c o n t r i b u t i o n s and b i o l o g i c a l uptake, hence : Q
N and combining
N
0
=
(N
+ Nf) - N
s
with equation
°
=
fïïô
