MEASUREMENT TECHNIQUES

Environmental Sciences

COMPARISON OF BENTHIC OXYGEN DEMAND MEASUREMENT TECHNIQUES

Thomas

V.

Belanger

DepartmentofEnvironmental ScienceandEngineering, Florida InstituteofTechnology, Melbourne,Florida32901

Abstract: Several common methods to measure benthic oxygen demandwere compared usingmeasuredandliteraturedata.Anin siturespirometertechnique, a laboratorycore-uptake techniqueanda laboratoryflow-through system techniquewerecomparedonsandandorganic sedimentinLakeWashington, Florida.Insituuptakeratesweresignificantlyhigher thancore- uptake rates, andthisdifferencewasgreater for highlyorganicsediment than sand sediment.

Measuredandliteratureuptakevaluesforin siturespirometerandcore-uptake techniques with organicsedimentwere comparedandresultedina relationship(r = .99;

P<

.01)of theform:in situ uptake (g ²/m²-hr) = .036 + 1.16coreuptake _(g 2/m²-hr). In many casesbothcore- uptake and in situ respirometer techniques under estimate oxygen consumption, due to dif- ficulties in obtaining correct water velocities over thesedimentsurface. In theflow-through systemstudies, sediment oxygen uptake varied considerably with flow rate anda significant logarithmic relationship(P<.01) was obtainedwith Lake Apopka, Florida organic sediment.

Measurement techniques thatsimulatefieldflows ^{are the} most accurate, such as theflow- through systemor thein situtunnel respirometertechnique.

The

principal oxygen sinks in aquatic systems are microbial

and macrophyte

metabolism in the water

column and

biological

and

chemical uptakeby bottomsediments. Sediment oxygen uptakehas receivedrelatively little attention

compared

to oxygen

demand

ⁱⁿ thewatercolumn, butsedi-

ment demand

can represent a significantpercentageofthetotal oxygenup- takein

some

aquaticsystems. In

some

rivers

and

streams,sedimentcan actas a stationaryoxygen sink

and

greatly affect the "oxygen sag" characteristics ofthe waterway.

Hanes and

Irvine (1968) indicatedthat oxygen uptakeby sedimentsin certainrivers

may

accountfor as

much

as50

%

ofthe totalox- ygen depletion from the watercolumn.

Sediment uptakerates obtained from a variety of insitu

and

laboratory

measurement

techniques range over 3 orders of magnitude (Table 1).

The

rates range from 0.001 g 2

/m

²-hr fordeep sea sediments (Smith

and

Teal, 1973) to 1.07 g 2

/m

²-hr for coral reef communities

(Odum and Odum,

1955).

The

lack of an accepted or approved

method

for measuringbenthic oxygen

demand,

however,

may

contribute to inaccuracies in the reported literature because different methods used to measure benthic oxygen de-

mand may

not be comparable.

Common

methods of measuring benthic oxygen

demand

have seldom been

employed

simultaneously in the

same

aquatic system for comparative purposes (Bradley and James, 1968; James, 1974;

Edberg and

Hofsten, 1973). In viewofthis, I

wanted

to: (1) statistic- allycorrelate several techniques usingmeasured

and

literaturedata,and ₍₂₎ discuss the feasibility ofeach method.

No.4, 1981] BELANGER

—

BENTHICOXYGEN 205

Fig. 1. LocationofLakeWashington.

Sampling —

^{Benthicoxygen}

^demand

measurements

were made

inAugust 1978 at sand (1.5% V.S.)

and

organic (20.5% V.S.) stations in

Lake

Washington, Florida.

Lake Washington

is the third uppermost of the naturallyconnectedlakes ofthe northward-flowingSt. JohnsRiver

and

lies

approximately 411 river

Km

^south of Jacksonville, Florida (Fig. 1).

An

in situ respirometer technique, a laboratory core-uptake technique

and

a laboratory flow-through system technique

were

tested.

Core-uptake

method —

Several sand

and

organic sediment cores

were

taken ateachstation, placedin astyrofoam container,

and

transferred to a portable laboratory. Divers obtained the sediment ^cores (10 to 20

cm

in depth) with 50.8

cm

long plexiglas tubes (11.40

cm

² openings). Care

was

taken to keep the sedimentwater interface in its natural state.

206 FLORIDASCIENTIST [Vol.44

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No.4, 1981] BELANGER

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208 FLORIDASCIENTIST [Vol.44

In thelab, thesedimentlevels

were

adjustedtoa 10

cm

depthbydiscard- ingsediment from thebottom ofthecore.

