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ORGANIC CARBON AND DEOXYGENATION I N THE PAMLICO RIVER ESTUARY

by

Graham J. Davis Mark M . B r i n s o n

and

W i l l i a m A . Burke

Department o f Biology E a s t C a r o l i n a U n i v e r s i t y

G r e e n v i l l e , N . C . 27834

The work by f u n d s n o l o g y ,

upon which t h i s p u b l i c a t i o n i s b a s e d was s u p p o r t e d i n p a r t p r o v i d e d by t h e O f f i c e o f Water Resources Research a n d Tech- U.S. Department of I n t e r i o r , through t h e Water Resources Research I n s t i t u t e of t h e U n i v e r s i t y of North C a r o l i n a , a s a u t h o r i z e d under t h e Water Resources Research Act of 1964, a s amended.

P r o j e c t No. A-088-NC

Agreement No. 14-34-0001-7070

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ACKNOWLEDGEMENTS

Many p e r s o n s c o n t r i b u t e d t o many a s p e c t s of t h i s s t u d y , and t h e i r a s s i s t a n c e i s g r a t e f u l l y acknowledged. We a r e e s p e c i a l l y i n d e b t e d t o Tom V i c a r s , who was r e s p o n s i b l e f o r much of t h e d a t a c o l l e c t e d d u r i n g t h e f i r s t y e a r of t h e s t u d y , and t o Martha J o n e s , who s u p e r v i s e d t h e l a b o r a t o r y a n a l y s e s of t h e n u t r i e n t s . We would a l s o l i k e t o e x p r e s s o u r a p p r e c i a t i o n t o t h e s t a f f of WITN-TV (Washington, N.C.) f o r t h e i r a s s i s t a n c e w i t h t h e c l i m a t o l o g i c a l d a t a , and t o B a r r y Adams and t h e Department o f N a t u r a l R e s o u r c e s and Community Development f o r much of t h e i n f o r m a t i o n c o n c e r n i n g f i s h kills. D r s . Bruce P e t e r s o n and John Hobbie were r e s p o n s i b l e f o r some of t h e c a l c u l a t i o n s a p p e a r i n g i n t h e r e p o r t , and Chip Duncan f o r much of t h e d a t a c o l l e c t e d f o r t h e T a r R i v e r . C a r o l Lunney h e l p e d w i t h many of t h e f i g u r e s .

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ABSTRACT

The d i s t r i b u t i o n , s o u r c e s and s i n k s o f o r g a n i c c a r b o n were s t u d i e d i n t h e Pamlico R i v e r E s t u a r y d u r i n g 1975-1977. The main s o u r c e of o r g a n i c c a r b o n was from p h y t o p l a n k t o n p r o d u c t i v i t y , which, t o g e t h e r w i t h o t h e r s o u r c e s from w i t h i n t h e e s t u a r y , p r o v i d e d 64% of t h e o r g a n i c carbon i n p u t s . The remaining 36% was mainly from t r i b u t a r i e s . O r g a n i c c a r b o n l o s s e s ( s i n k s ) were 80% a s w a t e r column r e s p i r a t i o n and 20% a s o u t f l o w . S e d i m e n t a t i o n and b e n t h i c r e s p i r a t i o n a r e undetermined l o s s e s .

T o t a l o r g a n i c c a r b o n (TOC) v a r i e d s e a s o n a l l y w i t h t h e h i g h e s t l e v e l s o c c u r r i n g d u r i n g t h e s u m e r months when f l o w r a t e s were low. A l s o , TOC n o r m a l l y d i s p l a y e d a g r a d i e n t w i t h t h e h i g h e s t l e v e l s c o n s i s t e n t l y i n t h e upper r e a c h e s n e a r Washington, N.C. Approximately 79% of t h e TOC in t h e e s t u a r y c o n s i s t e d o f d i s s o l v e d o r g a n i c c a r b o n (DOC) which a c c o u n t e d f o r most o f t h e s e a s o n a l v a r i a t i o n s i n TOC. L e v e l s of p a r t i c u l a t e o r g a n i c carbon (POC) were f a i r l y c o n s t a n t t h r o u g h o u t t h e y e a r w i t h t h e e x c e p t i o n of s p o r a t i c i n c r e a s e s i n p h y t o p l a n k t o n biomass.

The d r y and calm summers of 1976 and 1977 p r o v i d e d an e x c e l l e n t o p p o r t u n i t y t o s t u d y d e o x y g e n a t i o n i n t h e e s t u a r y . The n o r m a l l y s l u g - g i s h c i r c u l a t i o n of t h e Pamlico R i v e r depends p r i m a r i l y on wind stress f o r mixing, and b o t h wind speed and d i r e c t i o n a r e i m p o r t a n t i n d e t e r m i n i n g t h e e x t e n t of mixing. Deoxygenation was o b s e r v e d s p a n n i n g t h e e n t i r e l e n g t h of t h e e s t u a r y and w e l l i n t o Pamlico Sound, and h a s even been o b s e r v e d d u r i n g t h e w i n t e r i n a s s o c i a t i o n w i t h a n i n t e n s e a l g a l bloom. Ueoxygenation i s n o r m a l l y l i m i t e d t o t h e d e e p e r mid-channel r e g i o n s of t h e e s t u a r y e x c e p t i n t h e upper r e a c h e s where i t may e x t e n d t h r o u g h o u t much o f t h e w a t e r column and i n t o r e l a t i v e l y s h a l l o w a r e a s i n e x t r e m e

c a s e s .

The l a r g e s t i n p u t of o r g a n i c c a r b o n t o t h e e s t u a r y i s from produc- t i v i t y o f p h y t o p l a n k t o n which i s more r e a d i l y a v a i l a b l e t o organisms u t i l i z i n g oxygen t h a n t h e more abundant and l e s s r e f r a c t o r y i n p u t s from t r i b u t a r i e s . Because of t h i s , t h e r e g u l a t i o n of i n o r g a n i c n u t r i e n t s o u r c e s (mainly n i t r o g e n ) i s more i m p o r t a n t f o r c o n t r o l l i n g i n c r e a s e s i n o r g a n i c l o a d i n g t h a n a r e t r i b u t a r y i n p u t s of o r g a n i c c a r b o n , Although d e o x y g e n a t i o n depends on p h y s i c a l c o n d i t i o n s which r e s u l t i n s t r a t i f i - c a t i o n of t h e w a t e r column, e p i s o d e s of oxygen d e p l e t i o n may b e more s e v e r e and o c c u r more f r e q u e n t l y i f c o n d i t i o n s become more f a v o r a b l e f o r h i g h e r p h y t o p l a n k t o n p r o d u c t i o n .

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TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS

. . .

ii

. . .

ABSTRACT iii LIST OF FIGURES

. .

LIST OF TABLES

. . .

x

. . .

SUMMARYAFJDCONCLUSIONS

x i i

. . .

RECO~NDATIONS x i v

. . .

INTRODUCTION The Pamlico River E s t u a r y

. . .

1

MATERIALS

AND

METHODS

. . .

5

Sampling

. . .

5

A n a l y s e s

. . .

5

C a r b o n B u d g e t

. . .

10

RESULTS AND DISCUSSION

. . .

1 2 O r g a n i c Carbon

. . .

1 2 S t a n d i n g S t o c k

. . .

12

Allochthonous I n p u t s

. . .

25

. . .

Autochthonous I n p u t s 32

. . .

O u t p u t s o f O r g a n i c Carbon 38

. . .

O r g a n i c Carbon Budget and P e l a g i a l Carbon Flow 40 Deoxygenation

. . .

40

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TABLE

OF CONTENTS (continued)

Page

A P P E N D I C E S . . . 89

. . .

Appendix Abbreviations. 90

Appendix A. Areas of Estuarine Sectors and Tributary

. . .

Drainage Basins Used i n Calculations 92

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LIST

OF FIGURES

Page 1. L o c a t i o n o f t h e Pamlico R i v e r E s t u a r y i n e a s t e r n

North C a r o l i n a

. . .

3 2, L o c a t i o n of sampling s t a t i o n s f o r t h e E s t u a r i n e

O r g a n i c Carbon s t u d y

. . .

6 3 . L o c a t i o n o f sampling s t a t i o n s f o r t h e Carbon-Nutrient-

Biomass s t u d y .

. . .

7 4. O r g a n i c Carbon i n t h e Pamlico R i v e r E s t u a r y . Monthly

c o n c e n t r a t i o n s of o r g a n i c carbon from August 1975 t o

July1976 are graphed.

. . . , . .

13 5 . Monthly means o f h y d r o g r a p h i c and c a r b o n d a t a d u r i n g

t h e E s t u a r i n e O r g a n i c Carbon s t u d y

. . .

