SOUTHEASTERN NATURALIST 2005 4(3):487–512 Freshwater Mussel (Bivalvia: Unionidae) Assemblages of the Lower Cache River, Arkansas ALAN D. CHRISTIAN 1,* , JOHN L. HARRIS 1,2 , WILLIAM R. POSEY 1,3 , JOSEPH F. HOCKMUTH 1 , AND GEORGE L. HARP 1 Abstract - Freshwater mussel beds of the lower 68 km of the Cache River, AR, were delineated and sampled using diving and stratified random sampling methodology to determine species richness, density, size structure, and population and community numerical standing crop (CNSC). A total of 38 mussel beds were delineated, includ- ing 14 major beds (Mbeds) and 24 minor beds (mbeds). Twenty six species were collected, four of which were previously unknown from the Cache River. Amblema plicata, Megalonaias nervosa, and Plectomerus dombeyanus were the most abun- dant. Estimates of CNSC ranged from 3705 ± 1908 to 122,115 ± 24,194 individuals in Mbeds with mean densities ranging from 6.2 to 44.1 mussels/m 2 . Nine of 16 species with > 10 individuals had a unimodal size frequency distribution and the other seven had multi-modal distributions. This study found impressive mussel assemblages in the lower Cache River, previously thought to contain only refugial pockets of mussel assemblages. Further monitoring of some species is recommended based on lack of recruitment. Introduction Over 300 species of freshwater mussels occur in the continental United States (Turgeon et al. 1998); however, within the last 50 years this rich fauna has been decimated by impoundments, sedimentation, channelization, dredging, water pollution, and invasive species (National Native Mussel Conservation Committee 1998). Approximately 67% of the freshwater mus- sel species in the United States are imperiled or already extinct, and many have gone extinct in this century (Bogan 1997, Master et al. 1998, Williams et al. 1993). Freshwater mussels are ecologically important as a food source, improve water quality by filtering contaminants, sediments, and nutrients, and pro- vide an early warning for water quality problems (National Native Mussel Conservation Committee 1998, Vaughn and Hakenkamp 2001). Endangered species are indicator organisms, and require preservation of not only the species and its habitat, but of as much of the entire ecosystem as possible (Anderson 1993; Doppelt 1993; Richter 1993; Watters 1992, 1993). To conserve native freshwater mussels, the National Native Mussel Conservation Committee (1998) identified specific problems including the 1 Arkansas State University, Department of Biological Sciences, PO Box 599, State University, AR 72467. 2 Current address - Arkansas Highway and Transportation Department, Environmental Division, PO Box 2261, Little Rock, AR 72203. 3 Cur- rent address - Arkansas Game and Fish Commission, PO Box 6740, Perrytown, AR 71801. * Corresponding author - [email protected].
Transcript
SOUTHEASTERN NATURALIST2005 4(3):487–512
Freshwater Mussel (Bivalvia: Unionidae) Assemblages ofthe Lower Cache River, Arkansas
ALAN D. CHRISTIAN1,*, JOHN L. HARRIS
1,2, WILLIAM R. POSEY1,3,
JOSEPH F. HOCKMUTH1, AND GEORGE L. HARP
1
Abstract - Freshwater mussel beds of the lower 68 km of the Cache River, AR, weredelineated and sampled using diving and stratified random sampling methodology todetermine species richness, density, size structure, and population and communitynumerical standing crop (CNSC). A total of 38 mussel beds were delineated, includ-ing 14 major beds (Mbeds) and 24 minor beds (mbeds). Twenty six species werecollected, four of which were previously unknown from the Cache River. Amblemaplicata, Megalonaias nervosa, and Plectomerus dombeyanus were the most abun-dant. Estimates of CNSC ranged from 3705 ± 1908 to 122,115 ± 24,194 individualsin Mbeds with mean densities ranging from 6.2 to 44.1 mussels/m2. Nine of 16species with > 10 individuals had a unimodal size frequency distribution and theother seven had multi-modal distributions. This study found impressive musselassemblages in the lower Cache River, previously thought to contain only refugialpockets of mussel assemblages. Further monitoring of some species is recommendedbased on lack of recruitment.
Introduction
Over 300 species of freshwater mussels occur in the continental UnitedStates (Turgeon et al. 1998); however, within the last 50 years this rich faunahas been decimated by impoundments, sedimentation, channelization,dredging, water pollution, and invasive species (National Native MusselConservation Committee 1998). Approximately 67% of the freshwater mus-sel species in the United States are imperiled or already extinct, and manyhave gone extinct in this century (Bogan 1997, Master et al. 1998, Williamset al. 1993).
Freshwater mussels are ecologically important as a food source, improvewater quality by filtering contaminants, sediments, and nutrients, and pro-vide an early warning for water quality problems (National Native MusselConservation Committee 1998, Vaughn and Hakenkamp 2001). Endangeredspecies are indicator organisms, and require preservation of not only thespecies and its habitat, but of as much of the entire ecosystem as possible(Anderson 1993; Doppelt 1993; Richter 1993; Watters 1992, 1993).
To conserve native freshwater mussels, the National Native MusselConservation Committee (1998) identified specific problems including the1Arkansas State University, Department of Biological Sciences, PO Box 599, StateUniversity, AR 72467. 2Current address - Arkansas Highway and TransportationDepartment, Environmental Division, PO Box 2261, Little Rock, AR 72203. 3Cur-rent address - Arkansas Game and Fish Commission, PO Box 6740, Perrytown, AR71801. *Corresponding author - [email protected].
Southeastern Naturalist Vol. 4, No. 3488
lack of knowledge regarding current distribution and health of musselpopulations. Their suggestions included: 1) determining location, density,species composition, and status of existing mussel communities; 2) gather-ing historic distribution data and making it available; and 3) gatheringinformation on the occurrence and abundance of mussels valuable for thecommercial mussel industry. Such mussel surveys have proliferated inrecent years (e.g., Ahlstedt and Tuberville 1997, Miller et al. 1993, Will-iams and Schuster 1989).
