Manthe 1984 Aquacultural-Engineering

7/28/2019 Manthe 1984 Aquacultural-Engineering

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A quac u l t u r a l E ng i ne e r i ng 3 (1984) 119-140

L i m i t in g F a c t o r s A s s o c ia t e d w i t h N i tr i f i c a ti o n in C l o s e dB l u e C r a b S h e d d i n g S y s t e m s

Don P . Manthe , Rona ld F . Malone and Suni l Kumar

Department of Civil Engineering, Louisiana State University, Baton Rouge,Louisiana 70803, USA

A B S T R A C TA s tu dy o f" fac to r s l im i t ing crab dens i t i e s in c losed bh te c rab Callinectessapidus s h e d d i n g s y s t e m s w a s c o n d u c t e d u s i n g s c a l e d d o w n e x p e r i m e n t a lun i ts . N i t r if i c a t ion be ds, ac t i v a t e d c ar bon , do l o m i t e a nd p l an t s w e r e u s e dt o m a i n ta i n w a t e r q u a li ty . N i t r i t e ( N O 2 - N ) w a s f o u n d t o b e t h e m o s tc r it ic a l t ox i c e l e m e n t ac c um u l a t i ng i n t he s y s t e m as a r e s u lt o f t he n itri-f i c a t i on p r oc es s . Con c e n t r a t ions o f a ppr o x i m a t e l y 20 m g l it e r -1 NO , . - Nand above caused increased mor ta l i t y in in termol t c rabs . Mor ta l i t y inm o l t i ng c r abs was obs e r v e d a t c onc e n t r a t i ons a s l it t le a s 2 m g l it e r - tNO 2- N . D i s s o l v e d ox y ge n ( D O ) was i de n t i f i e d a s t he f ac t o r l i m i t ing thee f f i c i e nc y o f t he n i t r if i c a ti on beds. A s D O c on c e n t r a t i ons de c re as e d, tir era te o f n i t r i f i ca t ion s lowed , appare nt l y caus ing n i t r i f ica t ion to be inh ib i t edat th e n i t r i t e to n i t ra te convers ion s t ep . As n i t r i t e con cent ra t ions hwreased ,h igh mo r ta l i t i e s r esu l t ed , f i t r the r increasing th e loading in tir e s ys tem s andde pr e s s ing D O c onc e n t r a t ions , due t o t he h i gh B O D e x e r t e d by t he de adc rabs . E l e v a t e d c r ab popu l a t i ons we r e m a i n t a i ne d i n t he s y s t e m s wh e nae r a t ion an d f l o w i ncr e as e s we r e supp l i e d t o t he n i t ri f ic a t i on beds.

INTRODUCTIONProduction of softshell crabs in the state of Louisiana has decreaseddramatically over the last 20 years. The decline in the blue crab Cal l i -n e c t e s s a p i d u s has been principally attri buted to the deterioration ofthe coastal zones and acco mpany ing decline in water quality (Jaworski,

119Aquacul rura l Engineer ing 0144-8609/84/$03.00 Elsevier Applied SciencePublishers Ltd, England, 1984. Printed in Great Britain


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120 D. P. Manthe, R. F. Malone, S. Kurnar1 9 7 1 ). B e c a u s e o f t h e d e c l i n i n g n a t u r a l w a t e r q u a l it y , t h e p e r c e n t a g e o fs u c c es s fu l m o l t s ( e c d y si s) h a s d e c r e a s e d in o p e n w a t e r s h e d d i n g b o x e sa n d s o f t s h e l l cr a b f i s h e r m e n i n L o u i s i a n a h a ve b e e n f o r c e d t o t u r n t oo t h e r m e t h o d s to ' s h e d ' so f t sh e l l b l u e c ra b s (J a w o r s k i , 1 9 79 ). T h ev a lu a b le s o f ts h e l l f o r m o f t h e b l u e c r ab b r o u g h t p r i c es u p w a r d s o fU S $ 2 5 p e r d o z e n i n 1 9 8 3 . T h e p o t e n t i a l v al u e o f u s i n g c l o s e d r e c ir c u -l a ti n g s y s t e m s f o r t h e s h e d d i n g o p e r a t i o n s h a s b e e n d e m o n s t r a t e d b y af e w s u c c es s fu l c o m m e r c i a l o p e r a t o r s ( P e r ry e t a l . , 1 9 8 2 ) . T h e s e c l o s e ds y s t e m s r e d u c e l a b o r r e q u i r e m e n t s a n d e l i m i n a t e t h e e x p o s u r e o f c ra b st o m a n y d e l e t e r i o u s e n v i r o n m e n t a l c o n d i t i o n s d u r i n g t h e v u l n e ra b l em o l t i n g p e r io d . H o w e v e r , t h e w i d e sp r ea d a d o p t i o n o f t h is t e c h n o l o g yh a s b e e n i n h i b i t e d b y t h e l a c k o f e s ta b l i s h e d d e s i g n c r i te r i a a n d m a n a g e -m e n t g u i d e l in e s ( V a n G o r d e r a n d F r i t c h , 1 9 80 ).

