stream–floodplain connectivity and fish assemblage diversity ... · to evaluate the influence...
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Stream–floodplain connectivity and fish assemblagediversity in the Champlain Valley, Vermont, U.S.A.
S. M. P. SULLIVAN* AND M. C. WATZIN
Rubenstein School of Environment and Natural Resources, Rubenstein Ecosystem ScienceLaboratory, University of Vermont, 3 College Street, Burlington, VT 05401, U.S.A.
(Received 9 September 2007, Accepted 19 January 2009)
To evaluate the influence of main channel–floodplain connectivity on fish assemblage diversity
in floodplains associated with streams and small rivers, fish assemblages and habitat character-
istics were surveyed at 24 stream reaches in the Champlain Valley of Vermont, U.S.A. Fish
assemblages differed markedly between the main channel and the floodplain. Fish assemblage
diversity was greatest at reaches that exhibited high floodplain connectivity. Whereas certain
species inhabited only main channels or floodplains, others utilized both main channel and
floodplain habitats. Both floodplain fish a-diversity and g-diversity of the entire stream corridor
were positively correlated with connectivity between the main channel and its floodplain.
Consistent with these results, species turnover (as measured by b-diversity) was negatively
correlated with floodplain connectivity. Floodplains with waterbodies characterized by a wide
range of water depths and turbidity levels exhibited high fish diversity. The results suggest that
by separating rivers from their floodplains, incision and subsequent channel widening will have
detrimental effects on multiple aspects of fish assemblage diversity across the stream–floodplain
ecosystem. # 2009 The Authors
Journal compilation # 2009 The Fisheries Society of the British Isles
Key words: a, b and g-diversity; habitat use; incision; riverine landscapes; vertical adjustment;
widening.
INTRODUCTION
Streams and rivers are complex ecosystems that often harbour considerablebiodiversity (Naiman & D�ecamps, 1997; Araujo, 2002; Ward et al., 2002).Although influenced by many factors, stream and river biodiversity is directlyrelated to ecotonal changes in stream channel and hydrological connectionsbetween the channel and its adjacent floodplain (Gregory et al., 1991;Stanford & Ward, 1993). Whereas longitudinal changes and connectivity instream ecosystems are well described (Vannote et al., 1980), hydrological con-nections and processes of sediment movement, erosion and deposition also op-erate through lateral, vertical and temporal dimensions (Ward, 1989; Amoros
*Author to whom correspondence should be addressed at present address: School of Environment
and Natural Resources, Ohio State University, 2021 Coffey Road, Columbus, OH 43210, U.S.A. Tel.: þ1
6142927314; fax: þ1 6142927342; email: [email protected]
Journal of Fish Biology (2009) 74, 1394–1418
doi:10.1111/j.1095-8649.2009.02205.x, available online at http://www.blackwell-synergy.com
1394# 2009 The Authors
Journal compilation # 2009 The Fisheries Society of the British Isles
& Bornette, 2002). The lateral connections between stream channels and theirfloodplains create landforms of unique spatial arrangements (Richards et al.,2002) and myriad lotic, semi-lotic and lentic floodplain waterbodies. Habitatheterogeneity, extent and persistence vary with channel pattern and type offloodplain system, with each unique form providing a host of habitats formany taxa in various life stages (Amoros & Bornette, 2002). These connectionsoccur in lotic ecosystems of various sizes (Ward, 1989; Malard et al., 2000;Jungwirth et al., 2002; Ward et al., 2002), but have not been well studied instream and small river ecosystems relative to larger rivers.Undisturbed channels are in a state of dynamic equilibrium, constantly
undergoing a degree of natural vertical adjustment (Knighton, 1988; Simon,1992, 1995). Anthropogenic activities, however, can severely disrupt the naturalequilibrium, initiating responses described in channel evolution models(Schumm, 1977; Schumm et al., 1984; Simon, 1992, 1995). Channelization,for example, removes natural meander patterns and increases stream power,often leading to bed incision (i.e. degradation). Once the erodable material istransported downstream, stream power is shifted outward and an incisedstream reach begins to widen, causing bank failure, loss of competence andeventually an accumulation of sediment, bed aggradation and channel widening(Schumm, 1977; Schumm et al., 1984; Simon, 1995). As the channel aggrades,channel gradient is further reduced (Simon, 1992, 1995). Channel incision oftenimpairs the quality of floodplain habitats and separates a channel from itsfloodplain creating a channel in which bankfull flows do not reach overflowstage (Gore & Shields, 1995; Toth et al., 1995, 1998). The effects of bed aggra-dation and channel widening on fish communities, especially those in smallerstream–floodplain systems, remain largely undocumented.Geomorphic processes that alter the hydrological connectivity (surface or
subsurface) between the floodplain and its channel will change the habitat com-position and complexity of the floodplain and its waterbodies. Composition,diversity, distribution and many other characteristics of riverine fish communi-ties have been linked to the habitat heterogeneity and the distribution ofpatches in both the main channel (Gorman & Karr, 1978; Deacon & Mize,1997; Sutherland et al., 2002) and the floodplain (Copp, 1989; Ward et al.,1999; Grift et al., 2001). In particular, heterogeneity in the availability of largewoody debris (Lehane et al., 2002; Zika & Peter, 2002; Giannico & Hinch,2003) and temperature gradients (Ebersole et al., 2003; Giannico & Hinch,2003) have been shown to be particularly influential on fish communities.Changes in habitat heterogeneity, quality and extent can be expected to havesignificant effects on fish community composition and diversity acrossstream–floodplain ecosystems.In Vermont, U.S.A., as in many other areas across the world, human activ-
ities have impaired stream–floodplain ecosystems (Dynesius & Nilsson, 1994;Arthington & Welcomme, 1995; Sparks, 1995; Roth et al., 1996; Wang et al.,2001; Downes et al., 2002). In Vermont, the most significant anthropogenic ef-fects have occurred as a result of agriculture and development. These effectshave led to loss of riparian vegetation, streambank erosion, hydraulic modifi-cations, floodplain encroachment and channel straightening (VTDEC, 2001).
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Many of these activities have precipitated vertical channel adjustment, includ-ing both bed incision and aggradation.The present study focused on floodplain dynamics in streams and small rivers.
Recognizing the importance of floodplain connectivity to stream corridor bio-diversity at the reach scale (Richards et al., 2002), the goal of the research was tounderstand how the hydrological connectivity between the main channel and thefloodplain is related to fish assemblage diversity of the entire stream–floodplainecosystem. To this end, how geomorphic characteristics related to patterns of fishdiversity in both main channel and adjacent floodplains, and how habitat charac-teristics of floodplain waterbodies related to fish assemblage diversity in flood-plains were investigated.
MATERIALS AND METHODS
Twenty-four stream reaches (i.e. stream segments, including adjacent floodplainwaterbodies, if present) were investigated in the Champlain Valley of Vermont (44°359 N; 73° 229 W). Stream size varied from second to fifth order and represented dis-equilibrium, equilibrium and low gradient floodplain systems (Nanson & Croke, 1992)(Table I). Geomorphic characteristics of study reaches varied greatly across sites (TableII), as did floodplain waterbody characteristics (Table III). All reaches were at least 250m in length (>�10 bank-full width, i.e. width of a stream channel between the highestbanks on either side, following Harrelson et al., 1994; Montgomery et al., 1995). Four-teen stream reaches had waterbodies in their floodplains during the sampling period; 10reaches had no floodplain waterbodies.