This study was part of a larger project delineating, mapping, and sam-pling mussel beds along 1380 total km in 10 rivers and 185 total km in threeimpoundments and one natural (oxbow) lake in Arkansas from 1991–1997,in which standardized cost effective sampling protocols were developed andused for all systems. The objectives of the current study were to determinethe status of mussels in the lower 68 river km of the Cache River by 1)delineating and mapping current mussel beds, and 2) assessing musselpopulations and communities by using three levels of sampling effort in-cluding a random stratified sampling protocol.
Field-site Description
The Cache River originates on the western slope of Crowley’s Ridge inButler County, MO, and flows southwest and joins the White River nearClarendon, Monroe County, AR (Fig. 1). The watershed is approximately230 km long with a drainage area of > 5240 km2 (Smith 1996). The majorityof the floodplain ranges from 2–3 km wide (Kleiss 1996). The Cache Riveris located in the Western Lowlands portion of the Mississippi River AlluvialPlain (Royall 1988, Saucier 1974), a trough between the Appalachian upliftand the Ozark Highlands (Fenneman 1938). The alluvial layer or top stra-tum, averaging 6.0 m thick, is composed of sand, silt, and primarily claysized particles (Fisk 1944).
The main channel and tributaries in the upper third of the watershed werechannelized during the 1920s and 1930s to drain the land for agricultural use(US Army Corps of Engineers 1974). The channel upstream of Grubbs,Jackson County, AR, was straightened, and in one segment, a double chan-nel was constructed. The channelized segment of river drains 2072 km2 ofthe 5180 km2 basin. Nearly one third of the basin had been cleared forcropland by 1937 (MacDonald et al. 1979). Forest cover declined from 65%in 1935 to 15% in 1965 (Kress et al. 1996) and was replaced by row cropssuch as cotton, rice, and soybean, and pasture (State of Arkansas 1974).Ground water is used heavily for crop irrigation, and a decline in groundwater level has occurred in recent decades (Plafcan and Fugitt 1987). Agri-cultural runoff is the principal source of sediments and nutrients in the basin(Kleiss 1996). The Cache River can be considered a blackwater stream as ithas high sediment loads with high silts and clays and associated highturbidity that reduce the water clarity/visibility to near zero.
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 489
Figure 1. Major (M) and minor (m) beds of the lower 68 km of the Cache River, AR,from the State Highway 38 bridge to the confluence with the White River in Monroe,Prairie, and Woodruff Counties, AR.
Southeastern Naturalist Vol. 4, No. 3490
Downstream of Grubbs, the Cache River flows in a meandering naturalchannel. However, increased runoff from agricultural lands and upstreamchannelization have increased sediment deposition of the mid- and lowerreaches (Hupp and Morris 1990, Kleiss 1996). Discharge at Patterson,Woodruff County, AR, approximately 49 river km upstream of the begin-ning of the study area, ranges from a weekly average low flow of 3.0 m3/secto a seasonal high flow of 225 m3/sec (Smith 1996), with a mean annualdischarge is 35.7 m3/sec (Kleiss et al. 1989). Water levels fluctuate morethan 3.0 m during an annual cycle, and discharges range from 0 to >340 m3/sec (Kleiss 1996, Wilber et al. 1996).
Wheeler (1914) collected 19 mussel species from the upper Cache Riverat Nemo, Craighead County. Gordon et al. (1980), based primarily on reviewof museum holdings, listed 18 mussel species from the Cache River. Eco-logical Consultants (1983) collected at 19 sites along a 305 kilometer reachof Cache River and found 29 species (13 live, 16 shells only). Jenkinson andAhlsted (1988) sampled four sites in the Cache River watershed and col-lected 19 mussel species. Mauney and Harp (1979) collected 32 fish speciesin the Cache River, and 56 fish species occur in the drainage (Robison andBuchanan 1988).
Methods
A questionnaire was sent to each state-licensed mussel taker in Arkansasduring late 1990 to gain preliminary information on commercial mussel bedlocations. Additional information was compiled from interviews and fieldreconnaissance with commercial shellers between 1991 and 1994. Bothmapped commercial beds and unmapped potential mussel habitats weresearched by dive techniques supported by a Hookah System.
Initial searches of probable mussel habitats were from upstream todownstream to determine the limits of a potential bed. Divers estimatedthe number of live mussels within a 1-m2 area by feel as visibility wasnear zero. If the number of mussels was ≥ 10 /m2, the diver estimated thewidth of the bed by traversing the limits in 1 m increments. Additionaldownstream “transects” were conducted until the substrate was uninhab-ited by mussels, always consisting of at least three transects per areasearch. Total length of a bed was measured using a range finder. A recordof each site was compiled on a 7.5 minute topographic quadrat map andrecorded on a Global Position System. Water depth, determined by adepth finder, and river morphology (e.g., lateral scour pool or glide)(Fig. 2) were recorded. Substrate type, density of mussels, dimensions ofthe investigated bed, and species composition in each area was given bythe diver subsequent to each dive, and this information was used to deter-mine whether more intensive sampling was necessary.
Following the initial searches, an area was categorized by one of thefollowing three methods to determine if further sampling would be
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 491
conducted. If an area was determined to have mussel densities averaging< 10/m2, but with no limit as to total bed area, it was determined to be aqualitative-only area. This categorization required no further samplingand used the relative abundance, species composition, and general habitatinformation that were recorded from the initial searches obtained solelyby diver observations and estimations.
If an area was estimated to be > 500 m2 with sporadic densities> 10/m2, based on initial search estimates, or areas < 500 m2 with mean
Figure 2. Physical habitat arrangement, riffle, run, lateral scour pool, and glide, of atypical Cache River mussel bed in terms of depth profile (top) and longitudinalsequence (bottom).
