REGULATED RIVERS: RESEARCH & MANAGEMENT

Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

DOl: 10.1002/rrr.614

RESPONSES OF CONSUMERS AND FOOD RESOURCES TO A HIGHMAGNITUDE, UNPREDICTED FLOOD IN THE UPPER MISSISSIPPI

RIVER BASIN

MICHAEL D. DELONGa,*, JAMES H. THORpb,l, KIM S. GREENWOODb AND

MICHAEL C. MILLERca Large River Studies Center, Biology Department, Winona State University, Winona, MN 55987, USA

b Department of Biology, University of Louisville, Louisville, KY 40292, USAc Department of Biological Science, University of Cincinnati, Cincinnati, OH 45221, USA

ABSTRACT

The Mississippi and Missouri Rivers experienced flooding in 1993 that fell outside the annual predictable flood periodof spring and early summer. Flooding began in late June, peaked in late July (25232 m31s on the upper Mississippiand 21240 m3 Is on the Missouri) and remained at or near flood stage into October 1993. This study was performedto determine if disturbance by an unpredicted flood event would alter trophic dynamics of river-floodplain systemsby creating shifts in the composition of organic matter available to consumers. The Ohio River, which did not floodduring the same period, was examined for comparison. Stable isotopic ratios of carbon and nitrogen from samplescollected in 1993 and 1994 were used to characterize potential food sources and determine linkages between foodsources and invertebrate and fish consumers. Pairwise contrasts, performed separately for each river, indicated therewere few interannual differences in 1513C and 1515N of organic matter sources and consumers. Between sample period(flood year versus normal water year) trends in both flooded rivers were similar to between-year trends observed forthe Ohio River. Trophic structure of the Mississippi and Ohio Rivers was similar in both years, with fine andultra-fine transported organic matter and dissolved organic matter representing the major sources of organic matter.Overlapping isotopic signatures in the Missouri River made tracking of sources through the consumers difficult, butsimilarities in 15 13C and 1515N between years indicated trophic structure did not change in response to the flood. Theresults suggest that consumers continued to rely on sources of organic matter that would be used in the absence ofthe unpredicted 1993 flood. It is proposed that trophic structure did not change in response to flooding in theMississippi and Missouri Rivers because both rivers exhibited the same trends observed in the Ohio River. Copyright@ 2001 John Wiley & Sons, Ltd.

KEYWORDS: disturbance; flood; large rivers; Mississippi River; Missouri River; Ohio River; river-floodplain; stable isotopes;trophic structure

INTRODUCTION

River-floodplain systems are, by nature, flooding ecosystems. Most flood events are predictable becausethe seasonal timing is a response to annual climatological patterns: spring snowmelt in temperate regionsand the wet season in tropical/subtropical areas. Does flooding in these rivers and under thesecircumstances constitute a disturbance? Pickett and White (1985) described disturbance as any relativelydiscrete event in time that disrupts ecosystem, community, or population structure, and that changesresources, availability of substrate, or the physical environment. Resh et al. (1988) further defineddisturbance to include events that exhibit a frequency and intensity outside of predictable ranges. Inaddition to being a predictabl~ phenomenon, periodic flooding is an integral component of the ecologyof river-floodplain ecosystems. The connection of aquatic and terrestrial habitats through periodic,

* Correspondence to: Large River Studies Center, Biology Department, Winona State University, Winona, MN 55987, USA.I Current address: Department of Biology, Clarkson University, Potsdam, NY 13699-5805, USA.

Received 6 July 1999Revised 13 March 2000

Accepted 28 March 2000Copyright @ 2001 John Wi1ey & Sons, Ltd.

218 M.D. DELONG ET AL

predictable flooding is critical to successful reproduction of many riverine fishes (Welcomme, 1985;Bayley, 1995) and allows for a potential exchange of nutrients and energy (Edwards and Meyer, 1987;Winemiller, 1990; Polis et al., 1996). Furthermore, the interaction created by fluctuating water levels isinstrumental in shaping habitat structure and complexity of river-floodplain ecosystems (Sparks, 1995;Sparks et al., 1998). Using these definitions, therefore, one would conclude that the typical, predictableseasonal floods of river-floodplain ecosystems are not classic disturbances.

While 'predictable' flooding may not qualify as a disturbance, floods occurring outside the expectedtemporal and hydrologic range (unpredicted) could constitute a disturbance. Such an event occurred inthe upper Mississippi River drainage in 1993. Prolonged, above normal rainfall in the upper midwesternUSA in the spring and early summer, combined with above normal soil moisture, led to widespreadflooding in the upper Mississippi and lower Missouri River drainages during the summer/fall of 1993(Bhowmik et al., 1994; US Army Corps of Engineers, 1994). The magnitude of flooding on the upperMississippi River ranked among the top five floods for stage height and discharge since recording beganin the 19th century (US Geological Survey hydrological records), and was of long duration, with locationsalong both rivers reporting water levels above flood stage for 130-181 days (Bhowmik et al., 1994).Although the human and economic costs were well documented in the popular press and subsequentreports (e.g. US Army Corps of Engineers, 1994), only limited attention was given to potentialenvironmental impacts of this large-scale event (e.g. Galat et al., 1998; Miller and Payne, 1998; Sparksand Spink, 1998). Preliminary assessments have determined that many species of riverine fishes had aboveaverage spawning success in 1993 because of the greater availability of spawning habitat in the floodplain(Bhowmik et al., 1994; Raibley et al., 1997).