HABITAT SURVEYS
Except for high flow observations recorded in April and May, all habitat surveyswere conducted in June, July and August of 2002–2003. Along the length of each studyreach, habitat types were divided into main channel and floodplain habitat units basedon their position within the stream corridor (i.e. main channel habitats below bank-fullheight and floodplain habitats above bank-full height). Main channel habitat units weredesignated as pool, riffle or run based on flow pattern. In each floodplain, a thoroughsurvey of all waterbodies present (e.g. backwaters, floodplain ponds, marshes andoxbows) was conducted and their locations relative to the main channel mapped. Eachwaterbody was designated as a separate floodplain habitat unit.
Using measuring tapes and a stadia rod, the length and width of each samplinglocation was measured in order to calculate area for all sampling locations. Meandepth was also measured. Water temperature on the day of sampling was recordedonce in the morning and once in the afternoon, with three readings taken in variouslocations and depths in each habitat type. To identify vertically adjusting channels,bank-full width, flood-prone width (i.e. width of the stream corridor measured hori-zontally at a height of twice the maximum bank-full depth) and low-bank height (i.e.the height of the low bank relative to the elevation of the maximum bank-full depth)were measured. Bankfull stage was identified through observations during spring highflow and by using field indicators following regional geomorphic assessment protocols(VTDEC, 2003). From these measurements, width to depth, entrenchment and inci-sion ratios were calculated (Rosgen, 1996; VTDEC, 2003). Width to depth ratio(bank-full width divided by mean bank-full depth) is a channel relationship indepen-dent of stream size, key in understanding the distribution of available energy withinthe channel. During high flows, highly entrenched streams do not breach the channelonto the floodplain, moderately entrenched streams extend onto the floodprone areaand streams exhibiting little or no entrenchment access their floodplain at bank-fullflows; entrenchment ratio ¼ floodprone width divided by bankfull width. Incision
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
ratios (low-bank height divided by bank-full maximum depth) are considered a moresensitive measure of bed degradation than entrenchment ratios, targeting early stagedegradation. These ratio values were then used to determine if the channel was stable,incising, degrading or widening (VTDEC, 2003). Channel sinuosity was categorizedusing six categories ranging from low (<1�2) to very high (>1�5), using a combinationof field observations and orthophotos (VTDEC, 2003). Additionally, stage of channelevolution was assessed according to channel evolution models, using a combination offield indicators and incision, entrenchment and width-to-depth ratio values (Schumm,1977; Schumm et al., 1984; Simon, 1992, 1995).
All floodplains were classified as disequilibrium, equilibrium or low gradient accord-ing to Nanson & Croke’s (1992) conceptual classification of floodplains. Using this clas-sification as a guide, watershed position, valley confinement and dominant erosionaland depositional processes were used to identify floodplain type. Field indicators offloodplain characteristics (e.g. wet marks, vegetation type and extent and terrace form)at bank-full stage (i.e. height just below which water breaches banks onto floodplain)were used to determine if a reach was connected to its floodplain. Within each flood-plain, every floodplain waterbody type (e.g. habitat unit) was identified; its length,width, maximum and minimum mean depths (based on six measurements per flood-plain waterbody), as well as its distance to the nearest channel bankfull location were
TABLE I. Physical characteristics of streams and their floodplains at the 24 study reaches,Champlain Valley, Vermont, U.S.A.
Reachidentification
Streamorder*
Bank-fullwidth (m)
Geomorphiczone†
Floodplaintype‡
Allen Brook 2 10�1 Response Low gradientBeaver Brook 2 15�1 Source–transfer DisequilibriumBlack River 4 18�7 Response Low gradientBogue Brook 2 14�9 Source–transfer DisequilibriumBrowns River 4 26�8 Transfer EquilibriumFairfield River 2 10�1 Source–transfer DisequilibriumHuntington River 3 29�2 Transfer DisequilibriumLa Platte River 1 4 13�2 Transfer EquilibriumLa Platte River 2 4 18 Transfer EquilibriumLa Platte River 3 5 18�6 Response Low gradientLee River 2 9�8 Source EquilibriumLewis Creek 1 4 25 Transfer EquilibriumLewis Creek 2 4 14�65 Response Low gradientLittle Otter Creek 1 3 13 Response Low gradientLittle Otter Creek 2 3 21�2 Transfer EquilibriumMallets Creek 1 2 12 Transfer EquilibriumMallets Creek 2 2 9�9 Transfer–response EquilibriumMill Brook 2 14�1 Transfer EquilibriumMissisquoi River 3 31 Transfer EquilibriumNew Haven River 1 3 24 Transfer EquilibriumNew Haven River 2 3 51 Transfer EquilibriumRogers Brook 2 11�3 Transfer EquilibriumSouth Branch 2 6�5 Source DisequilibriumTyler Branch 2 34 Transfer Equilibrium
*Stream order based on USGS 1:24 000 topographic maps.
†According to Schumm’s (1977) zones of transport.
‡According to Nanson & Croke’s (1992) floodplain classification system.
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TABLEII.Geomorphic
characteristics
ofstudyreaches.Meander
pattern(basedonsinuosity:low
<1� 2,moderate
¼1� 2–1� 5
andhigh>1� 5).
