Resistance of riverine macroinvertebrate assemblages to hydrological

Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Volume 43, Issue 4 ISSN 1730-413X eISSN 1897- 3191

(402–417) 2014 Received: Accepted:

DOI: 10.2478/s13545-014-0159-2 Original research paper

Resistance of riverine macroinvertebrate assemblages to hydrological extremes Eliza Szczerkowska-Majchrzak *, Joanna Lik, Joanna Leszczyńska Department of Ecology and Vertebrate Zoology, Faculty of Biology and Environmental Protection, University of Łódź, ul. Banacha 12/16, 90-237 Łódź, Poland Key words: river, reservoir, flow disturbances, mosaic habitats, Chironomidae Abstract Macroinvertebrates were sampled in the lowland Drzewiczka River downstream from a dam reservoir and just below a whitewater slalom canoeing track. For over 20 years, pulse flow fluctuations of moderate intensity, an effect of two-three hour long releases of water per day to enable training of canoeists, induced a patchy mosaic in the tailwater riverbed compared to a natural site. After these regular disturbances, three accidental events of increased discharge of different magnitudes (three, five and sixteen times higher compared to a long-term median) occurred in two following years and we were able to investigate their impact on the habitat-specific processes. Two of the three events (in September 2000 and March 2001) had a minor effect on abiotic and biotic variables, while the third one (in February 2002, over 40 m3 s-1 discharge) destabilized the bed habitat, washing away the flood-sensitive macroinvertebrates of Ephemeroptera and Trichoptera. In the dominant benthic group, i.e. Chironomidae, varied resistance patterns were observed, depending on their mode of life and patch occupancy. In conclusion, biota in the Drzewiczka River have adapted their life history to long-term moderate flow disturbance, but the largest flood mobilized bed sediments together with most of their dwellers. *

Corresponding author: [email protected]

September 08, 2014 November 17, 2014

INTRODUCTION Pollution, fragmentation and flow regulation of the world’s rivers have been the subject of research for several decades. Reservoirs (over 45000 large, 15 m high dams worldwide, Robinson & Uehlinger 2008) impede the flux of water, sediments and nutrients, and can strongly alter the structure and dynamics of upstream and downstream environments and biota. The effects of impoundment have been described in many papers, including review papers (Ward & Stanford 1980, Armitage 1984, Petts 1984, Bednarek 2001, Feld et al. 2011). Damming of rivers usually leads to one of the two scenarios: first of all, this kind of flow disturbance allows homogenous communities to exist, with high densities of Oligochaeta, Chironomidae and Mollusca, while the density of macroinvertebrates increases or decreases, the species diversity is always reduced, as it is observed in many rivers below dams (Ward & Stanford 1980, Armitage 1984, Petts 1984, Grzybkowska et al. 1990). Such a scenario favors a more balanced flow, an increased stability of a riverbed, a lower amplitude of water temperature fluctuations, the increased availability of nutrients and, eventually, abundant zoobenthic food resources. The second scenario, with very high daily flow fluctuations, usually results from hydroelectric power generation, which depletes both the quality and quantity of benthos (Moog 1993, Nyman 1995). Benthic invertebrate assemblages are highly resistant to disturbance (Townsend et al. 1997), including their density recovery after flood events (Robinson et al. 2004), and it is known that assemblages also have some behavioral, morphological and physical traits protecting them against such events, as well as flow refugia to avoids floods (Palmer et al. 1995, Lancaster & Hildrew

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Response of riverine macroinvertebrates to floods| 403