15 6. D i s t r i b u t i o n of d i s s o l v e d o r g a n i c carbon a t 1 and 3 m

d e p t h s i n t h e Pamlico R i v e r d u r i n g 1975-1976

. . .

16 7. D i s t r i b u t i o n o f p a r t i c u l a t e o r g a n i c carbon a t 1 and 3

. . .

m d e p t h s i n t h e Pamlico River d u r i n g 1975-1976 17 8. P h y t o p l a n k t o n biomass a t s i x s t a t i o n s i n t h e P a m l i c o

R i v e r d u r i n g 1976-1977

. . .

19 9. P h y t o p l a n k t o n biomass as a f u n c t i o n o f p a r t i c u l a t e

. . . .

o r g a n i c c a r b o n i n t h e Pamlico R i v e r d u r i n g 1976-1977 21 10. C h l o r o p h y l l 5 as a f u n c t i o n of p h y t o p l a n k t o n biomass

i n t h e Pamlico R i v e r d u r i n g 1876-1977.

. . .

22 11. P a r t i c u l a t e n i t r o g e n a s a f u n c t i o n of phytoplankton

biomass i n t h e Pamlico R i v e r d u r i n g 1976-1977.

. . .

23 12. P a r t i c u l a t e phosphorus a s a f u n c t i o n o f p h y t o p l a n k t o n

biomass i n t h e Pamlico R i v e r d u r i n g 1976-1977.

. . .

24 13. D i s c h a r g e o f t h e T a r R i v e r d u r i n g 1975-1976.

. . .

26 14. C o n c e n t r a t i o n s o f d i s s o l v e d o r g a n i c carbon and p a r t i -

c u l a t e o r g a n i c c a r b o n a s a f u n c t i o n of d i s c h a r g e i n

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LIST

OF

FIGURES

( c o n t i n u e d )

Page

. . .

15

.

Discharge of Durham Creek d u r i n g 1975-1976 28

16

.

P a r t i c u l a t e o r g a n i c carbon a s a f u n c t i o n of d i s c h a r g e

. . .

i n Durham Creek d u r i n g 1975-1976 29

17

.

Dissolved o r g a n i c carbon a s a f u n c t i o n of p a r t i c u l a t e

. . .

o r g a n i c carbon i n Durham Creek d u r i n g 1975.1976 30 1 8

.

Phytoplankton p r o d u c t i v i t y a s a f u n c t i o n of phytoplank-

ton biomass i n t h e Pamlico River d u r i n g 1976.1977

. . .

33 19

.

Annual c y c l e of phytoplankton p r o d u c t i v i t y i n t h e

Pamlico River E s t u a r y d u r i n g 1976-1977

. . .

34 20

.

P e l a g i a l carbon flow i n t h e Pamlico R i v e r on 7 J u l y

1977

. . .

42 2 1

.

A s t y l i z e d drawing of t h e deoxygenation p r o c e s s i n t h e .

. . .

P a m l i c o R i v e r E s t u a r y 44

. . .

22

.

Hydrography of t h e Pamlico River on 7 August 1975 45 23

.

D i u r n a l hydrographic and wind d a t a a t Daymark 3 n e a r

. . .

I n d i a n I s l a n d on 12 August 1975 46

. . .

24

.

Hydrography of t h e Pamlico River on 2 1 June 1976 47

. . .

25

.

Hydrography of t h e Pamlico River on 24 June 1976 48

. . .

26

.

Hydrography of t h e Pamlico River on 29 June 1976 49 27

.

D i u r n a l d i s s o l v e d oxygen c u r v e s a t 0.5 m d e p t h a t

. . .

Daymark 9 n e a r B l o u n t ' s Bay on 30 June.. 1 J u l y 1976 50 28

.

D i u r n a l d i s s o l v e d oxygen c u r v e s measured 0.5 m from

t h e bottom a t Daymark 9 n e a r B l o u n t ' s Bay on 30 June..

. . .

1 J u l y 1976 5 1

. . .

29

.

Hydrography of t h e Pamlico River on 2 J u l y 1976 52

. . .

30

.

Hydrography of t h e Pamlico River on 5 J u l y 1976 53

. . .

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LIST OF FIGURES (continued)

Page

33

.

Hydrography o f t h e Pamlico River on

20

J u l y

1976

(A.M.).

. . .

57

34

.

Hydrography of t h e Pamlico River on

20

J u l y

1976

(P.M.).

. . .

58

35

.

Hydrography o f t h e Pamlico River on

6

August

1976

. . .

59

36.

Hydrography of t h e Pamlico River on

11

August

1976

. . .

60

37

.

Hydrography o f t h e Pamlico River on

24

August

1976

. . .

61

38

.

Hydrography of t h e Pamlico River on

27

August

1976

. . .

62

39

.

Hydrography o f t h e Pamlico River on

31

August

1976

. . .

63

40

.

Hydrography of t h e Pamlico River on

16

October

1976

. . .

64

. . .

.

41

Hydrography of t h e Pamlico River on

5

December

1976

66

42

.

Hydrography of t h e Pamlico River on

2

March

1977

. . .

67

43

.

Hydrography of t h e Pamlico River on

10

March

1977

. . .

68

44

.

Hydrography o f t h e Pamlico River on

23

May

1977

. . .

69

45

.

D i u r n a l hydrographic and wind d a t a a t Daymark

9

n e a r D l o u n t f s B a y o n 9

June1977

. . .

70

46

.

Hydrography of t h e Pamlico River on

27

June

1977

. . .

71

47

.

Hydrography of t h e Pamlico River on

7

J u l y

1977

. . .

72

48

.

Hydrography o f t h e Pamlico River on

13-14

J u l y

1977

. . .

74

49

.

Hydrography of t h e Pamlico River a t

0.5

m depth as

. . .

measured a t

50

s t a t i o n s on

13-14

J u l y

1977

75

50

.

Hydrography of t h e Pamlico River a s measured

0.5

m from t h e bottom a t

50

s t a t i o n s on

13-14

J u l y

1977

. . .

76

51

.

P l o t s of t y p i c a l summer and w i n t e r r e s p i r a t i o n dynamics i n t h e Pantlico River

. . .

77

52

.

P a r t i c u l a t e o r g a n i c carbon a s a f u n c t i o n o f t o t a l oxygen uptake a f t e r 5 days a t mean ambient water temperature

. . . .

78

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LIST OF FIGURES ( c o n t i n u e d )

53.

Management scheme f o r c o n t r o l of deoxygenation i n t h e

. . .

t h e Pamlico R i v e r . 80

Appendix A

1. S t a t i o n l o c a t i o n s f o r t h e E s t u a r i n e Organic Carbon s t u d y and c o r r e s p o n d i n g s e c t o r s used i n c a l c u l a t i n g t h e

. . .

t o t a l amount of o r g a n i c carbon i n t h e e s t u a r y .

93

2 . S t a t i o n l o c a t i o n s f o r t h e Carbon-Nutrient-Biomass s t u d y

and c o r r e s p o n d i n g s e c t o r s used i n c a l c u l a t i o n s i n v o l v i n g t o t a l p e l a g i a l r e s p i r a t i o n , phytoplankton p r o d u c t i o n and s e c r e t i o n , and s t a n d i n g s t o c k s of o r g a n i c carbon, phyto-

. . .

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LIST OF TABLES (continued)

Page Appendix B

1 . Data c o l l e c t e d during t h e Estuarine Organic Carbon

study (August 1975-July 1976)

. . .

97

2 . Data c o l l e c t e d during the Tributary Organic Carbon

study ( J u l y 1975-September 1976).

. . .

109

3 . Data c o l l e c t e d during t h e Carbon-Nutrient-Biomass

s t u d y (August 1976-July 1977)

. . .

110

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SUMMARY AND CONCLUS IONS

Organic carbon c o n c e n t r a t i o n s i n t h e Pamlico River Estuary v a r i e d w i t h season and w i t h l o c a t i o n s . Highest l e v e l s of t o t a l o r g a n i c carbon

(TOC) occurred d u r i n g t h e summer months when flow r a t e s were low. Highest c o n c e n t r a t i o n s of

TOC

were measured i n t h e upper reaches n e a r Washington, N.C., where t h e annual mean c o n c e n t r a t i o n was 9 . 3 m g / l i t e r . Annual mean TOC c o n c e n t r a t i o n s ranged between

7.3

and 7.9 m g / l i t e r f o r

t h e middle and lower s e c t i o n s of t h e e s t u a r y . Approximately 79X o f t h e

TOC c o n s i s t e d of d i s s o l v e d o r g a n i c carbon (DOC) which accounted f o r

o s t of t h e v a r i a t i o n i n TOC. Levels o f p a r t i c u l a t e o r g a n i c carbon (POC) were f a i r l y c o n s t a n t throughout t h e y e a r averaging between 1 . 6 and 2 . 0

g l l i t e r . E x c e p t i o n a l l y h i g h

PO@

c o n c e n t r a t i o n s were observed i n a s s o c i a - t i o n w i t h s p o r a t i c i n c r e a s e s i n phytoplankton biomass.