Incision,entrenchmentandwidth-to-depth
ratiosare
measuresoffloodplain
connectivity.Channel
evolutionstage[basedonSchumm’s
(1977)andSchumm
eta
l.’s(1984)evolutionmodelsforchannelized
streams]relatesto
stageofchannel
adjustment.Floodplain
connectivity
describes
thehydrologicalconnectivitybetweenthemain
channel
andthefloodplain,asdetermined
bysupportingobservations,presence
of
floodplain
waterbodiesandmeasurements(incision,entrenchmentandwidth
todepth
ratios).Floodplain
connectivity:none,nohydrological
connectivitybetweenthemain
channel
andthefloodplain;limited,reaches
withadegreeofhydrologicalconnectivity,thatmayormaynot
havesupported
floodplain
waterbodiesduringthestudyperiod;andhigh,reaches
withactivehydrologicalconnectivitybetweenthemain
channel
andthefloodplain
thatsupported
floodplain
waterbodiesduringtheentire
studyperiod
Reach
identification
Meander
pattern
Incision
ratio
Entrenchment
ratio
Width-to-
depth
ratio
Channel
evolutionstage
Floodplain
connectivity
Floodplain
waterbodies
present
Allen
Brook
Veryhigh
1� 6
15� 0
17� 0
IIIwidening
(earlystage)
None
No
Beaver
Brook
Moderate–high
1� 2
1� 3
30� 8
IVstabilizing
High
Yes
Black
River
High
1� 4
4� 3
50� 4
IIincision
(late
stage)
None
No
BogueBrook
Low
2� 3
1� 2
62� 1
IVstabilizing
None
No
BrownsRiver
Moderate
2� 3
5� 7
68� 7
IIIwidening
Lim
ited
No
FairfieldRiver
Low–moderate
2� 4
15� 0
19� 0
IIIwidening
(earlystage)
None
No
HuntingtonRiver
Low
2� 0
1� 1
53� 1
IVstabilizing
None
No
LaPlatteRiver
1Moderate–high
1� 3
11� 5
33� 0
Vstable
Lim
ited
Yes
LaPlatteRiver
2Moderate
1� 6
1� 9
36� 0
IVstabilizing
Lim
ited
Yes
LaPlatteRiver
3High
1� 0
1� 2
17� 7
Istable
High
Yes
Lee
River
Low–moderate
1� 3
1� 5
42� 6
IIIwidening
(earlystage)
None
No
Lew
isCreek
1Low
1� 7
1� 6
1� 7
Vstable
Lim
ited
No
Lew
isCreek
2Moderate–high
1� 5
10� 3
17� 3
IIincision
None
No
LittleOtter
Creek
1Moderate–high
1� 1
11� 7
15� 3
Istable
High
Yes
LittleOtter
Creek
2Low
1� 6
2� 9
41� 6
IVstable
Lim
ited
MalletsCreek
1High
1� 3
3� 0
22� 9
Istable
Lim
ited
Yes
1398 S . M. P . SULLIVAN AND M. C. WATZIN
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEII.Continued
Reach
identification
Meander
pattern
Incision
ratio
Entrenchment
ratio
Width-to-
depth
ratio
Channel
evolutionstage
Floodplain
connectivity
Floodplain
waterbodies
present
MalletsCreek
2Moderate
1� 6
2� 4
18� 7
Istable
High
Yes
MillBrook
Moderate
1� 2
8� 5
23� 5
IVstabilizing
High
Yes
MissisquoiRiver
Moderate
1� 7
3� 7
67� 4
IIIwidening
Lim
ited
Yes
New
Haven
River
1Moderate
2� 0
6� 3
80� 0
IIIwidening
Lim
ited
Yes
New
Haven
River
2Moderate
1� 5
3� 0
101� 0
IIIwidening
Lim
ited
Yes
RogersBrook
Moderate
1� 8
2� 6
22� 6
Vstable
High
Yes
South
Branch
Low–moderate
1� 8
4� 5
28� 3
IIIwidening
(earlystage)
Lim
ited
Yes
TylerBranch
Moderate–high
1� 5
4� 5
34� 0
IIIwidening
Lim
ited
Yes
FLOODPLAIN CONNECTIVITY AND FISH DIVERSITY 1399
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEIII.
Characteristics
offloodplainsatstudyreaches
withfloodplain
connectivity.Tem
perature
anddepth
ranges
representtheranges
of
thesefactors
across
allwaterbodies.Number
oflargewoodydebris(LWD)isthecumulativenumber
forallwaterbodies
Reach
identification
Number
of
floodplain
waterbodies
Floodplain
waterbodies
types
Tem
perature
range(°
C)
Meander
pattern
represented
by
waterbodies
Manner
of
hydrological
connectivity
Degrees
of
turbidity
Detritus
Number
of
LWD
Substrata
(>20%
)
Depth
range
(meanminim
um
depth–
mean
maxim
um
depth)(m
)
Total
water
surface
area
(m2)
Beaver
Brook1
6Floodchute,
pool,side
arm
18� 0�21� 0
None-low
Upstream
floodstage
Low–
moderateLow–
moderate
8Gravel,boul-
der,silt–clay,
organic,sand,
cobble
0� 03�0� 49
468� 7
LaPlatte
River
14
Marsh,
pond,
pool
21� 5�26� 0
None
Downstream
floodstage
and
adjacentto
channel
Low–high
Low–
moderate
15
Organic
0� 10�1� 85
1316� 3
LaPlatte
River
24
Pool,marsh,
sidearm
18� 0�31� 0
None-low
Downstream
floodstage,
upstream,
downstream
andadjacent
to channel
Veryhigh
Low–low
18
Cobble,silt–
clay,sand
0� 08�0� 35
666� 1
LaPlatte
River
32
Sidearm
,pool
21� 5�24� 0
None-
moderate
Downstream
and
floodstage
Low–
moderateModerate–
high
14
Organic
0� 05�0� 45
151� 0
LittleOtter
Creek
18
Marsh,
pool
23� 0�27� 0
None-
morderateFloodstageand
adjacentto
channel
Low–high
Low–high
17
Silt–clay,
organic
0� 05�0� 77
8017� 3
Mallets
Creek
13
Sidearm
,pool
22� 5�25� 0
None-low
Downstream
and
floodstage
Low–high
Low–
moderate
8Cobble,
organic,sand,
silt–clay,gravel
0� 06�0� 48
93� 1
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEIII.
Continued
Reach
identification
Number
of
floodplain
waterbodies
Floodplain
waterbodies
types
Tem
perature
range(°
C)
Meander
pattern
represented
by
waterbodies
Manner
of
hydrological
connectivity
Degrees
of
turbidity
Detritus
Number
of
LWD
Substrata
(>20%
)
Depth
range
(meanminim
um
depth–
mean
maxim
um
depth)(m
)
Total
water
surface
area
(m2)
Mallets
Creek
21
Pool
23� 0�24� 0
Low
Floodstageand
hillside
acquifer
Low
Moderate
13
Organic
0� 08�0� 20
42� 6
MillBrook
3Pond,
split
channel
15� 5�23� 0
None-
moderate
Floodstage,
spring,
upstream
and
downstream
floodstage
Low–
moderateLow–high
38
Organic,
sand
0� 11�0� 90
1572� 5
Missisquoi
River
2Sidearm
,flood
chute
20� 0�22� 5
Low–
moderate
Downstream
and
upstream
floodstage
Low–
moderateLow–
moderate
16
Cobble,
sand,
gravel
0� 10�0� 40
893� 8
New Haven
13
Pool,
sidearm
,flood
chute
17� 0�30� 0
None-
moderate
Floodstage
and
downstream
floodstage
Low–high
Low–high
27
Sand,
organic,
gravel
0� 025�0� 45
601� 2
New Haven
24
Pool
sidearm
,flood
chute,
marsh
23� 0�27� 0
Low–high
Floodstage,
upstream
and
floodstage
Low–
moderateModerate–
high
16
Silt–clay,
gravel,
sand,
organic
0� 05�0� 60
328� 7
Rogers
Brook
2Pool,
oxbow
16� 0�26� 0
None-
high
Floodstageand
downstream
Low–
moderateHigh
10
Sand,
organic
0� 02�0� 50
405� 0
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEIII.