Southeastern Naturalist Vol. 4, No. 3492
densities > 10/m2, the area was categorized as a minor bed (mbed) with anassociated second type of sampling. The subsequent mbed sampling con-sisted of five non-random 1-m2 quantitative samples taken in areas ofhighest density within the defined area. This allowed for cost-effectivesampling while obtaining non-random quantitative data in small or mar-ginally dense areas.
If an area was estimated to have densities > 10/m2, based on initial searchestimates, and an area > 500 m2, an area was categorized as a major bed(Mbeds), which resulted in a third type of sampling. These areas weresubsequently intensively sampled, with a sample size of 10–25 1-m2 quad-rats. Beds were divided into strata based on substrate composition, rivermorphology, and/or river depth (Fig. 1). The number of samples from aMbed was determined by total bed area: a minimum of ten 1-m2 samples for500–999 m2 areas; a bed with area between 1000–2500 m2 was sampled byone percent of the area (i.e., 10–25 samples); and a bed with area > 2500 m2
was sampled by twenty-five 1-m2 samples. Quadrat sample sites were deter-mined from a random numbers table. Furthermore, the number of samplestaken from each stratum was based on the proportion of stratum size to totalbed area, and a minimum of three samples was taken from each stratum forstatistical validity. The justification and effectiveness of this three level ofsampling has been addressed and found to be an effective and cost efficientmethod for sampling large, deep, blackwater rivers in Arkansas (Christianand Harris 2005).
A sample unit consisted of a 1-m2 quadrat constructed of a 2.5-cmweighted PVC pipe. Mussels and associated substrate were collected fromthe substrate, at a depth of ≈ 5 cm, placed in a dive bag, and sorted andidentified at the surface. Because visibility was zero, divers systematicallypicked the samples via feel and systematically went through the quadrattwo to three additional times to ensure complete collection of individuals.Nomenclature followed Turgeon et al. (1998). Measurements, obtainedusing dial calipers with 0.1 mm precision, were collected in accordancewith the commercial legal dimensions (i.e., either length or depth) set byArkansas Game and Fish Commission. Wet total mass was recorded ingrams using an electronic balance. Mussels were returned to the river nearwhere they were collected.
Total individuals, minimum density, maximum density, mean density,sample variance, and standard deviation were calculated for each musselspecies and quadrat sampled. Mussel population and community quantita-tive estimates and 95% confidence intervals of Mbed population (P) (i.e.,total numbers of a species) and total mussel community(C) numerical stand-ing crop (NSC) (i.e., total number of all species) were calculated using themodified Sampford method (Huebner et al. 1990).
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 493
Results
Survey of the lower 68 km of the Cache River required 94 person days tocomplete. Approximately 138 river reaches were explored by multiple divesresulting in sampling of 14 Mbeds and 24 mbeds (Fig. 1). No endangeredspecies were encountered in the study area.
LocationMost mussel beds began at the head of a lateral scour pool (sensu Bisson
et al. 1981) with point bars associated across from the high clay bank(Fig. 2). Upstream of the high clay bank and lateral scour pool, two adjacenthabitats were present: directly upstream of the lateral scour pool was ashallow but swift water area, i.e., riffle-run sequence (sensu Bisson et al.1981), and adjacent to this area was a small but distinct area of slowercurrent and high siltation, i.e., the secondary channel pool (sensu Bisson etal. 1981), that usually marked the beginning of substantial mussel densities.Moving downstream into the lateral scour pool, the current and water depthincreased. Concurrently, the thalweg (deepest portion of channel) narrowedand substrate was swept clear of encroaching sands. The thalweg substrate,which provided the most suitable mussel habitat, consisted of soft-to-hardclay and usually extended to the soft clay of the near ascending bank.Thalweg width increased and water depth decreased downstream of thelateral scour pool into the glide (sensu Bisson et al. 1981), where substrateusually consisted of sand.
Center channel substrate in the glide consisted of an ascending mid-channel bank of sand substrate grading into a soft or hard clay substrate(Fig. 2). On the opposite margin, an ascending near-shore bank of soft claymet the expanded clay substrate. After the widening of the thalweg, bedwidth decreased as the encroaching sands covered the clay substrate, leavinglittle suitable mussel habitat. Smaller beds consisted of only an encroachingsand-soft clay interface at the ascending near-shore clay bank, and this wasgenerally located in the thalweg, where depth increases slightly in compari-son to surrounding substrates. This narrow band, defined here as the transi-tion zone, was usually one meter or less in width, but often extended severalhundred meters downstream. Within this strip, mussel densities commonlyreached > 10/m2.
Species richness and relative abundanceTwenty-six species were identified from among 5686 specimens
(Table 1), and the results of other published surveys or compilations arealso summarized in Table 1. Before this study, Elllipsaria lineolata,Lasmigona complanata, Ligumia recta, and Truncilla donaciformis hadnot been reported from the Cache River drainage. A total of 39 speciesare now recorded from the Cache River watershed.
Amblema plicata , Megalonaias nervosa, Plectomerus dombeyanus, andQuadrula pustulosa comprised 77.5% of the specimens collected, and were
Southeastern Naturalist Vol. 4, No. 3494T
able
1. C
ache
Riv
er s
peci
es r
ichn
ess,
org
aniz
ed a
lpha
beti
call
y w
ithi
n su
bfam
ilie
s, f
rom
pre
viou
s st
udie
s, a
nd f
requ
ency
of
occu
rren
ce a
nd r
elat
ive
abun
danc
eof
spe
cies
for
Mbe
d an
d m
beds
in
this
stu
dy.
Gor
don
Eco
logi
cal
Jenk
inso
nM
beds
mbe
dsM
bed
% o
fm
bed
% o
fW
heel
eret
al.
Con
sult
ants
and
(n =
14)
(n =
24)
tota
lsM
bed
tota
lsm
bed
Spe
cies
(191
4)(1
980)
(198
3)A
hlst
edt
(198
8)fr
eque
ncy
freq
uenc
y(i
ndiv
idua
ls)
tot
al(i
ndiv
idua
ls)
tota
l
Am
blem
a pl
icat
a (S
ay)
xx
xx
1422
842
23.3
633
30.6
Cyc
lona
ias
tube
rcul
ata
(Raf
ines
que)
x
Ell
ipti
o di
lata
ta (
Raf
ines
que)
xx
77
250.