1993a, Sedell et al. 1990). In general, refugia are habitats whose biota show temporal or spatial resistance or resilience to environmental parameters (Sedell et al. 1990). According to Lancaster and Hildrew (1993b), Bournaud et al. (1998) and Lancaster (1999), these are fragments of river channels where negative influence of disturbance is lower compared to surrounding areas. This means that the riverbed materials are not evenly redistributed or are disturbed to a lower level, thus some of the invertebrates show higher stability. Potential refugia for invertebrates included lateral stream banks, large surface particles and patches covered by macrophytes (Matthaei et al. 1999, 2000), stream bryophytes (Korsu 2004), hydrodynamically dead zones (Lancaster & Hildrew 1993b) and the hyporheic zone (Dole-Oliver et al. 1997). This context concerns objects smaller than tributaries or catchments. Benthic invertebrate fauna, including Chironomidae as indicator organisms for continuous water monitoring, are suitable for a long-term analysis of both regular and accidental flow disturbance (Dallas 2000). This results, among others, from the fact that chironomid species composition differs qualitatively and quantitatively among microhabitats, and chironomid larvae are highly selective in their choice of habitat (Maasri et al. 2008). These dipterans reside in all types of fresh waters, both lotic and lentic ecosystems, owing to their broad ecological tolerance, represent more than half of the total number of riverine macroinvertebrate species (Rossaro 1991), and play a key role in the energy flow, both in natural riverine ecosystems as well as in the affected ones (Lindegaard 1989, Berg & Hellenthal 1991, Lindegaard & Brodersen 1995, Tokeshi 1995, Benke & Huryn 2010, Tang et al. 2010). This paper presents the results of our study which aimed at assessing the changes in abiotic parameters in response to artificial floods in the lowland river, and the resistance (measured as the relative lack of decrease in density) of macroinvertebrate assemblages, including chironomid ones, to accidental flow disturbances. Our study was conducted in the Drzewiczka River, the examined section of which was characterized by a very specific discharge regime caused not only by a dam reservoir but also by an operating whitewater slalom canoeing track of a montane type, located below the dam. In the course of these flow disturbances, we also wanted to define which of the five dominant habitats in this www.oandhs.org

tailwater reach of the Drzewiczka River may be referred to as refugia of macroinvertebrates. To address these problems, three different increased discharges were investigated during 18 months. This kind of water management was an excellent opportunity to examine not only the response of the whole course of the river (in particular the dominant habitats − patches) but also to learn about the type and the degree of biota resistance to the three spate events of different magnitude. STUDY AREA The lowland Drzewiczka River is the biggest right-bank tributary of the Pilica River (the Vistula system). The Drzewiczka River (20°28’ E and 51°27’ N) has its origin at 248 m a.s.l. and is 81.3 km long; its catchment area is 1,083 km2 and the slope ranges from 2.7-2.5‰ in the upper reaches to 0.8-0.7‰ in the middle and lower reaches (EMPHP, 2007). The Drzewiczka River flows through an agricultural land overgrown with meadows and pastures; the riparian trees were mainly Alnus glutinosa (L.) Gaertn, and Populus sp. The further details of the study area and parameters of water were given by Dukowska et al. (2007), Tszydel et al. (2009) and SzczerkowskaMajchrzak et. al. (2010). The dam reservoir, called the Drzewieckie Lake, is located between the 21st and 26th km from the river’s mouth in the fourth order

Fig. 1. Study area in the Drzewiczka River with plotted sampling habitats: P – pool habitat, S – stagnant habitat, M – macrophyte habitat, B – bank habitat, R– riffle habitat. The main fractions of the inorganic substrate of the riverbed are presented (after Przybylski, Zięba 2000, modified)

404 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska

stream section (Fig. 1). It has an area of 0.84 km2, and was constructed in 1932-1936, mainly to supply water to a metallurgic factory and for recreation. In 1980, the whitewater slalom canoeing track (abbr. WCT) of montane type, about 2 km long was built just below the dam reservoir. Due to the construction of the WCT, the hydrological regime in the tailwater became very changeable and clearly different from the natural one, particularly downstream from the dam and the track. Every day

in the afternoon, except winter, surface waters from the reservoir were released for 2-3 hours to enable training of canoeists. These specific pulse water fluxes below the dam and WCT induced a patchy mosaic in the riverbed with respect to morphometric and hydraulic parameters, amounts of benthic particulate organic matter as well as a density of periphyton. The study area was located downstream from the dam and WCT. Along a 160 m long river stretch, the following dominant five sampling