Continued
Reach
identification
Number
of
floodplain
waterbodies
Floodplain
waterbodies
types
Tem
perature
range(°
C)
Meander
pattern
represented
by
waterbodies
Manner
of
hydrological
connectivity
Degrees
of
turbidity
Detritus
Number
of
LWD
Substrata
(>20%
)
Depth
range
(meanminim
um
depth–
mean
maxim
um
depth)(m
)
Total
water
surface
area
(m2)
Sough
Branch
7Pond,
sidearm
,flood
chute
11� 0�23� 0
None-
moderate
Floodstage,
upstream
floodstage
and
downstream
floodstage
Low
Low–high
30
Sand,
organic,
silt–clay,
cobble,
gravel
0� 03�0� 22
876� 6
Tyler
Branch
1Pool
19� 0�21� 0
Low
Floodstage
Low
Low
4Sand
0� 02�0� 18
15� 0
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measured. Each waterbody’s sinuosity (low, moderate and high, based on meander pat-tern), connectivity to the main channel (i.e. upstream, downstream and floodstage), tur-bidity (i.e. low and moderate and high, based on visual estimates at four haphazardlyselected locations) and amount of detritus (i.e. low, moderate and high, based on driedmass of collections at four haphazardly selected locations) were described. Number oflarge woody debris pieces >0�1 � 1�0 m (Montgomery et al., 1995) were counted. Inorder to assess relative coverage and dominant substratum type, all substrata (i.e.organic, clay and silt, sand, gravel, cobble and boulder) that composed >20% of theoverall substratum were visually estimated.
FISH SURVEYS
Concurrent fish surveys along the main channel of each reach were conducted. Thefish assemblage was sampled at three to four locations, representing c. 15% of the wet-ted area at each reach. This sampling percentage has been shown to be effective in cap-turing all but the rarest species (Sullivan et al., 2006). After completing habitat maps ofthe main channel of each reach, sampling locations were selected that proportionallyrepresented major flow habitats (e.g. pool, riffle and run) present in the reach as a whole(VTDEC, 2004). The limitations of electroshocking in habitats with varying habitatstructure, turbidity and flow (Rodgers et al., 1992; Bayley & Dowling, 1993) compelledsampling to be made using a 1�22 � 12�19 m bag seine with 3�175 mm mesh weightedwith lead lines. A two-pass depletion method (Zippin, 1958) was used to collect themajority of fishes within the habitat unit (Sullivan et al., 2006). Floodplain fish assemb-lages were sampled separately from those of the main channel. Using the two-passdepletion method, the fish assemblage in each floodplain waterbody was sampled inde-pendently using the seine. All streams were wadeable with no water depths greater thancould be sampled by the seine; fishes from all depths present at a sampling locationwere represented by collections. Fishes from each collection were identified to species.Young-of-year fishes were excluded from the analysis. During processing, fishes wereheld in buckets and holding tanks with portable aerators. Once all fishes had been iden-tified, they were released at the site of capture.
Floodplain area across the reaches was not controlled for, despite marked differen-ces. The principal goal of the study was to relate geomorphic characteristics to hydro-logical connectivity and subsequently to patterns of fish assemblages. Hydrologicalconnectivity may influence fish communities by leading to the creation of uniquehabitat types as well as providing additional habitable area. Controlling for area wouldhave obscured this potential effect. Analytically, a paired design was used to explorefloodplain and channel comparisons, which helped control for additional characteristicsthat may vary across study reaches (e.g. area). Surface area of floodplain waterbodieswas considered in the ordination analyses – relating floodplain waterbody characteris-tics to fish assemblage diversity.
DATA ANALYSIS
Fish diversity within the stream–floodplain ecosystemFollowing Whittaker (1960, 1972), three components of fish species diversity were
distinguished: alpha (a), beta (b) and gamma (g). The use of all three components yieldsa more comprehensive look at spatial patterns of diversity across stream–floodplainecosystems (Cody, 1975; Gaston & Williams, 1996).
a-diversity corresponds to local or within-community diversity (Meffe et al., 2002). Inorder to develop a more comprehensive estimate of a-diversity, species richness (S), as wellas Shannon–Weaver’s H9 (Shannon & Weaver, 1963) and Simpson’s D�1 (Simpson, 1949)diversity indices were calculated. S is a widely used, straightforward and easily interpret-able measure of the number of species found in a given habitat type but is limited in itssimplicity. It is a static representation of diversity that provides little insight into the eco-logical mechanisms that govern biodiversity. S is also insensitive to the ecological
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placement of species (including rare species) (Hayek, 1994). Diversity indices provide addi-tional information about community composition, considering the relative abundances ofdifferent species. H9 is informational, accounting for overall number and evenness, whileD�1 is a dominance index, favouring common species in assigning masses.
These three diversity measures were used to estimate fish assemblage a-diversity inboth main channels and floodplains. These data were aggregated from all main channelsampling locations before calculating main channel S, H9 and D�1, and the same pro-tocol was followed with floodplain waterbodies for floodplain data.
b-diversity reflects between-community diversity and is related to the rate of spatialturnover of species. It is a measure of the difference in species composition betweenlocal assemblages along an environmental gradient, in the present case a two-habitatgradient (i.e. main channel to the floodplain). Although many measures have been pro-posed to assess b-diversity, Whittaker’s (1960, 1972) original measure continues to beused most frequently (Koleff et al., 2003), fulfilling the greatest number of criteria withthe least restrictions (Magurran, 1988). Whittaker’s (1960, 1972) beta (bW) is repre-sented by the formula: S a�1 � 1. This formula was used to assess the total numberof species that was unique to each of the habitat types (i.e. turnover rate betweenthe main channel and the adjacent floodplain waterbodies), aggregating sub-samplesbefore calculating diversity.
g-diversity incorporates the total diversity of a larger area, in the present study, theentire stream–floodplain ecosystem, and is a function of the within-habitat andbetween-habitat species diversity. g-diversity was measured by including all fish specieswithin the main channel and all associated floodplain waterbody habitats, calculating S,H9 and D�1 for fish assemblages. At some stream reaches, there were no active flood-plains (i.e. floodplains that did not support waterbodies during the study period).g-diversity, however, is a comprehensive measure of diversity across the entire ecosys-tem regardless of the condition of the component habitats. Therefore, the calculationsof g included all stream reaches irrespective of the condition of the floodplain.
Because fish diversity data were not normally distributed, non-parametric techniqueswere used to analyse the contribution of floodplain fish diversity to the total diversity ofthe fish assemblage of the stream–floodplain ecosystem (i.e. floodplain and main chan-nel). First, Wilcoxon signed-rank was used to test for potential differences in medianfish diversity values (as measured by S, H9 and D�1) between the main channel (a)and the entire stream–floodplain ecosystem (g) at each reach. Kruskal–Wallis was thenused to test for potential differences in (1) fish b-diversity among the three degrees offloodplain connectivity (i.e. full, limited and none) and (2) b-diversity between thosefloodplains that supported waterbodies and those that did not.
Fish diversity and floodplain characteristicsAfter logarithmic [ln (x þ 1)] and square (x2) transformations to normalize data and
eliminate heteroscedasticity (Snedecor & Cochran, 1967; Zar, 1984), principle compo-nent analysis (PCA) was performed on the environmental variables measured in flood-plain waterbodies that the authors had selected a priori as candidate predictors of fishassemblage diversity. For variables that represented a range (e.g. depth and tempera-ture), the average minimum was subtracted from the average maximum. Categoricaldata (e.g. floodplain waterbody type and turbidity) were coded as ordinal variablesfor input into the PCA (Hair et al., 1995; Vaughn & Ormerod, 2005). A sufficient num-ber of PCA axes to account for 80% of the total variance was retained (Rencher, 1995).Linear regression models were then built. The retained PCA axes were used as indepen-dent variables; each of the three measures of a-diversity, as well as fish assemblageb-diversity, were used as dependent variables. Variable additions proceeded until theF statistic for the change at the step fell below the P > 0�05 significance threshold.
