Nitrogen Recycling in the Chowan River

#

NITROGEN RECYCLING IN THE CHOWAN RIVER

1

Donald

W.

Stanley

,

Research Associate, Zoology 2

John E. Hobbie

,

Professor of Zoology Department of Zoology

N.C. State University Raleigh,

N. C.

27607

I Present address: Department of Environmental Sciences and

Engineering, University of North Carolina, Chapel Hill, North Carolina 27514.

3

L

Present address: The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543.

The work 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

by f u n d s p r o v i d e d by t h e O f f i c e o f Water R e s e a r c h and Technology,

U.S. Department o f t h e I n t e r i o r , t h r o u g h The U n i v e r s i t y o f

N o r t h C a r o l i n a Water R e s o u r c e s R e s e a r c h I n s t i t u t e , a s a u t h o r i z e d u n d e r t h e Water R e s o u r c e s R e s e a r c h Act o f 1 9 6 4 , a s amended.

Project No. B-077-NC

TABLE OF CONTENTS

Page

. . .

ACKNOWLEDGMENTS i v

. . .

ABSTRACT v

. . .

LISTOFFIGURES v i

. . .

LIST OF TABLES v i i i

. . .

SUMMARY AND CONCLUSIONS i x

. . .

RECOMMENDATIONS x i i i

I N T R O D U C T I O N . . . a * . . . . 1

. . .

A.Background . , 1

. . .

B

.

O b j e c t i v e s and Research P l a n 2

. . .

C

.

The Chowan River and I t s Drainage Basin

3

. . .

METHODS

. . .

9

. . .

A.Sampling 9

. . .

B

.

Carbon and t Nitrogen Uptake Experiments 9

. . .

C

.

N u t r i e n t Analyses and Algal Biomass 12

. . .

RESULTS AND DISCUSSION 1 4

. . .

A

.

Seasonal P a t t e r n s

14

. . .

1

.

Nitrogen Concentrations 1 4

2

.

Phytoplankton Composition and Biomass

. . .

23

3

.

Photosynthesis and I n o r g a n i c Nitrogen Uptake

. .

26 B

.

K i n e t i c s of Photosynthesis and Nitrogen Uptake

.

30

. . .

1

.

Temperature E f f e c t s 30

. . .

2

.

L i g h t E f f e c t s and Die1 Uptake ₃₀

. . .

3

.

Nitrogen Concentration E f f e c t s 35

. . .

a

.

Enrichment Experiments

35

. . .

.

b Nitrate-Ammonia I n t e r a c t i o n 39

. . .

.

C Annual Input and Output of Nitrogen 40

. . .

D

.

Annual Carbon and Nitrogen Uptake 46

. . .

.

E Nitrogen Recycling 52

REFERENCES

. . .

56

ACKNOWLEDGMENTS

This study was made possible by a grant from the North

Carolina Water Resources Research Institute to J.

E. Hobbie,

Zoology Department,

North Carolina State University, Raleigh.

Several people were associated with the project in various

ways. Bruce Dornseif, William Bowden, Charles Balducci, and

Adrianne Zlotowitz provided help with the sampling, field

experiments, and laboratory analyses. Mr. Grover Cook,

co-

ordinator of The ChowantRiver Study, cooperated in providing

work space for the project in

a

portable field laboratory

located near Winton,

N.C.

Dr. Jay Langfelder of the Center

for Coastal and Marine Studies at N.C. State University made

available to us other field laboratory and sleeping facilities.

Dr.

A.M. Witherspoon's laboratory provided estimates of algal

species composition and abundance in samples collected every

other week from the four sampling sites.

Dr.

Charles Daniels

of the U.S. Geological Survey provided river flow data. Total

Kjeldahl nitrogen analyses were performed in the laboratory

of Dr. Michael Overcash of the Biological and Agricultural

Engineering Department at N.C. State University. We aref

especially grateful to Dr. Richard Volk of

N.C. State Univer-

sity, in whose laboratory all the

samples were analyzed

by mass spectrometry.

Dr. John Miller served as principal investigator during

the last six months of the project, following J.

E. Hobbie's

ABSTRACT

The r e p e a t e d occurrence of nuisance a l g a l blooms i n t h e Chowan River d u r i n g t h e p a s t few summers may have been caused by i n c r e a s e d n i t r o g e n l o a d i n g i n t h e r i v e r . That p o s s i b i l i t y prompted t h i s study of t h e r e l a t i o n s h i p between n i t r o g e n and a l g a l growth i n t h e r i v e r .

The lower Chowan River, l o c a t e d i n n o r t h e a s t e r n North C a r o l i n a , i s a c t u a l l y a freshwater t i d a l e s t u a r y emptying i n t o Albemarle Sound. A s i s t y p i c a l f o r t h i s r e g i o n , d i s s o l v e d i n o r g a n i c n i t r o g e n c o n c e n t r a t i o n s i n t h e Chowan a r e high i n w i n t e r and l o w i n summer. T h i s p a t t e r n r e s u l t s from a combi- n a t i o n of high r a t e s of i n p u t from l a n d runoff i n t h e w i n t e r and high r a t e s of removal by r a p i d l y growing a l g a e i n t h e sum- mer. Dissolved organic n i t r o g e n

i s

t h e most abundant form of n i t r o g e n i n t h e r i v e r , and t h e c o n c e n t r a t i o n s decrease down- r i v e r , s u g g e s t i n g t h a t it i s transformed t o o t h e r forms w i t h i n t h e r i v e r .

Annual a l g a l production i n t h e r i v e r w a s around 100 g over 90% of which occurred between May and October, a p e r i o d when blue-green, d i n o f l a g e l l a t e and green a l g a e made up most of t h e a l g a l biomass. Annual i n o r g a n i c n i t r o g e n uptake, mea- s u r e d by l s ~ isotope t e c h n i q u e s , w a s

33

g NHq.-Nmm-2 and 1 2 g

-2

NO N g m

.

Carbon-nitrogen r a t i o s c a l c u l a t e d from t h e s e d a t a

3-

a r e low, probably because of n i t r o g e n a s s i m i l a t i o n by b a c t e r i a i n t h e samples and because of luxury uptake of n i t r o g e n by t h e phytoplankton.

During w i n t e r r a p i d f l u s h i n g r a t e s , low l i g h t i n t e n s i t i e s and low temperatures a r e t h e most important f a c t o r s l i m i t i n g a l g a l photosynthesis and n i t r o g e n uptake i n t h e r i v e r . During summer i n o r g a n i c n i t r o g e n became l i m i t i n g a s n i t r a t e and ammonia l e v e l s f e l l below

50

p g N l i t e r - ' , t h e c o n c e n t r a t i o n found

LIST OF FIGURES

Page

The Chowan River drainage basin in southeastern

Virginia and northeastern North Carolina. Sampling

stations are Edenhouse (E)

,

Colerain (C)

,

Harrells-

ville

(H),

andWinton(~)

. . .

4

Discharge for the Chowan River at Winton,

N.C. The

. . .

average rates for each day are plotted

6

Chowan surface water temperatures and depth of pene-

tration of 1% of the surface light, at the Winton

station.

. . .

8

Nitrate and ammonia concentrations at the Winton

and Harrellsville stations

. . . , . . . .

₁₅

Nitrate and ammonia concentrations at the Colerain

and Edenhouse stations,

. . .

16

Particulate nitrogen concentrations at the Winton

and Harrellsville stations

. . .

1 9

Particulate nitrogen concentrations at the Colerain

and Edenhouse stations.

. . .

20

Dissolved organic nitrogen concentrations at the

Winton and Harrellsville stations

. . .

21

Dissolved organic nitrogen concentrations at the

C o l e r a i n a n d E d e n h o u s e s t a t i o n s .

. . .

22

Seasonal variation in algal wet weight biomass and

species composition at the Winton and Harrellsville

s t a t i o n s . . . .

. . .

24

Seasonal variation in algal wet weight biomass and

species composition at the Colerain and Edenhouse

s t a t i o n s . . . , . . .

.