Fig. 2. Mean annual values of the environmental variables at dominant habitats before and after floods in September 2000, March 2001 and February 2002. If the means for a given variable were significantly different between ‘before’ and ‘after’, the levels of significance are indicated (ANOVA, df = 1; 8, *p < 0.05, **p < 0.01, ***p < 0.001). SI – Substrate Inorganic Index, BFPOM and BCPOM – benthic fine and coarse particulate organic matter, TFPOM and TCPOM – transported fine and coarse particulate organic matter, chlorophyll α – chlorophyll α concentration in periphyton. Additional explanations are provided in Fig. 1 Copyright© of Faculty of Oceanography and Geography, University of Gdańsk, Poland www.oandhs.ocean.ug.edu.pl

Response of riverine macroinvertebrates to floods| 405

habitats were sampled (Fig. 1, 2): PH – pool habitat – an area located on the left side of the river, close to the end of the WCT; a neighbouring island and a fallen tree made the water velocity swifter there; SH – stagnant habitat – a depositional area located along the left bank, with a very low flow and a large amount of fine and coarse particulate organic matter (POM), covered with emerged macrophytes; MH – macrophyte-dominated habitat – an area covered by vegetation that included large patches of Potamogeton lucens L. and Potamogeton crispus L., and small patches of Potamogeton pectinatus L. Cladophora glomerata (L.) Kutz filaments coated these macrophytes; BH – bank habitat – an area that extended along the left bank; RH – riffle habitat – an erosional, high-flow area along the right bank. MATERIALS AND METHODS For over 20 years, every day in the afternoon, except for winter, surface waters from the reservoir were released for two to three hours to enable the training of canoeists. We manipulated the discharge to assess the response of benthic macroinvertebrates at each habitat to longer floods (lasting more than 3 days) of different magnitudes. Three separate flood events were arranged and two samplings at each event were performed between 2000 and 2002 (during 18 months). The last event took place during the complete emptying of the reservoir, before dredging. During each event, we collected pre- and post-flood macroinvertebrate and environmental data to conduct a before-after control-impact analysis (BACI, pre- vs post-flood) (Bond & Downes 2003). Water velocity was measured using a water velocity meter (HEGA 1, Biomix); values of the discharge were calculated in a standard way. The long-term base-flow discharge in the river is about 2.6 m3 s-1. When regular (each day for several hours) events occurred, the discharge increased to 4.8 m3 s-1 in September 2000, 4.3 m3 s-1 in March 2001 and 4.5 m3 s-1 in February 2002. During our investigations, the discharge was additionally increased for several hours to the levels of 8.4 m3 s-1 (September 2000, ca. three times higher than the base-flow value), to 12.0 m3 s-1 (March 2001, ca. five times higher) and to 41.8 m3 s-1 (February 2002, ca. 16 times higher) (Fig. 3). www.oandhs.org

Fig. 3. Discharge of the Drzewiczka River in September 2000, March 2001 and February 2002 before and after the flood events

In each of the habitats, variables believed to have the greatest influence on the microdistribution of benthic fauna, water depth and water velocity, were measured. In each habitat, five samples (each consisting of 10 × 10 cm2 of river bottom) were collected before (B) and after (A) an induced water release, using a tubular sampler of 10 cm2 in a crosssectional area. The sampler was pushed through vegetation (if present) into the sediment to a depth of 15 cm. Invertebrates were manually sorted from the detritus and preserved in 10% formalin prepared with river water. All macroinvertebrates from quantitative samples were counted; these data were used to estimate the density in each sampling habitat. Chironomid larvae could rarely be identified to the species level from field samples on each sampling occasion; consequently, many species were identified from pupae’s exuviae obtained from laboratoryreared larvae which were obtained in additional field samples. The quantitative samples were also used to determine: 1) the particulate organic matter (POM) content in the bottom sediment. For this purpose, a 1-mm sieve was used to separate the following size classes of benthic POM: > 1 mm (coarse POM) and < 1 mm (fine POM), according to Petersen et al. (1989). POM was then dried at 60°C for two days, weighed, ashed at 600°C for two hours and reweighed. 2) the composition of particulate inorganic matter was determined according to Cummins (1962) and transformed into the single substrate index (SI) by summing up the midpoint values of size classes weighted by their percentage cover (Quinn & Hickey 1990).