Fish diversity and floodplain connectivityIncision, entrenchment and width to depth ratios were used to quantify connectivity of
the main channel to its floodplain. Because these data were non-parametric, Spearman’s
1404 S. M. P . SULLIVAN AND M. C. WATZIN
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
r was used to look for potential correlations between each of these three measurementand measurements of fish assemblage diversity across all 24 reaches. Because largerdrainage areas tend to support greater fish species richness (Angermeier & Schlosser,1989; Matthews & Robinson, 1998), potential correlations between stream sizeand watershed position (using bank-full width and stream order as proxies), and fishdiversity were also explored in order to test for potential effects of longitudinalgradient.
All statistical analyses were performed using JMP� 5.0 Statistical Discovery Software(SAS Institute; www.sas.com). Potential outliers were tested by using Cook’s D (Kleinbaumet al., 1998), however, none were found. All data were tested at the a ¼ 0�05 level.
RESULTS
FISH DIVERSITY WITHIN THE STREAM–FLOODPLAINECOSYSTEM
Median fish a-diversity of the main channel and median fish g-diversity (i.e.fish assemblage diversity of the main channel and floodplain) were significantlydifferent for all three measures: S (d.f. ¼ 23, P < 0�05), H9 (d.f. ¼ 23, P <0�001) and D�1 (d.f. ¼ 23, P < 0�001). At 10 of 24 study reaches, species(including some rare and uncommon ones) were found in the floodplain butnot in the main channel (Table IV). Kruskal–Wallis tests indicated that therewas a significant difference in fish b values among the three levels of floodplainconnectivity, with high connectivity exhibiting the lowest b value (0�496), fol-lowed by limited connectivity (0�544) and no connectivity (1�000; d.f. ¼ 2,P < 0�01). Because some reaches with limited floodplain connectivity did notsustain floodplain waterbodies during the course of the study, differences inb values were also tested between reaches with and those without floodplainwaterbodies. At reaches with floodplain waterbodies, median fish b-diversitywas 0�426. In contrast, reaches that had no floodplain waterbodies exhibitedb-diversity values of 1�000 (d.f. ¼ 1, P < 0�001).
FISH DIVERSITY AND FLOODPLAIN WATERBODYCHARACTERISTICS
PCA of the 12 physical floodplain waterbody measurements (n ¼ 14) identi-fied four axes of variation with eigenvalues that cumulatively accounted for>80% of the total variance (Table V). Because the second principal component(PC2, explaining 19�7% of the total variance) was the only significant predictorof floodplain fish a-diversity, only simple regression models were built (TableVI). PC2 had a mixture of positive and negative loadings (Table V). Depthrange shared the greatest amount of variance with the axis (r2 ¼ 0�55), althoughfloodplain type, number of substratum types �20% represented and number ofturbidity levels all exhibited r2 � 0�44. The strong positive association of this axiswith all three a-diversity measures of fish floodplain assemblages (accounting for38, 42 and 51% of the variance observed in S, H9 and D�1, respectively) indi-cated that depth range represented the strongest association with fish floodplaina-diversity.
FLOODPLAIN CONNECTIVITY AND FISH DIVERSITY 1405
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEIV
.Fishassem
blagediversity
measure’sspeciesrichness(s),Shannon–Weaver’s
H9andSim
pson(D
�1)indices
from
themain
channel
(MC)andthefloodplain
(FP)atallstudyreaches,alongwithfish
speciesuniqueto
thefloodplain
Reach
IDSMC
aH9MC
aD�1MC
SFP
aH9FP
aD�1FP
bg(S)
g(H
9)g(D
�1)
Fishspeciesfoundonly
inFP
Allen
Brook
71� 59
3� 905
0—
—1� 000
71� 59
3� 905
None
Beaver
Brook
10
1� 52
3� 237
50� 91
2� 07
0� 333
10
1� 51
3� 791
None
Black
Brook
10
1� 54
3� 408
0—
—1� 000
10
1� 54
3� 408
None
BogueBrook
91� 58
3� 387
0—
—1� 000
91� 58
3� 387
None
BrownsRiver
40� 77
1� 733
0—
—1� 000
40� 77
1� 733
None
FairfieldRiver
81� 58
4� 111
0—
—1� 000
81� 58
4� 111
None
Huntington
River
91� 35
2� 938
0—
—1� 000
91� 35
2� 938
None
LaPlatteRiver
114
1� 84
4� 088
15
2� 24
7� 21
0� 310
19
2� 33
6� 935
Hyb
og
na
thu
sh
an
kin
son
i,‡
Pho
xin
us
neo
ga
eus,
No
tem
igo
nu
scr
yso
lece
uca
s,P
ho
xin
useo
sand
Sca
rdin
ius
eryt
hro
ph
tha
lmu
s§LaPlatteRiver
215
1� 80
3� 519
51� 57
5� 76
0� 500
15
1� 93
3� 953
None
LaPlatteRiver
310
1� 72
4� 249
41� 24
5� 00
0� 857
13
1� 88
4� 697
Am
iaca
lva
,A
mei
uru
sn
ebu
losu
sand
Cyp
rin
us
carp
io§
Lee
River
51� 24
2� 523
0—
—1� 000
51� 24
2� 523
None
Lew
isCreek
18
1� 65
4� 235
0—
—1� 000
81� 65
4� 235
None
Lew
isCreek
28
1� 48
3� 943
0—
—1� 000
81� 48
3� 943
None
LittleOtter
Creek
111
2� 13
8� 267
13
2� 21
8� 14
0� 333
16
2� 41
10� 020
Lep
om
ism
acr
och
iru
s,P
imep
ha
les
no
tatu
s,R
hin
ich
thys
cata
ract
ae,
Lep
om
isg
ibb
osu
sand
Lep
om
ism
icro
lop
hu
sLittleOtter
Creek
27
1� 66
4� 344
0—
—1� 000
71� 66
4� 344
None
1406 S . M. P . SULLIVAN AND M. C. WATZIN
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
TABLEIV
.Continued
Reach
IDSMC
aH9MC
aD�1MC
SFP
aH9FP
aD�1FP
bg(S)
g(H
9)g(D
�1)
Fishspeciesfoundonly
inFP
MalletsCreek
18
1� 41
3� 346
71� 44
3� 34
0� 333
10
1� 49
3� 676
Cu
lea
inco
nst
an
s†and
No
tro
pis
volu
cell
us
MalletsCreek
211
1� 80
4� 605
10
10� 833
11
1� 89
5� 360
None
MillBrook
71� 22
2� 940
81� 63
3� 95
0� 467
11
1� 47
3� 443
P.n
ota
tusand
H.
ha
nki
nso
ni‡
Missisquoi
River
71� 44
3� 168
11
1� 52
3� 35
0� 333
12
1� 55
3� 356
C.
inco
nst
an
s,†
P.pro
mel
as,
P.n
eog
aeu
s*,
P.eo
s,N
otr
op
isst
am
ineu
sand
Sem
oti
lus
atr
om
acu
latu
sNew
Haven
River
17
1� 46
3� 611
61� 22
3� 29
0� 385
91� 52
4� 010
S.
atr
om
acu
latu
sand
P.p
rom
ela
sNew
Haven
River
28
1� 30
2� 829
81� 07
1� 17
0� 375
11
1� 54
3� 932
P.pro
mel
as,
P.neo
gaeu
s*and
Ma
rga
risc
us
ma
rga
rita
RogersBrook
14
1� 94
5� 170
12
2� 14
7� 24
0� 154
15
2� 22
7� 279
L.
gib
bo
sus
South
Branch
10
13
0� 85
2� 25
0� 500
30� 30
1� 156
S.
atr
om
acu
latu
sand
Note
mig
onus
crys
ole
uca
sTylerBranch
51� 18
2� 450
30� 97
3� 11
0� 250
51� 20
2� 550
None
Nofish
foundin
thefloodplain.