25

Midday rates of algal photosynthesis and nitrate

and ammonia uptake for surface samples from the

Winton and Harrellsville stations

. . .

27

~iddai

rates of algal photosynthesis and nitrate

and

ammonia uptake for surface samples from the

Colerain and Edenhouse stations

. . .

28

Effect of increasing temperature on rates of algal

photosynthesis and nitrate and ammonia uptake at

three Chowan River stations: Edenhouse (E), Colerain

Page

15. E f f e c t of i n c r e a s i n g l i g h t i n t e n s i t y on photosyn- t h e s i s and n i t r a t e and ammonia uptake a t t h e

. . .

C o l e r a i n s t a t i o n on

15

June 1975. 32 1 6 , Die1 p a t t e r n of p h o t o s y n t h e s i s and n i t r a t e and

ammo~ia uptake a t t h e Winton s t a t i o n on

31

August-

. . .

1 September 1975. 36

1 7 . E f f e c t of i n c r e a s i n g n i t r a t e and ammonia concen- t r a t i o n on t h e uptake r a t e s of t h e s e n u t r i e n t s a t

. . .

t h r e e Chovian s t a t i o n s 37

1 8 . E f f e c t of i n c r e a s i n g ammonia c o r ~ c e n t r a t i o n o n t h e

. . .

f r a c t i o n of D I N uptake t h a t i s n i t r a t e .

41

1 9 . Inflow, u p t a k e , and o u t f l c w of n i t r a t e and ammonia

i n t h e lotlTer Chowan River between Nove,mber 1974 and November 1975. S i z e of t h e b l o c k s i s p r o p o r t i o n a l

. . .

t o t h e q u a n t i t y of n i t r o g e n . 53

2 0 . T o t a l a n n u a l i n p u t , a s s i m i l a t i o n , and o u t p u t of n i t r o g e n f o r t h e lower Chowan R i v e r between 1

. . .

November 1974 and

31

October 1975

55

v i i

SUMMARY AND CONCLUSIONS

This p r o j e c t s t u d i e d t h e u t i l i z a t i o n and r e c y c l i n g of n i t r o g e n i n t h e lower Chowan R i v e r , North C a r o l i n a . The Chowan River Basin d r a i n s approximately 12,600 km2 i n south- e a s t e r n V i r g i n i a and n o r t h e a s t e r n North C a r o l i n a . One objec- t i v e was t o determine how s i g n i f i c a n t r e c y c l i n g o f t h e n i t r o g e n n u t r i e n t s i s f o r a l g a l growth i n t h e r i v e r . Second, we wished t o determine what f a c t o r s l i m i t a l g a l growth i n t h e Chowan a t d i f f e r e n t times of t h e y e a r . A f i n a l o b j e c t i v e was t o gen- e r a t e information t h a t could be used i n models t h a t would s i m - u l a t e b i o l o g i c a l processes ( e . g . , a l g a l growth) i n t h e r i v e r .

S p e c i f i c f i n d i n g s were:

1. I n t h e lower Chowan ( t h e s e c t i o n of r i v e r included i n t h i s study between Winton and t h e mouth a t Edenhouse) flow o f t e n exceeds 1 5 , 0 0 0 c f s i n winter but i s u s u a l l y l e s s t h a n 3,000 c f s i n summer when t r i b u t a r y inflow i s lowest. Thus, f l u s h i n g times f o r t h e lower Chowan a r e t y p i c a l l y 50 days o r more i n t h e summer, compared t o 1 0 days o r l e s s i n w i n t e r . Water temperatures range f r o m around

5 ' ~

i n January t o n e a r 3 0 ' ~ i n August. Light p e n e t r a t i o n i n t h e r i v e r water i s s o l i m i t e d by high t u r b i d i t y t h a t t h e depth of p e n e t r a t i o n of 1% o f t h e s u r f a c e l i g h t seldom exceeds 2 m.

2 . A t t h e f o u r s t a t i o n s sampled, n i t r o g e n l e v e l s were t y p i c a l of e s t u a r i n e systems i n t h i s r e g i o n . N i t r a t e c o n c e n t r a t i o n s were high i n w i n t e r and l o w during t h e summer. This s e a s o n a l p a t t e r n r e s u l t s f r o m a combination of high r a t e s o f i n p u t

from l a n d runoff i n t h e w i n t e r and high r a t e s of removal by r a p i d l y growing a l g a e i n t h e summer. Ammonia c o n c e n t r a t i o n s had l e s s seasonal v a r i a t i o n and were u s u a l l y lower t h a n t h e n i t r a t e c o n c e n t r a t i o n s . P a r t i c u l a t e n i t r o g e n l e v e l s remained high throughout t h e y e a r , b u t even i n t h e summer when a l g a l biomass was h i g h e s t over

50%

o f t h e p a r t i c u l a t e n i t r o g e n was d e t r i t a l . Although t h e d a t a on d i s s o l v e d organic n i t r o g e n

abundant form of n i t r o g e n i n t h e r i v e r and t h a t t h e concen- t r a t i o n s d e c r e a s e downriver. I t i s l i k e l y t h a t uptake and r e m i n e r a l i z a t i o n of DON o c c u r s i n t h e r i v e r .

3 .

Diatoms were t h e most abundant a l g a e d u r i n g t h e w i n t e r , whereas d i n o f l a g e l l a t e , blue-green and g r e e n a l g a e made up most of t h e biomass d u r i n g t h e summer. Measurements e v e r y o t h e r week d u r i n g 1975 showed t h a t a l g a l biomass

was

much h i g h e r i n summer t h a n w i n t e r . Even s o , t h e peak summer b i o - mass w a s no h i g h e r t h a n i n o t h e r l a k e s and e s t u a r i e s of t h e r e g i o n . However, i n 1975 blooms were n o t encountered i n t h e lower Chowan l i k e t h o s e which had o c c u r r e d i n t h e summers of 1972-1974

4 . S i m i l a r l y , a l g a l p h o t o s y n t h e s i s and n i t r o g e n uptake r a t e s were h i g h e r i n summer t h a n i n w i n t e r and were t y p i c a l f o r t h e r e g i o n . Annual a l g a l p r o d u c t i o n i n t h e r i v e r w a s around

100 g C a m W 2 , over 90% of which o c c u r r e d between May and Octo- b e r . I n nearby e s t u a r i e s where r a t e s were measured, t h e a n n u a l a l g a l p r o d u c t i o n w a s about t h e same as i t was i n t h e Chowan R i v e r d u r i n g 1975. Annual i n o r g a n i c n i t r o g e n u p t a k e , measured

by

1 5 ~

i s o t o p e t e c h n i q u e s , was

33

g N H ~ - N - ~ - ~ and 12 g NO - ~ * m - ~ .

3

Carbon-nitrogen r a t i o s c a l c u l a t e d from t h e s e d a t a a r e low, probably because of n i t r o g e n a s s i m i l a t i o n by b a c t e r i a and because of l u x u r y uptake of n i t r o g e n by t h e phytoplankton.

5.

R i v e r f l o w e x e r t s a v e r y s t o n g c o n t r o l on a l g a l biomass i n t h e Chowan R i v e r d u r i n g t h e w i n t e r . T h i s w a s i l l u s t r a t e d

v i v i d l y i n J u l y 1975 when a l g a l biomass d e c l i n e d r a p i d l y d u r i n g

a s h o r t - l i v e d p e r i o d of h i g h r i v e r f l o w . Biomass s u b s e q u e n t l y r o s e a g a i n t o normal summer l e v e l s as soon as t h e r i v e r f l o w r e t u r n e d t o normal.

6 .

Temperature and l i g h t f l u c t u a t i o n s a l s o cause p a r t of t h e h i g h s e a s o n a l v a r i a b i l i t y i n a l g a l growth i n t h e r i v e r , and b o t h of t h e s e f a c t o r s appear t o l i m i t a l g a l growth a t a l l

t i m e s of t h e y e a r . L i g h t l i m i t a t i o n i s e s p e c i a l l y s e v e r e i n w i n t e r because of h i g h t u r b i d i t y . Based on e x p e r i m e n t s , we

limitations, is responsible for a 6 to 10-fold seasonal change in the rates of algal growth and nitrogen assimilation. Both carbon and nitrogen uptake are also limited by low light inten- sities in the Chowan. Even on the brightest summer days, algal growth is only about 20% as rapid as it would be if light pene- trated all the way to the bottom of the river.