The

overlyingwater

was

siphoned

off

and

375

mL

ofmid-depthlakewater

were added

toeachcore.Thiswater containedsufficient oxygen

(>

5

mg/L)

toconductthe uptakeexperiments.

The

water

was

siphoned ^carefully into the cores to minimize sediment disturbance. Next, the styrofoam container

was

^filled with lake water to

keep the cores at field temperature

(30°C±2).

At the

same

time, several

empty

^cores

were

filledwith lake waterascontrol cores to estimaterespira- tion in the water column.

Everycore apparatus

was

^filledentirelywithwatertoinsuretheabsence ofairbubbles. After a30

min

incubationperiod, thedissolvedoxygeninthe water

was

measured with a YSI

Model

^51-

A

^dissolved oxygen meter at5-10

min

intervals.

The

water in the core

was

closed to the atmosphere with a rubber stopper to eliminate the effects ofatmospheric diffusion.

The

probe

was

positioned 10

cm below

the upper edgeofthe tube.

The

purpose ofthe waiting period

was

to avoid the initial rapid uptakeofoxygen byeasily ox- idized

compounds

that

may

have been exposed duringtheinitialcoringsteps

and

refilling procedure. Dissolvedoxygen

was

measured at 5-10

min

inter- vals forapproximately2hr.

The

average concentration ateachtimeinterval

was

plottedversus time

and

therate ofoxygenconsumption

was

determined from the slope of the curve.

The

average control core rate

was

subtracted from the average sediment core-uptake rate to determine the sediment ox- ygen uptakerate.

To

quantify therateson anarealbasis, the

volume

ofwaterover thesedi-

ment

in thecores (0.375 L)

and

the surfaceareaofthecoreopening (1.14 x 10^_3

m

²)

were

taken into consideration:

Sediment uptakerate

(mg

2

/m

²-hr)

=

2uptake(mg/L-hr) xwater

volume

x 1/sediment area

(1) = 329 x uptake rate (mg/L-hr).

in situ respirometer

— A

benthic respirometer described by

Fruh and

Davis, (1972)

was

constructed to measure in situ benthic oxygen de-

mand

(see Fig. 2). Dissolved oxygen

was

measured inside the uptake

chamber

withtheYSI

membrane

electrode.

A

stirringapparatus attachedto the electrode provided slight circulation within the

chamber and

aided in obtaining stable readings.

The

oxygen electrode

was

fixed in position, and the respirometer

was submerged and

inverted to allow airtoescape, before placingitonthebottom.

Once

in place, the apparatus

was

allowedtositun- disturbed for20

min

beforeany readingsweretaken to allowsettling tooc- cur to eliminate the effects of bottom disturbance. Dissolved oxygen measurements

were made

^{at 5-10}

min

intervals for80 to 100 min.

To

quan-

tify the rates on an ^areal basis, the sediment area covered by the respirometer (0.26

m

²), the

volume

of water over the sediment (110.8 L),

and

thewater

column

respiration were taken into account as follows:

Benthic 2

demand (mg

2

m

^2-hr)

⁼

[02uptake (mg/L-hr) -waterrespiration (mg/L-hr)] (water volume) x 1/respirometer area

No.4, 1981] BELANGER

—

BENTHIC OXYGEN ²⁰⁹

10.2cm

Sealed Rubber Stopper

D.O. Probe

+Stirring Mechanism

50.8cm

Fig. 2. Insiturespirometer.

(2)

=

_{426 [02 uptake} (mg/L-hr) ^- water respiration (mg/L-hr)]

Flow-Through

System

— ^The

respirometer

and

coremethodstomeasure benthic oxygen

demand

represent batch systems.

The

decrease in oxygen concentration in the overlying water

was

measured,

and

the results

were

used incalculating theoxygen uptake rates ofthesediments.

The

following problems, however, ^are involvedin usingbatch systemsto measurebenthic oxygen

demand, and

^these problems

may

affect the results.

1. Batch systems do not approximate natural conditions because of the lack of water flow.

2.

The

length of each experimental run is limited to the length of time required to deplete the initial dissolved oxygen concentration in theoverlying water.

3. Batch systemsdo not givethe sediments sufficient timetoacclimate

210 FLORIDASCIENTIST [Vol.44

to experimental oxygen levels under consideration since the oxygen concentration of the overlying water continually decreases.

For these reasons a continuous (flow-through) system

was

constructed (Fig. 3)

and

used to measure sediment oxygen uptake rates for comparison with batch uptake rates. This flow-through system ₍₁₎ better approximates natural flowconditions,an importantfactorinsediment oxygenuptake,

and

(2) permits the calculation of oxygen uptake rates along with uptake or release ofother elements of interest understeady state conditions.