714
0.7
Fus
cona
ia e
bena
(L
ea)
xx
62
160.
43
0.1
Fus
cona
ia f
lava
(R
afin
esqu
e)x
xx
1218
621.
748
2.3
Qua
drul
a m
etan
evra
(R
afin
esqu
e)x
21
3<
0.1
30.
1
Qua
drul
a no
dula
ta (
Raf
ines
que)
xx
86
250.
77
0.3
Qua
drul
a pu
stul
osa
(Lea
)x
xx
x14
2259
716
.510
45.
0
Qua
drul
a qu
adru
la (
Raf
ines
que)
xx
1319
218
6.0
834.
0
Meg
alon
aias
ner
vosa
(R
afin
esqu
e)x
x14
2458
716
.265
231
.5
Ple
ctom
erus
dom
beya
nus
(Val
enci
enne
s)x
xx
x14
2374
320
.524
711
.9
Ple
urob
ema
sint
oxia
(R
a fin
e squ
e )x
34
60.
24
0.2
Tri
togo
nia
v err
ucos
a (R
a fin
e squ
e )x
xx
x12
1611
63.
261
2.9
Uni
ome r
us t
e tra
lasm
us (
Sa y
)x
xx
Ano
dont
a su
borb
icul
ata
(Sa y
)x
Arc
ide n
s c o
nfra
gosu
s (S
a y)
xx
x6
515
0.4
130.
6
Las
mig
ona
c om
plan
ata
(Ba r
nes)
64
70.
24
0.2
Pyg
anod
on g
rand
is (
Sa y
)x
xx
x3
17
0.2
20.
1
Utt
e rba
c kia
im
bec i
llis
(S
a y)
xx
x
Ac t
inon
aias
lig
amen
tina
(L
ama r
c k)
x10
1865
1.8
101
4.8
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 495
Tab
le 1
, con
tinu
ed.
Gor
don
Eco
logi
cal
Jenk
inso
nM
beds
mbe
dsM
bed
% o
fm
bed
% o
fW
heel
eret
al.
Con
sult
ants
and
(n =
14)
(n =
24)
tota
lsM
bed
tota
lsm
bed
Spe
cies
(191
4)(1
980)
(198
3)A
hlst
edt
(198
8)fr
eque
ncy
freq
uenc
y(i
ndiv
idua
ls)
tot
al(i
ndiv
idua
ls)
tota
l
Ell
ipsa
ria
line
olat
a (R
afin
esqu
e)2
04
0.1
Lam
psil
is c
ardi
um (
Raf
ines
que)
xx
01
10.
1
Lam
psil
is h
ydia
na (
Lea
)x
x
Lam
psil
is s
iliq
uoid
ea (
Bar
nes)
x
Lam
psil
is t
eres
(R
afin
esqu
e)x
xx
x1
113
0.3
10.
1
Lep
tode
a fr
agil
is (
Raf
ines
que)
xx
xx
1211
802.
224
1.2
Lig
umia
rec
ta (
Lam
arck
)1
02
< 0
.1
Lig
umia
sub
rost
rata
(S
ay)
xx
Obl
iqua
ria
refl
exa
(Raf
ines
que)
xx
x11
973
219
0.9
Obo
vari
a ja
c kso
nian
a (F
rie n
soh)
x
Obo
vari
a ol
ivar
ia (
Ra f
ine s
que )
xx
Pot
amil
us o
hie n
sis
(Ra f
ine s
que )
xx
01
20.
1
Pot
amil
us p
urpu
ratu
s (L
ama r
c k)
xx
xx
49
90.
214
0.7
Tox
olas
ma
liv i
dus
(Ra f
ine s
que )
x
Tox
olas
ma
parv
us (
Ba r
nes)
xx
x
Tox
olas
ma
tex a
sens
is (
Le a
)x
x
Tru
ncil
la d
onac
ifor
mis
(L
e a)
30
3<
0.1
Tru
ncil
la t
runc
ata
(Ra f
ine s
que )
xx
x10
1310
02.
828
1.4
Vil
losa
lie
nosa
(C
onra
d)x
x
Tot
a l19
1827
1924
2336
1810
0.0
2068
100.
0
Southeastern Naturalist Vol. 4, No. 3496T
able
2.
Cac
he R
iver
Mbe
d st
ratu
m a
rea,
str
atum
den
sity
, an
d po
pula
tion
and
com
mun
ity
num
eric
al s
tand
ing
crop
est
imat
es (
± 9
5% c
onfi
denc
e in
terv
al [
CI]
),or
gani
zed
alph
abet
ical
ly w
ithi
n su
bfam
ilie
s.
1M4M
7M8M
12M
23M
30M
31M
33M
34M
35M
36M
37M
38M
Str
atum
1 a
rea
(m2 )
900
8000
560
600
426
460
500
350
200
250
750
360
500
800
Str
atum
2 a
rea
(m2 )
560
600
340
250
400
300
315
Str
atum
3 a
rea
(m2 )
440
Str
atum
4 a
rea
(m2 )
1860
Str
atum
1 d
ensi
ty/m
211
.613
.324
.815
.823
.819
.811
.023
.75.
712
.826
.517
.044
.118
.2
(S
D)
(8.
3)(7
.7)
(11.
2)(7
.9)
(8.4
)(1
5.5)
(6.0
)(1
1.7)
(4.2
)(6
.2)
(12.
6)(8
.2)
(34.
8)(1
0.5)
Str
atum
2 d
ensi
ty /
m2
44.4
14.5
23.5
19.5
6.4
33.8
12.6
(
SD
)(1
0.2)
(7.3
)(1
4.8)
(4.0
)(5
.1)
(14.
4)(3
.0)
Str
atum
3 d
ensi
ty/m
272
.3
(S
D)
(23.