Ecological interest in disturbance has focused on its role in shaping population, community, andecosystem structure. Lotic systems, in particular, have been studied extensively to improve the under-standing of the ecological role of disturbance (Ward, 1989; Poff and Ward, 1989). Temporal variation inthermal and chemical regimes as well as discharge patterns provide a means of scaling disturbances byfrequency and intensity (Resh et al., 1988). Much of the emphasis on disturbance in lotic systems has beenon changes in organismal abundance and community structure rather than on functional changes (Quinnand Hickey, 1990; Townsend and Hildrew, 1994; Townsend et al., 1997). Disturbance has been applied,at least theoretically, as a means of determining the stability of food webs, where stability of a food webis a measure of the ability of the system to recover or return to some equilibrium state (Pimm, 1982). Itshould be feasible to evaluate the ecological impact of a disturbance event on a food web by determiningif differences in the base source(s) of organic matter have been influenced or if trophic structure has beenaltered.

This study examined the effects of the flood of 1993 on components of the food web of the lowerMissouri and upper Mississippi Rivers. The central focus of this study was to determine if an unpredictedflood of high intensity and duration altered food web structure by shifting the availability or use ofpotential sources of organic matter. One approach to identify sources of organic matter assimilated intoa food web and to characterize trophic structure is the use of stable isotopes (Peterson and Fry, 1987;Winemiller and Polis, 1996). Stable isotopes have been successfully employed to examine food webs in avariety of aquatic ecosystems (e.g. Peterson and Fry, 1987; Angradi, 1994; Keough et al., 1996; Thorp etal., 1998).

Samples of potential sources of organic matter and representatives of invertebrate and fish consumerswere obtained to quantify ratios of stable isotopes of carbon and nitrogen to address the question of theeffects of the 1993 flood on trophic pathways. The lack of both stable isotope data and well-developedinformation on trophic structure in these rivers prior to the flood eliminated the possibility of directmeasures of effects. To circumvent this limitation, the lower Ohio River was sampled, which did notexperience flooding in 1993, at the same time as the flood-impacted rivers. Sampling was performed asclose to the flood period as possible (October 1993) and during a different season in the following year(July 1994). By taking this approach, the Ohio River essentially served as a 'control' that was notsubjected to the 'experimental' treatment (unpredicted, high intensity flood). Seasonal changes arecommonly expected in stable isotope studies because of changes in the isotopic composition of the sources

Copyright ((;) 2001 John Wiley & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

FLOOD RESPONSES IN RIVERS 219

(MacLeod and Barton, 1998; Barth and Veizer, 1999). The null hypothesis was 'no functional changes infood web structure would occur because ffooding would not affect the availability or composition of themajor food source(s) within the system'. If this hypothesis were to be supported, one would expect thepotential carbon sources to exhibit the same shifts in isotopic ratios from October 1993 to July 1994 inall three rivers with a concomitant shift in the consumers. In contrast, it was predicted that the flood didaffect trophic structure if sources and consumers from the Mississippi and Missouri Rivers exhibitedbetween-sample date shifts in isotopic ratios different from those witnessed in the Ohio River.

MA TERIALS AND METHODS

Study sites

Average annual discharge for both the upper Mississippi and lower Missouri Rivers were well in excessof historical annual averages (Table I). Discharge in 1994 more closely reflected historical patterns, as didaverage annual discharge of the Ohio River during both years of the study.

Samples for the upper Mississippi River were collected near the city of Louisiana, Missouri [riverkilometers (RK) 437-447; Figure 1]. This area is representative of the river-floodplain features of theupper Mississippi River and is characterized by a broad main channel with several side channels andbackwaters. Dominant floodplain vegetation includes silver maple (Acer saccharinum ), cottonwood(Popu/us sp.), and willow (Sa/ix sp.) in areas not converted to cropland. The predominant crop in the areais corn, which is found mostly behind protective levees. Bottom substrata consist primarily of sand andsilt although some gravel and cobble (mostly artificial riprap) is present.

The Missouri River is extensively modified for navigation, representing one of the most profoundlyaltered river systems in North America. Once characterized as a broad, extensively braided system, thelower Missouri River now has a single deep, narrow channel for most of its length. Interaction betweenthe river and floodplain has been further reduced by the construction of a levee system intended to controlflooding of adjacent farmland. Floodplain vegetation is comparable to that found at the upper MississippiRiver site. Cropland is normally located almost entirely behind levees, but was flooded following thefailure of levees upstream of and in the sample area. Samples were obtained near New Haven, Missouri(RK 123-133; Figure I). Bottom substrata are almost exclusively sand and silt. The only large particlespresent are rocks used to construct wing dams perpendicular to the shoreline.

Table I. Summary of discharge data (m3js) for the US Geological Survey gauging stations with historicaldata in closest proximity to sample sitesa

Data Mississippi River Missouri River Ohio River

Thebes, IL

07022000

RK 77

6068 ::!: 248

(1939-1996)

13 487

25232

(22 July)

Hermann, MO

06934500

RK 162

2528 :t 121(1958-1996)

5149

21 240(31 July)

Metropolis, IL

036511500

RK 1572

8353 :t 306(1928-1996)

8762

23 867(2 April)

S.E.