7. Nitrogen concentration, on the other hand, limits algal growth in the Chowan only during the summer. We base this

conclusion on evidence from nitrogen enrichment experiments which showed that as long as ammonia or nitrate levels in the river remain higher than 50 pg

liter-l,

nitrogen should not be limiting to algal growth. Summer was the only period when dissolved inorganic nitrogen levels fell below this concen- tration.

8.

Our nitrate and ammonia uptake data show that ammonia is used preferentially, especially when ammonia concentrations are high. As a result, nitrate becomes important as a nitro- gen nutrient only during the summer when ammonia levels are low.

9. We estimated total amounts of nitrogen entering and leaving the lower Chowan River between 1 November 1974 and 31 October 1975. Some 69% of the incoming nitrogen was dissolved organic nitrogen (DON), 18% was dissolved inorganic nitrogen (DIN)

,

and the remaining 13% was particulate nitrogen (PN). However, only about 64% of the incoming DON left the river at Eden- house. This loss was nearly matched by gains in DIN and PN, suggesting that some DON is assimilated by microorganisms or absorbed by sediments in the river and subsequently regener- ated as inorganic nitrogen.

10. The annual output of DIN from the Chowan watershed was 0.11 g ~ # m - ~ * ~ r - ' , which is similar to most watersheds in the Southeast. This indicates that at present the Chowan basin as a whole is not discharging unusually high quantities of nitrogen into the river.

i n f l o w and t h e amount of D I N a s s i m i l a t e d i n d i c a t e r a p i d

n i t r o g e n c y c l i n g w i t h i n t h e r i v e r d u r i n g summer. For example, d u r i n g June and August 1975,

75

t i m e s as much n i t r o g e n w a s a s s i m i l a t e d ( p e r month) as came i n . C l e a r l y , most of t h e n i t r o g e n t h a t i s a s s i m i l a t e d d u r i n g t h e summer comes from r e c y c l i n g p r o c e s s e s such as r e g e n e r a t i o n of D I N from dead o r g a n i c m a t t e r i n t h e water and sediments. U n f o r t u n a t e l y , t h e r e i s l i t t l e t h a t can be done t o slow down t h e s e p r o c e s s e s e x c e p t t o reduce t h e t o t a l amount of n i t r o g e n e n t e r i n g t h e r i v e r .

1 2 . During t h e summer f a c t o r s such as l i g h t , t e m p e r a t u r e , and flow a r e more f a v o r a b l e t o a l g a l growth t h a n a t any o t h e r time of t h e y e a r . T h e r e f o r e , because t h e summertime i n p u t of

"new" n i t r o g e n from t h e watershed i s s o low ( e , g . , 400 kg N e

day-') any a d d i t i o n a l i n p u t s h o u l d s i g n i f i c a n t l y i n c r e a s e t h e a l g a l biomass.

1. In the lower Chowan, most of the algal growth occurs in

the summer when dissolved inorganic nitrogen concentrations

are often analytically undetectable and when the rate of input

of nitrogen from upstream is at least an order of magnitude

lower than it is in winter. This is not a paradox, because

most of the summer nitrogen demand is met by recycling nitro-

gen already in the river water and sediments. Apparently,

most of this recycled nitrogen comes from the decomposition

of organic matter previously deposited in the river sediments.

Thus, it is probably not practical to try to use nitrogen

concentration alone as a criterion upon which to base regula-

tions for controlling eutrophication in the river.

2.

It has been estimated that the discharges from one nitro-

gen fertilizer plant on the banks of the lower Chowan amount

to about

200

to

500

kg ~#da~-'.

This is equivalent to less

than

10%

of the average daily input from the rest of the

river's watershed during the winter. However, during the

summer both the river flow and nitrogen concentrations are

so low that this one discharge nearly doubles the nitrogen

load in the river. Clearly, this example illustrates that in

the future additional inputs must

be

very closely regulated,

especially during the summer.

3.

The data suggest that one of the greatest threats to the

lower Chowan River is any action that would

decrease

the

flushing rate, especially during the summer when it is already

low. Conceivably, this could result from activities such as

construction of causeways or dams near the river's mouth that

would restrict water flow or circulation, or upstream removal

of large quantities of water during the summer.

4 .

Little is known about recycling of nutrients in rivers.

If the Chowan is at all typical, then we can say that

an

algal

bioassay technique is worthless for predicting the algal growth

potential in shallow rivers. This technique tells what the

growth p o t e n t i a l i s based on t h e t o t a l n u t r i e n t s i n s o l u t i o n . I n s h a l l o w r i v e r s , t h e r e g e n e r a t i o n of n i t r o g e n from t h e

sediments c a n a c t u a l l y provide

75

t i m e s more n i t r o g e n over t h e c o u r s e of t h e summer.

INTRODLTCTION

A . Background

Rapid and d r a m a t i c b i o l o g i c a l changes i n t h e lower Chowan R i v e r d u r i n g t h e e a r l y 1 9 7 0 ' s s t i m u l a t e d i n t e r e s t i n a d e t a i l e d s t u d y o f t h e e n t i r e r i v e r . I n p a r t i c u l a r , t h e n u i s a n c e a l g a l growths t h a t developed i n much of t h e r i v e r i n 1972 s e v e r e l y r e s t r i c t e d f i s h i n g , b o t h commercial and s p o r t , r e c r e a t i o n and n a v i g a t i o n .

Nuisance a l g a l growths, o r a l g a l "blooms", occur when a l g a e can o b t a i n l a r g e amounts of n u t r i e n t s , e s p e c i a l l y n i t r o - gen and phosphorus. These n u t r i e n t s come i n t o t h e Chowan

R i v e r from many s o u r c e s i n c l u d i n g wastewater d i s c h a r g e s by towns and o v e r l a n d runoff and d r a i n a g e from a g r i c u l t u r a l and urban a r e a s . I n a d d i t i o n , it w a s d i s c o v e r e d i n 1972 t h a t a new s o u r c e , a f e r t i l i z e r p l a n t on t h e banks of t h e Chowan a t T u n i s , North C a r o l i n a , w a s a d d i n g l a r g e amounts of n i t r o g e n t o t h e r i v e r . T h i s d i s c h a r g e was stopped by s t a t e a c t i o n , and no s e v e r e a l g a l blooms developed i n t h e r i v e r d u r i n g t h e n e x t few y e a r s . Then, i n t h e e a r l y summer of 1976 t h e n u i s a n c e blooms r e a p p e a r e d . A t about t h e same time it w a s d i s c o v e r e d t h a t h i g h n i t r o g e n water was s e e p i n g from t h e Tunis f e r t i l i z e r p l a n t i n t o t h e r i v e r . Thus, t h e r e i s c i r c u m s t a n t i a l evidence t h a t a n i n c r e a s e i n n i t r o g e n l e v e l s i n t h e Chowan i s respon- s i b l e f o r t r i g g e r i n g t h e n u i s a n c e blooms.