0)S
trat
um 4
den
sity
/m2
27.7
(
SD
)(1
8.2)
Pop
ulat
ion
esti
mat
e ±
95%
CI
by t
axon
Am
blem
a pl
icat
a17
1014
,720
57,0
0313
8063
8229
3523
9841
6287
623
7534
5029
9719
0018
40±
184
3±
471
9±
32,
391
± 1
580
± 2
055
± 2
592
± 1
026
± 1
804
± 7
14±
126
4±
199
2±
140
0±
125
6±
167
4E
llip
tio
dila
tata
2236
120
207
7757
7280
± 1
381
± 1
80±
305
± 1
72±
126
± 1
59 ±
180
Fus
c ona
ia e
bena
672
7750
150
5040
0±
670
± 1
72±
110
± 2
38±
112
± 7
21F
usc o
naia
fla
v a30
2412
031
317
026
657
113
150
189
250
640
± 6
67±
180
± 5
02±
216
± 2
67±
126
± 1
66±
238
± 1
71±
296
± 5
22M
egal
onai
as n
e rv o
sa11
7048
0038
0530
60 7
2604
2628
1279
1683
1200
4113
6300
3015
5550
2480
± 1
162
± 2
849
± 1
518
± 2
08±
185
5±
148
9±
621
± 1
169
± 9
06±
235
7±
473
8±
122
2±
417
7±
124
0P
lec t
ome r
us d
ombe
y anu
s36
0037
,120
9677
2940
3226
6898
5311
711
453
0017
2512
7810
,000
1760
± 3
669
± 1
0,53
1±
268
4±
254
8±
301
7±
719
9±
119
± 1
63±
163
± 2
899
± 1
326
± 1
610
± 9
43±
187
1P
leur
obem
a si
ntox
ia50
720
772
± 6
31±
730
± 1
59Q
uadr
ula
me t
ane v
ra16
916
0±
346
± 2
39Q
uadr
ula
nodu
lata
1920
558
5362
5772
100
± 1
791
± 6
00±
119
± 1
38±
126
± 1
59±
158
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 497
Tab
le 2
, con
tinu
ed.
1M4M
7M8M
12M
23M
30M
31M
33M
34M
35M
36M
37M
38M
Qua
drul
a pu
stul
osa
630
5440
44,3
3624
032
5019
2248
029
7940
011
321
0019
81
200
3120
± 6
71±
338
8±
10,
104
± 4
03±
214
3±
168
2±
485
± 1
323
± 5
10±
166
± 1
821
± 2
19±
707
± 1
661
Qua
drul
a qu
adru
la23
4018
,880
7196
420
307
255
320
471
362
1950
396
300
160
± 1
468
± 8
803
± 3
460
± 4
92±
481
± 5
65±
318
± 3
31±
211
± 2
005
± 4
84±
336
± 2
39T
rito
goni
a ve
rruc
osa
2294
180
320
962
320
412
5727
512
0034
215
0088
0±
162
± 2
88±
236
± 3
48±
477
± 6
38±
126
± 2
61±
151
0±
397
± 9
88±
565
Arc
iden
s co
nfra
gosu
s36
016
0011
212
085
150
± 3
50±
164
5±
230
± 2
69±
188
± 2
38P
ygan
odon
gra
ndis
270
960
112
± 2
86±
144
6±
230
Act
inon
aias
lig
amen
tina
281
360
839
918
160
5810
057
665
088
0±
407
± 5
70±
145
3±
102
5±
182
± 1
29±
139
± 3
19 ±
902
± 8
66E
llip
sari
a li
neol
ata
150
160
± 2
50±
359
Lam
psil
is t
eres
4160
± 3
307
Las
mig
ona
com
plan
ata
169
107
5311
472
50±
346
± 2
36±
119
± 2
52±
159
± 1
12L
epto
dea
frag
ilis
6720
2067
420
639
170
5857
668
150
414
150
160
± 3
382
± 1
132
± 4
50±
473
± 2
16±
129
± 1
40±
510
± 2
38±
327
± 1
58±
239
Lig
umia
re c
ta16
0±
239
Obl
iqua
ria
refl
e xa
4800
2419
107
8553
996
5710
5072
950
800
± 2
327
± 6
04±
236
± 1
88 ±
119
± 8
47±
126
± 1
010
± 1
58±
112
± 5
99P
otam
ilus
pur
pura
tus
60 ±
134
107
± 2
3611
3 ±
166
333
± 3
20
Tru
ncil
la d
onac
ifor
mis
320
169
60±
658
± 3
64±
134
Tru
ncil
la t
runc
ata
4800
2974
207
480
1200
286
100
1500
150
240
± 3
554
± 1
628
± 7
30±
547
± 7
11±
500
± 2
21±
875
± 2
40±
274
CN
SC
10,0
8010
6,24
012
2,11
594
8018
,818
17,1
8255
5013
,158
3705
13,3
3819
,875
10,0
8922
,050
14,5
60±
531
6±
25,
149
± 2
4,19
4±
334
7±
528
2±
798
7±
213
2±
376
1±
190
8±
409
7±
672
2±
300
3±
123
30±
484
4
Southeastern Naturalist Vol. 4, No. 3498T
able
3. C
ache
Riv
er m
bed
stra
tum
are
a, n
umbe
r of
sam
ples
, mea
n de
nsit
y, s
tand
ard
devi
atio
n, a
nd i
ndiv
idua
l sp
ecie
s po
pula
tion
den
siti
es.
2m3m
5m6m
9m10
m11
m13
m14
m15
m16
m17
m18
m19
m20
m21
m22
m24
m25
m26
m27
m28
m29
m32
m
Str
atum
1 a
rea
(m2 )
200
450
400
400
246
180
130
425
240
164
140
320
435
240
300
460
480
300
1000
200
250
305
400
150
Sam
ples
55
55
55
55
55
55
55
55
55
55
55
55
Mea
n de
nsit
y (
# /m
2 )11
.27.