T h e o p e r a t i o n o f a s u c c es s fu l c l o s ed c i r c u l a ti o n a q u a c u l t u r e s y s t e md e p e n d s o n t h e m a i n t e n a n c e o f a c c e p t a b le w a t e r q u a li ty . T h e a bi li ty o fb i o l o g i c a l f i l t e r s i n t h e c l o s e d s y s t e m s t o c o n v e r t a m m o n i a ( N H 3 ) ,w h i c h is t h e p r i n c i p a l e x c r e t o r y m e t a b o l i t e o f c r u st a c e a ( H a r t e n s t e in ,1 9 7 0 ) , t o t h e r e l a t iv e l y n o n - t o x i c n i t r a t e (NO3) b y b a c t e r i a l n i t r i f i c a -t i o n is w e ll s u m m a r i z e d b y S p o t t e ( 1 9 7 9 ) a n d W h e a t o n ( 1 9 7 7 ) .T h e o b j e c t iv e o f th i s s tu d y w a s to e x a m i n e t h e c a r r y in g c a p a c i t y o f as u c c e s s f u l f i lt e r c o n f i g u r a t i o n a n d t o i d e n t i f y f a c t o r s l i m i t i n g f i l te re f f i c i e n c y .

M A T E R I A L S A N D M E T H O D SS c a le d e x p e r i m e n t a l u n i t s w e r e d e v e l o p e d f r o m c ri ti ca l v o l u m e t r i c a n ds u r f a c e a r ea r e l a t i o n s h i p s d e r i v e d f r o m a s u c c e ss f u l c o m m e r c i a l s y s t e mo p e r a t e d b y M r C u l t u s P e a rs o n o f L a C o m b e , L o u i s ia n a ( M a lo n e e t a l . ,1 9 84 ). T h e u s e o f t h e 8 % s c al e e x p e r i m e n t a l s y s t e m s w a s w a r r a n t e d b yt h e n e e d t o l i m i t t h e c o s t as s o c i a te d w i t h a n t i c i p a t e d c r ab m o r t a l i t i e sa n d f o r r e p l i c a t i o n o f re s ul ts . T h e f o u r e x p e r i m e n t a l s y s t e m s e a c hc o n s i s te d o f t w o f ib e rg la ss ta n k s w i t h l e n g t h , w i d t h a n d d e p t h o f 6 f t ,3 f t a n d 1 f t ( 1 8 3 9 1 3 0 c m ) , r e s p e ct iv e l y . O f t h e t w o t a n k s u s e di n e a c h s c a l e d d o w n s y s t e m , t h e u p p e r t a n k w a s d e v o t e d t o h o l d i n gc r ab s a n d t h e l o w e r t a n k t o t h e v a r i o u s f i l t r a t io n u n i t s ( F ig . 1). T h ei n t e r i o r o f t h e u p p e r t a n k s w a s c o a t e d w i t h a s m o o t h w h i t e g e l c o a t.D i m e n s i o n s o f t h e e x p e r i m e n t a l s y s t e m s a re s h o w n i n T a b l e 1.


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Closed blue crab shed ding Lvsrems." l im itation and nitrif ica tion 121

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Descr ip t ion Length( : t ) Width D epth Water dep th Area( f t ) ( f t ) ( f t ) ( f t )

Volume(f t 3 )C r a b t a n k 6B i o l o g i c a l f i l t e r 1 . 2 5Sh e l l f ' fl t e r be d 1 .25D o l o m i t e f i l t e r

b e d 1 - 2 5C a r b o n f i lt e r

b e d 1 - 2 5Al ga e f ' tl t e r 0 - 6 67R e s e r v o i r c o m -

p a r t m e n t a 4 - 0 8 3

3 1 0 - 43 8 18 7 .87 53 1 0 - 9 5 8 3 . 7 5 3 . 5 93 0 - 2 5 - 3 . 7 5 0 . 9 3 83 O- 125 - 3-75 0 . 46 93 0 . 0 8 3 - 3 . 7 5 0 - 3 1 13 1 0 - 79 2 2 i . 5833 1 0 . 9 1 7 1 2 . 2 5 1 1 . 2 2 9

a I n c l u d e s h ea d c h a m b e r .


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C l o s e d b l u e c r a b s h e d d i n g s y s t e m s : l i m i t a t i o n a n d n i t r i f i c a t i o n 123line of the pump was fitted with a screened check valve to prevent lossof pump prime during power outages and uptake of large debris. Thewater was pumped through a ~ in (1- 6 cm) flexible plastic tubing to thespray nozzle in the upper crab tank.The experimental systems were set up during the spring of 1983 inan open air building at the Huey P. Long Fish Hatchery, located inLaCombe, Louisiana. Artificial seawater (Rila Sea Salts) was mixedwith local well water in the experimental systems at 5%0 salinity,duplicating the commercial system. Freshwater additions were onlymade to replace losses due to evaporation, sample collection andspillage. To duplicate the commercial system as closely as possible andto establish accelerated nitrification in the new systems (Bower andTurner, 1981), the biological and algae fdters o f each experimentalsystem were inoculated with media from the commercial system.Dominant forms of plants in the experimental systems included watermilfoil (Myriophyllum sp.) floating on top of the algae filter andattached filamentous green algae on the filter walls. No plant biomasswas harvested from the systems and the algae filters were provided witha constant light regime throughout the study.Crabs were obtained from a local commercial softshell crab fisher-man on a daily basis. These crabs were taken from Lake Pontchartrainby crab traps according to local practice. One experimental system wasdedicated to the function of holding incoming crabs. This holdingsystem was used to maintain constant crab populations in the systemsbeing used for active experiments. The majority of the crabs used forexperimental purposes were intermolt crabs (crabs between moltingcycles). In some cases, smaller populations of crabs experiencingmolting were placed in the systems to permit examination of the effectsof water quality fluctuations on the molting crabs. This approach wastaken since it was anticipated that mortalities associated with high riskexperiments would exceed our limited source of shedding crabs. Whennecessary, these shedding crabs were isolated from the general popula-tion by the vinyl coated wire enclosures. All crabs used in the studyranged in size from 10 to 15 cm across the carapace (top shell).Following local commercial practice, crabs in the closed sheddingsystems were not fed. Mortalities resulting from the experiments wereremoved continually and populations restored to constant levels once aday. Periodically, the entire population of crabs were replaced tominimize the effects of adaptation and selective processes. Debris


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124 D. P. Manthe, R. F. Malone, S. Kumarfrom the systems was periodically removed as accumulation becameapparent.