406 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska

Benthic samples of 50 cm2 were also collected at each habitat to estimate chlorophyll α concentrations. The benthic material was rinsed. The obtained solution was centrifuged and the supernatant was used to determine the concentration of chlorophyll α by spectrophotometry (Golterman et al. 1978). To determine the amounts of fine particulate organic matter in suspension (i.e. transported, TFPOM), triplicate water samples were collected in 10 dm3 plastic bags. These samples were filtered through Whatman GF/C glass-fibre filters (1.2 μm). And to estimate the amounts of the transported coarse organic matter (TCPOM), three nets of 1.5 m in length were mounted on 0.5 × 0.7 m frames; they were put into each habitat for ten minutes. All statistical analyzes were carried out using CCS Statistica (StatSoft 2011). Data were log (x+1) transformed to improve the normality of data distribution. A one- and three-way analyzes of variance (ANOVA) were used to examine spatial and temporal differences in the central tendency, i.e. the means of the amounts of benthic and transported organic matter, chlorophyll α, hydraulic parameters, and benthic densities. For three-way ANOVA, five sites (pool, stagnant, macrophyte, bank and riffle habitats), each sampled two times (before and after a flood event), on each of the three different sampling occasions (September 2000, March 2001 and February 2002) were used as factors. Resistance of macroinvertebrate taxa was assessed by comparing the percentage lack of reduction (% loss) in the invertebrate density before and after each flood (Robinson 2012). Two discrimination functions (DF 1 and DF 2) were used to determine which environmental variables (including food resources for macroinvertebrates) best separated the sampled riverine habitats before and after the flow disturbances during the three sampling times. The following parameters were taken into account: depth, water velocity, discharges, SI, temperature, all fractions of benthic POM, transported POM, and chlorophyll α. RESULTS Environmental parameters hydrological events

over

three

The floods resulted in larger differences in the water depth and water velocity in the Drzewiczka

River downstream of Lake Drzewieckie and WCT (Fig. 2). The river bed of the Drzewiczka River showed different stabilities in different habitats during flow disturbances (Fig. 2). In September 2000, differences in the SI index were noted at BH, while in March 2001 at PH. In February 2002, large increases in the discharge caused the relocation and resuspension of the whole riverbed sediment (except PH). Both the deposit (colmatation at SH, BH, and RH) and the scouring of sand were noted (at MH, Fig. 2). The results of ANOVA proved that water changes in the discharge caused translocation of the inorganic substrate. The redistribution of POM was also observed (TFPOM, Fig. 2); a high increase in the discharge in February 2002 produced greater amounts of TFPOM in the water column (except for the bank habitat) while lower discharge in other months of the research period created various amounts of this transported matter at given habitats. As an effect of erosion and deposition, different amounts of benthic fine POM and periphyton were observed at different habitats (Fig. 2). The discriminant function built based on the environmental variables was the most effective at classifying the data groups before and after disturbances. In DF1, the water velocity (MH and BH) or depth (PH) and temperature (SH and RH) exhibited the highest positive correlation, while the water velocity (PH), depth (MH and BH) and oxygen (SH and RH) had the highest negative correlation. In terms of DF2, the amount of TCPOM (PH), temperature (SH, BH and RH) or oxygen (MH) showed the highest positive correlation, while the depth (BH), the amount of TCPOM, temperature (MH) or oxygen (pool PH) showed the highest negative correlation. The environment ellipses of a given habitat (before and after) were close to each other in September (except BH), while during a higher discharge (in March), the spread of DA scores along DF1 became larger (except RH). Whereas in February, only ellipses of the stagnant habitat covered each other. The lowest explained amount of variation among the six groups of DF1 was noted for stagnant and riffle habitats, while the highest one was noted for the macrophyte habitat; for DF2, the maximum values were observed for stagnant items and minimum values for the macrophyte patch (Fig. 4). There was a significant 3-way interaction between habitats, flood events and time (before, after) for depth, water velocity, SI and the amount of TFPOM, TCPOM, BFPOM and chlorophyll α (Table 1).

