Effect of streambed sediment on benthic ecology

International Journal of Sediment Research 24 (2009) 325-338

Effect of streambed sediment on benthic ecology Xuehua DUAN1, Zhaoyin WANG2*, Mengzhen XU3, Kang ZHANG4 Abstract Benthic macroinvertebrates have been commonly used as indicator species for assessment of aquatic ecology. Streambed sediment, or substrate, plays an important role in habitat conditions for macroinvertebrate communities. Field investigations were done to study the benthic diversity and macroinvertebrate compositions in various stream substrata. Sampling sites with different bed sediment, latitude, and climate were selected along the Yangtze River, the Yellow River, the East River, and the Juma River, in China. The results show that benthic community structures found in different substrata clearly differ, while those found in substrata of similar composition and flow conditions but in different macroclimates are similar. The study, thus, demonstrates that the benthic macroinvertebrate community is mainly affected by substrate composition and flow conditions, but is generally unaffected by latitudinal position and macroclimate. Taxa richness of the macroinvertebrate community was found to be the highest on hydrophyte-covered cobbles, high on moss-covered bedrock, and low on clay beds and cobble beds devoid of plant biomass. Sandy beds are compact and unstable, thus, no benthic macroinvertebrates were found colonizing such substrata. Aquatic insects account for most of the macroinvertebrates collected in these rivers. Different insects dominate in different types of substrata: mainly EPT species (Ephemeroptera, Plecoptera, Trichoptera) in cobble, gravel, and moss-covered bedrock; and Chironomidae larvae in clay beds. The relation between the number of species in the samples and the size of the sampling area fits a power function of the species area. One square meter (1m2) is suggested as the minimum sampling area. A substrate suitability index is proposed by integrating the suitability of sediment, periphyton, and benthic organic materials for macroinvertebrates. The biodiversity of macroinvertebrates increases linearly with the substrate suitability index. Benthic taxa richness increases linearly with the suitability index. Key Words: Streambed sediment, Macroinvertebrates, Benthic ecology, Substrate suitability, Biodiversity

1 Introduction Freshwater benthic macroinvertebrates are animals with no backbones that are larger than 0.5 mm. Streambed sediment, also called substrate by ecologists, is the principal habitat and primary refuge for benthic invertebrates (Reice, 1985; Arunachalam et al., 1991). Many species of invertebrates feed on algae and bacteria, and some eat leaves and other organic matter entering the river. Macroinvertebrates are also important sources of food and energy for vertebrates such as fish. Because of their abundance and 1

Ph.D., Jiangyin Environmental Monitoring Station, Jiangyin 214431, China, Dept. of Hydraulic Engineering, Tsinghua University, E-mail: [email protected] 2 Prof., State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China, Tel: 86-10-62773448(O), *Corresponding author, E-mail: [email protected] 3 Doctoral candidate, State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China, E-mail: [email protected] 4 Doctoral candidate, State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China, E-mail: [email protected] Note: The original manuscript of this paper was received in Dec. 2008. The revised version was received in April 2009. Discussion open until Sept. 2010. International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