Station

USGS station number

Location on river

Average annual discharge ( I(Years of record)

Discharge calendar year 1993

Peak 1993 discharge(Date of peak)

RData were obtained by entering USGS station number after accessing http://waterdata.usgs.gov/nwis-w/IL/ for the Ohioand Mississippi River data and http://waterdata.usgs.gov/nwis-w/MO/ for the Missouri River. Although the peak flow forthe Ohio River was comparable to the peak discharges observed in the other rivers in 1993, the Ohio River peak was withinnormal range for maximum flows during the predictable high-water period. River kilometers (RK) are based on mileagefrom confluence with the Ohio River, Mississippi River, and from point of origin, respective to order in table. Averageannual discharge ( :t I S.E.) does not include 1993 data.

Copyright ~ 2001 John Wi1ey & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

220 M.D. DELONG ET AL.

Figure

100 km

General map of the region in which the study was conducted on the Mississippi, Missouri and Ohio Rivers. Approximatelocation of sites represented by large open circles

The Ohio River was not subjected to the flooding encountered in the Mississippi and Missouridrainages in 1993. Like the upper Mississippi, a series of 20 low-head dams are situated along the lengthof the Ohio to maintain a minimum channel depth of 2.74 m for navigation. These low-head dams do notfunction in flood control. Samples from the lower Ohio River were collected approximately 32-kmupstream (RK 1578-river kilometers are measured from origin to confluence for the Ohio) of theconfluence of the Ohio and upper Mississippi Rivers. Riparian vegetation consists of several species ofdeciduous trees, including sycamore (P/atanus occidenta/is), maple (Acer sp.), and willow (Sa/ix sp.),grasses, and agricultural crops (predominantly corn and soybeans) on higher land. Bottom substrataconsist of mostly sand in the main channel and sand/silt near shore. Stone revetments (erosion control)are present, but are not as common as in the other two rivers.

Sample methods

Samples, representing potential nutrient sources and consumers, were collected from all three rivers inOctober 1993 and July 1994 for analysis of stable carbon and nitrogen isotopes. Both the Mississippi andMissouri Rivers had only recently dropped below flood stage prior to collection of samples in October1993. Five replicates of each sample type were collected, when possible, from each river on both dates.Samples for coarse benthic organic matter (CBOM) and fine benthic organic matter (FBOM) from theOhio River were destroyed when a drying oven ignited. These samples were replaced with samplesobtained during the same sample period from a site located further upstream, yet still within thefloodplain portion of the lower Ohio River. Upon collection, samples were placed in plastic bags or vialsand stored on ice until they could be returned to the laboratory where they were dried at either 105°C for24 h (source organic matter) or 60°C for 48 h (consumers) and then ground into a fine powder with a

wig-l-bug@.Terrestrial sources of organic matter. Shed leaves from five to six species of terrestrial autotrophs (C3

and C4 plants) were collected from the floodplain forest floor at sites along each river. Specimens includedtwo to four numerically dominant and secondary tree species and two to three grasses and/or agriculturalcrops (primarily corn and soybeans). Isotopic ratios were determined separately for each species.

Aquatic sources of organic matter. Potential sources of organic matter from the river included: severalsize fractions of particulate transported organic matter (TOM), which represented organic mattersuspended in the water column (seston); size fractions of particulate benthic organic matter (BOM),representing organic matter retained in and removed from the bottom substrata of the river channel;dissolved nutrients (DN), which included dissolved organic carbon and dissolve organic and inorganic

Regul. Rivers: Res. Mgmt. 17: 217-234 (2001Copyright @ 2001 John Wi1ey & Sons, Ltd.

FLOOD RESPONSES IN RIVERS 221

nitrogen; benthic filamentous algae; and rooted vascular plants (aquatic macrophytes). Benthic algae werescraped or clipped from bottom substrata and aquatic macrophytes were clipped above the roots.Macrophytes were cleaned either in the field or the laboratory to remove most epiphytic algae. Particulatebenthic organic matter was collected from several depths along five cross-channel transects. Samples takenalong a transect were pooled and passed through I-mm and 53-l.1m sieves to partition coarse (CBOM) andfine (FBOM) benthic organic matter, respectively. Finer separation of benthic organic matter was notattempted because of the difficulty in acquiring a sufficient sample for isotopic analysis and because sandand silt in samples used for isotopic analysis can potentially damage the mass spectrometer (D. Schell,personal communication). Transported particulate organic matter (TOM) was collected from severaldepths with a water pump from two nearshore and two mid-channel stations along five cross-channeltransects. Samples within each transect were combined, resulting in five composite samples (one pertransect) per date per river. BOM and TOM samples from the Missouri River were taken within 100 mof the shoreline because of unsafe conditions resulting from continued high discharge and turbulence.

Water samples were passed through coarse (1 mm) and fine (0.1 mm) sieves to obtain CTOM ( > 1 mm)and FTOM (1-0.1 mm) size fractions, respectively. Water from these samples was then passed througha l-l.1m glass fiber filter to obtain ultra-fine transported organic matter (UTOM; 100-1 I.1m). In additionto representing fine and ultra-fine detritus, FTOM and UTOM also contain much of the phytoplanktoncomponent of transported organic matter. DN were obtained from glass-fiber filtered water by adjustingthe pH to 4.3 and bubbling with oxygen to remove inorganic carbon. Samples were evaporated at 105°Cin a drying oven, and the remaining residue was used for isotopic analysis.