626
.822
.410
.417
.614
.634
.823
.620
.011
.810
.822
.618
.015
.815
.811
.820
.414
.025
.011
.419
.618
.47.
8
SD
3.4
6.5
8.6
17.3
6.9
7.3
7.8
43.1
7.4
18.2
6.2
8.9
12.5
6.8
7.2
6.3
4.3
9.1
9.1
14.2
7.4
N/A
N/A
3.3
Pop
ulat
ion
dens
itie
s (#
/m2 )
by
taxo
n
Act
inon
aias
lig
amen
tina
0.2
0.4
1.2
2.2
0.2
0.2
0.2
1.6
5.2
0.4
0.2
1.0
0.2
0.6
1.0
2.4
0.4
2.6
Am
blem
a pl
icat
a3.
63.
211
.24.
26.
84.
42.
613
.28.
08.
44.
48.
66.
23.
81.
83.
210
.84.
06.
64.
05.
81.
8E
llip
tio
dila
tata
0.4
0.6
0.2
0.2
0.4
0.2
0.4
Fus
cona
ia e
bena
0.2
0.4
Fus
cona
ia f
lava
0.2
0.4
0.8
0.2
0.6
0.6
0.2
0.4
0.4
0.4
0.4
0.8
1.2
0.2
1.0
0.6
1.0
0.2
Meg
alon
aias
ner
vosa
0.2
2.2
4.4
0.4
0.6
6.0
4.8
25.0
2.2
4.6
0.8
0.4
3.2
10.4
8.0
6.0
5.4
1.2
4.4
8.0
5.2
11.4
11.2
3.4
Ple
ctom
erus
dom
beya
nus
4.6
1.4
14.6
1.2
0.6
0.4
0.6
4.6
1.4
4.2
0.6
2.0
0.8
1.6
1.8
1.2
0.8
0.6
0.8
1.0
1.2
2.6
0.2
Ple
urob
ema
sint
oxia
0.2
0.2
0.2
0.2
Qua
drul
a m
etan
evra
0.6
Qua
drul
a no
dula
ta0.
40.
20.
20.
20.
20.
2Q
uadr
ula
pust
ulos
a0.
80.
20.
63.
00.
40.
20.
42.
22.
20.
81.
00.
40.
20.
81.
01.
01.
81.
01.
80.
40.
40.
2Q
uadr
ula
quad
rula
1.6
1.2
0.8
1.0
0.8
1.0
0.6
0.4
0.6
1.4
0.4
0.4
0.8
1.8
1.0
0.4
0.4
0.2
1.8
Tri
togo
nia
v err
ucos
a0.
20.
20.
22.
20.
80.
40.
22.
40.
40.
80.
22.
20.
21.
40.
20.
2A
rcid
e ns
c onf
rago
sus
0.2
0.2
1.6
0.2
0.4
Las
mig
ona
c om
plan
ata
0.2
0.2
0.2
0.2
Pyg
anod
on g
rand
is0.
20.
2L
amps
ilis
car
dium
0.2
Lam
psil
is t
e re s
0.2
Lep
tode
a fr
agil
is1.
00.
40.
20.
40.
60.
80.
40.
20.
20.
40.
2O
bliq
uari
a re
fle x
a0.
21.
20.
40.
40.
20.
20.
20.
20.
2P
otam
ilus
ohi
e nsi
s0.
2P
otam
ilus
pur
pura
tus
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.8
Tru
ncil
la t
runc
ata
1.2
0.8
0.8
0.2
0.2
0.4
0.4
0.2
0.2
0.4
0.2
0.2
0.2
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 499
widespread with each species occurring in at least 36 of 38 sites sampled. Atthe other extreme, Lampsilis cardium, Ligumia recta, and Potamilusohiensis occurred at only one site, and they were represented by one, two,and three specimens, respectively.
Major beds. A total of 170 1-m2 quadrats from 14 Mbeds yielded 3621mussels. Mbeds ranged in size from 500–3420 m2 and were sampled by10–25 1-m2 quadrats representing 0.3–2.0% of the bed area (Table 2).Sampling located 7–18 species. Mean species richness per m2 rangedfrom 3.1–7.5.
Mean densities ranged from 6.2 to 44.1 mussels/m2 with an overall meanof 20.4 mussels/m2 (SD ± 10.4) (Table 2). A total of 24 species was identi-fied with richness ranging from 7–18. Mbeds possessed a variety of sub-strates including silt, sand, soft and hard clay, gravel, and gastropod shells.
Total number of mussels, CNSC, estimates ranged from 3705 ± 1908 to122,115 ± 24,194 at sites 33M and 7M, respectively (Table 2). Amblemaplicata, M. nervosa, P. dombeyanus, and Q. pustulosa, were present in all14 Mbeds. Amblema plicata was dominant in 3 of 14 beds and comprised23.3% of all mussels sampled in Mbeds (Table 2). Plectomerusdombeyanus dominated 5 of 14 beds and was second in overall abundancecomprising 20.5% of all mussels sampled in Mbeds (Table 2). Quadrulapustulosa and M. nervosa comprised 16.5% and 16.2% of total musselssampled in Mbeds, respectively.
Minor beds. A total of 120 1-m2 quadrats containing 2068 individualmussels was sampled from the 24 mbeds. Bed area varied from 130–1000 m2
,
and a variety of substrates were present including silt, sand, and soft andhard clay. Mean densities ranged from 7.6 to 34.8 mussels/m2 with an overallmean density of 17.2 mussels/m2. Twenty-three species were identified fromthe 24 mbeds with species richness ranging from 6–16 (Table 3).Megalonaias nervosa comprised 31.5% of the total, was the most abundantspecies sampled in mbeds, and was dominant in 13 of these beds. Amblemaplicata was a close second contributing 30.6% of the total and dominating innine beds. Plectomerus dombeyanus was most abundant in 2 beds andcontributed 11.9% of the total.