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crucial position as “middlemen” in the aquatic food chain, macroinvertebrates play a critical role in aquatic ecosystems. When benthic invertebrates die, they decay, leaving behind nutrients that are reused by aquatic plants and other animals in the food chain. Benthic macroinvertebrates have several primary advantages in bio-monitoring compared with the other groups: (a) they are ubiquitous (Lenat et al., 1980) and basically sedentary, thus, they can be collected easily; (b) they differ in their sensitivity to water pollution and can integrate environmental changes in physical, chemical, and ecological characteristics of their habitat over time and space (Milbrink, 1983); (c) they have long life cycles, and, thus, can provide information about the quality of a stream over long periods of time; (d) they may be affected by a variety of environmental disturbances impacting different aquatic ecosystems, allowing an analysis of a spectrum of responses to environmental stresses (Rosenberg and Resh, 1993). Due to these advantages, macroinvertebrates can be utilized to compute important bio-monitoring indices (Resh and Jackson, 1993), and are widely used as the indicator fauna for rapid stream ecology assessment (Plafkin et al., 1989; Smith et al., 1999; Karr, 1999). Benthic macroinvertebrates are highly adapted to a wide range of natural conditions in freshwater environments. Substrate is the primary refuge of benthic residents, and plays the role of the principal habitat of benthic invertebrates (Flecker and Allan, 1984; Cobb et al., 1992). The complex nature of fluvial geomorphology forms heterogeneous streambeds of unevenly distributed habitats (Wang et al., 2007). Many invertebrates show different preferences for substrata. The distribution pattern of benthic individuals and species occurrence are highly dependent on substrate size (Erman and Erman, 1984; Beisel et al., 1998; Buss et al., 2004). Reice (1980) and Beauger et al. (2006) also stressed the prominence of grain size as an important determinant for lotic macroinvertebrate community structures. Evans and Norris (1997) emphasized that rock dimensions gave the best discrimination of the community with regard to the physical environment. The substrate characteristics that strongly affect the resident macroinvertebrate community also include stability, heterogeneity, and compactness. Previous studies have consistently found that the biodiversity of invertebrates has positive correlations with the heterogeneity and stability of the streambed (Beisel et al., 2000), and is higher in a loose bed than in a compact bed (Cobb et al., 1992; Flecker and Allan, 1984). Verdonschot (2001) and Jowett (2003) also reported that benthic macroinvertebrate assemblages were greatly dependent on the streambed stability at the reach scale. Locations with unstable substrates or subject to the passage of saltating bedload are unlikely to be suitable habitats for most benthic species, either because food sources are not present where bedload movement occurs or because the substrate does not provide a secure platform for benthic invertebrates (Jowett, 2003). When substrata are disturbed, most benthic residents usually depart from the streambed and drift to downstream reaches (Waters, 1972). More complex substrata generally support more species (Minshall, 1984). Buss et al. (2004) spotlighted that each substrate supports a particular macroinvertebrate structure, corroborating that macroinvertebrates are not random assemblages of species (Melo and Froelich, 2001). Sediment transportation occurs on almost all stream beds, and affects the number of species and community of benthic invertebrates. This paper studies the effects of bed sediment composition and stability of the streambed on the biodiversity and community of macroinvertebrates based on field investigations. 2 Study sites Field investigations were conducted from June 2005 to May 2008 at selected sites in the Yangtze River, the Yellow River, the East River and the Juma River basins, in China. Thirteen sampling sites were selected, as shown in Fig. 1. The Yangtze River Basin has a sub-tropical climate with average annual air temperature between17 and 18 ℃, and receives 1,000-1,900 mm/yr of precipitation. The sampling sites in the Yangtze River Basin are located on the Jiuzhai Creek; the Fujiang River; the Shengou, Jiangjia and Xiaobaini ravines; and the Chexi and Shennongxi Streams. Jiuzhai Creek and the Fujiang River are tributaries of the Jialing River in the upper Yangtze River Basin. The Shengou, Jiangjia and Xiaobaini ravines are tributaries of the Xiaojiang River in southern China, which flows into the Jinsha River - an upstream reach of the Yangtze River. The Chexi and Shennongxi Streams are both tributaries of the Yangtze River in the Three Gorges reaches. - 326 -

International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

南

10 9

8 1 6 2

3

7

4 5 13

River Sampling site Lake

0

0

11 12

600

600

1200 km

1200 km

Fig. 1 Locations of the thirteen macroinvertebrate sampling sites 1 Jiuzhai Creek; 2 Fujiang River; 3 Jiangjia Ravine; 4 Shengou Ravine; 5 Xiaobaini Ravine; 6 Shennongxi Stream; 7 Chexi Stream; 8 Qingjianghe River; 9 Yellow River at Kenli; 10 Juma River; 11 Tributary of the East River at Yidu; 12 Xizhijiang River; and 13 East River at Yuanzhou

The Yellow River possesses a temperate climate with distinct seasons, abundant sunshine, and an average annual precipitation of 560 mm. The sampling sites in the Yellow River Basin are located on the Qingjianghe River and the mainstream of the Yellow River at Kenli County. Qingjianghe River is a second-order tributary of the Yellow River. The sampling reach of the Yellow River at Kenli had a relatively high suspended sediment concentration of 10 kg/m3, while the corresponding concentrations at the other twelve sampling sites all were very low. International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

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Table 1 Streambed sediment composition, geographical location and flow parameters of the sampling sites H V DO Sampling site Streambed sediment composition (m) (m/s) (mg/L) Juma River Cobbles and gravel (Dm=72mm); and 0.2~0.5 0.25~0.5 9.76 aquatic macrophyte

Yellow River Basin

Juma River

The East River Basin has a combination climate of sub-tropical and tropical, without clear seasons. The annual precipitation varies in a range of 1500-2400 mm and the average annual air temperature ranges from 21 to 22℃. The sampling sites in the East River Basin are located on a tributary at Yidu in the upstream reaches, on the Xizhijiang River, a first-order tributary in the middle reaches, and at Yuanzhou in the lower reaches of the river. The Juma River Basin has a semi-humid, temperate climate, with a wet and hot summer, a dry and cold winter, and a short spring and autumn. The annual precipitation is 587.6 mm, and the average annual air temperature is 12.5℃. The stream drains a catchment that mainly consists of mountains, numerous gullies and some agricultural and residential areas. The sampling site on the Juma River is located on the Shidu reach, and the riverbed is covered with gravel and cobbles with a slippery film of periphyton. In summer, macrophytes cover more than 80% of the riverbed. Samples of bed sediments were taken and analyzed. Table 1 lists the bed sediment composition, velocity, water depth, and the dissolved oxygen concentration at the sampling sites. The river bed substrata composition and flow parameters differed considerably at the thirteen sites. Figure 2 shows the cumulative size distributions of bed sediments at the sampling sites.