Consumers: zooplankton. Zooplankton samples were collected in near-bottom to surface vertical towswith 100-l.1m plankton nets from four stations (two nearshore and two main channel) along each of fivetransects for each river. Transect samples were pooled to create a single sample for each transect.Zooplankton were separated in the laboratory from particulate organic matter with a stereomicroscope.

Consumers: benthic invertebrates. Representatives of major invertebrate trophic groups (filterers,gatherers, grazers, omnivores, and predators) were collected from nearshore and mid-channel areas of allthree rivers. Invertebrates were collected from deep water with a Ponar grab sampler and from shallowwater by hand collection of rocks, snags, sediment, and detritus. Invertebrates were sorted in the field intosimilar taxonomic groups and were placed in small vials covered with a 0.50-mm mesh cap, and were heldin an aerated tank for 2 days to allow the alimentary tract to clear. Invertebrate samples were then frozenuntil they were returned to the laboratory where they were identified to the lowest practical taxonomiclevel, dried, and ground into a fine powder. Each replicate sample was a combination of several membersof the same taxon to ensure sufficient quantities for stable isotope analysis.

Consumers: fish. Fish representing major trophic groups ( detritivore, planktivore, omnivore, insectivore,and piscivore) were collected from multiple locations within each river. As a result of difficulties incollecting a single representative species for a feeding group, the five replicate samples sometimesconsisted of several species (e.g. largemouth bass Micropterus salmoides and sauger Stizostedion canadensefor piscivorous fishes). A separate stable isotope ratio was, however, still obtained for each species evenin these instances. Fish were collected by electroshocking, netting, and hook-and-line. White muscle frombelow the dorsal fin was removed and frozen in the field. Samples were dried and ground into a finepowder upon returning to the laboratory .

Chemical and comparative analyses

A minimum of 1 mg of powdered sample was needed for stable isotopic analysis. Dried and groundsamples of possible food sources and consumers were shipped to the University of Alaska at Fairbanks(laboratory of Dr Donald Schell) for determination of stable isotope ratios. Isotopic analyses wereperformed with a Europa 20-20 continuous flow mass spectrometer with a 0.2% reproducibility for bothcarbon (CO2) and nitrogen (N2). The 13C/12C and 15N/14N ratios (i.e. l513C and l515N, respectively) ofsamples were determined, with data expressed as the differences in parts per thousand (%0) betweensample ratios and those of standard compounds (Peedee belemnite C and atmospheric N).

Copyright @ 2001 John Wi1ey & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

222 M.D. DELONG ET AL

A drawback of making ecosystem comparisons with stable isotopes is location-specific differences inisotopic signatures (France, 1995). The same food source (e.g. benthic algae) can have different isotopicratios within each system; therefore, consumers may exhibit different isotopic ratios between ecosystemsalthough they are consuming the same form of organic matter. An analysis of variance approach wasrejected for this study because significant differences observed in comparisons between the flooded andunflooded rivers might be a result of river-specific differences rather than mechanistic differences resultingfrom the actions of the flood. The approach chosen to take to test the hypothesis was to compare samplesfrom the two flood-impacted rivers to a 'control'. The assumption was that if the flood had no effect onthe carbon and nitrogen isotopes of carbon sources and their consumers, then any changes or trendsobserved between the 1993 samples and those collected in 1994 for the Ohio River, should also be seenin the upper Mississippi and Missouri Rivers. This approach allowed the comparison of within-riverinterannual differences between rivers (i.e. if the isotopic ratio of this carbon source differed betweensample periods in the Mississippi or Missouri Rivers, did it also differ between sample periods in theOhio?) without creating any of the potential misinterpretations that may have resulted from taking astandard analysis of variance approach. Since the desired comparisons were known a priori, pairwisecontrasts were used to determine if differences existed between sample dates for respective sources andconsumers for each river. Contrasts were made using the general linear model (GLM) of SAS (SAS Inc.,

1994).Relationships between organic matter sources and consumers were also examined using standard

comparative and graphical approaches (Peterson and Fry, 1987; Fry, 1991), both of which rely on specificranges of association between organic matter source and consumer. For example, the ratio of 13C/12Cprovides a marker for identifying sources of organic matter, because it changes only slightly from sourceto consumer (1-2%0, Keough et al., 1996; :t 1%0, Peterson and Fry, 1987). Ratios of ISN/14N are usefulfor identifying trophic transfers. A <5 IsN enrichment of 3-5%0 has been commonly measured for eachincrease in trophic level because of selective excretion of lighter isotopes (DeNiro and Epstein, 1981; Fry,1988). Recent studies, however, have suggested that the level of IsN-enrichment from a lower to highertrophic level can be 2-3%0 (Fry, 1991; Meili et al., 1993; Keough et al., 1996; Hansson et al., 1997; Thorpet al., 1998). The combination of carbon and nitrogen stable isotopes greatly increases the ability todevelop relationships between food sources and consumers (Fry, 1991; Angradi, 1994).

RESULTS

Pairwise comparisons

Pairwise contrasts of organic matter sources from the upper Mississippi River indicated that the J 13Cof only benthic algae and C3 terrestrial plants differed significantly between 1993 and 1994 {Figure 2),whereas only UTOM differed significantly between sample periods in the Missouri River {Figure 3).FTOM and FBOM exhibited significant differences in J 13C between years in the Ohio River {Figure 4).Significant between-sample date differences in J 15N signatures in the Mississippi River were noted for C3terrestrial plants {Figure 2); however, there were no significant differences in J 15N among potential foodsources in the Missouri River {Figure 3). Between-year differences in J 15N were observed for C4 terrestrialplants, DN, and FTOM in the Ohio River {Figure 4).