T o t a l annual i n p u t of o r g a n i c carbon was e s t i m a t e d a t 108,124 tonnes and r o t a 1 annual l o s s e s a t 111,642 tonnes l e a v i n g a n unaccounted b a l a n c e

tonnes. The t o t a l i n p u t of o r g a n i c carbon was d i s t r i b u t e d a s alloehthonous 35.7%

(52.22

phytoplankton production, 11.X

phytoplankton s e c r e t i o n , 0.3X macrophyte production, 0.4% b e n t h i c primary t i o n , 0.1% periphyton production). Organic carbon l o s s e s ( s i n k s )

OX as w a t e r column r e s p i r a t i o n and 20%

as

e s t u a r i n e d i s c h a r g e . Sedimentation and b e n t h i c r e s p i r a t i o n were n o t d e t e m i n e d .

Water column r e s p i r a t i o n , t h e primary l o s s of o r g a n i c carbon from t h e e s t u a r y , was c o r r e l a t e d w i t h POC c o n c e n t r a t i o n which, i n t u r n , was c o r r e l a t e d w i t h phytoplankton b i ss. R e s p i r a t i o n r a t e s r a r e l y c o r r e - l a t e d w i t h DOC c o n c e n t r a t i o n s .

The d r y and calm sumners of 1976 and 1977 provided e x c e l l e n t oppor- t u n i t i e s f o r s t u d y i n g deoxygenation phenomena i n t h e e s t u a r y * Althou t h e most s e v e r e e p i s o d e s of oxygen d e p l e t i o n occurred d u r i n g the summer i n t h e upper h a l f of t h e e s t u a r y , i t h a s been observed spannin

e n t i r e l e n g t h of t h e e s t u a r y and i n t o Pamlico Sound. Dissolved oxygen d e p l e t i o n a l s o w a s observed during t h e w i n t e r i n a s s o c i a t i o n w i t h an i n t e n s e a l g a l bloom. Deoxygenation i s normally l i m i t e d t o t h e deeper mid-channel r e g i o n s o f t h e e s t u a r y e x c e p t i n t h e upper r e a c h e s where i t may extend throughout much of t h e w a t e r column and i n t o r e l a t i v e l y shallow a r e a s i n s e v e r e c a s e s . This r e g i o n r e c e i v e s high alloehthonous i n p u t s from t h e Tar River and i s t h e l o c a t i o n o f t h e most s e v e r e phytoplankton biomass d u r i n g t h e y e a r .

The f a c t o r s c o n t r i b u t i n g t o episodes of oxygen d e p l e t i o n a r e wind v e l o c i t y and d i r e c t i o n , s a l i n i t y ( d e n s i t y ) s t r a t i f i c a t i o n , and a supply of l a b i l e o r g a n i c carbon. Strong winds and storms i n i t i a t e mixing a f t e r a p e r i o d of deoxygenation and maintain an oxygenated water column u n t i l calm c o n d i t i o n s resume and a v e r t i c a l d e n s i t y g r a d i e n t i s r e - e s t a b l i s h e d .

The frequency and s e v e r i t y of deoxygenation d u r i n g c o l d e r months i s

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s i g n i f i c a n t l y reduced owing t o g e n e r a l l y b e t t e r mixed c o n d i t i o n s and h i g h e r throughflow of w a t e r from r u n o f f . Also, lower temperatures r e s u l t i n reduced r a t e s of r e s p i r a t i o n by organisms i n t h e w a t e r c o l and ~ sediments. Although t h e g r e a t e s t amounts of allochthonous o r g a n i c carbon a r e c o n t r i b u t e d t o t h e e s t u a r y by w i n t e r r u n o f f , much of t h i s

i s

presumed r e f r a c t o r y i n n a t u r e . Non-refractory ( b i o l o g i c a l l y l a b i l e ) o r g a n i c

carbon appears t o be

a

consequence of phytoplankton primary p r o d u c t i v i t y which i s g e n e r a l l y h i g h e s t i n t h e summer and i n t h e upper r e g i o n of t h e e s t u a r y . Winter and e a r l y s p r i n g phytoplankton b l o o m produce t h e l a b i l e o r g a n i c carbon source f o r deoxygenation p o t e n t i a l , b u t p h y s i c a l condi- t i o n s a r e seldom conducive f o r i t t o occur.

S i n c e t h e h i g h l y l a b i l e autochthonous carbon of primary production i s t h e primary i n p u t of o r g a n i c carbon t o t h e e s t u a r y , c o n t r o l of

n u t r i e n t s may b e t h e key t o c o n t r o l l i n g deoxygenation. Due t o t h e v e r y high phytoplankton biomass and a s s o c i a t e d p r o d u c t i v i t y l e v e l s

t h a t occur i n t h e region w e s t of Mauls P o i n t ( S e c t o r s 1, 2 and 3, Fig. 4 ) ,

t h i s a r e a may indeed be considered e u t r o p h i c . Anaerobic c o n d i t i o n s with subsequent mixing causing n u t r i e n t r e g e n e r a t i o n from t h e sediments may b e t h e d r i v i n g f o r c e behind t h e sunmer a l g a l blooms, b u t anthropogenic i n f l u e n c e s ( p r i m a r i l y a g r i c u l t u r a l runoff and domestic wastes) appear t o be r e s p o n s i b l e f o r t h e w i n t e r a l g a l blooms. With i n c r e a s e d u r b a n i z a t i o n o f t h e T a r - P a d i c o drainage b a s i n , and i n c r e a s e d commercial and r e s r e a -

t i o n a l development of t h e e s t u a r y i t s e l f , i e d i a t e c o n t r o l programs a r e necessary t o ensure t h a t more s e r i o u s e u t r o p h i c c o n d i t i o n s do n o t develop.

x i i i

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RECOMMENDATIONS

S i n c e a u t o c h t h o n o u s p r i m a r y p r o d u c t i o n was d e t e r m i n e d t o b e t h e l a r g e s t i n p u t o f o r g a n i c c a r b o n t o t h e P a m l i c o R i v e r E s t u a r y , and t h i s s o u r c e i s p r e d o m i n a n t l y l a b i l e , c o n t r o l o f n u t r i e n t s may b e more i m p o r t a n t i n c o n t r o l l i n g d e o x y g e n a t i o n t h a n t h e presum- a b l y more r e f r a c t o r y a l l o c h t h o n o u s i n p u t s . F o r t h i s r e a s o n , n u t r i e n t l e v e l s i n t h e e s t u a r y s h o u l d n o t b e z l l o w e d t o i n c r e a s e s i n c e t h e P a m l i c o R i v e r h a s a l r e a d y e x c e e d e d i t s c a p a c i t y t o a s s i m i l a t e i t s o r g a n i c l o a d , e s p e c i a l l y i n t h e u p p e r r e a c h e s . A management scheme f o r t h i s recommendation i s o u t l i n e d i n F i g u r e 53. T h i s f i g u r e

summarizes and e x t e n d s recommendations by o t h e r s a r i s i n g f r o m v a r i e d s t u d i e s o f t h e e s t u a r y o v e r t h e p a s t d e c a d e . S t r i c t l a n d u s e

r e s t r i c t i o n s and e d u c a t i o n o f f a r m e r s i n c a r e f u l f e r t i l i z e r a p p l i - c a t i o n p r o c e d u r e s a r e r e l a t i v e l y i n e x p e n s i v e m e a s u r e s which s h o u l d b e c o n s i d e r e d as i m m e d i a t e c o u n t e r m e a s u r e s t o t h e c o n t i n u i n g e u t r o - p h i c a t i o n o f t h e e s t u a r y . Long-term c o u n t e r m e a s u r e s s h o u l d i n c l u d e b e t t e r w a s t e t r e a t m e n t p r o c e s s e s a n d e n f o r c e m e n t of d i s p o s a l r e g u l a -

t i o n s a t a l l l e v e l s . Due t o t h e s l u g g i s h c i r c u l a t i o n o f t h e e s t u a r y , t h e c o n s t r u c t i o n o f any s t r u c t u r e s o r impoundments which may r e s t r i c t f l o w and c i r c u l a t i o n s h o u l d b e c a r e f u l l y s t u d i e d b e f o r e any a c t i o n i s t a k e n .

