Stream–floodplain connectivity and fish assemblage diversity ... · To evaluate the influence of main channel–floodplain connectivity on fish assemblage diversity in floodplains

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).

FLOODPLAIN CONNECTIVITY AND FISH DIVERSITY 1395

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Journal compilation # 2009 The Fisheries Society of the British Isles, Journal of Fish Biology 2009, 74, 1394–1418

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

1396 S. M. P . SULLIVAN AND M. C. WATZIN

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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.


# 2009 The Authors


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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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


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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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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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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


# 2009 The Authors


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


# 2009 The Authors


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.


# 2009 The Authors


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


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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).


# 2009 The Authors


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


# 2009 The Authors


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


# 2009 The Authors


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.


# 2009 The Authors


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


# 2009 The Authors


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.


# 2009 The Authors


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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Documents

Stream–floodplain connectivity and fish assemblage diversity ... · To evaluate the influence of main channel–floodplain connectivity on fish assemblage diversity in floodplains