Of the invertebrate taxa obtained during both years of the study, only the gastropod, Pleurocera sp.,differed significantly with regard to J 13C in the Mississippi River {Figure 2). The predaceous stoneflyAcroneuria sp. was the only invertebrate taxon to differ significantly between sample periods in theMissouri River {Figure 3). Three taxa, Corbicula jluminea {Asiatic clams), Dreissena polymorpha {zebramussels), and Stenonema/Stenacron {heptageniid mayflies), differed significantly between sample periodsin the lower Ohio River {Figure 4). Invertebrates obtained from all three rivers showed a trend towardslightly greater 13C enrichment in 1994 relative to 1993.

Copyright @ 2001 John Wi1ey & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

FLOOD RESPONSES IN RIVERS 223

20

18

16

14

12z

In

...1,0 10

8

6

4

2

0

-33 -30 -27 -24 -21

013C

-18 -15 -12 -9

22

20

18

16z

II)

'iJO14

12

10

8

-32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22

013C

Figure 2. Mean ( ::!: 1 S.E.) stable isotopic «(j 1JC and (j 1SN) signatures of potential food sources and consumers from the upperMississippi River. Values shown are for samples collected in both 1993 (denoted by 'a') and 1994 (denoted by 'b'). Sources andconsumers for which significant differences were found (p ~ 0.05) are in bold below with the variable «(j 1JC and (j 1SN) found todiffer in parentheses. Sources include: (I) benthic algae (OI3C); (2) C3 terrestrial plants (OI3C, oISN); (3) C4 terrestrial plants; (4)CHOM; (5) CTOM; (6) DN; (7) FHOM; (8) FTOM; and (9) UTOM. Consumers collected during both years include: (2) Argia sp.(OISN); (4) Heptagenia sp. (OISN); (5) Hydropsyche orris (OISN); (8) Neurocordulia sp./Dromogomphus sp.; (9) Oligochaeta; (10)Pleurocera sp. «(j 1JC); (12) Dreissena polymorpha «(j 1SN); (13) zooplankton; {14) Ictalurus punctatus; (15) Pomoxis nigromaculatus;

(16) Aplodinotus grunniens (OISN); (17) Dorosoma cepedianum; (18) Micropterus salmoides; and (19) Morone chrysops

Copyright ~ 2001 Jqhn Wiley & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001)

224 M.D. DELONG ET AL.

20

18

16

14

12z

8/)

.-/,0 10

8

6

4

2

0

-33 -30 -27 -24 -21

b13C

-18 -15 -12 -9

22

20

18

16z

II)

~14

12

10

8

-32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20

313C

Figure 3. Mean ( ::!: 1 S.B.) stable isotopic «(j 13C and (j 15N) signatures of potential food sources and consumers from the MissouriRiver. Values shown are for samples collected in both 1993 (denoted by 'a') and 1994 (denoted by 'b'). Sources and consumers forwhich significant differences were found (p ~ 0.05) are in bold below with the variable «(j 13C and (j 15N) found to differ inparentheses. Sources include: (2) C3 terrestrial plants; (3) C4 terrestrial plants; (4) CROM; (5) CTOM; (6) DN; (7) FROM; (8)FTOM; and (9) UTOM «(jl3C). Consumers collected during both years include: (I) Acroneuria sp. «(jl3C, olSN); (2) Argia sp.; (7)Polycentropis sp.; (Sa) Neurocordulia sp.; (Sb) Macromia sp.; (II) Stenonema sp. (Ol5N); (14) I. punctatus (Ol3C); (16) Aplodinotus

grunniens; (17) D. cepedianum; (18) M. salmoides; (19) M. chrysops; and (20) Pylodictis olivaris

Copyright ~ 2001 John Wiley & Sons, Ltd. Regul. Rivers: Res. Mgmt. 17: 217-234 (2001

FLOOD RESPONSES IN RIVERS 225

20 Sources

18

16

14268 ~12 09b

~z

on

...!.0 10~

d -

08b

Q6b ~8

6

~()j Sa4

~7b

2 ~

0

-33 -30 -27 -24 -21

013C

-18 -15 -12 -9

22

20

18

16z

In

~14

12

10

8

-32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20

313C

Figure 4. Mean ( :t 1 S.B.) stable isotopic (b" 13C and 15 15N) signatures of potential food sources and consumers from the lower OhioRiver. Values shown are for samples collected in both 1993 (denoted by 'a') and 1994 (denoted by 'b'). Sources and consumers forwhich significant differences were found (p ~ 0.05) are in bold below with the variable (15 13C and 15 15N) found to differ inparentheses. Sources include: (1) benthic algae; (2) C3 terrestrial plants; (3) C4 terrestrial plants (1515N); (4) CBOM; (6) DN (JI5N);(7) FROM (J1JC); (8) FrOM (JIJC, JI5N); and (9) UTOM. Consumers collected during both years include: (2) Argia sp. (JI5N);(3) Corbicu/a fluminea (JIJC, JI5N); (6) Lithasia sp.; (II) Stenonema sp.IStenacron sp. (JIJC, JI5N); (12) D. po/ymorpha (J1JC,JI5N); (13) zooplankton; (17) D. cepedianum (JIJC, JI5N); (18) M. salmoides; (21) Notropis atherinoides; and (22) Lepomis humilis