A s t u d y o f t h e c a u s e s and c o n s e q u e n c e s o f t h e a n n u a l w i n t e r H e t e r o -

----

c a t s a ( P e r i d i n i u m ) bloom i s n e e d e d , e s p e c i a l l y i n l i g h t o f t h e

7 - -

-

----

f a c t t h a t d e o x y g e n a t i o n o f t h e b o t t o m w a t e r s accompanied t h i s bloom d u r i n g March 1977.

P h y t o p l a n k t o n h a v e b e e n shown t o b e a n i m p o r t a n t s o u r c e o f c a r b o n i n a r e a s o f d e o x y g e n a t i o n . However, t h e r e l a t i v e i m p o r t a n c e o f r e s p i r a t i o n by p h y t o p l a n k t o n , and p h y t o p l a n k t o n c o n t r i b u t i o n s t o t h e DOC and POC p o o l s u t i l i z e d i n m i c r o b i a l r e s p i r a t i o n , n e e d t o b e f u r t h e r e l u c i d a t e d .

S e v e r a l f a c t o r s which were n o t s t u d i e d d u r i n g t h i s i n v e s t i g a t i o n may b e s i g n i f i c a n t i n t h e o r g a n i c c a r b o n b u d g e t o f t h e e s t u a r y and w a r r a n t f u r t h e r i n v e s t i g a t i o n . T h e s e i n c l u d e s e d i m e n t r e s p i r a t i o n ,

s e d i m e n t a t i o n r a t e s , r e l e a s e o f o r g a n i c m a t t e r by s e d i m e n t s , marsh i n p u t s t o t h e o r g a n i c c a r b o n p o o l , and g r o u n d w a t e r a n d s e p t i c t a n k s o r p t i o n f i e l d i n p u t s ( b o t h i n o r g a n i c n u t r i e n t and o r g a n i c c a r b o n l o a d i n g ) . E s t u a r i n e i n p u t f r o m h i g h m a r s h e s s h o u l d b e i n v e s t i g a t e d w i t h s p e c i a l r e f e r e n c e t o t h e l a b i l i t y o f p e a t which i s c o n t i n u o u s l y e r o d e d .

Our r e s u l t s s u b s t a n t i a t e t h e c o n c e p t t h a t c h l o r o p h y l l a -- i s a good i n d i c a t o r o f p a r t i c u l a t e o r g a n i c c a r b o n i n t h e s y s t e m w i t h i t s c o n c o m i t a n t c o n t r i b u t i o n t o d e o x y g e n a t i o n p o t e n t i a l .

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INTRODUCTION

Fundamental i n u n d e r s t a n d i n g t h e e c o l o g i c a l framework which s u p p o r t s an ecosystem is a knowledge o f t h e d i s t r i b u t i o n and c y c l i n g of carbon w i t h i n t h e system. S i n c e t h e c l a s s i c work o f B i r g e and Juday (1934), many i n v e s t i g a t i o n s have been undertaken t o understand t h e s e r e l a t i o n s h i p s . I n l a c u s t r i n e systems, t h e primary s o u r c e of

o r g a n i c carbon i s normally autochthonous primary p r o d u c t i o n (Lind, 1971; W e t z e l e t g . , 1972; B r i n s o n , 1973; Wetzel and Rich, 1973; Wetzel and O t s u k i , 1974; J o r d a n and L i k e n s , 1975). However, l i t t l e work h a s been done i n r e g a r d t o o r g a n i c carbon i n e s t u a r i e s . I n many e s t u a r i e s , t h e a l l o c h t h o n o u s i n p u t s are a b o u t e q u a l t o , o r even exceed t h e autochthon- ous i n p u t s ( T e a l , 1962; Copeland and Birkhead, 1973; Matson and Buck, 1977). I n e s t u a r i e s dominated by marshes, a l l o c h t h o n o u s o r g a n i c

d e t r i t u s o f t e n i s t h e primary l i n k between primary and secondary produc- t i o n , w i t h t h e major flow of energy a l o n g t h e " d e t r i t u s food chain" r a t h e r t h a n t h e "grazing food chain" (CJdm and de l a Cruz, 1967). However, autochthonous i n p u t s are becoming import a n t i n many e s t u a r i e s where a n t h r o p o g e n i c i n p u t s of i n o r g a n i c n u t r i e n t s a r e l e a d i n g t o

e u t r o p h i c c o n d i t i o n s w i t h i n c r e a s e d primary p r o d u c t i o n (Barlow e t a l . , 1963; Basu, 1965; Reuter, 1977). T h i s can a d v e r s e l y a f f e c t t h e a q u a t i c environment by i n c r e a s i n g t h e o r g a n i c l o a d i n g beyond t h e a s s i m i l a t o r y c a p a c i t y of t h e system. I n many e s t u a r i e s , t h i s h a s l e d t o deoxygen- a t i o n , o f t e n w i t h a s s o c i a t e d f i s h k i l l s ( C a r p e n t e r

g

-

.

&.,

1969; Klashrnan e t a1 1970; May, 1973; Knudson and B e l a i r e , 1975; S e g a r , 1977).

-

-a

,

P e r s i s t a n t summer deoxygenation h a s been r e p o r t e d i n t h e Pamlico R i v e r E s t u a r y f o r many y e a r s (Copeland e t a l . , 1969).

The l a r g e d r a i n a g e b a s i n of t h e Pamlico R i v e r e n s u r e s t h a t abundant aP1ochthonous o r g a n i c carbon w i l l b e c o n t r i b u t e d t o t h e s t a n d i n g s t o c k of o r g a n i c carbon i n t h e e s t u a r y . The importance o f t h i s s o u r c e can b e compared w i t h t h e o r g a n i c carbon of autochthonous o r i g i n . The ob j e c - t i v e o f t h i s s t u d y i s t o compare t h e s e a s o n a l v a r i a t i o n and t o t a l quan- t i t i e s of each of t h e s e s o u r c e s , and t o e v a l u a t e t h e i r e f f e c t on t h e d i s t r i b u t i o n of t h e s t a n d i n g s t o c k of o r g a n i c carbon i n t h e e s t u a r y . S i n c e r e c u r r e n t deoxygenation and a s s o c i a t e d f i s h die-off i n t h e Pamlico R i v e r a r e dependent on an a v a i l a b l e s u p p l y o f o r g a n i c carbon, i t i s i m p o r t a n t f o r management purposes t o u n d e r s t a n d i t s o r i g i n

( a l l o c h t h o n o u s v s . autochthonous) i n a d d i t i o n t o a n t h r o p o g e n i c i n f l u e n c e s on t h e s e s o u r c e s . A s energy flow and carbon flow a r e c l o s e l y p a r a l l e l , a b e t t e r u n d e r s t a n d i n g o f o r g a n i c carbon w i l l h e l p t o d e t e r m i n e t h e n a t u r e and a v a i l a b i l i t y o f consumer food s o u r c e s a s w e l l a s t h e c a p a c i t y of t h e a q u a t i c and s u r r o u n d i n g t e r r e s t r i a l watershed t o s u p p o r t t h i s e s t u a r i n e ecosystem.

The Pamlico River E s t u a r y

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h a l i n e e s t u a r y e x t e n d i n g a p p r o x i m a t e l y 60 km from Washington, N.C. t o t h e Pamlico Sound ( F i g . 1 )

.

Most of t h e l a n d s u r r o u n d i n g t h e e s t u a r y and i t s d r a i n a g e b a s i n i s f o r e s t and a g r i c u l t u r a l w i t h t o b a c c o , s o y b e a n s , and c o r n b e i n g t h e most i m p o r t a n t c r o p s . Along t h e l o w e r r e a c h e s of t h e e s t u a r y are w e l l - e s t a b l i s h e d h i g h s a l t marshes c o n s i s t i n g mainly of S p a r t i n a a l t e r n i f l o r a and Juncus r o e m e r i a n u s

.

The e s t u a r y v a r i e s i n w i d t h from a b o u t 0.5 km n e a r Washington, N.C. t o a b o u t 6.5 km a t i t s mouth and i s r e l a t i v e l y s h a l l o w w i t h an a v e r a g e d e p t h of a b o u t 3 m. S a l i n i t y r a n g e s from f r e s h w a t e r t o a b o u t 20 O/oo. S a l t w a t e r e n t e r s t h e e s t u a r y from t h e Pamlico Sound which i s connected t o t h e A t l a n t i c Ocean t h r o u g h numerous i n l e t s i n t h e O u t e r Banks. F r e s h w a t e r i n p u t i s by t h e many t r i b u t a r i e s , t h e Tar R i v e r b e i n g t h e most i m p o r t a n t i n terms of d i s c h a r g e . Lunar t i d e s a r e a l m o s t n e g l i g i b l e , a v e r a g i n g 15-20 cm. However, s t r o n g winds can o f t e n g e n e r a t e s e v e r e wind " t i d e s " which can s i g n i f i c a n t l y a l t e r t h e hydrography o f t h e e s t u a r y . S a l i n i t y s t r a t i - f i c a t i o n , o f t e n w i t h accompanying d e o x y g e n a t i o n , may become q u i t e

pronounced d u r i n g p e r i o d s of calm due t o a l a c k o f mixing. Temperature i n t h e e s t u a r y r a n g e s from O0 t o 3 2 ' ~ .