All systems were monitored for temperature, salinity, dissolvedoxygen, ammonia, nitrite and pH every day. Determinations of alka-linity levels and nitrate concentrations were undertaken on a weeklybasis. Mortalities and records of visual observations were continuallymainta ined. Routinely, a 1 liter sample water was taken from thereservoir compartment of each of the four experimental systems andanalyzed immediately for the laboratory determinations. Totalammonia as NH3-N was measured with an Orion 95-10 ammonia elec-trode in conjunction with an Orion model 701A digital ionalyzer.Nitrite as NO2-N was determined using a sulfanilamide-based colori-metric reaction and a Bausch and Lomb Spectronic 20. Both tests wereconducted in accordance with Standard Methods (APHA, 1980).Oxygen and temperature data from the systems were obtained with aYellow Springs Instruments (model 51 ) dissolved oxygen meter. Salinitywas measured with a refractometer (American Optical Corp.) and pHwith a Mini (model 47) pH meter. Nitrate (NOa-N) determinationswere undertaken by a modified hydrazine reduction (Spotte, 1979) andalkalinity by titration (APHA, 1980).

RESULTSOverv i ew

A total of four experiments were under taken during the spring sheddingseason of 1983. The four experimental units were numbered to permitidentification of system histories during interpretation of results.Figure 2 presents loading curve and critical nitrogen forms for System 4,which was used as a holding system during the first three experiments.This unit most closely approximates commercial operation of theclosed shedding systems, in contrast to the other units which werepurposely overloaded to test the limits of the system. The limitednumber of mortalities observed in this system were attributed toharvesting and handling operations and not to the accumulation oftoxic metabolites. These results illustrate the capability of this design toaccommodate highly variable crab densities normally associated withcommercial systems.


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F i g . 2 .

Closed blue cra b shed ding systems." l imita t ion and ni tr i f icat ion 125'I80 ~ System 46 0

40

3124 28 41t3 6 F

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5 9 13 17 21 25 29 5/ 3 7 II 15 t9 23

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3/24 2{} 4/ I 5 9 13 17 21 25 29 .5/3 7 II 15 19 2_3DAYA m m on ia and n i [ r i te leve ls assoc ia ted w i th c rab dens it ies in ho ld ing

system (System 4 ).

F i g u r e 3 il l u s tr a t es s u p p o r t i n g w a t e r q u a l i t y p a r a m e t e r s f o r t h ef i r s t t h r e e e x p e r i m e n t s . A s c a n b e s e e n f r o m t h i s i l l u s t r a t i o n , a l l t h r e ee x p e r i m e n t a l s y s t e m s b e h a v e d v e r y s i m il ar ly w h e n d i f f e r e n c e s in c r a bd e n s i t i e s a re t a k e n i n t o a c c o u n t . A l l s y s t e m s i n it i al l y s h o w e d a p H o f8 -3 a n d s l o w l y d e c l i n e d t o p H v a l u e s in t h e ra n g e 7 . 0 - 7 . 2 . T h i s d e c l i n ei n p H w a s a s s o c i a t e d w i t h a d e c l i n e i n a l k a l in i t y . T h e s e r e s u l t s i l l u s t r a t ea l im i t e d c a p a b i l i t y o f th e d o l o m i t e b l a n k e t s t o b u f f e r p H c h a n g esu n d e r t h e c o n d i t i o n s o f th is e x p e r i m e n t . T h e s e o b s e r v a t i o n s a re c o n -s i st e n t w i t h t h e o b s e r v a t i o n s o f B o w e r a n d T u r n e r ( 1 9 8 1 ) , w h i c hd i s c u s s e d t h e li m i t e d a b i l i t y o f c a l c a r e o u s f i l tr a n t s t o m a i n t a i n p Ha b o v e 8 . 0 . S i m i l ar o b s e r v a t i o n s w e r e a ls o m a d e o n c o m m e r c i a l o p e r a -t i o n s u s i n g t h e s a m e d e s i g n a n d w a t e r s o u r c e s ( M a l o n e e t a l . , 1 9 8 4 ) .T h e s e s y s t e m s d i s p l a y e d a l k a li n i ti e s a s l o w a s 3 0 m g l i te r -~ C a C O 3 a n dp H v a l u e s in t h e r a n g e o f 7 . 0 a f t e r 2 y e a r s ' o p e r a t i o n w i t h n o w a t e rc h an g e s. T h e r e is s o m e d i s a g r e e m e n t o n p H v a l u es f o r o p t i m u m n it ri -f i c a t i o n r a t e s , b u t it a p p e a r s t h a t N i t r o s o m o n a s h a s a h i g h c o n s t a n to x i d a t i o n r a t e b e t w e e n p H 7 -0 an d 9 . 0 , w h i l e N i t r o b a c t e r c o n v e r s i o n