*Uncommon.
†Rare.
‡Veryrare,ofspecialconcern.
§Exotic(accordingto
VTANR,2008).
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
FISH DIVERSITY AND FLOODPLAIN CONNECTIVITY
Incision (x ¼ 1�6), entrenchment (x ¼ 5�2) and width to depth ratios (x ¼ 37�7)(Table II), as well as bank-full width (x ¼ 18�9 m) and stream order (x ¼ 2�9)(Table I) all exhibited wide ranges. These measurements of channel geometrywere used as potential indicators of the degree of floodplain connectivity. Resultsfrom Spearman’s r analysis showed a number of significant correlations betweenincision ratio and fish assemblage diversity, and between width-to-depth ratio andfish diversity, but none between entrenchment ratio and fish diversity (Table VII).
TABLE V. Results of the principal components analysis, including loadings, the pro-portion of the variance (r2) shared with the PC axes and the eigenvalues and per cent ofthe variance captured by each axis. PC2, in bold, was the only axis that was significant in
the regression analysis presented in Table VI
PCI PC2 PC3 PC4
Loading r2 Loading r2 Loading r2 Loading r2
Depth range (m) 0�05 0�01 0�48 0�55 0�34 0�17 �0�18 0�03Floodplain type �0�11 0�06 0�43 0�44 �0�05 0�00 0�35 0�13Number of connectivity
types0�29 0�42 0�10 0�03 �0�51 0�37 �0�19 0�04
Number of detritus levels 0�33 0�52 0�07 0�01 0�20 0�06 0�55 0�33Number of floodplain
waterbodies0�33 0�52 0�00 0�00 0�42 0�25 0�07 0�01
Number of floodplainwaterbody types
0�30 0�44 �0�05 0�01 0�30 0�13 �0�41 0�18
Number of largewoody debris
0�34 0�56 0�09 0�12 �0�35 0�18 0�30 0�09
Number of substrata(>20%)
0�29 0�40 �0�44 0�45 0�18 0�04 �0�15 0�03
Number of turbiditylevels
0�20 0�20 0�44 0�47 �0�06 0�01 �0�37 0�15
Sinuosity range 0�35 0�59 �0�30 0�21 �0�03 0�00 0�20 0�04Temperature range (° C) 0�33 0�52 �0�01 0�00 �0�38 0�21 �0�17 0�03Total surface area (m2) 0�36 0�65 0�28 0�19 0�14 0�03 0�09 0�01Eigenvalue 4�90 2�36 1�43 1�07Variance (%) 40�80 19�70 11�93 8�93
TABLE VI. Significant linear regression models for fish assemblages
Model Variable Coefficient r2 F statistic
Species richness (S) Intercept 7�21(P < 0�05) PC2 1�68 0�38 7�35Shannon–Weaver index (H9) Intercept 1�36(P < 0�05) PC2 0�26 0�42 8�61Simpson’s index (D�1) Intercept 4�06(P < 0�01) PC2 1�06 0�51 12�46
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
Results suggested that as incision ratios increased, floodplain fish species richnessdecreased, b-diversity increased (i.e. high species turnover) and fish diversityacross the stream corridor decreased. As width-to-depth ratios increased, fishassemblage diversity of both main channel and of the entire stream–floodplainecosystem decreased. No significant relationships were indentified between bank-full width and fish g (Spearman’s r ¼ 0�251, 0�251 and 0�242; S, H9 and D�1,respectively; P > 0�05) or stream order and fish g (Spearman’s r ¼ 0�013,�0�157, �0�181; S, H9 and D�1, respectively; P > 0�05).
FISH–HABITAT ASSOCIATIONS
Stream corridor fish assemblages differed in number of species, assemblagecomposition and diversity (Table IV). The distribution and frequency of fishspecies in main channel and floodplain waterbodies suggest four divisions ofhabitat use in the study area (Fig. 1): main channel (i.e. species found onlyin the main channel), main channel–floodplain (species not only found withgreater frequency in the main channel but also found in floodplain waterbod-ies), floodplain–main channel (species not only found with greater frequency infloodplain waterbodies but also found in the main channel) and floodplain(species found only in floodplain waterbodies).The most common species found in main channels were blacknose dace Rhi-
nichthys atratulus (Hermann) (found at 71% of reaches), common shiner Luxiluscornutus (Mitchill) (75%), creek chub Semotilus atromaculatus (Mitchill) (58%)and white suckers Catostomus commersoni (Lac�epede) (67%). In contrast, thesesame species were found at 33, 15, 42 and 25% of floodplains, respectively. Eventhough there was no one species found in all floodplains, there were a few foundmore commonly in floodplains than in main channels. For example, both fines-cale Phoxinus neogaeus (Cope) and northern redbelly dace Phoxinus eos (Cope)were found at c. �1�7 the frequency in floodplains than in main channels. Sim-ilarly, brook sticklebacks Culea inconstans (Kirtland) were collected at �2 the fre-quency in floodplains than in main channels and brown bullhead Ameiurusnebulosus (Lesueur) at �3 the frequency. In contrast, largemouth bass Micropterussalmoides (Lac�epede) and banded killifish Fundulus diaphanus (Lesueur) werefound at equal frequencies in main channels and floodplains.
TABLE VII. Spearman’s r correlations for geomorphic measurements of main channel–floodplain connectivity and fish assemblage diversity measures (n ¼ 24)
Habitat variable Fish assemblage variable r P
Entrenchment ratio Floodplain a-diversity (S) 0�1780 >0�05Entrenchment ratio b-diversity �0�1249 >0�05Entrenchment ratio g-diversity (S) �0�0801 >0�05Incision ratio Floodplain a-diversity (S) �0�6445 <0�001Incision ratio b-diversity 0�5340 <0�01Incision ratio g-diversity (S) �0�4711 <0�05Width depth ratio Main Channel a-diversity (H9) �0�4765 >0�05Width depth ratio Main Channel a-diversity (D�1) �0�5974 <0�01Width depth ratio g-diversity (D�1) �0�5122 <0�05
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There were a number of species that were never found outside the mainchannel. These included brown trout Salmo trutta L., longnose sucker Catosto-mus catostomus (Forster), rosyface shiner Notropis rubellus (Agassiz), slimy scul-pin Cottus cognatus Richardson and yellow perch Perca flavescens (Mitchill)among others (Fig. 1). In contrast, bowfin Amia calva L., brassy minnowsHybognathus hankinsoni Hubbs, carp Cyprinus carpio L. and rudd Scardiniuserythrophthalmus (L.) were found only in floodplain waterbodies.