D i v e r s i t y o f p h y t o p l a n k t o n , z o o p l a n k t o n , b e n t h i c i n v e r t e b r a t e s and a q u a t i c p l a n t s i n t h e Pamlico R i v e r i s c h a r a c t e r i s t i c a l l y low. The p h y t o p l a n k t o n a r e dominated by d i n o f l a g e l l a t e s w i t h m a j o r blooms o f - -

-

H e t e r o c a p s a ( P e r i d i n i u m ) triq;etrum d u r i n g t h e w i n t e r and Gymnodinium, Gyrodinium and P o l y k r i k o s d u r i n g t h e summer (Hobbie, 1971). Zooplankton a r e dominated by t h e copepod, A c a r t i a t o n s a ( P e t e r s , 1968). Durinp t h e summer, t h e c t e n o p h o r e , Mnemiopsis l e y d i i , becomes i m p o r t a n t i n t h e food c h a i n ( M i l l e r , 1970). I n t h e o l i g o h a l i n e p o r t i o n o f t h e e s t u a r y , Rangia c u n e a t a i s t h e dominant b e n t h i c i n v e r t e b r a t e whereas Macorna

b a l t h i c a dominates t h e s a l t i e r r e g i o n s n e a r Pamlico Sound (Tenore, 1970). V a l l i s i n e r i a americana comprises t h e l a r g e s t p o r t i o n o f t h e submersed masrophytes w i t h o t h e r s p e c i e s i n c l u d i n g Ruppia m a r i t i m a , Potomogeton p e r f o l i a t u s v a r . b u p l e u r i o d e s , and Najas g u a d a l u p e n s i s (Davis and B r i n s o n ,

1976)

.

F i s h s p e c i e s a r e abundant and i n c l u d e , among o t h e r s , t h e s o u t h e r n f l o u n d e r , American e e l , c r o a k e r , s p o t , s p e c k l e d and g r a y t r o u t , and b l u e f i s h . M i g r a t i n g s t o c k s i n c l u d e s t r i p e d b a s s , a l e w i f e , s h a d and h e r r i n g . Commercial h a r v e s t i n g o f b l u e c r a b s , shrimp and e e l s i s i m p o r t a n t e c o n o m i c a l l y , b u t much o f t h e e s t u a r y h a s been c l o s e d t o s h e l l f i s h i n g f o r many y e a r s .

The p r i m a r y i n d u s t r y a l o n g t h e e s t u a r y c o n s i s t s of a l a r g e p h o s p h a t e mining o p e r a t i o n l o c a t e d a b o u t midway down t h e e s t u a r y (T.G.I., F i g . 2) which d i s c h a r g e s s i g n i f i c a n t amounts o f phosphate mining w a s t e s i n t o

t h e e s t u a r y . S t u d i e s of t h i s e f f l u e n t have i n d i c a t e d t h a t i t does n o t s i g n i f i c a n t l y a l t e r t h e b i o t a o f t h e e s t u a r y ( C a r p e n t e r , 1971) and t h a t n i t r o g e n i s most o f t e n t h e element l i m i t i n g p r i m a r y p r o d u c t i o n ( H a r r i s o n and Hobbie, 1974). Another p h o s p h a t e mining p l a n t i s c u r r e n t l y under c o n s t r u c t i o n n e a r South Creek.

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km.

R R I V E R -

PAMLlCO RIVER ESTUARY

HATTERAS

ATLANTIC OCEAN

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N.C.

However, increases i n urbanization, i n d u s t r i a l i z a t i o n and agri- suPtura1 i n t e n s i t y along the estuary and i n its drainage basin have

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MATERIALS AND METHODS

Sampling

E s t u a r i n e Organic Carbon Study (August 1975-July 1976).-- A t monthly i n t e r v a l s , w a t e r samples were c o l l e c t e d w i t h a Van Dorn w a t e r sampler a t d e p t h s o f 1 and 3 m a t 22 s t a t i o n s a l o n g t h e Pamlico R i v e r ( F i g . 2 ) . These samples were analyzed i n d u p l i c a t e f o r p a r t i c u l a t e o r g a n i c carbon

(POC) and d i s s o l v e d o r g a n i c carbon (DOC). Hydrographic measurements d u r i n g t h i s s t u d y i n c l u d e d temperature, s a l i n i t y and d i s s o l v e d oxypen. T r i b u t a r y Organic Carbon Study ( J u l y 1975-September 1976).-- Water samples were c o l l e c t e d a t 0.5 m d e p t h from t h e T a r River a t Grimesland, N.C. and

from Durham Creek a t Edward, N.C. ( F i g . 1 ) . D u p l i c a t e samples were a n a l y z e d f o r POC and WC. Sampling i n t e r v a l s were i r r e g u l a r and were chosen t o encompass a wide r a n g e o f flow.

Carbon-Nutrient-Biomass Study (August 1976-July 1977)

.--

Water samples were c o l l e c t e d on a l t e r n a t e months a t 0.5 m from b o t h t h e s u r f a c e and bottom a t s i x s t a t i o n s a l o n g t h e Pamlico R i v e r ( F i g . 3 ) . Hydrographic measurements i n c l u d e d temperature, s a l i n i t y , c o n d u c t i v i t y and d i s s o l v e d

oxygen, and w a t e r samples were c o l l e c t e d and analyzed i n t r i p l i c a t e f o r POC, DOC, p h y t o p i ments, and r e s p i r a t i o n r a t e s . L e v e l s o f n i t r a t e , n i t r i t e , ammonium, t o t a l K j e l d a h l n i t r o g e n , d i s s o l v e d K j e l d a h f n i t r o g e n , t o t a l phosphorus, d i s s o l v e d phosphorus, d i s s o l v e d r e a c t i v e phosphate, sodium, potassium, calcium and magnesium were measured. Phytoplankton p r o d u c t i v i t y and s e c r e t i o n , and pH and a l k a l i n i t y were a l s o measured on s u r f a c e (0.5 m) w a t e r samples a t each s t a t i o n . Q u a r t e r l y measurements were made o f phytoplankton biomass w i t h dominant s p e c i e s i d e n t i f i e d .

D i u r n a l S t u d i e s (1975-19771.-- D i u r n a l s t u d i e s of phytoplankton produe- t i v i t y were conducted on f i v e o e c a s s i o n s . Some i n c l u d e d a l l a n a l y s e s of t h e Carbon-Nutrient-Biomass s t u d y .

A d d i t i o n a l Hydrographic S t u d i e s (1975-1977).-- S p e c i a l h y d r o g r a p h i c s t u d i e s of t h e e x t e n t and s e v e r i t y of t h e deoxypenation i n t h e e s t u a r y were made, Sampling d a t e s and s t a t i o n s v a r i e d a c c o r d i n g t o t h e s e v e r i t y of deoxygenation.

Analyses

Hydrography.-- Measurements o f t e m p e r a t u r e , s a l i n i t y and c o n d u c t i v i t y were made w i t h a YSI Model 33 S-C-T meter. Dissolved oxygen was

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Carbon.-- Water samples t o b e analyzed f o r o r g a n i c carbon were s t o r e d on i c e f o r t r a n s p o r t back t o t h e l a b o r a t o r y , where n e c e s s a r y f i l t r a t i o n was normally w i t h i n 1 2 h of c o l l e c t i o n . F i l t r a t i o n f o r t h i s and o t h e r d e t e r m i n a t i o n s w a s w i t h Gelman t y p e A/E g l a s s f i b e r f i l t e r s (0.2 Um

a v e r a g e p o r e s i z e ) . POC d e t e r m i n a t i o n s were made by dichromate o x i d a t i o n ( S t r i c k l a n d and P a r s o n s , 1972). DOC w a s analyzed on a Beckman Model 915 Organic Carbon Analyzer a f t e r t h e f i l t e r e d water was a c i d i f i e d and

purged w i t h n i t r o g e n g a s f o r 1 5 min.