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226 M.D. DELONG ET AL

Four invertebrate taxa (Argia sp., Heptagenia sp., Hydropsyche orris, and D. polymorpha) from theupper Mississippi River demonstrated significant differences in 1515N, with all four considerably more15N-enriched in 1994 than in 1993 (Figure 2). Two taxa, Acroneuria sp. and Stenonema sp., differedbetween years in the Missouri River, with both taxa being more 15N-depleted in 1994 (Figure 3). Thelower Ohio River followed the pattern observed in the Missouri River, with invertebrates having 1515Nvalues that were more 15N-depleted in 1994. Four taxa possessed 1515N signatures that differed signifi-cantly between years in the Ohio. These differences included suspension feeders (C. fluminea and D.polymorpha), a scraper/collector (Stenonema sp.), and a predator (the damselfly Argia sp.).

13C signatures did not differ among Mississippi River fish species collected during both sample periods(Figure 2). Only channel catfish, Ictalurus puncta/us, showed a difference large enough to be detected assignificant among fishes collected in the Missouri (Figure 3). Fish also tended to show no change in theOhio River, with only gizzard shad, Dorosoma cepedianum, showing a significant difference (Figure 4).Few significant differences were observed in 1515N values between sample periods. Only freshwater drum,Aplodinotus grunniens, which was slightly more 15N-depleted in 1994, had a 1515N signature that differedbetween years in the upper Mississippi River (Figure 2). No differences were detected among fish takenfrom the Missouri River (Figure 3). D. cepedianum, with a higher 1515N in 1994, was the only fish topossess a nitrogen signature that differed significantly between years in the Ohio River (Figure 4).

Organic matter source:consumer relationships

Consumer <513C values covered a broad range in the upper Mississippi River ( -32 to -22%0),overlapping several potential sources of organic matter. Incorporation of <51sN values, however, helpedclarify trophic relationships (Figure 5). Previous studies have stated that a change of :t 1 %0 in <513C canbe expected as organic matter transfers from one trophic level to the next (Peterson and Fry, 1987). Shiftsin <51sN are more pronounced with a typical enrichment of 3-5%0 when moving from the lower to the nexthigher trophic level (DeNiro and Epstein, 1981). Recent studies have suggested, however, that IsN-enrich-ment from one trophic level to the next highest level can be less than 3%0 (Fry, 1991; Meili et al., 1993;Keough et al., 1996; Hansson et al., 1997; Thorp et al., 1998). Evidence of IsN-enrichment < 3%0 wasapparent in the upper Mississippi River within both sample periods. Piscivores, such as Micropterussalmoides (largemouth bass), had, on average, <51sN values only 2-3%0 greater than insectivorous fishes.Isotopic ratios of the zooplanktivore Dorosoma cepedianum and crustacean zooplankton also followed thispattern, with an enrichment of about 2- 3%0 from prey to consumer. As has been observed in other stableisotope investigations of trophic pathways, it can be assumed that lower trophic exchanges would exhibitsimilar levels of IsN-enrichment relative to higher trophic 1evels.

Although <513C of some invertebrate consumers was within 1-2 %0 of the floodplain vegetation,invertebrate <51sN is 6-8%0 greater than the <51sN of C3 terrestrial plants and its associated forms ofparticulate organic matter (CROM and FROM). This makes it unlikely that terrestrial vegetationcontributed directly to a substantial proportion of the trophic groups in the Mississippi River eitherduring the 1993 flood or in the following year. It does appear that some allochthonous organic mattermoved through the food web as FROM to oligochaetes and other detritivores. Comparison of <513C and<51sN of CTOM to that of invertebrate consumers suggested that a relationship existed in both years(Figure 5). It is very unlikely, however, that CTOM served as a major food source. Although amounts ofCTOM were not quantified, the effort required to filter a sufficient amount (1 mg) for stable isotope~

Figure 5 (next page). ;5 IJC and ;5 15N values for consumers and potential organic matter sources of the lower Mississippi River,October 1993 and July 1994. Potential food sources are shown as boxes delineated by the mean::!: I S.D. Consumers are representedby open circles (mean::!: I S.D.). Numbers correspond to the following consumers: (1) Acroneuria sp.; (2) Argia sp.; (3) Corbiculafluminea; (4) Heptagenia; (5) Hydropsyche sp.; (6) Lithasia sp.; (9) Oligochaeta; (10) Pleurocera; (11) Stenonema/Stenacron; (12) D.polymorpha; (13) zooplankton; (14) I. punctatus; (15) Pomoxis nigromaculatus; (16) Aplodinotus grunniens; (17) D. cepedianum; (18)M. salmoides; (19) M. chrysops; (23) Stizostedion canadense; (24) Pisidiidae; (25) Cheumatopsyche sp.; (26) Moxostoma spp.; (28)Hirudinea; (29) Quadrula sp.; (30) Campeloma sp.; (31) Chironomidae; (32) Cyprinus carpio; and (33) Hexagenia sp. A compositemean of M. chrysops, Lepomis macochirus, and P. nigromaculatus was created (27) for 1994 data because these taxa overlapped

extensively

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227FLOOD RESPONSES IN RIVERS

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228 M.D. DBLONG ET AL.

analysis reflected the extremely low abundance of this size fraction. Moreover, most CTOM sampleshad 1515N values closer to C3 terrestrial plants, putting CTOM well outside of the 3-5 %0 range of15N-enrichment that would suggest a trophic relationship in this system.