I n response t o t h i s s i t u a t i o n , t h e North C a r o l i n a Depart- ment of N a t u r a l and Economic Resources (NCDNER) i n 1973 author-

ized a thorough i n v e s t i g a t i o n of t h e Chowan R i v e r . T h i s Chowan R i v e r Study h a s two primary o b j e c t i v e s t 1) t o determine t h e e f f e c t s of p o i n t and non-point s o u r c e d i s c h a r g e s on w a t e r q u a l i t y i n t h e r i v e r ; and 2 ) t o develop mathematical models f o r a water q u a l i t y management p l a n t h a t w i l l p r e d i c t t h e

impact of i n c r e a s e d d i s c h a r g e s and new d i s c h a r g e s i n t h e Chowan b a s i n .

flow measurements by t h e U . S . Geological Survey, and s p e c i a l - i z e d p r o j e c t s by u n i v e r s i t y r e s e a r c h e r s . These cooperative p r o j e c t s included a study o f t h e e f f e c t s of a q u a t i c macro- phytes on t h e s t o r a g e and r e l e a s e of n i t r o g e n and phosphorus, a study of t h e kinds and numbers of a l g a e i n t h e r i v e r , and a study of t h e u t i l i z a t i o n and r e c y c l i n g o f n i t r o g e n n u t r i e n t s i n t h e r i v e r ( d a t a presented i n t h i s r e p o r t ) , I n a d d i t i o n , two modeling p r o j e c t s have u t i l i z e d information from t h e s e s t u d i e s t o develop mathematical s i m u l a t i o n s of t h e b i o l o g i c a l dynamics o f t h e Chowan as a f f e c t e d by i n c r e a s e d n u t r i e n t

l o a d i n g .

B . O b j e c t i v e s and Research P l a n

One o b j e c t i v e of t h i s study was t o determine t h e e x t e n t of r e c y c l i n g and r e g e n e r a t i o n of n i t r o g e n n u t r i e n t s i n t h e Chowan R i v e r . A t h r e e - s t e p procedure was followed i n pursuing t h i s goalr

1. The i n p u t of n i t r o g e n i n t o t h e lower Chowan River was c a l - c u l a t e d by m u l t i p l y i n g n u t r i e n t c o n c e n t r a t i o n s i n t h e water by r i v e r flow. These d a i l y e s t i m a t e s were summed t o give t o t a l monthly and t o t a l annual i n p u t .

2 . The a s s i m i l a t i o n of ammonia and n i t r a t e by a l g a e and b a c t e r i a i n t h e r i v e r water was measured a t two-week i n t e r v a l s , s t a r t i n g i n A p r i l

1975.

The d a t a were i n t e g r a t e d over time and space t o give t o t a l a s s i m i l a t i o n i n t h e lower Chowan (Winton t o Edenhouse segment) per month and p e r y e a r .

3.

Recycling r a t e s were c a l c u l a t e d by comparing a s s i m i l a t i o n r a t e s t o i n p u t r a t e s OM a s h o r t - t e r m (monthly) and a long- term ( a n n u a l ) b a s i s .

A second o b j e c t i v e was t o study t h e r e l a t i o n s h i p s between l i g h t i n t e n s i t y , temperature and i n o r g a n i c n i t r o g e n concen- t r a t i o n and a l g a l growth i n t h e r i v e r . F a c t o r i a l experiments were conducted which r e l a t e d both t h e r a t e s of a l g a l growth

intensities and nutrient concentrations. Analysis of the data from these experiments permitted us to draw some con- clusions as to which factors were controlling algal growth in the river at various times of the year.

Finally, the third objective was to generate information that could be used in models that would simulate biological processes (e.g., algal growth) in the river. The factorial experiments described above fulfilled this objective in that they gave quantitative relationships between the biological processes (e.g., rate of nitrogen uptake by algae) and environ- mental conditions (e.g., water temperature, light, and nitro- gen concentration)

.

C. The Chowan River and Its Drainage Basin

The Chowan River drains approximately 12,000 km2 in southeastern Virginia and northeastern North Carolina. The Chowan River proper extends 80 km from the confluence of the Blackwater and Nottoway Rivers near the North Carolina-Virginia border south to its mouth in Albemarle Sound near Edenton,

North Carolina (Fig. 1). Another important tributary is the Meherrin River which empties into the Chowan about 20 km below the Blackwater-Nottoway confluence.

Because Albemarle Sound is sheltered from the Atlantic Ocean by a series of barrier islands, lunar tides in the

lower Chowan seldom exceed 15 cm, These barrier islands also prevent seawater from intruding into the western half of

Albemarle Sound; thus the entire Chowan is freshwater. Wind is an important factor affecting flow rates and direction in the lower Chowan. Between Winton and the mouth at Edenhouse, the river's cross-sectional area increases by nearly 10-fold, yet the volume of flow is estimated to increase by only 10% since about 10% of the total watershed lies between Winton and Edenhouse. As a result the Chowan flows very

Fig. 1. The Chowan River drainage basin in southeastern Virginia and northeastern North Carolina.

i n f l o w , s o u t h e r l y winds may a c t u a l l y r e s u l t i n upstream f l o w f o r s h o r t p e r i o d s ( C , D a n i e l s , p e r s o n a l communication).

T y p i c a l l y t h e r i v e r flow i s h i g h e s t i n w i n t e r (around 15,000 c f s ) and l o w e s t i n summer ( o f t e n l e s s t h a n 1 , 0 0 0 c f s )

.

During 1975 t h e r e was one o u t s t a n d i n g e x c e p t i o n t o t h i s p a t t e r n .

I t occurred i n J u l y when t h e f l o w exceeded 50,000 c f s f o r a

s h o r t p e r i o d d u r i n g u n u s u a l l y r a i n y weather ( F i g . 2 ) . Under normal f l o w c o n d i t i o n s f l u s h i n g times f o r t h e lower Chowan can range

from l e s s t h a n 1 0 days d u r i n g t h e w i n t e r t o over 50 days d u r i n g t h e summer, I n 1975 t h e average f l o w r a t e was 6 , 8 4 9 c f s and t h e average f l u s h i n g time was 26 days (Table

1).

The average d e p t h of t h e lower Chowan i s about

4 m

and t h e w a t e r

i s

u s u a l l y w e l l mixed. I t i s h i g h l y t u r b i d because o f suspended d e t r i t u s and s t r o n g l y c o l o r e d from humic m a t e r i a l . The 1% l i g h t l e v e l o f t e n

i s

l e s s t h a n 1 m deep and seldom

exceeds 2

m

( F i g .

3 ) .

Water t e m p e r a t u r e s range from about

5 ' ~

i n J a n u a r y t o n e a r l y 30°C i n August ( F i g .

3 ) .

2 The Chowan b a s i n h a s a s p a r s e p o p u l a t i o n (20.6/km )

,

t o t a l i n g o n l y 260,000, of which n e a r l y 80% l i v e i n r u r a l a r e a s . The r e g i o n a l economy

i s

p r i m a r i l y a g r i c u l t u r a l , w i t h tobacco

Table 1. Summary of morphornetric and hydrographic p a r a m e t e r s f o r t h e lower Chowan R i v e r

Drainage a r e a (kmL)

2

R i v e r a r e a (km )

3

Volume ( m )

Mean depth ( m )

3

Annual d i s c h a r g e ( m )

TEMPERATURE

("c)

METHODS

A . Sampling

Between November 1974 and November 1975 s u r f a c e water samples were c o l l e c t e d e v e r y two weeks from p i e r s a t f o u r s t a t i o n s a l o n g a

65

km s t r e t c h of t h e lower Chowan R i v e r be- tween Winton and t h e r i v e r ' s mouth a t Edenhouse ( ~ i g . 1)

.

Within 2 h a f t e r c o l l e c t i i ~ n , t h e s e samples were t a k e n t o a f i e l d l a b o r a t o r y a t Winton where a l l n i t r o g e n and carbon a s s i m i l a t i o n experiments were conducted and where n u t r i e n t and a l g a l biomass sub-samples were t a k e n . The n u t r i e n t sam- p l e s were f r o z e n w i t h d r y i c e and s t o r e d f r o z e n . The a l g a l

samples were p r e s e r v e d w i t h L u g o l t s a c e t i c a c i d solution. A t each sampling s t a t i o n t h e s u r f a c e w a t e r t e m p e r a t u r e was meas- ured w i t h a mercury thermometer and s u b - s u r f a c e l i g h t i n t e n - s i t y was measured a t 0 . 2 m d e p t h i n t e r v a l s by means of a

submarine quantum meter (Lambda I n s t r u m e n t s Co. Model LI-185). T h i s i n s t r u m e n t measures o n l y p h o t o s y n t h e t i c a l l y a v a i l a b l e r a d i a t i o n ( i . e .