Nutrients.-- A l i q u o t s of w a t e r were immediately f i l t e r e d f o r n u t r i e n t s and s t o r e d i n t h e d a r k on i c e f o r t r a n s p o r t back t o t h e l a b o r a t o r y where t h e y were f r o z e n w i t h i n 8 h of c o l l e c t i o n f o r l a t e r a n a l y s i s .

P a r t i c u l a t e f r a c t i o n s were e s t i m a t e d by s u b t r a c t i n g t h e d i s s o l v e d f r a c - t i o n from t h e t o t a l . K j e l d a h l d i g e s t i o n s (EPA, 1976) were used f o r t h e n i t r o g e n d e t e r m i n a t i o n s , and n i t r a t e was determined by t h e

W

spec- t r o p h o t o m e t r i c method (APHA, 1971). Ammonium-nitrogen a n a l y s i s w a s by t h e i n d o l p h e n o l method ( S c h e i n e r , 1976) e x c e p t f o r t h e 11 August 1976 Carbon-Nutrient-Biomass r u n where an Orion e l e c t r o d e was used, The azo- s p e c t r o p h o t o m e t r i c method (EPA, 1976) was used f o r n i t r i t e . Phosphorus a n a l y s e s i n v o l v e d c o n v e r s i o n of phosphorus t o o r t h o p h o s p h a t e by p e r s u l - f a t e d i g e s t i o n , and subsequent c o l o r i m e t r i c d e t e r m i n a t i o n o f s o l u b l e o r t h o p h o s p h a t e (EPA, 1976).

Cations.-- Water t o b e analyzed f o r sodium, potassium, c a l c i u m and mag- nesium was immediately f i l t e r e d , a c i d i f i e d , and f r o z e n on d r y i c e f o r

t r a n s p o r t back t o t h e l a b o r a t o r y . C a t i o n s were a n a l y z e d on a Perkin- Elmer Model 403 Atomic Absorption Spectrometer (Perkin-Elmer, 1973). Phyt0pigments.-- Water t o be analyzed f o r phytopigments was s t o r e d i n

t h e d a r k on i c e f o r t r a n s p o r t back t o t h e l a b o r a t o r y w i t h f i l t r a t i o n w i t h i n 1 2 h of c o l l e c t i o n . The samples were s t o r e d i n a d e s i c c a t o r and f r o z e n u n t i l , a n a l y z e d , D e t e r m i n a t i o n s of c h l o r o p h y l l

5

( c o r r e c t e d and u n c o r r e c t e d f o r p h a e o p h y t i n ) , c h l o r o p h y l l

b,

c h l o r o p h y l l 2, c a r o t e n o i d s , and phaeopigments were made a c c o r d i n g t o S t r i c k l a n d and P a r s o n s (1972). A l l a n a l y s e s were made w i t h i n 30 days of c o l l e c t i o n .

pH and A l k a l i n i t y . - - Water samples f o r pH and a l k a l i n i t y were s t o r e d i n t h e d a r k a t mean ambient w a t e r t e m p e r a t u r e u n t i l measured ( w i t h i n 4 h ) . D e t e r m i n a t i o n s of pH were w i t h a Corning Model 10 pH m e t e r , w h i l e a l k a l i n -

i t y a n a l y s e s were by t i t r a t i o n w i t h 0.02 N s u l f u r i c a c i d t o pH 4.5 ( S t r i c k l a n d and P a r s o n s , 1972).

P r o d u c t i v i t y and S e c r e t i o n . - - Water samples f o r p e l a g i a l phytoplankton ~ r o d u c t i v i t y and s e c r e t i o n were s t o r e d i n t h e d a r k a t ambient water t e m p e r a t u r e - u n t i l i n c u b a t i o n ( w i t h i n 6 h )

.

P r o d u c t i v i t y d e t e r m i n a t i o n s were w i t h t h e l i g h t / d a r k 14c method a c c o r d i n g t o S t r i c k l a n d and P a r s o n s

(19721, and s e c r e t i o n was analyzed a c c o r d i n g t o Beman (1976). I n t o d u p l i c a t e l i g h t b o t t l e s and a d a r k b o t t l e (125 ml) c o n t a i n i n g a s u r f a c e

(25)

f o r a 2 h period (1300-1500 EST) d u r i n g t h e Carbon-Nutrient-Biomass

study, and f o r 4 h p e r i o d s beginning

a t

0400 EST during d i u r n a l s t u d i e s . Also, during a d i u r n a l s t u d y i n both summer and w i n t e r , t h e b o t t l e s were incubated on s t a t i o n a t 0.5 m, 1.5 m, and 2.5

m

t o e s t i m a t e produc-

t i v i t y v a r i a t i o n s w i t h depth. Following i n c u b a t i o n , t h e samples were f i l t e r e d and washed w i t h f i l t e r e d ambient water. The samples were then placed i n 20 ml of a toluene-based c o c k t a i l f o r counting. Counting was w i t h

a

Packard Model 3320 Tri-Carb Liquid S c i n t i l l a t i o n Spectrometer, and amount o f quenching (due t o c h l o r o p h y l l ) was measured on each sample using t h e e x t e r n a l s t a n d a r d i z a t i o n method (Packard Instrument Co.).

Quenching was g e n e r a l l y i n s i g n i f i c a n t (15% i n extreme c a s e s ) and r e p o r t e d v a l u e s are uncorrected.

Determinations of s e c r e t i o n (and a u t o l y s i s ) by phytoplankton were made on t h e f i l t r a t e from t h e p r o d u c t i v i t y b o t t l e s . Two m l of t h i s f i l t r a t e were p l a c e d i n a s c i n t i l l a t i o n v i a l , and a l l i n o r g a n i c carbon was removed through a c i d i f i c a t i o n and s t o r a g e i n a vacuum d e s i c c a t o r f o r

24 h. For counting, 1 8 ml of a toluene-based c o c k t a i l was added. Phytoplankton Biomass.-- Phytoplankton samples were c o l l e c t e d 0.5 m

from both t h e s u r f a c e and bottom, and f i x e d w i t h a modified Lugol's s o l u t i o n c o n t a i n i n g 1 0 g 12, 21.85 g sodium a c e t a t e , 20 g potassium i o d i d e , and 200

m l

water. This modified s o l u t i o n g i v e s b e t t e r p r e s e r - v a t i o n of coccolithophores (Hobbie, 1971). For determining t h e biomass of t h e phytoplankton, t h e U t e d h l sedimentation method according t o Lund e t a l . (1958) was used. F i f t y em3 of sample were allowed t o s e t t l e

f o r 24 h i n t o a counting chamber. Random f i e l d s were then counted on a Wild M40 i n v e r t e d microscope u n t i l

a t

l e a s t 100 i n d i v i d u a l s of t h e most abundant s p e c i e s were counted. This g i v e s approximately +20% a t t h e 95% confidence l e v e l (Lund

et&.,

1958). The counts

were

converted t o

numbers p e r l i t e r by c o n s i d e r i n g t h e amount of

sample

s e t t l e d , t h e a r e a of t h e counting chamber, and t h e a r e a of t h e counting chamber counted. These numbers

were

converted t o volume p e r

l i t e r

by m u l t i p l y i n g by t h e volume of t h e organism, and f i n a l l y t o biomass by assuming a s p e c i f i c g r a v i t y of one, References used i n i d e n t i f i c a t i o n included Campbell

(1973), P r e s c o t t (1970), Whitford and Schumacher (1969), Wolle (1894), P a r a g a l l o (1965), Cleve-Euler (1951), Vinyard (1974), Vinyard (1975), and Wood and Lutes.

Respiration.-- Water samples t o be analyzed f o r r e s p i r a t i o n rates were s t o r e d i n t h e dark on ice f o r t r a n s p o r t back t o t h e l a b o r a t o r y . The samples were incubated a t mean ambient w a t e r temperature a t time of c o l l e c t i o n . Oxygen r e a d i n g s were taken d a i l y f o r t h e f i r s t 5 days and every second o r t h i r d day t h e r e a f t e r . Dissolved oxygen r e a d i n g s were made w i t h a

YSI Model 54A oxygen

meter

w i t h an e l e c t r o d e equipped w i t h

a

(26)

described by Ogura (1975).

Carbon Budget

Standiug Stock of Organic Carbon i n t h e Estuary.-- Carbon concentrations f o r each s t a t i o n and depth within a s e c t o r were averaged t o r e p r e s e n t t h e carbon concentration f o r t h a t s e c t o r . This concentration was then multi- p l i e d by t h e volume of t h e s e c t o r , determined p l a n i m e t r i c a l l y ( a t

mean

low water), t o give t h e amount of carbon i n t h e s e c t o r . Sector volumes a r e given i n Appendix A. Addition of s e c t o r standing s t o c k s gave t h e standing s t o c k of carbon i n t h e e s t u a r y .

