CONCLUSIONS

Once i n i t i a t e d , t h e i n - s i t u impac t s t u d y was a non -man ipu la t i ve

exper iment . The p lanned i n t e r g r a t e d e f f o r t c o u l d n o t be e f f e c t i v e l y

a l t e r e d w i t h o u t a m a j o r m o d i f i c a t i o n i n t i m e and funds, c o n t r a r y t o

t h e Manning s t u d y . Weather c o n d i t i o n s needed t o be i d e a l d u r i n g t h e

d redg ing o p e r a t i o n and f o r an approx imate 24 hour p e r i o d f o l l o w i n g ,

t o e f f e c t a r easonab le p robab i 1 i ty o f p r o j e c t e d r e s u l t s . A1 1 owance

a l s o had t o be made f o r t h e commissioning ( i n t h i s case, s u b c o n t r a c t -

i n g ) o f a commercial d redge r wh ich reduced t h e degree o f a b s o l u t e

c o n t r o l . Many o f t h e o b s e r v a t i o n s were f rom n e c e s s i t y , v i s u a l and/

o r p resumpt ive b u t were t h o r o u g h l y d iscussed and judged p r i o r t o

making d e c i s i o n s o r recommendations. The up-bay sampl i ng s i t e s , f o r

example, were examined o n l y on one occas ion each and d u r i n g a p e r i o d

o f cons ide red h i g h s a l i n i t y , wh ich would r e f l e c t t h e s t r u c t u r e o f t h e

b e n t h i c community. A t t h e Broome I s l a n d s i t e t h e sed iment t r a p d a t a

appeared t o be t h e most r e l i a b l e even though background p o i n t source

was u n d e f i n a b l e . I n o u r e s t i m a t i o n , however, t h e o p e r a t i o n was a

q u a l i f i e d success and t h e s e v e r a l weeks p r e p a r a t i o n f o l l o w e d b y an i n -

t e n s i v e i n - s i t u e x e r c i s e went e s s e n t i a l l y as p lanned.

The sediment t r a p d a t a e f f e c t i v e l y separa ted t h e component plumes

and p r o g r e s s i v e l y i n d i c a t e d d e p o s i t i o n r a t e s . T h i s da ta a l s o i n -

d i c a t e d a marked i n c r e a s e i n d e p o s i t i o n r a t e s when h y p o t h e t i c a l l y

e x t r a p o l a t e d t h rough t h e p rox ima l zone between t h e 15 f o o t s t a t i o n and

t h e dredged a rea ( F i g . 2 ) . The dredge plume and ambient background

suspensions t e n d toward n e a r convergence if t h e curves a r e r e g r e s s i v e l y

CORE Metadata, citation and similar papers at core.ac.uk

Provided by Aquatic Commons

t r a c e d sugges t i ng t h e overwhelmi ng e f f e c t o f t h e suspens ion r e -

s u l t i n g from t h e d redg ing . T rapp ing e f f i c i e n c y was s i g n i f i c a n t l y

i d e n t i c a l compa i r ing between and among s t a t i o n s . A l though t h e r e was

n e a r l y t w i c e t h e t o t a l amount o f m a t e r i a l t r apped i n t h e bo t tom cups

as compaired t o t he sum o f t h e m i d d l e and t o p cups, b o t h between

and among t h e s t a t i o n s percentage-wise was n e a r l y i d e n t i c a l . Deposi-

t i o n a t t h e 15 f o o t s t a t i o n was one -e i gh th t o t h r e e - s i x t e e n t h of an

i n c h i n dep th (3-4 mm).

The sediment d isp lacement s t akes proved r e l a t i v e l y i n e f f e c t u a l

when compaired t o t he sed iment t r a p s a l t h o u g h t h e s t a k e s r e q u i r e d an

i n o r d a n a n t e f f o r t i n f a b r i c a t i o n , deployment and a q u i s i t i o n . I t i s

p o s s i b l e t h a t p o s i t i o n i n g t h e s takes nea re r t h e dredge zone ( l e s s t h a n

15 f e e t ) c o u l d have p r o v i d e d a d d i t i o n a l d a t a on heavy p a r t i c u l a t e

f a l l o u t . We assumed, however, t h a t t h e p o t e n t i a l f o r damage t o t h e

commissioned dredge f rom i m p l a n t e d dev i ces and t h e r e s u l t a n t t o t a l

r ees tab l i shmen t e f f o r t wou ld be p r o h i b i t i v e m o n i t a r i l y . I t i s a l s o

l i k e l y t h a t t he s t akes wou ld have proved t o be v a l u a b l e i n a reas o f

g r e a t e r c u r r e n t v e l o c i t i e s and/or s u b s t r a t e s c o n s i s t i n g m o s t l y o f

f i n 2 p a r t i c u l a t e s .

The s u b s t r a t e p a r t i c u l a t e f r a c t i o n a n a l y s i s i n d i c a t e d n o

measurable s i g n i f i c a n t change i n t h e impac t zone due t o d e p o s i t i o n

o f m a t e r i a l from t h e dredge zone.