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126 D. P. Manthe. R. F. Malone, S. Kumar

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3 /2 4 2 8 4 / I 5 9 1 3 1 7 2 1 2 5 2 9 5 / 3 7 I I 1 5 1 9 2 3 2 7D A YFig. 3. Supporting water quality parameters for Experiments 1-3 (pH, alkalinity,nitrate, temperature).

rates are satisfactory between 6.5 and 8.5 (Wheaton, 1977). Submergedfilters can be acclimated to lower pH values if given time (Haug andMcCarty, 1972). It is concluded that the dolomite bed is sufficient formaintenance of pH above 7-0 and that this pH apparently does notadversely affect the crabs. In fact, the lower pH may be beneficial inthat it reduces ammonia toxicity due to the equilibrium reactionbetween NH~ and NH 3 (Wheaton, 1977; Spotte, 1979).


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Closed b lue crab shedding sys tems: l im i ta t ion and n i t r i f i ca t ion 127Temperatures in the four systems were dramatically influenced by

the open air nature of the systems and rapidly came to equilibriumwith the ambient temperature. All systems therefore displayed identicaltemperatures. During the course of the study, the temperature rangedfrom a low of 1 IC to a high of 27C. The rate of nitrification increaseswith temperature. Wild e t a l . (1971) in their studies found that nitri-fication rates increased in the range 5-30C. For this reason, compar-able experiments were run simultaneously when possible. However, thistemperature variation must be recognized when experimental resultsare interpreted .

Nitrate levels in the experimental systems increased throughout thecourse of the study. The rates of increase parallel the loading historiesof each system. Values over 140 mg liter -t NO3-N were observed bythe end of the study period. Nitrate toxicity was not thought to be afactor even at the high concentrations observed. Nitrate is generallynot toxic to marine organisms, even at elevated levels (Hirayama, 1974;Siddall, 1974). Commercial systems monitored at this time experiencedupwards of 360 mg liter -~ NO3-N with no deleterious effects (Malonee t a l . , 1984). Clearly, the plant filter (despite heavy plant growth) failedto significantly influence the accumulation of nitrate in these systems.

Dissolved oxygen (DO) concentrations for the first three experi-ments are shown in Fig. 4. These values were obtained from measure-ments in the holding compar tments of each system. DO will be discussedin greater detail for each experiment.Experiment no. 1Experiment no. 1 was under taken to determine reasonable crab densitiesfor acclimation of the nitrification beds in the biological filter units.After inoculation of the filters, each system was populated with inter-molt crabs at varying densities. Systems 1, 2 and 3 were loaded with15, 30 and 50 crabs, respectively. During the start up period of eachsystem, the submerged biological filters demonstrated the classicalnitrogen development curve expected by immature biological filters(Wheaton, 1977). In newly established culture systems, the filter beddoes not contain enough bacteria to carry out its purifying activity;thus toxic substances accumulate in the system (Hirayama, 1974).A reduction of ammonia in the system during start up signifies theestablishment of a N i t r o s o m o n a s population, which converts ammonia


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i | i i i i i i i i i28 29 ]43 5 / t 2 3 4 5 6 7 8Experiment 3 Sys I. . . . . . S ys . 2..... . . .. . . .. S y s 3

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(~ , i i t = i , i i I i i i i i : i5 /11 12 13 14 15 16 17 18 19 2 0 21 22 2 3 24 25 2 6 27 28 29 30DAYFig. 4. Dissolved oxygen concentrations for Experimen ts 1-3.

t o n i t r i t e an d cau s es a b u i l d u p i n t h e n i t r i t e con cen t ra t i on . As t h eN i t r o b a c t e r p o p u l a t i o n b e c o m e s e s t a b li sh e d , n i t r it e c o n c e n t r a t i o n sdecrease , as the N i t r o b a c t e r convert n i t r i t e to n i t rate .

F i g u re 5 s h o w s t h e d e v e l o p m e n t o f t h e b io l o g ic a l fi lt e rs u n d e r t h ed i f f ere n t crab d en s i ti e s . S ys t e m s 1 an d 2 ( l oad ed l igh t t o m ed i u m )e x p e r i e n c e d n o d i f f i c u l t y w i t h t h e f il te r s o r t h e cr ab p o p u l a t i o n s a n dt h e s y s t e m s r e a c h e d e q u i l i b r i u m w i t h o u t i n c i d e n c e . M o r t a l i t y d i d n o te x c e e d 7% i n e i th e r sy s t e m . N o n e o f t h e m o r t a l i ti e s in t h e s e t w os y s t e m s w a s a t t ri b u t e d t o t o x i c e l e m e n t s . S y s t e m 3 , w h i c h w a s l o a d e dh e a vi ly , e x p e r i e n c e d u p w a r d o f 5 m g l it er - t a m m o n i a w i t h n o a p p a r en te f f e c t s . H o w e v e r , a s a m m o n i a d e c r e a s e d w i t h t h e s u b s e q u e n t r i s e i nn i t r i t e , s y s t em f a i l u re an d f u n c t i on a l m ort a l i t y i n crab s was ob s erved .


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C l o s e d blue crab sheddi ng systems." l imi tati on and nirr~.yicarion 1 2 90 i

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E x p erim ent 1 - a m m o n ia a n d n it ri te co n cen tra tion s o f S y s t em s 1 - 3during start-up period.