Standing stocks of organisms were determined by multiplying t h e mean biomass of an organism a t a s t a t i o n (surface and bottom) by t h e volume of t h e s e c t o r represented by t h e s t a t i o n . Summation of t h e s e c t o r standing s t o c k s gave t h e standing s t o c k e s t i m a t e f o r the estuary.

Total Tributary Input of Organic Carbon t o t h e Estuary.-- Carbon concen- t r a t i o n s were p l o t t e d a g a i n s t discharge (from U.S.G.S., 1976, 1977) f o r both t h e Tar River and Durham Creek, ;he two gauged t r i b u t a r i e s of t h e Pamlico River. Discharge f o r t h e Tar River a t Grimesland, N.C. was assumed t o be proportional t o the measured discharge a t Tarboro, N.C.

where t h e gauging s t a t i o n i s located (Fig. 1). The r e s u l t i n g c o r r e l a t i o n s were used t o give mean concentrations of t h e s e t r i b u t a r i e s (based on mean discharge during t h e study p e r i o d ) , which were then m u l t i p l i e d by t h e t o t a l water i n p u t t o give t h e annual import of organic carbon t o t h e e s t u a r y by t h e s e t r i b u t a r i e s . By dividing t h e annual import of t h e s e t r i b u t a r i e s by t h e a r e a of t h e i r r e s p e c t i v e drainage b a s i n s , a value

representing t h e annual input p e r u n i t a r e a of drainage basin was ohtained. Since the Tar River and Durham Creek represent t h e two extremes of mean organic carbon concentration (based on preliminary d a t a of 14 t r i b u t a r i e s during J u l y 1975), t h e s e values were averaged t o give a mean value f o r annual import of organic carbon p e r u n i t a r e a of drainage basin f o r the o t h e r , non-gauged t r i b u t a r i e s . This value w a s then m u l t i p l i e d by t h e drainage basin a r e a s of these t r i b u t a r i e s , a s determined by Harrison and Hobbie (1974), t o give the annual input of organic carbon t o t h e e s t u a r y by t h e s e t r i b u t a r i e s .

Input of Organic Carbon by Phytoplankton.-- The p r o d u c t i v i t y and s e c r e t i o n r a t e s a t 0.5 m depth a t each s t a t i o n were f i r s t extrapolated t o d a i l y r a t e s based on d i k a l p r o d u c t i v i t y and s e c r e t i o n curves which were

i n t e g r a t e d by c u t t i n g and weighing. The p r o d u c t i v i t y and s e c r e t i o n values baaed on 2 h incubations were m u l t i p l i e d by f a c t o r s of 4.12 and 4.47,

(27)

by m u l t i p l y i n g t h e mean d a i l y p r o d u c t i v i t y and s e c r e t i o n r a t e s f o r each depth a t a s t a t i o n by t h e volume o f t h e s e c t o r a t each c o r r e s p o n d i n g d e p t h . These were t h e n m u l t i p l i e d by 365 days t o g i v e an annual v a l u e . Swnrnation of t h e s e a n n u a l s e c t o r p r o d u c t i o n and s e c r e t i o n v a l u e s gave t h e a n n u a l p a r t i c u l a t e p r o d u c t i v i t y and s e c r e t i o n i n p u t s t o t h e e s t u a r i n e o r g a n i c carbon p o o l ,

Output o f Organic Carbon through R e s p i r a t i o n . - - R e s p i r a t i o n r a t e s were measured a s t h e t o t a l oxygen uptake d u r i n g t h e i n i t i a l 24 h of t h e r e s p i r a t i o n experiment,

The

mew s u r f a c e and bottom v a l u e s a t each s t a t i o n were averaged t o g i v e t h e mean r e s p i r a t i o n f o r t h a t s t a t i o n . T h i s v a l u e w a s t h e n m u l t i p l i e d by t h e volume of t h e c o r r e s p o n d i n ? s e c t o r

t o g i v e t h e t o t a l r e s p i r a t i o n f o r t h a t s e c t o r . T h i s was p u t on an annual b a s i s t o g i v e a n n u a l oxygen uptake p e r s e c t o r . S u m a t i o n of t h e s e c t o r v a l u e s g i v e s t o t a l e s t u a r i n e uptake o f oxygen p e r y e a r , which was t h e n c o n v e r t e d t o carbon u n i t s by m u l t i p l y i n g by 0.300 ( R y t h e r , 1956).

(28)

RESULTS

AND

DISCUSSION

Organic Carbon

Standing Stock.-- The d i s t r i b u t i o n of organf c carbon was surveyed i n t h e Pamlico

River

d u r i n g t h e E ~ t u a r i n e O r a n i c Carbon s t u d y , where the 22

e p r e s e n t e d

11

s e c t o r s (Fig.

4

and Table 1). Means of hydro- asurements and carbon c o n c e n t r a t i o n s d u r i n g % h i

p l o t t e d i n Fig.

5.

T o t a l o r g a n i c carbon (

e s t u a r i n e w a t e r s w i t h t h e h i g h e s t c o n c e n t r a t i o n s i n t h e when flow r a t e s were bow. There w a s a 'POC g r a d i e n t f o r

e h i g h e s t c o n c e n t r a t i o n s i n t h e paren t l y a s s o c i a t e d w i t h t h s a l i n i t y on s e t t l i n g r a t e s . he a r e a of B l o m t h Bay (Se of t h e o r g a n i c carbon e n t e r i n g f r

t h a t uptake of o r g a n i c mat mag b e

an

important s i n

g s t o c k s f TOC i n t h e P

7,277 tonnes (approximately 25.7

g/m2)

.

Dissolved o r g a n i c carbon accounted f o r

mst

of t i o n s s f o r g a n i c carbon w i t h h i g h c o n c e n t r a t

r

anoflths and low c o n c e n t r a t i o n s d u r i n g w i n t e r (Fig. 6 ) . Levels of DOC were o f t e n n s a l i n i t y , b u t d u r i n g p e r i o d s of Pow flow t e n between t h e upper r e a c h e s and t h e

t h a t a large p o r t i o n o f t h e DOC i n t h e e s t u a r y o r i g i n a t e

r u n o f f . Crawford (1971) noted a s i m i l a r r e l a t i o n s h i p f o r DOC i n South Creek, and Wapp e t a l . (1977) o b t a i n e d s i m i l a r r e s u l t s i n a L s u i s m a e s t u a r y . The average s t a n d i n g s t o c k of D8C i n t h e P %co River was 5,742 tonnes (approximately 20.3 g/m2), which averaged 79

These c o n c e n t r a t i o n s compare f a v l y w i t h e a r l i e r s t u d f Creek (Crawford, 1971) and t h e P co River ( H a r r i s o n , 1973).

(29)
(30)
(31)

-

S u r f a c e ( l m )

O * - * * - o B o t t o m (3m)

(32)

A S O M D J F M A M J J

Month

Month

(33)
(34)

compare f a v o r a b l y w i t h c o r r e c t e d d a t a o b t a i n e d by P e t e r s (1968) n e a r I n d i a n I s l a n d (Sector 8).

Standing s t o c k s of TOC, WC and POC were c a l c u l a t e d from t h e

six-

s t a t i o n Carbon-Nutrient-Biomass s tudy f o r comparison w i t h t h e 22 s t a t i o n s t u d y of t h e p r e v i o u s year. These s t a n d i n g s t o c k s were 8,790 tonnes

(31.1 g / n 2 ) , 7,114 tonnes (25.2 g/m2) and 1,676 tonnes (5.9 g/m2) respec- t i v e l y . T y p i c a l l y , DOC was 81% and POC 19% of TOC g i v i n g a DOC:POC r a t i o of 4 . 2 : l . These v a l u e s a r e s i m i l a r t o t h o s e of t h e E s t u a r i n e Organic Carbon s t u d y where t h e r a t i o was 3.9:1, b u t a r e n o t a s a c c u r a t e s i n c e they a r e based on fewer samples.