E lementa l a n a l y s i s ev idenced no unusual amount o f t o x i c substances

i n t h e s u b s t r a t e s t e s t e d . When c o n s i d e r i n g t h e t r a n s f e r o f energy

w i t h i n a b e n t h i c cornmuni ty, however, resuspens ion o f m a t e r i a l s can

e f f e c t t h e t o t a l communi t y . Suspended m a t e r i a1 c o n t a i n i n g p o t e n t i a l

p o l l u t a n t s can r e s u l t i n t h e concen t ra ted d e p o s i t i o n o f these p o l l u -

t a n t s by f i l t e r feeders t h rough s e l e c t i v e s o r t i n g and f e e d i n g

niechani sms. I n faunal d e p o s i t feeders cou l d, t h e r e f o r e , r e c e i v e l a r g e

amounts t h rough t h e i r normal f eed ing process.

V i s u a l obse rva t i ons t h r e e days f o l l o w i n g t h e d redg ing and down-

s t ream f rom t h e dredge zone i n d i c a t e d no a1 t e r a t i o n o f t h e bottom,

no s i g n i f i c a n t accumula t ions o f d i s p l a c e d subs t ra tes , o r b u r i a l o f

marke t s i z e d o y s t e r s and c u l t c h m a t e r i a l s . Oys te r m o r t a l i t i e s w i t h i n

t h e con f i nes o f t h e dredged p l o t appeared t o be 100 pe rcen t due t o

b u r i a l and o n l y a few clam s h e l l s were e v i d e n t a t t h e sediment i n t e r -

face.

B e n t h i c i n f auna l p o p u l a t i o n s were n o t s i g n i f i c a n t l y reduced i n

e i t h e r t h e impac t o r dredge zones when compared t o c o n t r o l s . The

March 1981 sampl ing i n d i c a t e d a d i s t i n c t s i m i l a r i ty i n t h e p o p u l a t i o n

i n a l l sampl i n g s i t e s . The dredged p l o t , however, was sma l l when con-

s i d e r i n g commercial d redg ing o p e r a t i o n s . Reestab l ishment was a p p a r e n t l y

r a p i d w i t h i n t h e dredged s i t e r e s u l t i n g f rom an o p p o r t u n i s t i c c o l o n i z a -

t i o n response and /o r a m in ima l e f f e c t t o t he r e s i d e n t p o p u l a t i o n s

w i t h i n t h e s i t e .

Clam p o p u l a t i o n s i nc reased i n t h e impact , dredge and c o n t r o l s i t e s

i n t he March and June samples. There was, however, a s i g n i f i c a n t de-

c rease i n t he September sarr~ples a t a l l t h e s i t e s i n d i c a t i n g a h i g h

r e c r u i t m e n t o f j u v e n i l e clams b u t a low s u r v i v a b i l i t y r a t e t o market

s i z e . The h y d r a u l i c d redg ing had no measurable n e g a t i v e impac t on

j u v e n i l e c lam p o p u l a t i o n s .

Dominant spec ies i n a l l t h e a r e a s sampled were those commonly

a s soc ia t ed with s h e l l s u b s t r a t e s i n t h e mid-Bay a r e a . The ma jo r i ty of the

dominant spec ies c o l l e c t e d were s o f t - s u b s t r a t e burrowers and may no t have

been adverse ly a f f e c t e d by t h e dredging. In most cases t h e s e small animals

washed through t h e dredge and were i n suspension f o r a s h o r t per iod . Preda-

t i o n i s a seasonal cons idera t ion and in a l l p robab i l i ty not a s e r i o u s f a c t o r

during t h e October dredging.

Hydraulic e s c a l a t o r dredging r e s u l t e d i n t h e t o t a l d i s rup t ion of a

l a r g e port ion of t h e ep iben th ic community and caused a s h o r t term d i s t u r b -

ance in t h e infaunal benthic community, in c o n t r a s t t o spo i l d isposa l where

na tu ra l bottoms and a l l r e s i d e n t populat ions a r e completely covered w i t h

deposi ted s p o i l . Sediment suspended from any source decreases a s d i s t ance

from t h e bed inc reases and a l s o decreases w i t h increased v e l o c i t i e s . A

maximum deposi t ion-concent ra t ion e f f e c t would, t h e r e f o r e , occur i n an area

o f low flow and approaching t h e sediment i n t e r f a c e .

Recommendations

Comparing t h e present s tudy t o t h e Manning s tudy (Manning 1956) i t i s

ev ident t h a t l a r g e s c a l e va r i a t ion e x i s t s between geographical a r e a s i n t h e

Chesapeake Bay. These v a r i a t i o n s a r e evidenced i n c u r r e n t p a t t e r n s , t i d a l

ampli tudes, sediments , s a l i n i t y , e t c . The physical environment is re -

f l e c t e d in t h e biol ogi cal community. Base 1 ine environmental c h a r a c t e r i z a -

t i o n would se rve t h e present r e c l a s s i f i c a t i o n s i t u a t i o n and provide a

reference f o r f u t u r e cons idera t ion .

Due t o t h e v a r i a b i l i t y between a r e a s , dec is ions t o r e c l a s s i f y bottom

should be based on s i t e - s p e c i f i c physical and b io logica l obse rva t ions .