Yangtze River Baisn

0.1~0.7

8.67

0~1.0

0.3~1.0

10.3

0.2~0.7

11.83

0.3~0.5

11.01

Fine sand (Dm=0.07mm), flow with suspended sediment of about 10 kg/m3

Qingjianghe River

Bedrock, moss

Shennongxi Stream

Cobbles (Dm=158mm), aquatic epiphytic 0.2~0.6 moss Bedrock, aquatic epiphyte covered, 0~0.5 shading by riparian vegetation S*: Boulders and huge stones; S*:0.1~0.5; P*: Gravel bed, aquatic grass P*:0.1~2 Fully developed step-pool systems, S*:0.1~0.3; S*: Boulders and cobbles; P*:0~0.8 P*: Aquatic grass, gravel and fluid mud Gravel and cobbles 5-60 mm; sand and 0.05~0.15 gravel filling in the interstices of the cobbles A moving layer of sand and gravel on 0~0.1 the bed; unstable bed Clay, macrophytes growth 0.2~5.0

Chexi Stream Jiuzhai Creek Shengou Ravine Jiangjia Ravine Xiaobaini Ravine Fujiang River East River Basin

0.5~0.7

Yellow River at Kenli

Xizhijiang River

Clay and silt (Dm =0.008 mm); sparse hydrophytes; pollution-control project under construction on the bank at Coarse sand (Dm=0.6 mm)

0.1~0.3

S*:0.3~3.0; P*:0~0.5 S*:0.6~1; P*:0~0.3

10.0 9.30

0.4~1.2

8.40

0.3~1.2

8.40

<0.2

9.2

0.05~0.1

8.0

East River 0.1~1.0 0~1.0 7.9 Yuanzhou 0.1~0.7 0.1~0.4 8.2 Tributary of the East Cobbles, with small amounts of silt and River at Yidu sand (Dm=210 mm) Note: Dm - Median diameter of bed sediment at sampling sites; H - Water depth; V - Local flow velocity; DO - Dissolved Oxygen concentration; S*- Step; P*- Pool

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International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

p < D (%) ,

100 Xizhijiang River

80 60 40

Juma Xiaobaini River Ravine Jiangjia Ravine

Yellow River estuary

20 0 0.001

Fig. 2

0.01

Shennongxi Stream

East River at Yuanzhou

0.1

1 D (mm)

10

East River at Yidu 100

1000

Size distribution curves of bed sediments at the sampling sites

3 Study methods 3.1 Sample size To reduce the effect of sample plot size on taxa richness and biodiversity, a field investigation on the relation between the sampling area and the number of species collected was conducted to select a reasonable sampling area. Study sites with apparently consistent neighboring environments were chosen along a 100m stretch of the Juma River. The size of the sampling area is plotted against the number of species collected in Fig. 3. Overall, the number of species increases with the sampling area despite somewhat scattered data points, which may be due to sampling method limitations and varying local conditions at the different sites. However, as the area sampled increases, the rate at which new species are encountered slows down for sampling areas between 1 m2 and 1.5 m2. The mathematical regression equation is S=26A0.3, i.e., the number of species and the sampling area exhibit a power-exponent relation consistent with the viewpoints of most ecologists such as Arrhenius (1921). The fluctuation in the number of species for sampling areas between 1 m2 and 2 m2 is small. In order to ensure the reliability of research results, and considering actual workloads, 1 m2 is suggested as the minimum sampling area. This value should be larger where the density of macroinvertebrates is low.