,

400

-

700 nrn wavelength band)

,

which is equal t o about 50% _{of t h e t o t a l s o l a r r a d i a t i o n ( S t r i c k l a n d}

1 9 5 8 ) .

B , Carbon and N i t r o g e n Uptake Experiments

Phytoplankton p h o t o s y n t h e s i s w a s e s t i m a t e d by t h e c a r - bon-14 t e c h n i q u e o f Steemann-Nielsen ( 1 9 5 2 ) . Samples o f r i v e r water were i n c u b a t e d i n 1 2 5 - m l ground-glass-stoppered b o t t l e s w i t h 1 t o

3

m l of a 2 > ~ i / m l s o l u t i o n of N~H"CO

3

* A f t e r incu-

i ~ a t i o n t h e phytoplankton were k i l l e d w i t h L u g o l ' s a c e t i c a c i d s o l u t i o n ( ~ o l l e n w e i d e r 1974) and f i l t e r e d onto 0 . 4 5 y m pore s i z e membrane f i l t e r s ( M i l l i p o r e H A ) . The f i l t e r s were d r i e d and s t o r e d u n t i l t h e i r r a d i o a c t i v i t y could be determined w i t h

a Beckman p l a n c h e t t e c o u n t e r .

the

Saurebindungsvermogen

(SBV)

indicator undergoes a dis- tinct color change from green to clear gray to red. During the titration nitrogen gas bubbles through the sample in a sintered glass funnel, so as to hold the C02 tension low, thus forcing a complete shift in the carbonate-bicarbonate- carbon dioxide equilibrium towards carbon dioxide. Carbonate alkalinity was calculated as follows:

-

( H C ~ ~ - H C ~ ~ ) * (N) a (1,000)

meq,liter,

A '

-

_v

where

H C ~ "

equals the ml of acid used to titrate the sample, b

HC1 equals the ml of acid used to titrate a blank of 50 ml of boiled, distilled water; N is the normality of the acid used, and v equals the volume (ml) of sample titrated.

12

Available carbon (

Cavail)

was then calculated as

The factors fl, f2, and f which vary with pH, are used for

3

'

calculating concentrations of C02, HCO;, and CO=

3

' Tables of values for the factors at different pH leve3.s can be found in Karlgren's (1962) paper.

A

glass electrode pH meter (Corning Model 10) was used to measure pH of the Chowan samples.

Finally, photosynthesis rates were calculated as followsr

where

14~assim

equals the corrected counts per minute of the sample and the count per minute of isotope added

to

the bottle is I4cadd. The factor 1.06 corrects for the heavier carbon-

The

1 5 ~

t r a c e r technique w a s used t o measure uptake of ammonia and n i t r a t e i n samples of r i v e r w a t e r . The procedure was similar t o t h a t used by Dugdale and Goering ( 1 9 6 7 ) .

F i r s t , a known amount of 15N-labeled Na15N0 o r 1 5 ~ ~ 4 ~ 1 w a s

3

added t o t h e w a t e r i n 1 - l i t e r ground-glass-stoppered b o t t l e s . The amount of l a b e l e d compound normally added w a s

14

y g 1 5 N

as ammonia o r n i t r a t e . A f t e r i n c u b a t i o n , t h e 1 5 N samples were k i l l e d w i t h L u g o l ' s a c e t i c a c i d s o l u t i o n and f i l t e r e d o n t o Whatman GF/C g l a s s - f i b e r f i l t e r s , which were t h e n d r i e d and

s t o r e d i n a d e s s i c a t o r . L a t e r , p a r t i c u l a t e m a t e r i a l on t h e f i l t e r s w a s c o n v e r t e d t o molecular n i t r o g e n by Dumas combus- t i o n ( ~ a r s d a t e and Dugdale 1 9 6 5 ) . The

1 5 ~ ~ ' 4 ~

r a t i o of t h e r e s u l t i n g N2 was determined w i t h a mass s p e c t r o m e t e r , t h e r a t i o w a s converted t o atom p e r c e n t 1 5 N , and t h e enrichment over t h e normal atom p e r c e n t I 5 N ( 0 . 3 7 ) of t h e o r g a n i c m a t e r i a l w a s c a l c u l a t e d , The amount of n i t r o g e n t a k e n up w a s c a l c u -

l a t e d a s f o l l o w s ;

mg N * l i t e r - ' * h - l = (PN) ( A f )

.

( h ) ( A i )

The t o t a l amount of p a r t i c u l a t e n i t r o g e n (PN) on t h e f i l t e r was determined by a method d e s c r i b e d below. The atom p e r c e n t e x c e s s 15N of t h e ammonia o r n i t r a t e f r a c t i o n i n t h e sample a t t h e beginning of t h e experiment j u s t a f t e r t h e i s o t o p e w a s added i s

Ai.

The atom per cent e x c e s s 1 5 N i n t h e p a r t i c u l a t e n i t r o g e n ( P N ) on t h e f i l t e r a t t h e end of t h e experiment i s A f .

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

14c

and i n c u b a t i o n s were c a r r i e d o u t s i m u l t a n e o u s l y i n a s e t of f o u r t e m p e r a t u r e - c o n t r o l l e d w a t e r b a t h s p l a c e d o u t d o o r s a t t h e Winton l a b o r a t o r y s i t e . Two of t h e t a n k s were h e l d a t t h e ambient r i v e r t e m p e r a t u r e , one

loOc

h i g h e r and one 1 0 ' ~ l o w e r . Five l i g h t l e v e l s

( l o o % ,

55%,

33%,

12%, and

400 nm t o 700 nm. U s u a l l y , i n c u b a t i o n s were begun a t 1000 and ended a t 1400 EST.

P h o t o s y n t h e s i s w a s measured on e v e r y sampling d a t e

between November 1974 and November 1975, b u t n i t r o g e n uptake w a s measured only between A p r i l and November 1975. Each p h o t o s y n t h e s i s experiment c o n s i s t e d o f : (two r e p l i c a t e s of each l i g h t i n t e n s i t y ) X ( t h r e e t e m p e r a t u r e s ) X ( f o u r s t a t i o n s ) , f o r a t o t a l of 120

1 4 ~

samples. The 1 5 N experiments always i n c l u d e d a s e t of samples c o n s i s t i n g ofr (two r e p l i c a t e s of two l i g h t i n t e n s i t i e s ; ambient and d a r k ) X ( 1 5 ~ 0 and

l 5 N ~ 4 )

3

X ( 4 s t a t i o n s ) run a t ambient r i v e r t e m p e r a t u r e . A v a r i e t y of e x t r a

1 5 ~

experiments were conducted from time t o t i m e . On some o c c a s i o n s , f o r example, samples were i n c u b a t e d a t a l l t h r e e t e m p e r a t u r e s . Sometimes uptake r a t e v s . n u t r i e n t con- c e n t r a t i o n w a s determined by i n n o c u l a t i n g s e t s of samples w i t h v a r i o u s amounts of t h e i s o t o p e , u s u a l l y

3

p g , 7 p g ,

14

y g , and 42 p g 1 5 N .

C . N u t r i e n t Analyses and Algai Biomass

Soon a f t e r t h e i n c u b a t i o n s were begun, samples f o r 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 n i t r o g e n , p a r t i c u l a t e n i t r o g e n , and a l g a l biomass were p r e p a r e d . Whatman GF/C g l a s s - f i b e r f i l t e r s were used t o s e p e r a t e p a r t i c u l a t e and d i s s o l v e d n i t r o - gen samples. These f i l t e r e d samples, a l o n g w i t h samples of whole w a t e r , were q u i c k - f r o z e n ( u s i n g d r y i c e ) i n p o l y e t h y l e n e b o t t l e s and s t o r e d f r o z e n u n t i l n u t r i e n t s could be a n a l y z e d . The f i l t e r s were d r i e d and s t o r e d i n a d e s s i c a t o r . P l a n k t o n samples were p r e s e r v e d w i t h L u g o l ' s a c e t i c a c i d s o l u t i o n .