The

flow-through apparatus consisted of4 airtight

chambers

placed in series

and

connected by 0.95

cm

inside-dia rubber hose

and CPVC

pipe, with appropriate fittings. Three of the

chambers

were constructed from

Army

surplus

ammunition

boxes

which were

painted on the inside with epoxy paint.

One chamber was

built usingplexiglas so that turbidity of the waterat different flow rates could beobserved.

The

total

volume

ofthe 4

chambers was

99.6 L,

and

thebottom surface area

was

0.34

m

^2.

A

submersible

aquarium pump was

placed in a closed plasticgarbage bucket (waterreservoir)

and

connectedtothefirst tankwith a rubber hose. Influent flow rates

were

varied by adjusting the position of pinchvalves on the recyclelinethat

was

connected tothe influentflowline (Fig. 3)^.

A

1.27

cm

insidediameterrubberreturn flowlineentered thewater reservoir so that the set-up

was

essentially a closedsystem.

An

air stone at-

tachedto plastic tubing

was

placedinthewater reservoir so that theoxygen contentoftheinfluentwatercouldbevariedby bubblinginnitrogenorair.

Sediment oxygen uptake studies using the flow-through system with

Lake Apopka,

Florida sediment generally indicated^that benthicoxygen

demand was

independent ofoverlyingdissolvedoxygen at concentrations above ap- proximately 2.5

mg/L,

but

below

thatlevel uptakerates

were

dependent on oxygen

and

decreased rapidly (Belanger, 1979). Thisrelationshipis

shown

in Fig. 4.

Observation of fluoresceindyein the clear plexiglas

chamber

atvarious flowrates indicatedthat mixingin the

chambers was

complete

and

noshort circuiting

between

influent

and

effluent points occurred. Fluorescein dye

was

also used tosimulate the range ofwatervelocities (.05 ^- 1.0 ft/sec) ob- tained from measurements in

Lake Washington and

the upper St. Johns River, Florida

and

these velocities

were

then correlated toflowrates (L/hr) fromthelabsystem. It

was

feltthisrange

would

encompass mostin situflow rates over the sediment-water interface.

Water

sample collection points

were

placedon theinfluent

and

returnflowlines. Dissolvedoxygenconcen- trations

were

determined in duplicate by Winkler titrations. Flocculent organicsediment

was

collectedwith aPonar dredge, placedinsealedplastic buckets,

and

transported to the laboratory.

Sand

sediment

was

collected with box corers

and

carefully transferred to the flow-through

chamber

to

keep the sediment-water interfaceintact.

Sand and

organic sediments were put in the flow-through

chambers

to a depth of 12.7

cm and

covered with

Lake Washington

waterthat hasbeenfilteredthroughglasswool. Bothsedi-

No.4, 1981] BELANGER

—

BENTHICOXYGEN 211

Effluent Sample Point

^^>

Large Chamber

Fig. 3. Flow-throughsystem.

etu_ZfioVTi^

Influent FlowLine

-150

2 3 4 5 6 7

DISSOLVED OXYGEN CONCENTRATION(mg/U

Fig. 4.Sediment oxygen uptakerateasfunction of overlying dissolvedoxygen concentration forLakeApopkastation2sedimentwith flowratesvaryingfrom 138to240L/hr.

212 FLORIDASCIENTIST [Vol.44

ment

types

were

allowed to equilibrate for approximately 7 da before the

pump was

turnedon,

and

the

pump was

run at leastone daybeforesamples

were

taken.

During

this waiting period concentration gradients should be reestablished

and

in situ sediment conditions

would

be simulated in the organic sediment.

A

simpleequation

was

developed

and

usedtocalculatesediment oxygen uptake for the flow-through system. This equation can only be

employed when

thedetentiontime attheparticularflowrateinusehas beenequalled orsurpassed. Usuallya flowrate

was

selected

and

allowedtoequilibrate for at least a day before samples

were

taken.

The

equation used was:

(6)

D _B ⁼ PQ

_I -

DQ _{F -BOD}

DT

D.T.

where Dg ⁼

netoxygen uptake rate (mg/hr-m²_),

DOj =

influent dissolved oxygen concentration (mg/L),

DOp ⁼

effluent dissolved oxygen content (mg/L), D.T.

=

detention time (hours),

BODj} j ⁼ BOD

^of overlying water for a particular D.T.