Number of species

35 30 25 20 15

Recursivecurve curve recursive

10

Measureddata data measured

5 0 0

0.5

1

1.5

2

2

Fig. 3

Sampling area (m ) Number of species in the samples as a function of the sampling area

3.2 Materials and sampling methods Due to the complexity and diversity of the benthic habitat, field sampling usually requires a combination of quantitative and qualitative collection methods (Wang and Yang, 2001). Semi-quantitative samples were taken at appropriate depths of 0.15 m of the substrate with a kick-net for water depth less than 0.7 m. A D-frame dip net was used to qualitatively sample along stone surfaces and in plant clusters. For water depth greater than 0.7 m, replicates were sampled by using a Peterson grab sampler with an open area of 1/16 m2. Within each sampling area, three or four benthic samples spaced at least 2 m apart were collected. Bed materials greater than 64 mm located at the substrate-water interface (i.e. surface cobbles) were removed, and they were scrubbed with a wire brush in order to select invertebrates and debris, and International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

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subsequently they were discarded. Following the removal of surface cobbles, finer bottom materials were agitated and the debris and macroinvertebrates were blocked within the kick net. The debris and invertebrates were rinsed through a sieve (mesh size = 0.5 mm), then the collected material was placed in plastic sample containers and preserved in 10% formaldehyde. Substrata composition and flow parameters including water depth, local flow velocity and dissolved oxygen concentration, were also measured and recorded. All macroinvertebrates were picked out of the samples, identified and counted under a stereoscopic microscope in the laboratory. All species were identified, most to family or genus level (Liang et al., 1995; Liu et al., 1979; Morse et al., 1994). Oligochaeta, Hirudinea, Hydrachnidia and early-instar insects were identified only to higher taxonomic levels. The biomass of specimens at each sampling site was measured using an electronic balance with a precision of 0.0001g. 3.3 Methods of analysis Eight biotic indices were applied in order to evaluate the biodiversity of the macroinvertebrate communities in the sampling sites. These measures are defined in Table 2. Table 2 Item Taxa richness, S Density, D Weight biomass, W Shannon-Wiener (1949), H´ Modified Shannon-Wiener (Wang, 2003), B Pielou evenness (1975), J

Indices used in the evaluation of biodiversity Definition Note The number of species in the sample A widely-used measure of biodiversity The number of benthic individuals per unit area The weight of fresh femur of the total individuals per unit area S H΄ integrates both richness and H ' = − ∑ ( ni / N ) ln( ni / N ) evenness, a common diversity index i =1 An index integrating richness, evenness S B = − ln N ∑ (ni / N )ln(ni / N ) and total abundance i =1 The ratio of the measured diversity and ' ' ' J = H /H = H / ln S the maximum diversity max

Margalef richness (1957), dM

d M = ( S − 1) / ln N

Simpson diversity (1949), d S

d S = 1 − ∑ ( ni / N ) 2

S

The degree of taxa richness of the bio-community The opposite of dominance

i =1

Notes: S－number of species; N－total number of individual specimens; ni －the number of individuals in the ith species; H’max －the maximum diversity, i.e. the diversity when the community is completely evenly distributed, H’max=lnS.

4 Results 4.1 Structure and composition of macroinvertebrate community The compositions of the macroinvertebrate communities collected from the sampled substrata showed significant differences (Table 3). No individuals were collected at the Kenli reach of the Yellow River or at the Yuanzhou reach of the East River. Taxa richness for the Xiaobaini Ravine was also zero. Thus, these sites are not listed in Table 3. In total, thirty-three species were collected from the Juma River, belonging to four phyla and twenty-eight families. Twelve species were collected from the Xizhijiang River sampling site, belonging to three phyla and seven families. Ten species were collected from the Fujiang River. Sixteen and twenty-eight species were collected from the tributary of the East River at Yidu and from the Shennongxi Stream site, respectively. Seventeen and nineteen species were collected from the Chexi Stream and the Qingjianghe River, respectively. Both Chexi and Qingjianghe have bedrock riverbeds. Eight species were collected from the Jiangjia Ravine. Eighteen and twenty-nine species were collected from the Jiuzhai Creek and the Shengou Ravine, respectively, both of which have step-pool structures. - 330 -

International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

Table 3 Composition of macroinvertebrate community (number of individuals in each group per unit area) Sampling sites Taxa Arthropoda Chironomidae Tipulidae Ceratopogonidae Simuliidae Stratiomyidae Tabanidae Other Diptera Hydropsychidae Brachycentridae Polycentropodidae Hydroptilidae Philopotamidae Leptoceridae Rhyacophilidae Gomphidae Aeshnidae Libellulidae Cordulegasteridae Zygoptera Ephemeridae Caenidae Baetidae Siphlonuridae Heptageniidae Leptophlebiidae Ephemerellidae Plecoptera Hemiptera Psephenidae Elmidae Hydrophilidae Dryopidae Dytiscidae Megaloptera Acariformes Decapoda Gammaridae Lepidoptera Mollusca Corbiculidae Anodonta Lymnaeidae Planorbidae Melaniidae Hydrobiidae Physidae Viviparidae Bithyniidae Ampullariidae Mytilidae Annelida Oligochaeta Hirudinea Platyhelminthes Unidentified