Percent harvestable and size structureThe summary of the minimum legal dimensions, count, and summary
statistics of Mbed mussel species with n ≥ 10 is located in Table 4. Threeof the species, Arcidens confragosus, Leptodea fragilis, and Truncillatruncata, are considered non-commercial species by the Arkansas Gameand Fish Commission. Within Cache River Mbeds, a mean of 33.0% ofindividuals of harvestable species were of legal harvest size, whereas themean percent legal harvestable mussels for mbeds was 38.1%.
Size frequency histograms are presented for 16 species with > 10specimens measured (Figs. 2–7). Nine species exhibited unimodal size
Southeastern Naturalist Vol. 4, No. 3500
frequency distribution. These were Actinonaias ligamentina, Amblemaplicata, Elliptio dilatata, Fusconaia flava, Obliquaria reflexa, P.dombeyanus, Quadrula nodulata, Q. pustulosa, and T. truncata. Recruit-ment was indicated by low numbers of small individuals for A. plicataand P. dombeyanus.
Figure 3. Size frequency distribution (length) of Cache River Elliptio dilatata (a),Plectomerus dombeyanus (b), and Tritigonia verrucosa (c).
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 501
Figure 4.Size fre-quency dis-t r i b u t i o n(length) ofC a c h eR i v e rA m b l e m aplicata (a),Fusconaiaebena (b),Fusconaiaflava (c),and Meg-a l o n a i a snervosa (d).
Southeastern Naturalist Vol. 4, No. 3502
Four species exhibited bimodal or multimodal size frequency distribu-tions (Figs. 3c, 4d, 5c, 7c). Tritigonia verrucosa ranged from 50–119 mm
Figure 5. Size frequency distribution (depth) of Cache River Quadrula nodulata (a),Quadrula pustulosa (b), and Quadrula quadrula (c).
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 503
in depth and had a bimodal distribution (Fig. 3c). One major mode wasfrom 90–120 mm, and a minor mode was present at 55–75 mm length.Less than 25% of T. verrucosa individuals were of legal harvest size (Table 4).
Figure 6. Size frequency distribution (depth) of Cache River Arcidens confragosus (a).
Table 4. Minimum legal dimensions, percent legal harvestable, and median sizestructure (with minimum and maximum sizes) of Cache River Mbed mussel spe-cies, organized alphabetically within subfamilies, with counts ≥ 10. NC = nocommercial value.
MinimumSize harvest Percent Median size
Species parameter size (mm) harvestable (min–max ) (mm)
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 505
The size frequency distribution of Cache River M. nervosa indicatedthree distinct cohorts (Fig. 4d). About 60% of the 535 Mbed M. nervosaindividuals were of legal harvest size (Table 4). Quadrula quadrula sizedistribution was bimodal with a depth range from 22–94 mm (Fig. 5c).Very low numbers of Q. quadrula were of legal harvest size (Table 4).Leptodea fragilis exhibited evidence of multiple recruitment cohortswithin Cache River beds (Fig. 7c) and L. fragilis is not harvested com-mercially in Arkansas.
Discussion
LocationPhysical habitat is often considered a template for potential distribution
of aquatic organisms (Allan 1995). In this study, we did not measure physi-cal characteristics in each of the different habitats, so we can only speculateon the factors that may influence the distribution of mussels in differenthabitat. Our observation of mussel beds being located below a run in theoutside bend side of the lateral scour pool into the glide area may representan area that is the most stable during high flow events compared to moreunstable and aggregating riffle areas, as pools are areas of convergent flowand bed scour whereas riffles are areas of divergent flow and aggregation ofbedload (Frissell et al. 1986).
Bed and species distributionEcological Consultants (1983) reported the mussel fauna of the Cache
River as a few refugial populations scattered along the river. However, oursurvey indicates the Cache River has a significant mussel fauna similar toother streams in the Mississippi Delta in terms of bed density (beds/km),density (individuals/m2), CNSC, and species richness. For example, meandensity of both major and total beds was much higher in the Cache Riverthan other Delta streams like the St. Francis, White, and Black rivers (Chris-tian 1995, Posey 1997, Rust 1993). The mean density for Cache River Mbedswas similar to Mbeds in the Black (Rust 1993), White (Christian 1995), andSt. Francis (Posey 1997) rivers. Mean CNSC for Cache River Mbeds was27,589, whereas mean St. Francis, Black, and White river CNSCs were23,664, 31,287, and 45,600, respectively (Christian 1995, Posey 1997, Rust1993). Average species richness/bed in the Cache was lower than in theWhite, Black, and St. Francis beds. However, Cache River average bed areawas smaller than the Black and White river average bed areas, while beinglarger than the average St. Francis River mussel bed (Christian 1995, Posey1997, Rust 1993). All of these rivers are deltaic blackwater streams innortheast Arkansas with similar physical and chemical characteristics,
Figure 7 (opposite page). Size frequency distribution (length) of Cache RiverActinonaias ligamentina (a), Lampsilis teres (b), and Leptodea fragilis (c).
Southeastern Naturalist Vol. 4, No. 3506
including substrates, thereby potentially providing similar habitats andniches for biological interactions.
Although listing new species was not the primary objective of this study,4 species were recorded for the first time from the Cache River drainage.Thirty-nine species are now known to have occurred in the drainage histori-cally. Two other taxa, Fusconaia selecta Wheeler and Lampsilis clarkiana(I. Lea), have been reported from the Cache River (Johnson 1980 andEcological Consultants 1984), but we have not included them because theyare now considered ecophenotypes or junior synonyms (Turgeon et al.1998). We believe more rigorous phylogenetic analyses (e.g., Lydeard andRoe 1998) are required to determine the taxonomic status of these problem-atic forms in this drainage.
The difference in number of species identified in this study versus otherArkansas Delta streams may be explained by the relationship of speciesrichness as a function of drainage area and fish richness as reported byWatters (1992, 1993) for the Ohio River drainage. As drainage area in-creases within a river system, the number of fish species increases. TheBlack River drains 22,165 km2 (Rust 1993), whereas the Cache River onlydrains 5227 km2. This may be one of the reasons that the Black Rivercontains 50 species of mussels, with the Arkansas portion supporting 34species (Oesch 1984, Rust 1993), whereas the Cache River historicallysupported only 39 species, with 26 species in this study.