DISCUSSION
The pulsed nature of stream flow governs the cyclical phases of floodplainexpansion and contraction. In northern temperate regions, expansions typi-cally coincide with spring snowmelt and autumn rains, whereas contractionsoccur during the drier summer months and during winter when availablewater is stored as ice. Although the seasonal variation in the flux of bothsurface and subsurface water changes the composition and configurationof floodplain waterbodies, the ‘shifting mosaic’ of floodplain habitats canprovide for a diversity of species in undisturbed systems, at least over eco-logical time scales (Bormann & Likens, 1979; Ward et al., 2002). As anthro-pogenic disturbances to stream channels increase in both severity and extent,however, hydrological connectivity, and the nature and persistence of flood-plain waterbodies will be altered, with profound implications for stream fishcommunities.
Main channel Main channel–floodplain Floodplain–main channel Floodplain
Salmo trutta (brown trout) Fundulus diaphanus (banded killifish) Hybognathus hankinsoni (brassy minnow)Notropis bifrenatus (bridle shiner) Culea inconstans (brook stickleback) Amia calva (bowfin)Hybognathus regius (eastern silvery minnow) Ameiurus nebulosus (brown bullhead) Cyprinus carpio (carp)Notropis atherinoides (emerald shiner) Phoxinus neogaeus (finescale dace) Scardinius erythrophthalmus (rudd)Percina caprodes (logperch) Micropterus salmoides (largemouth bass)Catostomus catostomus (longnose sucker) Phoxinus eos (northern redbelly dace)Esox lucius (pike)Notropis rubellus (rosyface shiner)Cottus cognatus (slimy sculpin)Micropterus dolomieu (smallmouth bass)Cyprinella spiloptera (spotfin shiner)Perca flavescens (yellow perch)
Fundulus diaphanus (banded killifish)Rhinichthys atratulus (blacknose dace)Lepomis macrochirus (bluegill sunfish)Pimephales notatus (bluntnose minnow)Salvelinus fontinalis (brook trout)Luxilus cornutus (common shiner)Semotilus atromaculatus (creek chub)Pimephales promelas (fathead minnow)Notemigonus crysoleucas (golden shiner)Micropterus salmoides (largemouth bass)Rhinichthys cataractae (longnose dace)Notropis volucellus (mimic shiner)Margariscus margarita (pearl dace)Lepomis gibbosus (pumpkinseed sunfish)Lepomis microlophus (redear sunfish)Ambloplites rupestris (rock bass)Notropis stramineus (sand shiner)Notropis hudsonius (spottail shiner)Etheostoma olmstedi (tessellated darter)Catostomus commersoni (white sucker)
Cool, less turbid water; range of substrata,mostly gravel and cobble, range of flowvelocities, persistent hydrological connectivity
Wide temperature range; fine substrata; sluggish,turbid water, sporadic hydrological connectivity
Main channel Floodplain
FIG. 1. Schematic of the four principle fish divisions of habitat use found in study area, based on presence
or absence and frequency data from each study reach. Fishes were only considered ‘present’ if
greater than three individuals were found at the reach. Fish species found in both main channel–
floodplain and floodplain–main channel columns were found at equal frequencies in main channel
and floodplain habitat.
1410 S. M. P . SULLIVAN AND M. C. WATZIN
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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418
FISH ASSEMBLAGE DIVERSITY, FLOODPLAINS ANDCHANNEL ADJUSTMENT
Although floodplain waterbodies may only exist seasonally, the presentresults affirm their importance in enhancing the diversity of fish assemblagesin stream corridors (Copp, 1989; Bravard et al., 1997; Fisher & Willis, 2000).In the present study, the contribution of floodplain fish assemblages led tosignificantly higher fish assemblage diversity at reaches that exhibited highfloodplain connectivity. Simply because of the additional habitat area pro-vided by floodplain waterbodies to the stream–floodplain ecological unit, thisresult, at least in part, follows well-established aquatic species–area relation-ships that indicate that larger areas support more species (Angermeier &Schlosser, 1989; Matthews & Robinson, 1998). In exploring the contributionof floodplain fish assemblages to the total fish assemblage of the stream cor-ridor, no control for area was made to illustrate the importance of hydrolog-ical connectivity to the development and persistence of floodplain habitat,that both provides additional area as well as unique habitat types. Bank-fullwidth and stream order were used as proxies for stream size and watershedposition, neither of which was correlated with fish diversity. Therefore, therewas no evidence to suggest that longitudinal gradient influenced the diversityresults, although it is has been shown to influence fish community diversityin other studies (Vannote et al., 1980; Rahel & Hubert, 1991; McClellandet al., 2006).Floodplain fish assemblages not only influenced the number of species but
also the evenness of species distribution and influence of dominant species,as evidenced by similar results across all three diversity measures (i.e. S, H9
and D�1). The presence of species not found in adjacent main channels under-scores the nature of fish assemblage composition in floodplains. Species turn-over was observed to be significantly higher in reaches that had nofloodplain connectivity than in those stream reaches with limited and high lev-els of floodplain connectivity, a pattern consistent with isolated and fragmentedfloodplain channels (Tockner et al., 1999a). This low observed community sim-ilarity between the main channel and the floodplain fish assemblages directlyaddresses the importance of preserving natural hydro-morphological dynamicsin stream–floodplain ecosystems from a functional standpoint.Any geomorphic adjustments that alter hydrological connections might be
expected to affect the floodplain. Channel incision and widening were stronglycorrelated with fish assemblage diversity of the main channel, of the floodplainand of the stream–floodplain ecosystem as a whole. Stream reaches with thehighest relative width to depth ratios were negatively correlated with mainchannel a-diversity as expressed by H9 and D�1, and of stream corridor g-diver-sity as expressed by D�1. Because D�1 is a dominance index, its associationwith width to depth ratio implies a decrease in the number of individuals ofdominant species in both the main channel and the floodplain in widened rea-ches. In general, widened reaches were dominated by large gravel bars, werespatially separated from riparian vegetation, often had leaning trees falling intothe water and had many overhanging banks. In these highly widened reaches,floodplain waterbodies tended to occur in depressions in juvenile terraces; they
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often had no shading and were subject to flow disturbance at moderate flowlevels. Therefore, although many widened reaches did sustain some degree offloodplain connectivity, the floodplain waterbodies were limited in qualityand extent and did not support diverse fish assemblages.In both cases (i.e. incision and widening), physical and hydraulic disturbance
may be the mechanistic link governing the development and persistence ofwaterbody habitats suitable for high floodplain fish diversity. Incision ratioproved to be a repeated proxy for floodplain connectivity: higher incision ratioswere correlated with not only lower numbers of fish species in both the flood-plain and across the stream corridor but also with higher species turnover. Nostreams with incision ratios >2 exhibited floodplain connectivity. Floodplainsof these reaches probably only experience physical disturbance during extremeflood events. Conversely, the floodplains of widened reaches are highly suscep-tible to slight increases in flow and probably experience consistent exchanges ofsediment, debris and biota.