Juma River

Chexi Stream

48 8 4

212 36 1 3

4 1

70

Shennongxi Stream 105 14

20 108

2 8

82 2

5

1

Shengou Ravine

Jiuzhai Creek

Jiangjia Ravine

138 2 35 9 1

65 9

0.083 0.25

3

Xizhijiang River 11

East River at Yidu

Qingjianghe River

21

195 3 11

Fujiang River

2 3 15 144

1 4

4 9 5

5 3 15

3

14 2

1

5

3

90 14 191 57 2 13 1

3 3 12

7 1

792

53

7

118 76 5

76 1 11 2

9 4

38 6 9

117 11 44 1 3 1

41 18

1 27

29

29

27

1 92

1 26 1 7 1 5

0.167 0.167

2 4

1

1 41 1 85

2

121 0.083

15 16 111

10

2

30 10

1

5 32 2

1

5 30 45

10 58 2 1 7

5 3 5

5

20

0.083 9 15

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1

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Table 4 lists the values of the individual density and biomass amounts at each site. It can be found that the density and biomass are the highest on moss-covered or hydrophyte–covered substrata. Except for the Fujiang River site, aquatic insects account for more than 60% of the total number of individuals collected. Exact percentages are 68.6, 73.1, 95.9, 67.6, 67.6, 80, 90.5, 81.2, and 98.2％ for the Juma River, Xizhijiang River, East River at Yidu, Jiuzhai Creek, Shengou Ravine, Jiangjia Ravine, Qingjianghe River, Shennongxi Stream and Chexi Stream sites, respectively. These high percentages indicate that aquatic insects account for most of the total benthos. The dominant population among the aquatic insects differed in different substrata, with EPT species (Ephemeroptera includes Ephemeridae, Caenidae, Baetidae, Siphlonuridae, and Heptageniidae in Table 3; Plecoptera; Trichoptera includes Hydropsychidae, Brachycentridae, Polycentropodidae, Hydroptilidae, Philopotamidae, Leptoceridae, and Rhyacophilidae in Table 3) constituting more than half of the total aquatic insects in cobbles, gravel, and moss-covered bedrock, while mainly Chironomidae larvae occur in clay beds. Photographs of typical macroinvertebrates found in different substrata are shown in Fig. 4. Table 4 Site Juma River Shennongxi Stream Chexi Stream Jiuzhai Creek Fujiang River East River at Yidu Qingjianghe River

a.1

Values of macroinvertebrate density and biomass for the sampling sites Density Biomass Density Site (ind/m2) (g/m2) (ind/m2) 754 14.1894 Shengou Ravine 398 488 1.4476 Jiangjia Ravine 0.83 1410 1.2346 Xiaobaini Ravine 0 552 6.3964 Xizhijiang River 26 125 / Kenli (Yellow River) 0 97 1.9528 Yuanzhou (East River) 0 423 /

Biomass (g/m2) 6.5029 0.0081 0 3.4785 0 0

a.2 a.3 b.1 b.2 c.1 c.2 (a) Cobble bed: a.1-Perlidae, a.2-Ephemeridae, a.3-Leptoceridae; (b) Clay bed: b.1-Oligochaeta, b.2-Chironomidae; (c) Bedrock: c.1- Baetidae, c.2 – Psephenidae (Scale note: one grid=1mm) Fig. 4 Typical macroinvertebrates in different substrata

4.2 Biodiversity comparisons Values for bio-assessment indices are presented in Table 5. Indices of S , H ' , B , d M , and d S , attain their highest values at the Juma River site. Taxa richness for the Shennongxi Stream is the third highest. Values of S , H ' , B , and d M are high at the Shengou Ravine and the Jiuzhai Creek sites, both of which have step-pool sequences. This demonstrates that hydrophyte-covered cobbles and streambeds with step-pool structures support the highest biodiversity. Except for the Juma River, the Shengou Ravine, and the Jiuzhai Creek, biodiversity for the Qingjianghe River is the highest. The lowest evenness value (J = 0.5) for the Chexi Stream site results from the dominance of Baetidae larvae, and it leads to a low value of Shannon-Wiener index ( H ' ). However, the water in the Chexi Stream has no pollution, and the abundance of pollution-sensitive species at the site is higher than at the other twelve sites. Macroinvertebrates collected from Chexi Stream also possessed a high taxa richness (S) and Margalef value (dM), showing high biodiversity. Values of the biotic indices for the tributary of the East River at Yidu are lower than those for the Juma River, Shennongxi Stream, Chexi Stream and Qingjianghe River, which all have aquatic plant growth. Values of taxa richness (S) and individual density (D) are low for the Fujiang River site. The evenness value of macroinvertebrates (J) collected from the Xizhijiang River site is high (0.93), which makes - 332 -