N i t r a t e and n i t r i t e were a n a l y z e d by s t a n d a r d methods o u t l i n e d i n S t r i c k l a n d and P a r s o n s ( 1 9 6 8 ) . Ammonia w a s mea- s u r e d by a m o d i f i c a t i o n of t h e Solorzano method (Solorzano 1969)

,

as d e s c r i b e d i n L i d d i c o a t e t a l . (1975)

.

P a r t i c u l a t e n i t r o g e n on t h e f i l t e r s was measured i n a Coleman N i t r o g e n A n a l y z e r * Between May and November 1975 t o t a l Kjeldahl n i t r o -

i n Standard Methods (1971). Values f o r d i s s o l v e d organic n i t r o g e n ( D O N ) were obtained by s u b t r a c t i n g PN and NH4 f r o m

t h e TKN v a l u e s . Plankton samples were counted by t h e

Utermohl sedimentation technique (Lund e t a l . 1958) w i t h an i n v e r t e d microscope. Algal bio,mass was c a l c u l a t e d from t h e number of i n d i v i d u a l s per l i t e r and t h e esti,mated average

RESULTS AND DISCUSSION

A. Seasonal Patterns

1. Nitrogen Concentrations

Nitrate levels in the Chowan River are high in the late

winter and low during the summer. For example, at Colerain

the nitrate concentration was around 400

s g

N *

liter-' during

January and February 1975 (Fig.

5).

By July and August the

nitrate had decreased to less than 10 a g ~aliter-l.

Then, in

September and October 1975 the nitrate began to increase

again, apparently climbing toward another winter peak, The

same seasonal pattern occurred at the three other sampling

stations (Figs.

4

and

5 ) ,

but the winter-to-summer decline

was less pronounced at the upstream stations than at the

river mouth.

There are at least two factors which cause nitrate levels

to fluctuate seasonally in the Chowan. The first is varying

upstream input of nitrate. Gilliam and Lutz (1972) studied

the nitrogen in the ground water of the Coastal Plain and

found that the nitrate in the runoff greatly increased as soon

as the crops were harvested and the ground water was recharged

by the

fall

rains. Thus, input

of

nitrate from the Chowan

Basin is much greater in winter than in summer. The second

factor affecting nitrate levels in the river is biological

activity of micro-organisms (algae and bacteria) in the water

and sediments. This will

be

discussed later in more detail,

but can

be

summarized here. Because environmental factors

affecting algal growth (e.g.,

temperature and light intensity)

are more favorable in summer, more nitrate is removed from

the water by the growing algae in summer than in winter.

Nitrate levels in the Chowan are similar to those in

other coastal systems of the Mid-Atlantic Region. For example,

Hobbie repeatedly found highest nitrate during the winter

WINTON

HARRELLSVILLE

AMMONIA

NITRATE

r

I

NITRATE '. - - -. ..

.-

---

River e s t u a r i e s (Hobbie 1974, Hobbie 1975). I n both e s t u a r i e s n i t r a t e was very l o w during t h e summer. T h i s same s e a s o n a l p a t t e r n was noted f o r t h e Chesapeake Bay by Carpenter e t a l .

(1 969)

,

although t h e r e t h e winter c o n c e n t r a t i o n s were some- what higher t h a n i n t h e North C a r o l i n a estuaries, presumably because o f considerable p o l l u t i o n i n t h e upper p a r t o f t h e bay.

O f a l l t h e n i t r o g e n f r a c t i o n s measured i n t h e Chowan, n i t r i t e w a s by f a r t h e l e a s t abundant. Concentrations ranged from undetectable e m ,l e s s t h a n 1 p g N S l i t e r - I ) t o only about 1 4 >g N * l i t e r - ' , Furthermore, t h e r e were no s e a s o n a l

o r s p a t i a l t r e n d s i n t h e n i t r i t e c o n c e n t r a t i o n s . The d a t a a r e n o t presented g r a p h i c a l l y , but a r e included i n Appendix A .

S i m i l a r l y , Hobbie (1975) found no more than about 20 p g N D

l i t e r - ' a s n i t r i t e a t any time i n t h e Neuse River E s t u a r y . Indeed, n i t r i t e i s n o t common i n n a t u r a l w a t e r s except i n s i t u a t i o n s where ammonia i s being oxidized t o n i t r a t e o r where n i t r a t e i s being d e n i t r i f i e d t o ammonia. Even t h e n , however, n i t r i t e c o n c e n t r a t i o n s do n o t u s u a l l y b u i l d up, s i n c e t h e s e t r a n s f o r m a t i o n s a r e r e l a t i v e l y r a p i d and complete,

I n comparison t o n i t r a t e , ammonia l e v e l s i n t h e Chowan showed l e s s s e a s o n a l v a r i a t i o n and were u s u a l l y lower t h a n t h e n i t r a t e c o n c e n t r a t i o n s . A t C o l e r a i n t h e range was from around 100 pg

liter-I

i n January t o l e s s t h a n 1 0 p g liter-I

i n J u l y ( F i g .

5 ) .

A s i n t h e case o f n i t r a t e , t h e s e ammonia l e v e l s seem t y p i c a l f o r n a t u r a l systems i n t h i s r e g i o n (Hobbie 1974, 1975; Carpenter e t al. 1969) ,

Most o t h e r l a k e s and e s t u a r i e s t h a t have been s t u d i e d a l s o show l i t t l e o r no c l e a r - c u t s e a s o n a l p a t t e r n i n ammonia concerArations. This f a c t i s c i t e d a s evidence t h a t ammonia i s r a p i d l y r e g e n e r a t e d i n t h e water and sediments of a q u a t i c systems. I n a subsequent s e c t i o n we w i l l p r e s e n t o t h e r e v i - dence from

1 5 ~

t r a c e r s t u d i e s t h a t t h i s i s indeed t h e case i n t h e lower Chowan River a t some times of t h e y e a r ,

n i t r o g e n and d e t r i t a l n i t r o g e n i n p a r t i c l e s t h a t a r e r e t a i n e d on a f i l t e r t h a t h a s a pore s i z e of about

0 . 5

,urn. P a r t i c u l a t e n i t r o g e n l e v e l s were r e l a t i v e l y high during w i n t e r and summer i n t h e Chowan and lower i n s p r i n g and f a l l ( F i g s .

6

and 7 ) . Highest v a l u e s were around 400 s g N - l i t e r - ' and t h e lowest were about 75 g g N o l i t e r - ' . Thus, PN i s an important n i t r o g e n f r a c t i o n i n t h e r i v e r because it r e p r e s e n t s a s i g n i f i c a n t

p a r t of t h e t o t a l n i t r o g e n i n p u t and because it i s continuously being broken down i n t o i n o r g a n i c forms, p r i m a r i l y ammonia,

t h a t can be used by t h e phytoplankton. U n f o r t u n a t e l y , t h e r a t e of t h i s breakdown remains unknown.

Apparently, most of t h e PN i n t h e r i v e r i s d e t r i t a l . During t h e 1975 summer t h e average a l g a l biomass was around 1 5 mg wet wgtmliter-' ( s e e n e x t s e c t i o n ) . T h i s is e q u i v a l e n t t o approximately 100 pg N l i t e r - ' , assuming a carbon-nitrogen r a t i o of 7 r l and a wet weight-to-carbon conversion of 0 . 0 5

( S t r i c k l a n d 1958). Thus, s i n c e t h e summer PN l e v e l s were

o f t e n over 200 p g ~ m l i t e r - l , a t l e a s t 50% o f i t must have been d e t r i t a l . During t h e w i n t e r , almost a l l of t h e PN w a s d e t r i t a l s i n c e a l g a l biomass was very low then ( e q u i v a l e n t t o l e s s t h a n

1 0 p g N - l i t e r - ' ) compared t o t h e t o t a l PN c o n c e n t r a t i o n s , which were 1 0 0 p g N * l i t e r - ' o r more ( ~ i g s .

6

and 7)

.