FLOODPLAIN WATERBODIES AND FLOODPLAIN FISHASSEMBLAGES
In exploring which characteristics of floodplain waterbodies were mostimportant in maintaining high levels of floodplain fish diversity, waterbodieswith multiple depths were found to support higher fish a-diversity as measuredby both simple S and multifactor indices. Others have also reported this result.For example, Winemiller et al. (2000) found water depth to be a key predictorof fish diversity in oxbows. Likewise, greater variation in turbidity levels andfloodplain types were positively correlated with fish a-diversity. In contrastto the main channel, where turbidity levels tend to be spatially constant, indi-vidual floodplain waterbodies may exhibit marked differences in turbidity lev-els, even when in close physical proximity to one another. Furthermore,channel side arms, floodplain pools and ponds, flood chutes, abandoned ox-bows, marshes and other floodplain waterbodies provide a host of characteris-tics atypical of the main channel. A suite of these waterbodies acrossa floodplain offers an array of distinct patches, each differing in flow regime,shading and structure.Heterogeneity of substrata was expected to increase fish diversity, therefore,
the negative association between substratum types and fish diversity appearscontradictory. Whereas seven of the 14 active floodplains in fact only had twoor less substratum types representing at least 20% of the waterbody, this resultis probably an artefact of the floodplain-level resolution. A waterbody-level anal-ysis would be required to further explore the relationship between substratumtypes, coverage and fish assemblage diversity. To account for the potential effectof species–area relationships (Angermeier & Schlosser, 1989) on the presentfloodplain diversity results, the area of floodplain waterbodies was included inthe analysis. Area, however, was not an influential variable on PC2, suggestingthat a waterbody’s area is not a driving factor in habitat use, not necessarilyequating with accessibility and suitability to floodplain fish species.
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FISH–HABITAT ASSOCIATIONS
The majority of species collected were found in both main channel andfloodplain habitats. Whereas some of these species, such as P. eos, P. neogaeusand C. inconstans used only floodplain habitats; others including various sun-fishes, shiners and minnows used only main channel habitats. Of the speciesthat were typically found in both main channel and floodplain waterbodies,a generalist feeding strategy and a comparatively high tolerance of environmen-tal variation were common traits.For example, R. atratulus and S. atromaculatus were two species commonly
found in both main channel and floodplain habitats. Rhinichthys atratulus pre-fer small streams of relatively high gradient (11�4–23�3 m km�1; Burton &Odum, 1945; Werner, 1980), although they tend to select areas of low flowwithin these streams and are commonly found over a variety of substrata(Gibbons & Gee, 1972). Semotilus atromaculatus are an even more ubiquitousspecies, found in high relative abundance in almost all stream sizes (Gerking,1945). Both species have broad dietary requirements. Rhinichthys atratulus typ-ically feed on insect larvae, plant material and small worms and crustaceans(Werner, 1980). As a generalized carnivore, S. atromaculatus are notably adapt-able in diet and food sources range from benthic macroinvertebrates in juve-niles to crayfish, molluscs, worms and even other minnows in larger fish(Werner, 1980; Pflieger, 1997).The widespread presence of these and other generalist species in floodplain
waterbodies suggests that these species are habitat opportunists, takingadvantage of the increase in available habitats created by floodplain connec-tions as it occurs. The use of floodplain habitats by these species also suggeststhat floodplain waterbodies may be important refuge areas for commonstream fishes during times of stress (e.g. high flows, drought and temperatureextremes).
Luxilus cornutus is another stream species with broad environmental require-ments, inhabiting both warmwater and coldwater streams. Even though L. cor-nutus was the most common species found in the main channel in the studyarea, they were only found in 15% of floodplain study reaches. Catostomuscommersoni was found also more extensively in main channels than in flood-plain habitats, despite the species’ tolerance of a wide range of environmentalconditions and use of many potential food sources (Werner, 1980). Its predom-inance in main channel habitats may confirm its preference of gravel substratawith current (Werner, 1980).
Salmo trutta, N. rubellus and C. cognatus were never found in floodplainwaterbodies. Their need for clear, cool water, rapid flow and gravel and cobblesubstrata (Wydoski & Whitney, 1979; Aadland, 1993; Holton & Johnson, 1996)restricts their use of sluggish and often turbid floodplain waterbodies. A varietyof piscivores, including pike Esox lucius L., smallmouth bass Micropterus dolo-mieu Lac�epede and P. flavescens were also found only in the main channel.Their dietary requirements, as well as intolerance for low dissolved oxygen con-centrations and high temperatures (Werner, 1980) probably encourages use ofmain channel habitats.
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In contrast, species including A. calva, H. hankinsoni, C. carpio and S. eryth-rophthalmus were found only in floodplain waterbodies, reflecting their use ofaquatic vegetation and low flow variability, as well as their tolerance of lowdissolved oxygen levels, high turbidity and a wide range of temperatures(Edwards & Twomey, 1982; Becker, 1983; Koel, 1997).The distinct groups of species that appeared to prefer main channels, and
those that preferred floodplain waterbodies indicates a clear division of habitatuse within the stream–floodplain ecosystem. Whereas some species, such asR. atratulus and S. atromaculatus, opportunistically inhabit floodplain waterbod-ies, the present samples suggest floodplain habitats are essential for other spe-cies such as A. calva and H. hankinsoni. Other species, such as P. neogaeus andP. eos seemed to prefer the habitat and resources provided by floodplains butcommonly inhabit main channels when available and necessary. Across the 14reaches with active floodplains, three rare (i.e. imperiled, at high risk of extinc-tion due to very restricted range and often <20 populations showing steep de-clines) and uncommon (i.e. vulnerable, at moderate risk of extinction due torestricted range and often <80 populations with recent and widespread de-clines) (VTANR, 2008) species of fish in Vermont were found: H. hankinsoni,C. inconstans and P. neogaeus. The data, therefore, suggest that ChamplainValley fish assemblages in stream–floodplain ecosystems may be divided intofour groups of habitat use: main channel, main channel–floodplain, flood-plain–main channel and floodplain (Fig. 1).Stream and river impairment, particularly channelization, has been pervasive
across many regions of the world, but the effects on the entire stream–floodplainecosystem are still not as well documented. Channelization causes extensive bedincision and separation of streams from their floodplains. Subsequent channelwidening reshapes the channel–floodplain relationship (Brierley et al., 1999).Floodplains have been recognized as biodiversity hotspots because of their eco-tonal nature (Tockner et al., 1999b; Araujo, 2002), yet a full understanding ofhow fishes and other aquatic biota depend on exchanges between the mainchannel and the floodplain is still unknown. Until a better understanding ofthe links between stream channel geomorphic processes and the habitats avail-able in both main channel and floodplain environments is developed, this ques-tion will remain unresolved.The importance of floodplain waterbodies as habitat units that contribute to
fish assemblage diversity of the entire stream corridor has been documented.Both bed incision and channel widening were found to restrict fish diversityof stream–floodplain ecosystems. These relationships suggest intact channelmorphologies are critical for providing heterogeneous and persistent floodplain(and main channel) habitats for fish communities, and illustrate how animproved understanding of links between biota and physical structure ofstream–floodplain ecosystems can provide important insights into fish-habitatrelationships.
Funding for this project was provided by the National Centre for Environmental(NCER) STAR Programme, EPA, grant number R83059501-0. Special thanks areextended to B. Ellrott and E. Royer for their assistance in the field.
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