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values of the Shannon-Wiener ( H ' ) and Simpson (dS) indices high (2.30 and 0.88, respectively). However, taxa richness (S), density (D), and value of modified Shannon-Wiener index (B) for the Xizhijiang River are all low, which demonstrates that the biodiversity level for the site is very low. Values of these indices are the lowest at the Jiangjia Ravine, except the null results for the Xiaobaoni Ravine, the Yellow River at Kenli and the East River at Yuanzhou. Table 5 Site Juma River Shennongxi Stream East River at Yidu Shengou Ravine Jiuzhai Creek Qingjianghe River Chexi Stream Jiangjia Ravine Fujiang River Xizhijiang River Xiaobaini Ravine East River at Yuanzhou Yellow River at Kenli

Results of the bio-assessment indices for the sampling sites S J H’ B dM 33 0.70 2.65 17.56 6.49 28 0.71 2.28 14.11 4.36 16 0.67 1.85 8.46 3.28 29 0.72 2.60 15.54 4.68 18 0.84 2.29 14.47 3.27 19 0.71 2.08 12.58 2.98 17 0.50 1.57 11.37 3.59 8 0.92 2.02 -0.38 2.61 10 0.82 1.88 9.08 1.86 12 0.93 2.30 7.49 3.38 0 / 0 0 0 0 / 0 0 0 0 / 0 0 0

dS 0.90 0.86 0.75 0.89 0.85 0.83 0.65 0.86 0.79 0.88 0 0 0

The bio-assessment indices in Table 5 reveal that for the natural rivers, benthic biodiversity is the highest on cobbles with hydrophyte growth, high on moss-covered bedrock, low on cobbles devoid of plant biomass and clay beds, and the lowest on sandy beds. No invertebrates were found colonizing sandy beds, which demonstrates that sand is very inhospitable to invertebrates in large rivers. Because of the high compactness and extreme mobility of sand substrate, invertebrates have great difficulty in finding places to live and shelter within it. Biodiversity increases with the increase in particle size, reaching a maximum value for cobble-size bed. Biodiversity subsequently decreases somewhat as the particle size continues to increase to that of boulders and bedrock. The result is consistent with that of Grubaugh et al. (1997), who found that annual production of macroinvertebrates was relatively low in cobble substrata devoid of plant biomass (mosses and hydrophytes), greater in bedrock habitats and greatest on hydrophyte-covered cobbles. 4.3 Substrate suitability index Substrata can be categorized into two classes: inorganic substrate (streambed sediment) and organic substrate (periphyton, benthic organic material). The composition, stability, and heterogeneity of inorganic substrata greatly influence macroinvertebrate assemblage structure at the sample scale (Downes et al., 1995). Streambed sediment is always classified into six groups according to the grain size of the dominant substrate: bedrock, large stone (>200 mm), cobble (20~200 mm, further classified into cases of hydrophyte presence and hydrophyte absence), gravel (2~20 mm), sand (0.05~2 mm), silt and clay (<0.05 mm). The availability of periphyton food sources can distinctly influence the distribution and abundance of benthic invertebrates (Jowett and Richardson, 1990). Rabeni and Minshall (1977) concluded that detritus is a primary factor influencing the distribution of benthic invertebrates. Accordingly, research concerning the effect of streambed substrate on macroinvertebrates should consider both inorganic substrate and organic substrate. In this paper a substrate suitability index is proposed as follows: Ns= a Nsi = a [b1×(stones + bedrock) %+ b2×cobbles %+ b3×gravel %+ b4×sand %+ b5×(silt + clay)%]

(1)

where a is the suitability of benthic organic material, Nsi is the suitability of streambed sediment, bi is the suitability value of each sediment group, % is the percentage of each sediment group in each sample. The value of a was assigned based on the conclusion of Jowett and Richardson (1990). It was found that substrate with a slippery film of periphyton appeared to be the best habitat, clean substrate the worst, and substrate with an evident layer of periphyton was intermediate. Therefore, a is assigned as follows: a =1 for clean substrate, a = 1.5 for substrate with evident periphyton growth or with lots of benthic organic International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

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detritus, and a = 2 for slippery substrate. Values of bi were assigned based on the results of the section “Biodiversity comparisons” from this paper. The suitability values of stones and bedrock, cobbles, gravel, sand, and silt and clay were assigned as b1 = 4, b2 = 5, b3 = 2, b4 = 1, b5 = 3, respectively. The bi value of each substrate type is a modified form of the original IFIM (In-stream Flow Incremental Methodology) substrate codes (Bovee, 1982), adding the effects of silt and clay and considering the suitability of each substrate. Values of the inorganic substrate suitability, N si , and the substrate suitability index, N s , for the sampling sites are plotted against taxa richness, S, in Fig. 5. Points also are included in Fig. 5 for the sampling sites from previous work. It can be seen that S has positive relation with both N s and N si . However, the data points for S ~ N si are much more scattered than those for S ~ N s . Furthermore, the correlation coefficient of S versus N s is significantly larger than that of S versus N si . This result stresses the important role of periphyton and benthic organic material in shaping macroinvertebrate structures. The fluctuations of the data points in the two figures result from the complexity of the habitat environment, i.e. it is difficult to study the effect of a single factor, such as substrate, on benthic macroinvertebrates for natural streams and rivers.