(determined from regression equation of

DO

vs.

time for five-day

BOD),

V ⁼ volume

ofoverlyingwater (liters),

and

A ⁼

surface area ofsediment

(m

²)

.

Results

and

Discussion

—

Overlying water temperatures in each

method were

similar

and

kept at

30°C(±2°C).

Core

and

in situ

measurements

were made

on the

same

day for each sediment type so that direct comparisons could be

made.

Sediment for the flow-through system

was

collectedonthese days,also, butit

was

allowedto equilibrate for a

week

beforemeasurements

were made. The

resultsfrom each

method

areinTable

2.

A summary

ofthe flow-through system uptake results is in Table 3.

Results from the comparative study indicate that in situ oxygen uptake rates aresignificantly higherthancore uptakerates

and

that this difference

is greater with organic sediment than with sand sediment.

The

higher up- take rates in flocculentorganic sediments areprobably related tosediment disturbance

and

resuspension. Core-uptake

and

in situ respirometer results

from replicate measurements in

Lake Washington

indicated greater variability in uptake rates fromorganic than in thosefrom sand sediments.

In fact, uptake rateswith sand sediment did not varyatall withthese tech- niques (Table 2). Uptakerates inthe flow-throughsystem, however, varied considerablywithflowratebut alsovariedtoa greater extent inthe organic sediment (Table 3). Highest uptake rates

were

obtained with organic sedi-

ment

at high flow rates.

Core

uptake rates with sandy sediment

were

ap- proximately the

same

astheflow-through uptakeforthe

same

sediment type at low flows (1 to 3 hr detention time).

However,

core uptake rates with

No. 4,1981] BELANGER

—

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214 FLORIDASCIENTIST [Vol.44

100 125 150

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90 80 70

•

•

60 50 40

•

•

D_B=-/49.l3+53.74InFlow

r2

=0.59, n=22, p/L.OI

30 ^•

20

m

J •

• i

75 100 ^125-

FLOW (l/hr)

Fig. 5. Sediment oxygen uptakerate asa function offlowrate for (a) LakeApopkanorth midlakeand(b)southmidlake sediment withoverlyingoxygenconcentrations greaterthan2.5 mg/L.

organicsedimentapproximatedflow-through uptakeathigher flows(0.75to 1 hr detentiontime). Insitu uptakeby sand approximatedflow-through up- takewith a detentiontimeofapproximately0.75 hrorless. Inorganicsedi-

ment, thein situuptake approximated aflow-through uptake^at adetention time of0.50 hror less.

In acomparisonofthe core-uptake, in situ respirometer,

and

in situ tun- nel respirometer techniques in English streams, James (1974)

showed

that

No.4,1981] BELANGER

—

BENTHIC OXYGEN 215 the core uptake technique

and

in situ respirometer

method

^both underestimatedoxygenconsumption asdetermined by tunnel respirometers

and

mass balancecalculations,with resultsfromthelaboratorycore-uptake technique having the greatest percent deviation. Other workers have sug- gested thatuptakerates fromcoremethods arelower thanthosefromin situ

methods (Edberg

and

Hofsten, 1973; Rolley

and Owens,

1967; Bradley

and

James, 1968). Theseresults are consistent with findings in thisstudy. In all cases the

low

values for core

and

in situ methods appear to be caused

by

a difficulty inobtaining thecorrectrangeofwatervelocitiesover thesediment surface. Results from using the laboratory flow-through system with

Lake

Apopka, Florida organicsediment (55 to

66%

V.S.)indicatedthatsediment oxygen uptake rate (Dg) varied considerablywith flow rate.

A

statistically significant (r

=

.94, .75;

p<.01)

logarithmic relationship of the form

Dg ⁼

-a + b In

Flow was

obtained with

Lake Apopka

sediment from 2 locations (Belanger, 1979). This relationship is

shown

in Fig. 5. Inshallow streams, a large in situ tunnel respirometer as

employed

by James (1974)

may

eliminate the problemofsimulating actualflowsinthelab.

The

insitu tunnel respirometer, however, isnot applicablein

many

aquaticsystems, in- cluding lakes, deep canals, estuaries

and

deep rivers. In these situations a flow-through lab system such as

shown

in Fig. 3

may

be thebest

method

if

the water velocitiesover the sediment can be approximated. Careful atten- tion must be givento theflow rate intheflowthroughsystem, however, as unrealisticallyhighratescanoverestimatesedimentoxygen uptakedueto in- creased chemical oxygen

demand

from disturbed sediments.

Table4.

A

comparisonofsedimentoxygen uptakemeasurementtechniques_(g 2/m²-hr).