Q u a r t e r l y samples f o r phytoplankton biomass were c o l l e c t e d d u r i n g t h e Carbon-Nutrient-Biomass study f o r comparison w i t h POC. The q u a r t e r l y c y c l e of phytoplankton biomass i s shown i n Fig. 8. I n d i v i d u a l phyto- plankton biomass samples ranged from 0.253 g w e t weight/rn3 t o 23.184 g w e t weight/m3 and were dominated by d i n o f l a g e l l a t e s . On 16 October 1976, a v e r y h i g h biomass was measured a t S t a t i o n 1 a s s o c i a t e d w i t h a bloom of Gymnodinium and Gyrodinium, and t h e r e was a g r a d i e n t of phytoplankton biomass downstream. On 2 March 1977, t h e l a r g e s t biomass was recorded

a t S t a t i o n 2 corresponding t o blooms of Heterocapsa ( P e r i d h i m ) t r i q u e - todinium r o t u n d a t m . This bloom became even more i n t e n s e

-

d u r i n g t h e weeks following t h i s sampling (M. White and D. Kornegay,

p e r s o n a l communication). -tIigh n i t r a t e c o n c e n t r a t i o n s , probably a s s o c i a t e d w i t h a g r i c u l t u r a l runoff flowing down t h e Tar R i v e r , were observe

t h a s d a t e upstream from t h i s bloom a t S t a t i o n

I.

Diatoms (mostly Melosira, N i t z c h i a and Navicula f l a r i n g i n from t h e Tar River) made up most o f t h e biomass a t S t a t i o n 1 on t h i s d a t e . Due t o t h e l a c k of mixing a s s o c i a t e d w i t h s a l i n i t y s t r a t i f i c a t i o n i n t h i s a r e a , t h e s e organisms probably s e t t l e d r a p i d l y t o t h e bottom being r e p l a c e d by t h e m o t i l e d i n o f l a g e l l a t e s a t S t a t i o n s 2 and 3. Diatoms (predominantly S k e l e t o n m a costatum, Pleurosigma sp. and Coscinodiscus sp. ) a l s o became imoortant downstream ( a t S t a t i o n s 5 and 6) where wind stress and r e s u l - taht mixing coupled w i t h d e c r e a s i n g t u r b i d i t i e s probably added t o t h e growth of t h i s p o p u l a t i o n .

On 23 May 1977, t h e phytoplankton biomass throughout t h e e s t u a r y w a s r e l a t i v e l y low, perhaps due t o n u t r i e n t d e p l e t i o n by t h e w i n t e r blooms. Blue-green a l g a e were dominant i n numbers b u t , due t o t h e i r

small

s i z e , were i n s i g n i f i c a n t i n t h e o v e r a l l biomass. A r e l a t i v e l y h i g h biomass was recorded a t S t a t i o n 1 a s s o c i a t e d w i t h high numbers of

Gymnodinium and G l e n o d i n i m . On 7 J u l y 1977, t h e r e was a v e r y h i g h biomass of phytoplankton a t S t a t i o n 1 a s s o c i a t e d w i t h h i g h numbers of Gymnodinium and Gyrodinium. The l a r g e c o l o n i a l d i n o f l a g e l l a t e , P o l y k r i k o s , became important i n t h e lower reaches.

A n a l y s i s of phytopigments on t h e two d a t e s n o t sampled f o r phyto- p l a n k t o n biomass d u r i n g t h e Carbon-Nutrient-Biomass s t u d y i n d i c a t e t h a t bio~unss was high on

11

August 1976 and low on 5 December 1976. Hobbie

(35)

16 October 1976

h , , O r n , l .

C"ln*m"l.

Cl,,,~h"i.

C I O . O I * l .

23 May 1977

22

7 July 1977

S

STATION

Figure 8 . Phytoplankton biomass a t s i x s t a t i o n s i n the Pamlico River Estuary during 1976-1977. Samples were c o l l e c t e d 0 . 5 m

from both the surface (S) and bottom (B). S t a t i o n l o c a t i o n s are shown i n F i g . 3 .

(36)

August-September and January-March and low biomasses d u r i n g October- December and l a t e April-early June. The discrepancy between t h e s t u d i e s in October i s probably due t o t h e unseasonably calm, dry autumn weather d u r i n g 1976 which extended summer s t r a t i f i c a t i o n .

Phytoplankton biomass during 1976-1977 c o r r e l a t e d w e l l w i t h POC (Fig. 9 ) , and phytoplankton carbon accounted f o r an annual mean of

26%

of t h e t o t a l POC w i t h t h e q u a r t e r l y averages ranging from 9% t o 53%. E s t u a r i n e s t a n d i n g s t o c k of phytoplankton based on t h e s e c o l l e c t i o n s

averaged 2,025.5 tonnes, which, assuming phytoplankton carbon i s 13% of

w e t weight ( S t r i c k l a n d , 1960),

is

approximately

263.3

tonnes of carbon (0.93 g c/m2). Chlorophyll

=

c o r r e l a t e d w e l l w i t h phytoplankton biomass (Fig. 101, and phytoplankton c a r b o n : c h l o r o p h y l l ~ r a t i o s averaged 52:l w i t h

a

range of 9 : l t o 111:l ( c h l o r o p h y l l = c o r r e c t e d f o r phaeophytin) and 47:l w i t h a range o f 9 : l t o 8 5 : l (uncorrected f o r phaeophytin). P a r t i c u l a t e n i t r o g e n and phosphorus a l s o c o r r e l a t e d w e l l w i t h phyto- plankton biomass (Figs. 1l and 12) i n d i c a t i n g t h a t t h e s e parameters may be u s e f u l a s biomass i n d i c a t o r s i n t h e e s t u a r y . Due t o t h e g r e a t v a r i a b i l i t y of phytoplankton biomass i n t h e Pamlico River (Hobbie, 1971),

and o u r l i m i t e d sampling only a t mid-river s t a t i o n s , c a r e should be used

i n

e x t r a p o l a t i o n o f t h e s e r e l a t i o n s h i p s .

B a c t e r i a l biomass was e s t i m a t e d d u r i n g t h e Carbon-Nutrient-Biomass study on 7 J u l y 1977 (James C. Anderson, unpublished d a t a ) using t h e e p i - f l u o r e s c e n c e technique (Daley and Hobbie, 1975). There was a g r a d i e n t downstream w i t h i n d i v i d u a l biomass samples ranging from 0.374 t o 1.320 g wet weight/m3. B a c t e r i a l carbon accounted f o r

3.3%

of t h e t o t a l POC. Standing s t o c k of b a c t e r i a was e s t i m a t e d a t 469.3 tonnes (wet weight) which, assuming b a c t e r i a l carbon i s e q u a l t o 7.9% of t h e wet weight

(Ferguson and Rublee, 1976), c o n v e r t s t o 37.1 tonnes of b a c t e r i a l carbon (131 mg c/m2). Biomass of b a c t e r i a i n t h e w i n t e r h a s n o t been measured i n t h e Pamlico River. Crawford (19711, however, s t u d i e d uptake r a t e s of glucose and amino a c i d s i n t h e e s t u a r y and found maximum h e t e r o t r o p h i c a c t i v i t y during July-September w i t h minimum r a t e s during t h e w i n t e r .

The zooplankton p o p u l a t i o n s of t h e Pamlico River Estuary were

s t u d i e d by P e t e r s (1968) during 1967 and 1968, and by Hobbie

eta.

(unpub- l i s h e d d a t a ) during 1971 and 1972, The dominant zooplankter was A c a r t i a tonsa which accounted f o r 73% of t h e organisms p r e s e n t (Hobbie e t a l . ,

-

unpublished d a t a ) . The mean d e n s i t y of t h i s s p e c i e s f o r seven c o l l e c t i o n s from August 1966 t o June 1967 was 3,101/m3 ( P e t e r s , 1968). Assuming an average dry weight of 8 pg p e r organism based on a Patuxent River s t u d y of t h e organism (Heinle, 1966), t h e average biomass of A c a r t i a a d u l t s and copepodids was 24.8 mg dry weight/m3. To account f o r t h e n a u p l i i which were n o t sampled due t o sampling n e t s i z e , an average r a t i o of 4.66 ( n a u p l i i t o a d u l t s and copepodids) and an average weight of 1.5 pg p e r organism was used (Heinle, 1966). T h i s g i v e s an average annual biomass of 21.6 rag d r y weight/m3. Thus, t h e o v e r a l l mean biomass of A c a r t i a f o r t h e y e a r was 46.4 mg d r y weight/m3. Using s i m i l a r c a l c u l a -

(37)

Particulate Organic Carbon

(

POC)

-

mg/liter

(38)

Phytoplankton

Biomass

-

g/m3

(39)

5 10 15

Phytoplankton Biomass

-

mg/liter

(40)

Phytoplankton Biomass [mglliterl

Figure

Figure 1 .
Figure 5. Monthly means of hydrographic and carbon data a t  1 and 3 m during the Estuarine Organic Carbon study (August 1975
Figure 6. D i s t r i b u t i o n  of dissolved organic carbon ( m g l l i t e r )  a t  1 and 3 m depths i n  t h e  Pamlico River Estuary during 1975-1976
Figure 8. Phytoplankton biomass a t  s i x  stations i n  the Pamlico River Estuary during 1976-1977
+7

References

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