F u n c t i o n a l m o r t a l i ty w a s d e t e r m i n e d b y c r a bs s h o w i n g s i gn s o f d is tr e ss ,l ac k o f m o v e m e n t , l o ss o f e q u i l i b r iu m , p a r al ys is o f a p p e n d a g e s a n d ,f in a ll y , d e a t h . F u n c t i o n a l m o r t a l i t i e s w e r e a t t r ib u t e d t o t o x i c e f f e c t so f n i tr it e . A t c o n c e n t r a t i o n s g r e a te r t h a n 2 0 m g l it er -~ N O 2 - N , S y s t e m3 e x p e r i e n c e d m o r t a l i t ie s a s h i g h a s 2 0 % p er d a y a n d t h e b e h a v i o r o ft h e s u r v i v i n g c ra b s w a s a c u t e l y a f f e c t e d . L a r ge r cr a b s w e r e o b s e r v e d t ob e m o r e s e n s i ti v e t o t h e n i tr i te t o x i c i t y t h a n c r a b s o f a s m a l l er s iz e .A s t h e n i t ri te c o n c e n t r a t i o n d e c r e a s e d , c r ab m o r t a l i ty , i f a n y , d e c l i n e d


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130 D. P. Manthe, R. F. Malone, S. Kumarand became non-functional in character as before. In all systems, thebiological filters took approximately 30 days to stabilize to the pointthat both nitrite and ammonia were both rapidly oxidized to insigni-ficant levels. The length of this condit ioning period is consistent withthe observations of Bower and Turner (1981), and significantly shorterthan that of Hirayama (1974). However, differences in salinity,temperature and loading regimes make direct comparisons difficult.Experiment no. 2Experiment no. 2 was intended to test the response of systems accli-mated at various crab densities to heavy shock loadings. Followingcompletion of Experiment no. 1, the three experimental systems wereincreased in population to 100 intermolt crabs plus a small experimentalpopulation of three crabs entering ecdysis (molting). Figure 6 shows theresponse of the systems to the increased crab densities. All systemsdisplayed a small accumulat ion of ammonia in response to the increasedcrab densities. The timing of the appearance of an ammonia peak wasinversely related to the number of crabs used to condition the filters.The filters quickly adjusted to the increased ammonia loading anddropped to baseline conditions. Nitrite increased and remained at highlevels throughout the experiment. Although a slight nitrite recoverywas noted in each system in the latter part of the experiment, thisproved to be temporary and the systems remained in the failure state.All systems failed and suffered heavy functional morta lity when nitriteconcentrations rose above 20 mg l it er -~ NO:-N. The relationshipbetween nitrite levels and mortalities is most clearly illustrated by theresults of System 1. Crab populations in Systems 1, 2 and 3 sufferedtotal daily mortalities as high as 14, 30 and 30%, respectively. Decreasedmolting success was also observed in crabs entering ecdysis at concen-trations greater than 2 mg liter -~ NO2-N. These mor tal ity levels areclearly above those which can be tolerated by a commercial operationin areas where crab harvesting limits production. All systems weretherefore considered in a state of complete failure. After 12 days thesystems showed no signs of recovery and the experiment was terminated.This experiment provided further evidence linking the functionalmortalities with the accumulation of nitrite within the system. It alsoillustrated that the number of crabs used to acclimate the system wasnot critical. The ability of the systems to carry the increased crab


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Fig. 6 .

Closed blue c r ab shedd i n g sy st ems : l im i t a t i o n and n i t r i f i c a t i o n 131

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fo l lowin g shock loading .

p o p u l a t i o n s w a s u l t i m a t e l y l i m i t e d b y a f a c to r c o m m o n t o a ll t h es y s t e m s . T h e l im i t i n g f a c to r a l so m o s t a d v e r se l y a f f e c te d t h e o x i d a t i o no f n i t r it e t o n it r a te w i t h l it t l e i m p a c t o n a m m o n i a o x i d a t i o n . It w a so b s e r v e d i n th i s e x p e r i m e n t t h a t D O c o n c e n t r a t i o n s in t h e e f f l u e n t so f t h e b i o l o g i c a l f il te r s d u r i n g p e r i o d s o f s y s t e m f a il u r e w e r e s ig n i-


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132 D . P. M a n t h e , R . F . M a l o n e , S. K u m a rficantly depressed. Al though DO levels averaged 5.6 mg liter -~ in thecrab tanks, levels in the effluent of the biological filters averaged only2.0 mg liter -~. This suggested that lack of dissolved oxygen may belimiting the nitrifying activity of the filters.

Experiment no. 3In this experiment, crab densities in Systems 2 and 3 were reducedto populations of 75 and 85 crabs, respectively, to dete rmine if thesepopulations could be supported. Aeration was added to System l inan a ttempt to increase the efficiency of the nitrification bed, whilemaintaining 100 crabs in the system. Aerat ion was accomplished byplacing four air stones in the head chamber and two in the reservoircompartment. As can be seen in Fig. 7, all systems continued in a stateof failure. Heavy mortality continued to characterize the systems.It is believed that mortal ities of this magnitude significantly increasedloading of both carbonaceous and nitrogenous BOD to the biologicalfilters. This increased BOD loading neutralized the benefits of aerationin System 1. Although the DO concentrations in the head chamber ofSystem 1 averaged 6-1 mg liter -~, the exit DO concentra tions of thebiological filter remained depressed, averaging 1.8 mg liter -t. Sensitivityof the systems to oxygen limitations is demonstrated in System 1; ason 27 May, the aeration was inadvertently discontinued for a 24 hperiod. After this occurrence, a large increase in ammonia concentra-tion dominated the system. Systems 2 and 3 rapidly recovered in3 days when crab densities were reduced and removed.Experiment no. 4This final experiment was under taken to examine the effect of increasedoxygen supplies on the nitrification abilities of the biological filters.One of the systems (System 2) was supplied aeration by means of fourair stones placed in the head chamber and the flow rate of the systemwas increased to 10 liters min -~. No modif ications were made in theother system (System 4). Both of the experimental systems were thenloaded with crab populations of 75 crabs each. Figure 8 shows thelevels of ammonia and nitrite in both systems during this period. Theunmodified system realized system failure with mortality losses as high


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Closed blue cr a b shedd i n g s yst ems : l im i t a t i o n and n i t r i f i c a t i o n 1 3 3

Fig, 7 .