The closest affiliation of potential sources of organic matter to primary consumers in the MississippiRiver, based on carbon and nitrogen enrichment along the food chain, was that of FTOM and DN tothe majority of benthic consumers. This association appeared to include the Hydropsychidae, Bivalvia,Chironomidae, and Heptageniidae, all of which are major components of the invertebrate communityof large rivers (Beckett and Miller, 1982; Beckett et at., 1983; Delong and Payne, 1985), and zooplank-ton. Direct or indirect consumption of dissolved organic matter by aquatic invertebrates has been welldocumented (Richardson and Mackay, 1991; Porter, 1996; Ciborowski et at., 1997). The level ofenrichment of 13C and 15N within invertebrate predators (represented by Argia sp.) and insectivorousfishes suggested that primary consumers using organic matter originating from FTOM and DN werethe major food source for higher trophic levels. This enrichment pattern followed through to the finaltrophic level, the piscivorous fishes. Examination of stable isotopic data, therefore, indicated thatFTOM and DN were major sources of organic matter for consumers during both sample periods.

Although there were also few differences in 1513C and 1515N in comparison of samples from theflood year to those of the non-flood year, evaluation of the relationship between sources and con-sumers paints a very different picture in the Missouri River (Figure 6). The pronounced overlap of theisotopic ratios of the various forms of organic matter made it difficult to make clear linkages. Initialexamination of the data might suggest that benthic algae were an important part of trophic dynamicsin the Missouri River. Benthic algal samples from the Missouri River consisted of encrustingcyanobacteria that were scraped from woody debris. Although great care was taken, it is probablethat some contamination with wood particles occurred during sample collection. This may account forthe broad range of 15 13C for benthic algae. Despite the fact that the results of stable isotopic analysisof the Missouri River were inconclusive with regard to developing a strong linkage with a specificorganic matter source, the limited number of significant differences detected by statistical analysissuggested that trophic dynamics of the Missouri River did not shift in response to the flood of 1993.

Between-year comparisons for the Ohio River, like the Mississippi, did not reveal marked significantdifferences in 1513C and 15 15N among either consumers or potential food sources (Figure 7). Further-more, many of the trends evident in the flooded Mississippi River were also apparent in the unfloodedOhio. Consumer 1513C values covered abroad range ( -32 to -22%0), overlapping several potentialsources of organic matter. Evaluation of consumers again indicated that 15N-enrichment among thesegroups to be around 2%0. As was the case with the Mississippi River, this placed FTOM and DN, inaddition to UTOM, as the sources in sufficient range of primary consumers to aCCOunt for expectedlevels of 15N-enrichment. Both UTOM and DN fell within this range during both years. FTOM, onthe other hand, had a higher 1513C and a lower 15 15N in 1993. This discrepancy was not apparent inanother Ohio River floodplain site located further upstream (Delong and Thorp, unpublished data)nor was it evident at a site in the upper Ohio River (Thorp et at., 1998). The limited number ofusable samples of FTOM from the lower Ohio River may account for this difference; nevertheless, itis still significant that. the consumers examined appeared to be most closely allied to UTOM and DN,for which the isotopic signatures did not change significantly between years.

Figure 6 (next page). t513C and t51sN values for consumers and potential organic matter sources of the Missouri River, October 1993and July 1994. Potential food sources are shown as boxes delineated by :tl S.D. Consumers are represented by open circles(mean :t 1 S.D.). Numbers correspond to the following consumers: (I) Acroneuria sp.; (2) Argia sp.; (4) Heptagenia sp.; (5)Hydropsyche sp.; (7) Polycentropus sp.; (9) Oligochaeta; (II) Stenonema/Stenacron; (14) I. punctatus; (16) A. grunniens; (17) D.cepedianum; (18) M. salmoides; (19) M. chrysops; (20) Pylodictis olivaris; (22) L. macrochirus; (28) Hirudinea; (31) Chironomidae;

(32) C. carpio; (34) Perlidae; (35) Physella sp.; (36) Corydalus sp.; (37) Baetis sp.; and (38) Hypophthalmichthys nobilis

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229FLOOD RESPONSES IN RIVERS

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230 M.D. DELONG ET AL.

DISCUSSION

Comparisons of the statistical analyses reveal that interannual trends observed for sources and consumersfor the two flood-impacted rivers were similar to the interannual trends of the unflooded Ohio River.Even when significant differences were noted among potential food sources or consumers for either theMissouri or Mississippi Rivers, a concomitant difference was evident in the Ohio River. For example,differences noted in the c515N of Stenonema sp., D. polymorpha, and Argia sp. in the Mississippi Riverwere matched by significant differences for the same or similar taxa in the Ohio River. Consistencies introphic structure among all three rivers were also supported by examination of organic matter:consumerlinkages. Both the Ohio and upper Mississippi River demonstrated similar structure in trophic pathwaysin 1993 and 1994. It was predicted that the flood affected trophic structure if the stable isotope ratios ofsources and consumers from the Mississippi or Missouri Rivers exhibited interannual shifts different fromthose observed in the Ohio River. Because what shifts that did occur in the flood-impacted riverscomplimented shifts in isotopic ratios observed in the Ohio River, as well as the general lack of shifts instable isotope ratios, it can be assumed that the patterns observed in all three rivers were naturalresponses rather than flood-induced effects.