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Response of riverine macroinvertebrates to floods| 407

Fig. 4. Classification of particular habitats on the basis of environmental variables using the first two discriminant functions: DF1 and DF2. Samples were collected three times, in September 2000, March 2001 and February 2002, each time before and after high flow fluctuations. Percentages of variance explained by the first and second function are given at the end of each axis

Macroinvertebrate assemblages versus hydraulic stress Zoobenthos reached very high densities at the riffle in March (over 75,000 ind. m-2) and slightly lower ones in February 2002 (over 51,000 ind. m-2) before significant changes in the discharge took place, while the minimum values of the benthic abundance were recorded after the disturbances (Fig. 4). The benthic assemblage was dominated either by www.oandhs.org

Oligochaeta (mainly gathering collectors) or by Chironomidae (all trophic groups, except shredders) in all habitats. Other benthic groups, such as Ephemeroptera, gathering collectors (mainly Caenis at SH and MH and Baetis at MH and RH), Trichoptera (Hydropsyche, filtering collectors and Psychomyia pusilla (Fabricius), periphyton scrapers dwelling at RH, BH, and PH), were common in different habitats (Fig. 5). The relative sharp decrease in the density of macroinvertebrates occurred with increase in flood

408 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska Table 1 Summary results of the three-way ANOVA comparing river parameters between (1) five habitats (pool, stagnant, macrophyte, bank and riffle habitats), (2) flood events (September 2000, March 2001, February 2002) and (3) sampling periods (before, after) and five habitats (pool, stagnant, macrophyte, bank and riffle habitats); df value for error = 120, additional explanation in Figure 2 Variable depth water velocity SI temperature chlorophyll α BFPOM BCPOM TFPOM TCPOM

interaction df (effect) F p F p F p F p F p F p F p F p F p

1 4

2 2 24.50 0.000 455.06 0.000 55.84 0.000 0.02 1.000 14.53 0.000 23.44 0.000 62.34 0.000 62.32 0.000 3248.62 0.000

3 1 224.62 0.000 271.90 0.000 0.54 0.586 32.17 0.000 326.57 0.000 31.79 0.000 28.91 0.000 101.47 0.000 2191.39 0.000

1*2 8 295.39 0.000 24.33 0.000 5.56 0.020 15.82 0.000 4.15 0.044 3.15 0.079 2.82 0.096 60.77 0.000 1269.72 0.000

4.86 0.000 27.19 0.000 5.29 0.000 0.01 1.000 8.59 0.000 5.20 0.000 3.53 0.001 63.50 0.000 2103.79 0.000

1*3 4

2*3 2 4.65 0.002 2.23 0.070 7.34 0.000 0.04 0.998 2.67 0.035 1.76 0.141 0.80 0.529 69.52 0.000 896.28 0.000

1*2*3 8 30.66 0.000 6.24 0.003 8.99 0.000 4.62 0.012 3.319 0.040 4.56 0.012 0.34 0.711 54.81 0.000 856.76 0.000

3.37 0.002 2.55 0.013 4.02 0.000 0.02 1.000 6.61 0.000 3.28 0.002 0.39 0.923 14.38 0.000 825.81 0.000

Fig. 5. Plot of mean macroinvertebrate density at each habitat before and after the floods. See text and Figs. 1 and 2 for additional explanations

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Response of riverine macroinvertebrates to floods| 409

Resistance of chironomids

magnitude, indicating their resistance was greater to smaller floods (3- and 5-fold increase in discharge) than to the largest flood (respectively over 16-fold increase). The total macrobenthic densities were significantly depleted only at PH in September and at PH and RH in March. However, after the high-flow event in February, the abundance of organisms living in the studied habitats dramatically decreased because a large portion of stream invertebrates were physically eliminated from the riverbed of patches SH, BH and RH. The more sensitive taxa to large floods included Ephemeroptera (both Baetis and Caenis) and Trichoptera (Hydropsychidae and Psychomyidae), which were almost depleted from their preferred macrophyte habitat. This pattern concerns all habitats except the pool where rather small changes in this insect abundance were observed (Fig. 5). There was a significant 3-way interaction between habitats, flood events and time (before, after), indicating that the density of Trichoptera, Ephemeroptera, Chironomidae (except Prodiamesinae) was significantly different (Table 2). A whole range of possibilities were recorded for Oligochaeta. At pool (PH), stagnant (SH) and macrophyte (MH) patches, statistically significant increases in the abundance were noted after floods while at the riffle habitat (RH), no changes in the density were observed; while at the bank habitat (BH), a decrease in the abundance was recorded (Fig. 5).