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a e r a t i o n ( S y s t e m 1 ) a n d h e a v y l o a d i n g ( S y s t e m s 2 a n d 3 ) .

a s 5 5 % . T h e m o d i f i e d s y s t e m w i t h a e r a t i o n a n d i n c r e a s e d f l o w ra tes h o w e d i n i t ia l i n c r e a s e s o f a m m o n i a a n d n i tr i t e , b u t r e c o v e r e d a n dd e c r e a s e d a f te r se v e ra l d a y s . I n t h e m o d i f i e d s y s t e m , b i o l o g i c a l f i l te re f f l u e n t s a v e ra g e 4 . 2 m g l it e r - t D O d u r i n g t h i s e x p e r i m e n t . P e r f o r m -


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17/22

7 -

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5

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Fig. 9.

Closed b lue crab shedding sys tems: l im i ta t ion and n i t r i f i ca t ion 135

HE.AvY LOADING-AERATION/FLOW NCREASE ~~ ST4,BLE~ SYSTEMS

~ , ~ L IGHT LOADING - NORMAL SYSTEM9

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-)HEAVY LOAOtNG-AERATION [

O ~ S Y S T E M S1 FAILUREHEAVY LOAOING-NORMAL SYSTEM

CRAB TANK lEAD BIOLOG ICAL ALGAE RESERVO IR COMPARTMENTC H A M ~ F , L T E R F ~LTER

Oxygen traces through experimental system s with differing crab densitiesand system modifications.

a e r a t i o n p r i o r t o t h e b i o l o g i c a l f i l t e r , w e r e i n c r e a s e d c r a b d e n s i t i e sm a i n t a i n ed . O x y g e n t ra c es t h r o u g h t h e v a r i o u s c o m p o n e n t s o f t h es y s t e m s r e v e a le d d r a m a t i c d e c r e a s e s i n D O c o n c e n t r a t i o n s t h r o u g ht h e b i o l o g ic a l f i lt er . F i g u r e 9 d e m o n s t r a t e s v a r i o u s o x y g e n t r a c est h r o u g h t h e e x p e r i m e n t a l s y s t e m s i n s t a te s o f e q u i l i b r iu m a n d s y s te mf a il u r e, t n t h e s y s t e m s , n o r m a l a e r a t i o n i s s u p p l i e d b y t h e s p r a y i n g o fw a t e r i n t o t h e c r a b t a n k . A l l t r a c e s r e v e a l t h a t t h e h i g h e st o x y g e nd e m a n d i n t h e s y s t e m s i s i n c u r r e d b y t h e b i o l o g ic a l f il te r. S y s t e m s i na fa il u re s t a t e a re d e m o n s t r a t e d b y D O t r a c e s i n c u r re d b y t h e h ea v yl o a d i n g o f b o t h a n a e r at e d a n d a n o r m a l s y s t e m w i t h o u t m o d i f i c a t i o n .S t a b l e s y s t e m s p r e s e n t e d i n c l u d e a l ig h t ly l o a d e d s y s t e m w i t h o u tm o d i f i c a t i o n a n d a h e av i ly l o a d e d s y s t e m w i t h m o d i f i c a t i o n s c o n s is ti n go f a e r a t i o n a n d i n c r e a s e d f l o w . In t h e s t a t e o f s y s t e m f a i l u re , D Oc o n c e n t r a t i o n s d e c r e a s e d t o a s l o w a s 1 m g l i te r - t a f t e r t h e b i o l o g i c a lf il te r. D O m e a s u r e m e n t s o f t h e e f f l u e n t o f t h e b i o l o g i c a l f il te r s w e r et a k e n i n t h e s t a n d i n g w a t e r ly i n g o v e r th e a c t i v a t e d c a r b o n l a y e r. T h e s ew a t e r s u n d o u b t e d l y e x p e r i e n c e d s o m e re a e ra t io n p r i or to m e a s u r e-m e n t . T h u s , o x y g e n l ev e ls w i t h i n t h e f i lt er s w e r e p r o b a b l y m u c h l o w e r .