Sediment Study Core In Situ

Flow-Through Tunnel English River

Mud

Bradleyand

James,'1968

0.04 0.10

0.05 0.24

0.22 1.09

English Riverand Lake

Mud

James, 1974 0.15 0.92 0.39 0.22 0.07 0.22

0.16 1.13 0.40 0.35 0.13

0.22 1.59

0.42 Eutrophic

EnglishLake

Mud

withNewlyDeposited Algae

Edberg and Hofsten, 1973

0.05 0.06

0.08 0.10

LakeWashington, Florida

Belanger, 1979

Organic Sand

0.18 0.20

0.27 0.24

216 FLORIDASCIENTIST [Vol.44

I.2CH-

.20 .40 .60 .80 1.00 1.20

Core Uptake( g

02/m

²-hr)

Fig.6. Comparisonofcore-uptake andin siturespirometermeasurementtechniques.

A

listing ofsediment uptake rates from studies

where

several measure-

ment

techniques

were employed

is in Table 4. Uptake rates are presented primarily for organic sediment, asthe lack ofdata for othersediment types prevented statistical comparisons. In situ respirometer

and

core-uptake techniques

were

significantly (r = 0.99;

P<0.05)

correlated (Fig. 6). Tun- neluptakeresults

were compared

withcore-uptakeratesfromdatacollected from significantly flowing English rivers

and

lakes by James (1974)

and

a r

valueof0.79 (P<0.05)

was

obtained (Fig. 7). Although a lack ofdataexists forthetunnel respirometer, probablyduetothedifficultyofconstructingit,

the in situ tunnel respirometer isexpected to givethebest resultsbecause it

measures sediment oxygen uptake under in situ environmental conditions.

Core-uptake results, then, significantlyunderestimate sediment oxygen up- take.

The

"best" approaches forflowing systems

would

betosimulateactual measured field flows in the lab or, if possible, use the tunnel respirometer technique in the field. Fororganic sediment systems that exhibit littleflow or circulation, thein siturespirometertechniqueis

recommended

or thecore

No. 4,1981] BELANGER

—

BENTHIC OXYGEN 217

Tunnel = 1.33(Core

)+

0.33

r= 0.79

n=5

h:

—

^{i .} 1 ^

—

H

To

.20 .40 .60 .80 1.00 I.

Core Uptake (g

02/m

²-hr)

Fig.7. Comparisonofcore-uptakeandtunnel respirometermeasurementtechniques.

technique with the Correction factor for conversion to in situ respirometer rates presented in Fig. 6. Although differences between the methods are generally less on sand sediments than organic sediments (Table 4),

more

comparative data should be collected on sand sediment to adequately establish this fact. Further data are also needed for comparisons with the tunnel respirometer, as only five data points

were

available in this study (Fig. 7).

218 FLORIDASCIENTIST [Vol.44

LITERATURE CITED

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dissert. Univ. ofFlorida, Gainesville, Florida.

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Bradley, R. M., andA.James. 1968. Anewmethodforthemeasurementofoxygenconsump- tioninPolluted Rivers._J.WaterPollut. Cont. Fed. 15:24-35.

Carey, A. G.,Jr. 1962. Anecologicalstudyoftwobenthicanimal populationsinLongIsland Sound. Ph.D. dissert. Yale Univ.,

New

Haven,Connecticut.

. 1967. Energetics of thebenthos of LongIsland Sound. Bull. Bingham Oceanog.

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Edberg, N., and V. Hofsten. 1973. Oxygen uptakeofbottom sedimentsstudied in situ and

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Edwards. R. W., andH. Rolley. 1965. Oxvgen consumption ofriver muds. _J. ofEcology.

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Fruh, E. G., and E. M. Davis. 1972. Limnological investigations of Texas impoundments

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B-020-TEX, Univ. Texas.

Hanes,N.B.,andR. L. Irvine. 1968.

New

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andD. Fenton. 1968. Aninstrumentformeasuringsubtidalbenthicmetabolism insitu. Limnol. Oceanog., 13:537-540.

Riley,G.A. 1956.OceanographyofLongIslandSound,1952-54. IX.Productionandutilization oforganicmatter. Bull. BinghamOceanog. Collect. YaleUniv., 15:324-344.

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Steiner, G.R.,Caroll,W.E.,Patyrak,R.

C,

andE.G. Fruh. 1972. Dissolvedoxygensinks inthehvpolimnionofa subtropicalimpoundmentPp. 5-1-5-26. InFRUH,E. G.,andE.

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