Dissolved organic n i t r o g e n (DON) makes up a high per- centage ( F i g s . 8 and 9) o f t h e t o t a l n i t r o g e n i n p u t t o t h e Chowan. Between June and November 1975 t h e c o n c e n t r a t i o n s averaged approximately 600 p g Noliter-' and ranged from 200

t o 1 2 0 0 sg N - l i t e r - ' . Often t h i s w a s 1 0 t o 100 times t h e amount of d i s s o l v e d i n o r g a n i c n i t r o g e n p r e s e n t (ammonia, n i t r i t e , and n i t r a t e ) . Except d u r i n g June and August DON

c o n c e n t r a t i o n s were a l s o h i g h e r , by a f a c t o r of 2 t o

5

times, t h a n t h e PN l e v e l s . However, there w a s no c l e a r p a t t e r n i n t h e DON l e v e l s . Likewise, Hobbie found t h a t i n t h e Neuse River and Pamlico River e s t u a r i e s DON was u s u a l l y t h e most abundant f o r m o f n i t r o g e n (range 70

-

350 y g N w l i t e r - ' ) b u t t h a t t h e c o n c e n t r a t i o n s d i d n o t appear t o c o r r e l a t e very w e l l

I I I I

I

I I I

I

0 0 0 0 0 0 0 0

0 0 0

z

0 0 0 0

e m -

*

ca

*

N

F

M

A

M

J

J

1974

1975

DON

DON

I

2 . Phytoplankton Composition and Biomass

I n t h e Chowan, diatoms made t h e g r e a t e s t c o n t r i b u t i o n

t o t h e a l g a l biomass d u r i n g t h e w i n t e r , whereas d i n o f l a g e l l a t e s , b l u e - g r e e n , and g r e e n a l g a e formed most of t h e summer b i o -

mass ( F i g s . 10 and 11)

.

A t t h e r i v e r ,mouth (Edenhouse s t a - t i o n ) , f o r example, diatoms, mostly s p e c i e s of M e l o s i r a ,

u s u a l l y accounted f o r over 75% of t h e t o t a l biomass i n samples c o l l e c t e d between November and May. Following a r a p i d d e c l i n e i n diatom abundance i n t h e l a t e s p r i n g , blue-green a l g a e

( e . g . , M i c r o c y s t i s , Anabaena, Aphanozomenon) and d i n o f l a g e l l a t e s ( P e r i d i n i u m ) began t o predominate, t o g e t h e r making up as much as 80% of t h e biomass from June through October. Green a l g a e

(Coelosphaerium, M i c r o a s t e r i a s , Scenedesmus), and Eugleno- p h y t a were p r e s e n t i n s m a l l e r q u a n t i t i e s throughout t h e y e a r , and Chrysophyta were observed i n a few samples.

T h i s p a t t e r n

i s

about t h e same as t h a t which Whitford (1958) d e s c r i b e d as b e i n g t y p i c a l f o r North C a r o l i n a C o a s t a l P l a i n l a k e s and ponds. He n o t e d t h a t t h e r e seemed t o be a c o r r e l a t i o n between t h e winter-to-summer s h i f t from diatoms t o g r e e n and b l u e - g r e e n a l g a e and changes i n w a t e r t e m p e r a t u r e , b u t c a u t i o u s l y added t h a t t h e i n t e r a c t i o n of many f a c t o r s

d e t e r m i n e s t h e r e l a t i v e abundance of any one s p e c i e s . A l g a l biomass i n t h e r i v e r was l o w e s t d u r i n g w i n t e r (average l e s s t h a n 1 m g 0 l i t e r - ' wet weight) and h i g h e s t i n summer ( a v e r a g e about 15 m g * l i t e r - ' wet weight)

.

However, a s shown i n F i g s . 10 and 11

,

s e v e r a l of t h e summer samples con- t a i n e d l a r g e numbers of one o r two s p e c i e s which r a i s e d t h e t o t a l biomass t o as h i g h as 37 m g * l i t e r - I ( e . g . ,

30

J u n e 1975 a t C o l e r a i n s t a t i o n ) . I n t h i s c a s e a s p e c i e s of P e r i d i n i u m accounted f o r more t h a n h a l f t h e t o t a l wet weight. L a t e r , i n August, l a r g e numbers of P e r i d i n i u m a l o n g w i t h two s p e c i e s

of b l u e - g r e e n s ( i n t h e g e n e r a Synechoccus and Aphanozomenon) produced u n u s u a l l y h i g h biomass peaks of about 21 and 1 9 mga l i t e r - ' a t H a r r e l l s v i l l e and Winton r e s p e c t i v e l y ( F i g . 1 0 ) .

d 1 I I I

I

I I

L

BIOMASS COLERAIN

BA

4

N D I J F M A M J J A S O N

1974

1975

N

D I J

F M A M J J A S O N

1974

1975

l a k e s and e s t u a r i e s i n North C a r o l i n a , t h e Chowan biomass l e v e l s d u r i n g 1975 were n o t u n u s u a l . A f t e r s u r v e y i n g 6 9

n a t u r a l l a k e s and inpoundments i n t h e s t a t e , Weiss and

Kueneler (1976) found t h a t most had between 1 and

6

rng.liter-' of a l g a l biomass d u r i n g t h e summer. Also, a l t h o u g h t h e s e a s o n a l p a t t e r n s a r e d i f f e r e n t , t h e Chowan and t h e Pamlico R i v e r

E s t u a r y have s i m i l a r q u a n t i t i e s of phytoplankton. I n t h e Pamlico t h e wet weight biomass a v e r a g e s around

5

t o 10 mge l i t e r - ' e x c e p t d u r i n g b r i e f p e r i o d s i n t h e l a t e w i n t e r when blooms of P e r i d i n i u m t r i q u e t r u m r a i s e t h e biomass t o around 50 r n g * l i t e r - ' (Hobbie 1972)

.

I t s h o u l d be n o t e d , however, t h a t d u r i n g t h e c o u r s e of t h i s s t u d y we d i d n o t e n c o u n t e r any blooms i n t h e Chowan l i k e t h o s e which o c c u r r e d i n t h e summers of 1972-1974, However, r e c e n t l y C . Balducci ( p e r s o n a l communication) c o l l e c t e d samples a t t h e C o l e r a i n s t a t i o n d u r i n g t h e e a r l y summer of 1976 when a n a l g a l bloom covered t h e water s u r f a c e w i t h a v i s i b l e l a y e r

of b l u e - g r e e n a l g a e , mostly Anabaena and M i c r o c y s t i s s p e c i e s . I n t h e s e s u r f a c e samples, t h e a l g a l biomass was a s h i g h as 100 m g . l i t e r - l .

3 .

P h o t o s y n t h e s i s and I n o r g a n i c Nitrogen Uptake

PHOTOSYNTHESIS

N

D I J F M A M J J A S O N

Fig.

12.

Midday rates of algal photosynthesis and nitrate and

ammonia uptake for surface samples

from

the Winton

PHOTOSYNTHESIS

AMMONIA & NITRATE UPTAKE

There w a s one u n u s u a l d i p i n t h e summertime a l g a l photo- s y n t h e s i s which w a s most n o t i c e a b l e a t t h e two u p - r i v e r

s t a t i o n s , Winton and H a r r e l l s v i l l e , A t t h e s e s t a t i o n s , t h e r a t e s f e l l r a p i d l y d u r i n g J u l y 1975 t o about 10% of t h e May and June r a t e s , b u t t h e n i n c r e a s e d a g a i n i n August t o t h e

t y p i c a l summertime r a t e s ( a b o u t 125 pg

c

* l i t e r m 1 ah-')

.

T h i s s h o r t - l i v e d b u t p r e c i p i t o u s d e c l i n e d i d n o t occur a t Eden- house and C o l e r a i n ( F i g .

1 3 ) $

where, i n s t e a d , t h e h i g h r a t e s of p h o t o s y n t h e s i s p e r s i s t e d throughout J u l y and August. The most l i k e l y cause of t h i s anomaly w a s t h e u n u s u a l l y h i g h r i v e r flow, which peaked a t n e a r l y 50,000 c f s i n mid-July 1975.