Habitat preference may also provide insight into the large populations ofcertain species in the Cache River. Amblema plicata and Q. pustulosa arewidespread species that inhabit a variety of substrates and river conditions.However, P. dombeyanus and M. nervosa, two other widespread species,prefer more sluggish waters (Oesch 1984), which is a condition typical of theCache River.
Mauney (1974) identified the presence of many fish hosts for three ofthe four most abundant mussel species in this region. These included:Pomoxis annularis Rafinesque, P. nigromaculatus Lesueur, Lepomismacrochirus Rafinesque,Ictalurus punctatus Rafinesque, and Micropterussalmoides Lacepede, for A. plicata; Dorosoma cepedianum Lesueur, P.annularis, P. nigromaculatus, I. punctatus, L. macrochirus, M. salmoidesand Aplodinotus grunniens Rafinesque, for M. nervosa; and I. punctatusand P. annularis for Q. pustulosa. The fish host for P. dombeyanus isunknown (Oesch 1984, Watters 1994).
The upper portion of the study area contained the majority of the beds,but was less than half of the area surveyed. This region consisted ofrelatively homogeneous habitat types and bed substrates that provided acontinuous series of mussel communities. The soft substrates of these bedswere ideal habitats for P. dombeyanus and M. nervosa, which were found intheir highest abundance in Mbeds of this region.
A.D. Christian, J.L. Harris, W.R. Posey, J.F. Hockmuth, and G.L. Harp2005 507
Mbeds in the middle portion of the study area were located directlydownstream of an expanded reach of the river that resembled an impound-ment. These beds were the largest in area and estimated CNSC within theCache River study area. Minshall et al. (1985) discussed the roles thatnaturally impounded reaches may have in altering the stream conditions.These natural impoundments seem to cause increased density and speciesrichness directly below their outflow. We hypothesize that these impoundedareas may hold large amounts of organic materials (e.g., algae) or increase infish richness in these areas by supporting more lentic fish, both of whichcould possibly increase mussel abundance and richness. Furthermore, bed7M was approximately a one hour boat ride from the nearest access, whichmay decrease shelling pressure at this site, thus resulting in high density andCNSC estimates.
Two beds were located within the lower portion of the study area,which was channelized in the early 1970s. These beds were heavily siltedand contained fewer species than other beds in this river. Many of thespecies found in this region, including A. plicata, Pyganodon grandis(Say), A. confragosus, M. nervosa, P. dombeyanus, Q. pustulosa and Q.quadrula, are found in slow-moving waters with mud or silt substrates(Oesch 1984).
Mauney and Harp (1979) found that the channelized sections of theCache River were characterized by large numbers of D. cepedianum,Cyprinus carpio Linnaeus, P. annularis, and A. grunniens. Amblema plicata,P. grandis, A. confragosus, and M. nervosa all use two or more of thesefishes as hosts, which may partially explain their abundance in this region(Oesch 1984). Truncilla donaciformis, one of the new species for the CacheRiver, was collected from three beds in the middle and lower regions, andone of its fish hosts (Oesch 1984), A. grunniens, is present in the CacheRiver (Mauney and Harp 1979).
Overall, mbeds provided additional distributional, species composition,and density data that are needed to properly manage freshwater mussels.These mbeds usually mirrored the Mbeds in species percent abundance andprovided percent legal information within their perspective regions. How-ever, due to smaller sample size, species diversities may not have been asaccurately portrayed as in Mbeds.
Size frequency and percent harvestableOur size frequency distributions from Mbeds indicated distinctive size
cohorts and that most species exhibited one dominant cohort with other minorcohorts. Payne and Miller (1989) reported that in Ohio River Fusconaiaebena, dominant cohorts existed every few years. Payne and Miller (2000)related that dominant cohort to high flow events in the Ohio River.
The sampling method we used in this study was biased toward the adulthabitat. For all species, the number of younger individuals is far smaller than
Southeastern Naturalist Vol. 4, No. 3508
expected. Yeager and Cherry (1994) noted juvenile mussels deposit feedingand that juvenile mussels exhibit habitat partitioning with adults. Payne etal. (1997) noted that searching a quadrat by sight and/or feel risks missingsmall, young individuals. More accurate total substrate removal is morelabor intensive and requires more equipment to process samples in the field.However, this study indicates the feel/sight search method performed byexperienced divers can determine if recruitment is occurring.
Currently, the primary influence on percent legal harvestable musselsfor the Cache River is harvest pressure. A harvest of 209,010 kg of shellsfrom the Cache River was reported for 1990–1994 (Todd 1994). A de-crease in harvest pressure was observed during 1992 and 1993, but thepercent legal harvestable mussels observed in 1993 still reflected the in-tense harvest pressure of 1991 (Todd 1994). The high percent legallyharvestable M. nervosa, the most valuable commercial shell in the CacheRiver, is probably due to poor shell quality. Many of the valves had 2.5 cmor more of erosion near the umbo. Other species such as A. ligamentina, A.plicata, E. lineolata, F. ebena, and Q. pustulosa, had similar erosion,precluding their harvest.
The 38 major and minor mussel beds defined in this study with theirmodest species richness, and moderate to high densities, represent substan-tial mussel assemblages within the lower 68 km of the Cache River. Mostspecies had dominant cohorts and recruitment.
Acknowledgments
Funding for this project was provided by the US Fish and Wildlife Service, theUS Army Corps of Engineers, Memphis and Little Rock Districts, and the ArkansasGame and Fish Commission. We thank B. Bennett for field and technical assistancein obtaining GPS points for bed locations and R. Reed for creation of the GIS-basedmap. Field assistance was graciously provided by L. Christian and L. Thompson.This manuscript was greatly improved by comments provided by guest editor K.Brown and two anonymous reviewers.
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