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136 D. P . Manthe , R . F . Malone , S . KumarNitrification processes require 4.0 -4.6 mg liter -t oxygen to com-

plete ly oxidize 1 mg NH3-N to NO3-N, depending on the culture age(Wheaton, 1977). N i t r o s o m o n a s consumes, by stoichiometric calcula-tions, 3.02 mg oxygen per mg NH3-N oxidized to NOv.-N, as comparedto 1-02 nag of oxygen consumed b y N i t r o b a c t e r converting 1 mg NO2-Nto NO3-N (Wheaton, 1977). If oxygen is not present in the requiredstoichiometric amount, the nitrification rate decreases. In fact, nitriteconcentrations can be increased through dissimilatory reduction ofnitrate (Spotte, 1979; Mevel and Chamroux, 1981). Haug and McCarty(1972) reported that in upflow submerged filters, NH3-N oxidationrates were the greatest at the bottom of the filter, where ammoniumconcentrations were the highest. This could explain the increase andbuild-up of high levels of nitrite found in our study. If oxygen was at acritical stoichiometric level, oxygen concentrations could be depletedat the bottom of the filter in the process of oxidizing ammonia tonitrite. As near anoxic conditions occurred in the filter, the rate ofNO2-N oxidat ion to NO3-N would be severely decreased. This wouldcreate a kinetic 'bottleneck' and result in elevated toxic nitrite con-centrations.During the study, nitrite proved to be the most toxic form of nitrogento the blue crab. Ammonia concent rations did not correlate well withobservations of functional mortalities. Nitrite concentrations in therange 20 mg liter- ' NO2-N produced functional mor tal ity and adverseeffects on the experimental intermolt crabs. Armstrong e t a l . (1976)and Wickins (1976) reported similar nitrite toxicity levels in prawns.The molting crabs experienced decreased molting success at lowerconcentrations of nitrite. The authors observed crabs dying duringmolting at concentrations as low as 2 mg liter -~ NO2-N. These lowerconcentrations were also supported by observations from commercialsystems monitored at the time of this study (Malone e t a l . , 1984).Commercial production of softshell crabs similarly decreased as thenitrite concentra tions in these systems rose to near 2 mg liter -z NO2-N.These field observations clearly indicate that nitrite toxicity was thefactor limiting crab densities in the experimental systems.The mean nitrite and DO concentrations for~varying percent mor-tality ranges are presented in Table 2. As nitrite concentrations increaseand DO concentrations decrease in the closed systems, mortalities tendto rise. It is believed tha t both temperature and the highly erratic BODloading resulting from mortalities, induced significant variability in


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C l o s e d b l u e cr ab s h e d d i n g s y s t e m s : l i m i t a t i o n a n d n i t r i f i c a t i o n 13 7T A B L E 2M ean DO and Nitri te Co ncentrations Associated w ith Varying Percent T otal

Mortality LevelsT o t a l m o r t a l i t y n D i s s o l v e d o . v yg e n N i t r i t e

ra t e (% ) (m g l i t e r - t) , { rng l i t e r -1 NOv.-N),2 2

0-5 I 26 6.41 5-45-2 0 66 3.66 17-34> 2 0 12 2.88 26.92

n = Num ber of observa t ions .

s y s t e m r e s p o n s e s . I t w a s g e n e r a l l y o b s e r v e d t h a t i n c re a s in g t h e t e m p e r a -t u r e d e c r e a s e d t h e c a r ry i n g c a p a c i t y o f t h e s e s y s t em s . T h i s w o u l d b ee x p e c t e d , d u e t o t h e e f f e c t s o f t e m p e r a t u r e o n t h e s a t u r a t i o n l ev els o fd i s so l v e d o x y g e n a n d o n t h e m e t a b o l i c r a te s o f b o t h t h e n i t r i f y in gb a c t e r i a a n d t h e c r a b s . M o r t a l i ti e s a d v e r s e l y a f f e c t e d t h e s y s t e m s b yi n c re a s in g t h e t o t a l o x y g e n d e m a n d o n t il e b i o l o g i c a l f i lt e rs ( d e c a yo f r e l ea s e d o r g a n i c s ) a n d b y i n c r e a si n g a m m o n i a l o a d i n g s t h r o u g hm i n e r a l i z a t i o n . A l t h o u g h B O D d o e s n o t in h i b i t t h e n i t r i f i c a ti o n r a ted i r e c t l y ( W i l d e t a l . , 1 9 7 1 ) , i t d o e s c o n s u m e o x y g e n f o r b r e a k d o w np r o c e s s e s . T h i s w o u l d t e n d t o d r i v e ti le D O in t h e s y s t e m l o w e r a n dn i t r i f i c a t i o n r a t e s w o u l d b e i m p a i r e d . A s to x i c l ev e ls o f n i tr o g e ni n c r e a se d b e c a u s e o f l o w e r e d n i t r i f i c a t io n r a te s , i n c r e a s e d c r ab m o r -t a li t ie s w o u l d r e s u l t . T h i s w o u l d a d d t o t h e B O D l o a d a n d f u r t h e rc o n t r i b u t e t o t h e d e c l i n e o f w a t e r q u a l i t y in t h e s y s t e m s . S y s t e m s t h a tw e r e s e v e r e ly o v e r l o a d e d r e c o v e r ed o n l y a f t e r cr a b s w e r e r e m o v e df o r a s h o r t t i m e ( 3 - 5 d a y s ) .

I n t h e G u l f o f M e x i c o w h e r e s o f ts h e l l c r ab p r o d u c t i o n is o f t e nl i m i t e d b y t h e a v a i l a b i l i t y o f m o l t i n g b l u e c r a b s , a t o t a l d a i l y m o r t a l i t yr a t e o f le ss t h a n 5 % is r e q u i r e d t o m a i n t a i n t h e v i a b i l i t y o f a c o m m e r -c ia l o p e r a t i o n . T h e d e m a n d f o r g o o d w a t e r q u a l i t y in t h e c l o se ds y s t e m s is t h e r e f o r e h ig h . C l e a rl y , t h e r e s u lt s o f t h is s t u d y i n d i c a t e t h en e e d f o r t h e d e v e l o p m e n t o f s p ec i fi c d e si g n c ri te r ia t o m e e t t h e n e e d sf o r t hi s t y p e o f s y s t e m . F u r t h e r r e s ea r c h s h o u l d b e d i r e c t e d t o w a r d st h e d e v e l o p m e n t o f f i lt e r d e s ig n s t h a t a l le v i at e t h e n i t r it e a c c u m u l a t i o n si n t h e s e s y s t e m s .