T h i s w a s 10 t o 20 t i m e s t h e normal flow f o r t h a t time of y e a r and w a s t h e h i g h e s t flow d u r i n g t h e e n t i r e 1975 c a l e n d a r y e a r , ( F i g . 2 ) . Consequently, t h e r e w a s a s e v e r e washout of t h e

a l g a e i n t h e W i n t o n - H a r r e l l s v i l l e r e g i o n . F a r t h e r down-river t h e c r o s s - s e c t i o n a l a r e a of t h e Chowan i n c r e a s e s much more r a p i d l y t h a n t h e volume of water c a r r i e d , s o t h a t flow r a t e s do n o t v a r y n e a r l y s o much a t t h e mouth as f a r t h e r u p r i v e r . T h i s probably e x p l a i n s why t h e washout was n o t n o t i c e a b l e a t t h e C o l e r a i n and Edenhouse s t a t i o n s . T h i s example i l l u s t r a t e s t h e s t r o n g e f f e c t t h a t f l o w r a t e can have on t h e q u a n t i t y of a l g a e i n a r i v e r l i k e t h e Chowan. Another example of t h e e f f e c t w a s r e p o r t e d by Hobbie (1971) f o r t h e Pamlico R i v e r E s t u a r y , where t h e 1968 P e r i d i n i u m bloom w a s t e m p o r a r i l y washed o u t of t h e r i v e r f o l l o w i n g heavy r a i n s .

I n a d d i t i o n t o r i v e r f l o w r a t e t h e r e a r e s e v e r a l o t h e r i m p o r t a n t f a c t o r s t h a t i n f l u e n c e d t h e s e a s o n a l p a t t e r n of a l g a l growth i n t h e Chowan. Among t h e s e a r e s o l a r r a d i a t i o n , w a t e r t e m p e r a t u r e and n u t r i e n t a v a i l a b i l i t y , a l l of which w i l l be examined q u a n t i t a t i v e l y i n a l a t e r s e c t i o n of t h i s r e p o r t .

A s e x p e c t e d , n i t r o g e n uptake by t h e p l a n k t o n f o l l o w e d t h e same s e a s o n a l p a t t e r n a s p h o t o s y n t h e s i s . There i s no n i t r o g e n uptake d a t a between November 1974 and March

1975.

our understanding of t h e s e a s o n a l c y c l e and c a l c u l a t i o n s of t o t a l _{annual a s s i m i l a t i o n of n i t r o g e n i n t h e r i v e r . For} example, assuming t h a t carbon and n i t r o g e n uptake r a t e s approximated t h e Redfield e t a l . (1963) C I N atomic r a t i o of 7 r 1 , we e s t i m a t e d wintertime v a l u e s f o r n i t r a t e and ammonia uptake t o be l e s s than 1 jag N e l i t e r - l - h - ' on most days. By mid-summer, t h e measured r a t e s had r i s e n t o about 1 0 y g N 9

l i t e r - ' ah-' of ammonia and 4 pg N a l i t e r - ' sh-' o f n i t r a t e .

Then a s a l g a l growth r a t e s and biomass decreased during Sep- tember and October, n i t r o g e n uptake r a t e s a l s o decreased.

B. K i n e t i c s of Photosynthesis and Nitrogen Uptake

1. Temperature E f f e c t s

Te,mperature f l u c t u a t i o n s caused p a r t of t h e g r e a t sea- s o n a l v a r i a b i l i t y i n a l g a l growth r a t e s i n t h e Chowan. Experi- ments showed t h a t both photosynthesis and n i t r o g e n uptake

i n c r e a s e d a s water te,mperature i n c r e a s e d . For example, i n one experi,ment i n August 1975, photosynthesis approximately doubled (Q10=2 . 0 ) between 8 ' ~ and 18Oc. I n concurrent e x p e r i - ments, ammonia and n i t r a t e uptake r a t e s t r i p l e d (Q10=2.8-3.3)

( F i g . 1 4 ) . The Q l O v s seemed t o remain about t h e same over t h e whole range o f temperatures o c c u r r i n g i n t h e r i v e r d u r i n g t h e y e a r . Therefore, we conclude t h a t temperature a l o n e , i n t h e absence of s e v e r e l t m i t a t i o n by o t h e r f a c t o r s , w a s respon- s i b l e f o r a 6-to-10 f o l d s e a s o n a l d i f f e r e n c e i n t h e r a t e of a l g a l growth and n i t r o g e n a s s i m i l a t i o n .

2 . L i g h t E f f e c t s and Die1 Uptake

A t y p i c a l s e t of d a t a f o r photosynthesis and n i t r o g e n uptake v s . l i g h t a t d i f f e r e n t te,mperatures a r e shown i n F i g .

PHOTOSYNTHESIS

I I I I I

A M M O N I A UPTAKE

NITRATE UPTAKE / E

C

Q,,

= 3.3

E

Q,,

=

3.0

4

8

12

16

20

24

28

TEMPERATURE (OC

)

Fig.

14.

Effect of increasing temperature on rates of algal photosynthesis and nitrate and ammonia uptake at three Chowan River stations: Edenhouse

(E),

Colerain

PHOTOSYNTHESIS

15 June, 1975

( Colerain 9

AMMONIA

&

NITRATE UPTAKE

(@olerain)

15 June, 1975

e

Nitrate

20%

SOLAR RADIATION

(

ly

minute-'

)

Fig. 15. Effect of increasing light intensity on photosynthesis and nitrate and ammonia uptake at the Colerain

water t e m p e r a t u r e . S i m i l a r l y , ammonia and n i t r a t e uptake r a t e s f i r s t i n c r e a s e d w i t h i n c r e a s i n g l i g h t t o a s a t u r a t i o n l e v e l and t h e n d e c l i n e d a t h i g h e r i n t e n s i t i e s . Here a g a i n , t h e degree of l i g h t i n h i b i t i o n w a s i n v e r s e l y r e l a t e d t o l i g h t i n t e n s i t y ( d a t a n o t shown)

.

Our d a t a f o r n i t r o g e n uptake v s . l i g h t a r e similar t o o t h e r s t h a t have been p l o t t e d f o r marine phytoplankton

assemblages and f o r s i n g l e s p e c i e s of a l g a e i n c u l t u r e . The c u r v e s t a k e t h e shape of r e c t a n g u l a r h y p e r b o l a s , up t o t h e p o i n t of l i g h t s a t u r a t i o n . They may be d e s c r i b e d by t h e f o l l o w i n g e q u a t i o n r

where V i s t h e r a t e of n i t r o g e n uptake ( p g N l i t e r - ' * h - l )

,

Vd i s t h e dark uptake r a t e , Vmax i s t h e maximum, l i g h t s a t u r a t e d u p t a k e , and I i s l i g h t i n t e n s i t y . The l i g h t i n t e n s i t y c o r r e - sponding t o

$vmaX

i s

1 0 , 5 :

it i s c a l l e d t h e l i g h t h a l f - s a t u - r a t i o n c o n s t a n t (MacIssac and Dugdale 19'72).

Chowan I

0 . 5 v a l u e s , e s t i m a t e d from g r a p h s l i k e F i g .

15,

u s u a l l y were around 0 . 0 5 ly a min-'

.

MacIssac and Dugdale (1 969)

r e p o r t e d I

0 . 5

v a l u e s around 0.005

-

0.02 lyomin-' f o r phyto-

p l a n k t o n i n t h e t r o p i c a l P a c i f i c Ocean, b u t Bates (1976) found t h a t I0 v a r i e d , depending on t h e p r e v i o u s l i g h t h i s t o r y of t h e a l g a l c e l l s . Shade-adapted c e l l s of some s p e c i e s had a

lower I

0 . 5

t h a n sun-adapted c e l l s .

N e v e r t h e l e s s , even on cloudy days n i t r o g e n uptake a t t h e s u r f a c e of t h e Chowan must be l i g h t s a t u r a t e d , whereas l i g h t i n t e n s i t y must almost always l i m i t t h e u p t a k e i n t h e deeper w a t e r s o f t h e r i v e r , where l i g h t l e v e l s f a l l below 0 . 0 5 l y * min-l. For example, i f t h e s u r f a c e i n t e n s i t y were 1 . 0 l y s min-'