Taxa richnes

50 unpolluted Unpollutedsites sitesininour ourprevious previouswork work

40

r = 0.84

sites Sitesininthis thisstudy study

30 20 10 0 0

2

4 6 Substrate suitability index, N s

8

10

(a)

Taxa richnes

50 unpolluted Unpollutedsites sitesininour ourprevious previouswork work

40

Sitesininthis thisstudy study sites

30

r = 0.70

20 10 0 1

Fig. 5

2 3 4 Inorganic substrate suitability coefficient, N si

5

(b) Correlations of taxa richness with (a) inorganic substrate suitability coefficient, N si , and (b) substrate suitability index, N s (r: correlation coefficient for all data)

5 Discussion 5.1 Climate Many researchers have reported that macroinvertebrate species richness is negatively correlated with latitude, with though low correlation coefficients (Oswood, 1989; Miserendino, 2001). Sampling sites on the Juma River, on the Shennongxi Stream and in Yidu township in the East River basin, all had cobble substrata and similar flow conditions, such as water depth, water temperature, and local flow velocity. - 334 -

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However, the Juma River, the Shennongxi Stream, and the tributary of the East River at Yidu sites differed in their geographical locations, lying in the northern, central, and southern parts of China, respectively. The three sites presented visibly different macroclimates. Unexpectedly, values of taxa richness for the three cobble bed sites increased with latitude, opposite to the general hypothesis. The differences of the invertebrate communities among these three sites were studied via Sorensen’s similarity index. The definition of this index is SI=2c/(x+y), in which x is the number of species in X community, y is the number of species in Y community, and c is the number of mutual species in communities X and Y (Ma et al., 1995). Table 6 lists the values of the similarity index among the macroinvertebrate communities for these three sites. Table 6

The similarity of benthic communities among three cobble substrata sites Juma River Shennongxi Stream

Yidu town site

0.47

Shennongxi Stream

0.47

0.47

Fifteen families were collected at both the Yidu site in the East River basin and at the Shennongxi Stream site, ten of which occurred in the Juma River sample. Moreover, for the remaining five families, three from the Shennongxi sample and two from the Yidu sample, also have been identified in samples from previous field investigations on the Juma River. The similarity coefficient values are all close to 0.5, indicating a relatively high level of similarity among the bio-communities at the three cobble substrata sites. Thus, from this study it can be deduced that benthic macroinvertebrate communities are mainly affected by the substrate composition with little influence from latitudinal position and macroclimate. Macroinvertebrate communities are dependent on the local conditions and have low correlation with macroclimate. 5.2 Homogeneity and compaction of substrata Compared with homogeneous streambeds, substrata exhibiting a wide range of particle sizes create a physically more complex and heterogeneous habitat. Such environments can provide more suitable conditions for more species to live, and therefore, support a greater variety of benthic species. The diversity of benthic invertebrates is directly proportional to the availability of different micro-habitats. The cobble beds on the Juma River, on the Shennongxi Stream and at the Yidu site in the East River basin, with particle sizes ranging from 3-9 cm, 1-20 cm, and 2.5-50 cm, respectively, all create high-quality habitat environments for invertebrates. In contrast, sandy beds are compact and the interstices between sand particles are too small for benthic macroinvertebrates to move and live within them. Thus, sandy beds only supply a low diversity of habitats and can only support a low biodiversity. The zero abundance observed at the Kenli reach of the Yellow River may also be related to the characteristics of its sand, the so-called iron sand. The high compaction degree of such sand makes it unsuitable for invertebrates to colonize. Therefore, sandy beds in large rivers are the worst substrate for benthic macroinvertebrates. Bedrock is also structurally simple substrate, and its high biodiversity is due to the moss growth providing the major food resources. 5.3 Stability of substrata The composition of invertebrates depends upon the stability of habitat, and biodiversity increases with habitat stability. Any form of substrate instability has an adverse impact on invertebrates (Beisel et al., 1998; Cummins, 1979). Sandy beds are unstable and subject to rapid erosion and deposition, leaving invertebrates attempting to colonize them insufficient time to reproduce. Hubert et al. (1996) concluded that the enhancement of sediment deposition in small high-plain streams influences on macroinvertebrate assemblages. Clay substrate is loose and rich in deposited organic debris, and this in combination with aquatic plants as food for aquatic fauna would support a relatively high biomass. However, the individual density at the Xizhijiang River sampling site was low, due to intensive erosion and deposition caused by the flood that occurred prior to the actual investigation. Moss-covered bedrock can supply a stable substrate for bio-communities, but resident benthic organisms are subject to being flushed out by the lotic flow, thus, only those with a great ability to cling are suited to survive in such places. Step-pool sequences International Journal of Sediment Research, Vol. 24, No. 3, 2009, pp. 325–338