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138 O . P . M a n t h e , R . F . M a l o n e , S . K u m a rC O N C L U S I O N S

T h e f o l l o w i n g c o n c l u s i o n s h a v e b e e n d r a w n f r o m t h e r e s u lt s o f t h iss t u d y :

1. T h e f i l t e r c o n f i g u r a t i o n t e s t e d h e r e m a i n t a i n e d s a t i s f a c t o r y w a t e rq u a l i t y w h e n o p e r a t e d w i t h c r a b d e n s i t i e s o f 5 0 c r a b s o r l es s. T h es y s t e m s r e s p o n d e d q u i c k l y t o v a ri ab le c r ab p o p u l a t i o n s w i t h in t h isr a n g e .

2 . A n n u a l w a t e r c h a n g e s w o u l d b e w a r r a n te d w i t h t h is s y s t e m toa l le v i at e l o n g - t e r m b u i l d - u p o f n i t r a t e a n d l o n g - t e r m d e c re a s e s i na l k a l i n i t y .

3 . T o x i c a c c u m u l a t i o n s o f n i t r it e w e r e i d e n t i f ie d a s t h e f a c t o r l im i t -i n g c ra b d e n s i t i e s i n t h e s e s y s t e m s .

4 . I n t e r m o l t c r a b s w e r e a d v e r s e l y a f f e c t e d b y n i t r i t e le v el s i n t h ev i c i n i t y o f 2 0 m g l i t e r - t a n d m o l t i n g c r a b s s e e m e d t o b e a d v e rs e l ya f f e c t e d b y l ev e ls o f n i t r i t e a s l o w a s 2 m g l i te r -~ .

5 . F a i l u r e o f t h e b i o l o g i c a l f i lt e r to p r o v i d e r a p i d o x i d a t i o n o fa m m o n i a a n d n i t r it e w a s a t t r i b u t e d t o a D O l i m i t a t i o n in t h eb io log ica l f i l t e r .

6 . I n c r e a se d t e m p e r a t u r e s a n d h i g h m o r t a l it i e s a g g ra v a te d t h e o x y g e nd e f i c i e n c y i n b i o l o g i c a l f il te r s a n d a d v e r s e ly a f f e c t e d s y s t e mp e r f o r m a n c e .

A C K N O W L E D G E M E N T ST h i s r e s e a r c h w a s s u p p o r t e d b y t h e L o u i s i a n a S e a G r a n t C o l le g e P r o -g r am . C o l l a b o r a t iv e s u p p o r t w a s p r o v i d e d b y t h e M i s s is s ip p i -A l a b a m aS e a G r a n t C o n s o r t i u m . T h e s e p r o g r a m s a r e e l e m e n t s o f t h e N a t i o n a lS e a G r a n t C o l le g e P r o g r a m , u n d e r t h e d i r e c t i o n o f N O A A , U S D e p a rt -m e n t o f C o m m e r c e . T h e c o l l a bo r at iv e w o r k w a s u n d e r th e d ir e c t i o n o fM r s H a r r i e t P e r r y a t t h e G u l f C o a s t R e s e a r c h L a b o r a t o r y i n O c e a nS p r i n g s , M i s s i s s i p p i . C r a b s w e r e s u p p l i e d a n d i n v a l u a b l e t e c h n i c a la d v ic e g i ve n b y M r C u l t u s P e a rs o n t h r o u g h h is c o m m e r c i a l o p e r a t i o n inL a C o m b e , L o u i s ia n a . D o n a t i o n o f f i l te r t a n k s b y P l a n t a t i o n F i b e rg l aso f B u s h , L o u i si a n a , is a c k n o w l e d g e d . H o u s i n g a c c o m m o d a t i o n s a n dr e s e a r c h f a ci li ti e s in L a C o m b e , L o u i s i a n a , w e r e p r o v i d e d b y t h e L o u i s i a n aD e p a r t m e n t o f W il dli fe a n d F i sh e ri es .


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Closed blue crab shedding systems: limitation and nitrification 139R E F E R E N C E S

A P H A ( 1 9 8 0 ) . Standard Methods for the Examination of Water and Wastewater,15 th e dn , A me r i c a n W a te r W or ks A ssoc i a t i on , W a te r P o l lu t i on Con t r o l F e de r a -t i on a nd A me r i c a n P ub l i c H e a l th A ssoc i a t i on , W a sh ing ton D C.

A r m s t r ong , D . A . , S t e phe ns on , M. J . & K n igh t , A . W . ( 1976 ) . A c u te t ox i c i t y o fni tr i te to larvae o f the giant Malaysian praw n Macrobrachium rosenbergii.Aquaculture, 9 , 3 9 - 4 6 .

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140 D. P. Manrhe, R. F. Malone, S. KumarW he a ton . F . W . ( 1977 ) . Aquacultural Engineering, W ile y -ln t e rsc i e nc e , N e w Y or k .Wickins , J . F . (1976[) . Th e to le rance of warm -wate r p rawns to r ec i rcu la ted wa te r .

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Manthe 1984 Aquacultural-Engineering