The results of one-way ANOVA (Fig. 5) show that three or five-fold increases in the discharge caused flushing away of some macroinvertebrate groups, including also Chironomidae from particular habitats: Chironomini at PH and BH in September 2000, at SH and MH in March 2001, Tanytarsini at BH in September, Tanypodinae at PH and RH in March, Prodiamesinae at MH in March and Orthocladiinae at PH and RH in March. However, during the highest flow event in February 2002, only Chironomini were flushed away at PH, while at other habitats, the spate eliminated most of the chironomid taxa, including psammophilous Chironomini. Despite this fact, less than 40% (at MH) and about 60% (at BH) of pelophilous Chironomini (Microtendipes chloris (Meigen)) were able to survive in these habitats though erosive forces occurred during the floods. Mobile predators, such as Tanypodinae, were mainly dwellers of the stagnant habitat and, depending on the flood event, occurred also in the macrophyte habitat (in February 2002). The most common were Procladius sp. and Ablabesmyia monilis (Linnaeus). Diamesinae and Prodiamesinae rarely occurred in the investigated range. The former subfamily was represented by Potthastia longimana (Kieffer); occasionally the larvae of this species were dwellers of MH, BH and RH habitats, mainly in March. Table 2

Summary results of the three-way ANOVA comparing density of macroinvertebrate groups between (1) − five habitats (pool, stagnant, macrophyte, bank and riffle habitats), (2) − flood events (September 2000, March 2001, February 2002) and (3) − sampling periods (before, after) and five habitats (pool, stagnant, macrophyte, bank and riffle habitats); df value for error = 120, additional explanation in Figure 2 Variable Oligochaeta Ephemeroptera Trichoptera Chironomidae Tanypodinae Prodiamesinae Diamesinae Orthocladiinae Chironominae Chironomini Chironominae Tanytarsini Others Total

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interaction df (effect) F p F p F p F p F p F p F p F p F p F p F p F p

1 4

2 2 17.01 0.000 4.72 0.001 44.66 0.000 35.48 0.000 8.01 0.000 1.31 0.270 8.08 0.000 37.74 0.000 21.24 0.000 4.48 0.002 18.08 0.000 56.65 0.000

3 1 10.52 0.000 4.96 0.009 3.40 0.037 40.04 0.000 22.86 0.000 1.08 0.342 22.50 0.000 33.65 0.000 4.14 0.018 15.48 0.000 5.33 0.006 29.84 0.000

1*2 8 2.95 0.089 17.25 0.000 1.40 0.239 95.78 0.000 24.82 0.000 1.76 0.187 1.29 0.258 52.62 0.000 55.35 0.000 57.17 0.000 1.20 0.275 26.87 0.000

1*3 4 3.78 0.001 3.13 0.003 7.15 0.000 19.83 0.000 4.13 0.000 0.92 0.501 13.01 0.000 7.79 0.000 23.24 0.000 6.49 0.000 3.73 0.001 15.00 0.000

2*3 2 0.47 0.756 0.30 0.878 3.22 0.015 2.86 0.026 1.17 0.329 1.01 0.407 6.44 0.000 2.35 0.058 9.48 0.000 0.28 0.890 3.01 0.021 5.10 0.001

1*2*3 8 2.72 0.070 14.50 0.000 5.55 0.005 29.49 0.000 6.55 0.002 0.99 0.375 5.11 0.007 28.56 0.000 6.18 0.003 11.95 0.000 0.76 0.472 6.546 0.002

1.57 0.142 3.04 0.004 4.28 0.000 8.72 0.000 2.56 0.013 0.98 0.456 3.02 0.004 5.50 0.000 4.51 0.000 3.32 0.002 2.73 0.009 6.70 0.000