These observat ions inc lude:

Physical Observations

1 . Sediment grain s i z e ana lys i s - p a r t i c u l a t e s i z e determins length

of time in suspension and/or t r anspor t mode.

2 . Water col umn - incl udes measurements o f current v e l o c i t i e s and

d i r e c t i o n s a f f e c t i n g dispersion r a t e s and eventual impact of de-

posi ted mater ia ls - s a l i n i t y - behavior of f i n e pai - t icula tes in

suspension a r e dependent on the presence of ionizing s a l t s i n t he

water along w i t h composition, s i z e and shape (~ig.~).

3 . Geographical f ea tu res of area t o be r e c l a s s i f i e d - consol idat ion

o r compaction of f r e s h l y deposited sediments occurs slowly and

material i s 1 ike ly t o be resuspended both by t i d a l and wind in-

duced events. Distr ibution and t r anspor t of resuspended sediments

a r e d i r e c t l y re la ted t o the tu rbu len t nature of the flow. For

example, sanding o r t r anspor t o f heavy p a r t i c u l a t e s m i g h t occur

i n a reas of high s t r e s s , p a r t i c u l a r l y in unconsolidated bed sedi -

ments.

Biological Observations

1 . Determinations should include estimated dens i t i e s o f o y s t e r and

clam populat ions, cul tch mater ia ls a t t h e i n t e r f a c e and the benthic

infaunal communi t i e s . Economical l y harvestabl e populat ions of

clams should be present t o j u s t i f y r e c l a s s i f i c a t i o n with i t s pro-

found a1 t e r a t i o n of bottom s u b s t r a t e . The potent ia l f o r natura l

oys te r propagation shoul d be minimal o r non-existan t .

2 . The p o s s i b i l i t y o f the uptake and magnification of tox ic material

by t h e biological comrnuni t y in areas of suspected contaminants

should be examined. The pulsed resuspension of such mater ia l would

r e s u l t in the t r anspor t of these contaminates over a wide a r e a .

We would l ike t o t h a n k the following persons for the i r advice

and technical assistance in the execution of th is project.

Department of Natural Resources, Tidewater Fisheries Administra-

t ion, Mr. Peter Jensen, Mr. William Outen, Mr. Roy Scott; University

of Maryland, Chesapeake Biological Laboratory, Dr. Joseph Cooney, Dr.

Richard Smucker, Captain Martin 0 ' Berry, Captain Will iam Keefe, Mr.

Michael Reusing, Mr. John Crane, Mrs. Janet Crane, Mr. George Nichols,

Mr. William Cap1 ins, Mr. John Wilson, Mr. Larry Lubbers; Summer Interns

Miss Brenda Donlin, Miss Cynthia Schwenk; Commissioned personnel Mrs.

Linton Beaven, and Mr. James Lathroum.

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MIDDLE BAY PROPOSED RECLASSIFICATION SITES

W i t h the r ec l a s s i f i c a t i on of four s i t e s in the mid-Bay region pending,

i t was f e l t t h a t some basic baseline data should be obtained before dredging

commenced. These areas could then be examined following r ec l a s s i f i c a t i on t o

determine i f problems exis ted. The s i t e s se lected were sampled f o r benthic

community s t ruc tu r e , hydrography, sediment gra in s i ze s and elemental analyses

of the s i l t c lay f r ac t i ons .

The s i t e s evaluated were se lected by Tidewater Fisheries personnel . The

four s i t e s were Hol l i cu t t s Noose, Queen Anne's County; 'Three S i s t e r s , Anne

Arundel County; Buoy Rock, Kent County; and Tolley's-Thomas Point in Anne

Arundel County. W i t h the exception of the To1 ley's-Thomas Point s i t e , the

sampling s t a t i ons were a l so se lected by the management personnel. Sediment

samples were obtained by van Veen grab a t the Hol l icut ts Noose, Three S i s t e r s ,

and the Buoy Rock s i t e s . Cores were obtained a t the Tolley's-Thomas Point s i t e

a f t e r a su i t ab l e method had been developed fo r coring i n sand-shell subs t r a t e s .

In the laboratory a l l the samples from the four s i t e s were i den t i ca l l y processed.

Method appl icat ion as previously described f o r the Broome Is1 and sample t r e a t -

ments were a l so i den t i ca l .

Holl i cu t t s Noose sediment displayed a mean pa r t i cu l a t e f ract ion grain s i z e

considerably smaller than the other s i t e s invest igated (Fig. I j ) . The Three

S i s t e r s s i t e mean displayed a grain s i z e l a rger than the other s i t e s sampled by

van Veen grab and corresponded favorably with the Tolley's s i t e when comparing

la rger grain s i z e f rac t ions (Fig. 4a - 89) . The Tro l ley ' s s i t e , however,

produced a g r ea t e r percentage of material smaller than .063 micrometers which

may have resul ted from variance in sampling method. Species d ive r s i t y and

dominance between s i t e s could resul t from seasonal variance, subs t ra te preferance

and from difference i n s a l i n i t y (Environmental data - a l l s i t e s ) .