Why would a flood falling outside the normal high-water period not create a fundamental change in thetype of organic matter driving the food web of large river ecosystems? Failure to reject the null hypothesisthat there would be no functional shift in trophic structure because of the flood does not rule out thepossibility that increased inputs of organic matter sources used by primary consumers under 'typical'conditions may have entered the food web during the flood. Rather than changing the composition oforganic matter available to consumers, the flood may have served to alter or enhance the availability ofthe forms of organic matter already present and used in the rivers. Inundation of the floodplain has beensuggested, at least for tropicai rivers, to provide increased availability of terrestrial litter (Junk, 1984;Soares et al., 1986; Junk et al., 1989) and autochthonous organic matter (Wissmar et al., 1981;Araujo-Lima et al., 1986; Hamilton et al., 1992). Inundation of the floodplain during 1993 eventually leadto the formation of a broad moving littoral zone that remained in place during the summer and into theearly fall in some areas of both rivers. The presence of this expansive slow-moving littoral during thesummer would coincide with a period of increased primary production in the upper Mississippi River(Grubaugh and Anderson, 1989) and possibly the Missouri River (Galat et al., 1998), although ourobservations indicated that phytoplankton and benthic algal abundances were low in the Missouri.Continued flooding in some areas during litterfall would also have made new terrestrial leaf litteravailable during later stages of the flood (Grubaugh and Anderson, 1989). It may be possible, then, thatconsumers responded to flooding in the summer of 1993 by taking advantage of greater availability offood resources typically used within the system.

Is it possible that changes in trophic structure resulting from the 1993 flood were not detected in thisstudy because recovery from the flood extended beyond the samples collected in July 1994? The answerto this question is an obvious yes. Pre-nood stable isotope information for potential food sources andconsumers is lacking for all three rivers; therefore, it was impossible to take a before and after perspectivein this investigation. Therefore, it was possible that the lack of differences between 1993 and 1994 werebecause the two rivers had still not recovered trophically from the effects of the flood. Despite this caveat,it is still contended that the evidence presented strongly suggests that the rivers had recovered from theflood. The strongest case from the data was that each river followed the same trends as the unfloodedOhio River. If there had been marked changes in trophic pathways, there should have been pronounceddeviations from what was seen on the Ohio.

Figure 7 (next page). O 13C and 0 ISN values for consumers and potential organic matter sources of the lower Ohio River, Oc~ober1993 and July 1994. Potential food sources are shown as boxes delineated by :t I S.D. Consumers are represented by open cIrcles(mean :t I S.D.). Numbers correspond to the following consumers: (2) Argia sp.; (3) Corbicula fluminea; (6) Lithasia sp.; (8)Neurocordulia sp.; (10) Pleurocera sp.; (11) Stenacron sp.; (12) D. polymorpha; (13) zooplankton; (14) I. punctatus; (17) D.cepedianum; (18) M. salmoides; (21) N. atherinoides; (22) L. humilis; (28) Hirudinea; (30) Campeloma sp.; (31) Chironomidae; (33)Hexagenia sp.; (39) Enallagma sp.; (40) Etheostoma caeruleum; (41) M. salmoides/Stizostedion canadense composite sample; (42)

Orconectes sp.; (43) Gammarus fasciatus; (44) Leptodea sp.; and (45) Potamila sp

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231FLOOD RESPONSES IN RIVERS

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232 M.D. DELONG ET AL.

Sparks et al. (1990) observed that the flood of 1973 had little effect on the biota of the upperMississippi River. There was no measurable change in total biomass and species composition of benthicinvertebrates. Additionally, aquatic macrophyte beds remained stable from before to after the flood(Steffeck et al., 1985). Sparks et al. (1990) pointed out, however, that the 1973 flood should not be viewedas a disturbance because its timing overlapped the expected period of seasonal flooding. What of the floodof 1993, an event that occurred well outside the predicted time frame for flooding in the upper Mississippiand its tributaries? Studies relating to the 1993 flood revealed that the reproductive success and growthof many fishes exceeded what had been observed during normal or low water years in the Illinois andupper Mississippi Rivers (Bhowmik et al., 1994; Bartels, 1995). There was also no detectable effect onrecruitment and densities of unionid mussels (Miller and Payne, 1998). Impacts on aquatic organismsranging from beneficial to minimal were also evident in the Missouri River following the 1993 flood(Galat et al., 1998). The findings of past studies relating to large-magnitude flood events in the upperMississippi River system, therefore, strengthen the conclusion that the flood served to enhance rather thanchange the trophic structure within these river-floodplain ecosystems.

ACKNOWLEDGEMENTS

The authors wish to thank the following people for their assistance with sample collection: ChristineRaak, Kari Marley, Jason Maurer, Joseph-Marley, Richard Lehtinen, and Greg Moeller (Winona State).The authors also appreciate the contributions of Andrew Casper and students in the University ofLouisville's Large River Program, and Lane Stocker of the University of Cincinnati. David Galat andthree anonymous reviewers provided helpful editorial comments. This project was supported primarily byan NSF ecosystem SGER grant (DEB 9320303) to J. H. Thorp, M.D. Delong, J.D. Wehr, and M.C.Miller. Additional support was provided through professional improvement funds from Winona StateUniversity (MDD). This paper represents contribution number 99-01 of the Large River Studies Center,Winona State University.

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