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can enhance the flow resistance and stabilize the streambed and banks. Streams having step-pool sequences or cobble substrate are stable, can provide multiplayer living spaces, and are favorable refuges for a great variety of benthic species. Clearly, the stability of substrata is an important determinant in shaping macroinvertebrate communities. 5.4 Aquatic plants Aquatic plants, such as mosses, aquatic grasses, and algae, are also important in determining the composition of the benthic bio-community. Plants provide food, habitat, and refuge for invertebrates; can stabilize substrata; enhance access of filter-feeding invertebrates to food resources suspended in the water column, and mitigate perturbations due to spates (Beisel et al., 1998). In addition to providing habitats for many species, aquatic grasses also generate a low velocity canopy. High levels of periphyton cover explains why individual density and the biodiversity of invertebrates are higher at the Juma River site than those at the other two cobble substrate sites. Epiphytic mosses on the bedrock surface of the Chexi Stream site create substantial quantities of oxygen via photosynthesis, which brings about the frequent appearance of aerobic individuals. Streams with aquatic plants exhibit high habitat diversity. The absence of essential aquatic plants on the substrata greatly reduces the total abundance and biodiversity of invertebrates. 6 Conclusions Aquatic insects account for most of the macroinvertebrates found in the surveyed rivers, with different dominant species on different substrata. In particular, EPT species (Ephemeroptera, Plecoptera, Trichoptera) are the main residents on cobbles, gravel, and moss-covered bedrock, and Chironomidae larvae are dominant in clay beds. Structures of benthic communities differ in different substrata, and those in substrata of similar composition have similar structures, even at distinctly different latitudinal locations. This study demonstrates that the benthic macroinvertebrate community is mainly affected by the substrata composition, and in general is little affected by latitude and macroclimate. Taxa richness of macroinvertebrates is the highest on hydrophyte-covered cobbles; high on moss-covered bedrock; low on cobble substrate devoid of plant biomass and clay beds; and the lowest on sandy beds. Taxa richness in sandy beds is very low or zero, since sand is compact and unstable, thus, unsuitable for colonization by macroinvertebrates. Streambed substrate characteristics, including homogeneity, compaction, stability, and the presence of aquatic plants, also strongly affect macroinvertebrate communities. An improved substrate suitability index was developed by integrating the effects of both sediment and organic substrata. Biodiversity of macroinvertebrates increases linearly with the improved substrate suitability index. The findings of this study stress the important role of periphyton and benthic organic material in shaping macroinvertebrate structures. Acknowledgements This research was supported by the National Science Foundation of China (Grant No. 50779027) and the National Key Project of Scientific and Technical Supporting Programs funded by the Ministry of Science and Technology of China (No. 2006BAB04A08). References Arrhenius O. 1921, Species and area. The Journal of Ecology, Vol. 9, pp. 95–99. Arunachalam M., Madhusoodanan Nair K. C., Vijverberg J., and Kortmulder K. 1991, Substrate selection and seasonal variation in densities of invertebrates in stream pools of a tropical river. Hydrobiologia, Vol. 213, pp. 141–148. Beauger A., Lair N., Reyes-Marchant P., and Peiry J. -L. 2006, The distribution of macroinvertebrate assemblages in a reach of the River Allier (France) in relation to riverbed characteristics. Hydrobiologia, Vol. 571, pp. 63–76. Beisel J. N., Usseglio-Polatera P., Thomas S., and Moreteau J. C. 1998, Stream community structure in relation to spatial variation: The influence of mesohabitat characteristics. Hydrobiologia, Vol. 389, pp. 73–88. Beisel J. N., Usseglio-Polatera P., and Moreteau J. C. 2000, The spatial heterogeneity of a river bottom: A key factor determining macroinvertebrate communities. Hydrobiologia, Vol. 422/423, pp. 163–171. Bovee K. D. 1982, A guide to stream habitat analysis using the instream flow incremental methodology. Instream Flow Information paper No. 12 FWS/OBS-82-26.U.S. Fish and Wildlife Service. Biological Services Program, Fort Collins, CO. - 336 -

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