410 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska

The latter subfamily, represented by Prodiamesa olivacea (Meigen), rarely occurred at MH, and sporadically at SH and BH (Fig. 5, 6). Orthocladiinae were very abundant, except in September, especially before the spate events. They were dominant at RH, and, to a lesser degree, at other habitats, excluding SH, where they occurred rarely. The most important taxa of this subfamily were Cricotopus spp., Eukiefferiella gracei (Edwards), Tvetenia clavescens (Edwards) and Synorthocladius semivirens (Kieffer) (Fig. 5, 6). Chironominae Chironomini were represented by many species, but the most abundant were

pelophilous M. chloris, Polypedilum convictum (Walker) (at MH, BH, RH) and, to a lesser degree, Stictochironomus histrio (Fabricius) (at BH). These two latter taxa were associated with fine inorganic substrate and rather small abundance of FPOM. The second tribe of Chironominae, Tanytarsini, was represented mainly by Micropsectra notescens (Walker) at SH, MH and BH habitats. The density of this tribe was similar to that of Tanypodinae (Fig. 5, 6). The three-way ANOVA was applied to compare habitats, floods and time (before, after) indicating that only the density of M. notescens was insignificantly different (Table 3).

Fig. 6. Mean density of dominant chironomid taxa at the studied habitats before and after spates in September 2000, March 2001 and February 2002. Densities of P. longimana and P. olivacea (Prodiamesinae) are omitted because each of these species is the main component of Diamesinae and Prodiamesinae (over 95% of the total sub-family densities, Fig. 5). See text and Figs. 1 and 2 for additional explanations Copyright© of Faculty of Oceanography and Geography, University of Gdańsk, Poland www.oandhs.ocean.ug.edu.pl

Response of riverine macroinvertebrates to floods| 411 Table 3 Summary results of the three-way ANOVA comparing density of Chironomidae between (1) five habitats (pool, stagnant, macrophyte, bank and riffle habitats), (2) flood events (September 2000, March 2001, February 2002) and (3) sampling periods (before, after) and five habitats (pool, stagnant, macrophyte, bank and riffle habitats); df value for error = 120, additional explanation in Figure 2 Variable Procladius Ablabesmyia monilis Cricotopus spp. Eukiefferiella gracei Synorthocladius semivirens Tvetenia calvescens Microtendipes chloris Polypedilum convictum Stictochironomus histrio Micropsectra notescens Others

interaction df (effect) F p F p F p F p F p F p F p F p F p F p F p

1 4 21.35 0.000 13.80 0.000 27.39 0.000 28.92 0.000 11.94 0.000 29.91 0.000 17.49 0.000 9.46 0.000 12.58 0.000 3.38 0.012 20.65 0.000

2 2 6.90 0.001 21.83 0.000 41.50 0.000 14.09 0.000 4.29 0.016 42.74 0.000 0.36 0.696 0.07 0.932 2.41 0.094 5.05 0.008 38.36 0.000

DISCUSSION Disturbances versus benthic fauna The moderate flow fluctuations in the Drzewiczka River (cumulative effect of damming and WCT functioning) led to the formation of a mosaic of bed patches with different benthic fauna. These fluctuations (over 20 years) had a great effect on macrobenthic assemblages. They enabled a high number of species to develop and coexist by preventing a strong dominance of a few species (Dukowska et al. 2007, Szczerkowska-Majchrzak et al. 2010). This finding is consistent with that observed in other rivers (Lake 2000, Matthaei & Townsend 2000, Palmer et al. 2000). Furthermore, the macroinvertebrate density in this river reached a high degree of similarity to that of the natural lowland Grabia River (Poland), maintained in undisturbed conditions for many decades, whose geomorphological characteristics (including slope) resemble those of sub-mountain rivers. The hydromorphology of this natural river also results in a mosaic riverbed with dominant gravel habitats characterized by high macroinvertebrate density, diversity and production (Grzybkowska 1989, Grzybkowska & Witczak 1990). In the Drzewiczka River, however, the diversity of some insects was lower due to the disappearance of more sensitive

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3 1 5.50 0.021 16.38 0.000 49.31 0.000 26.11 0.000 0.80 0.374 40.42 0.000 19.07 0.000 7.76 0.006 0.29 0.593 17.50 0.000 82.44 0.000

1*2 8 3.65 0.001 3.16 0.003 7.85 0.000 3.48 0.001 4.66 0.000 5.83 0.000 16.93 0.000 8.16 0.000 5.65 0.000 3.27 0.002 12.04 0.000

1*3 4 4.51 0.002 1.37 0.248 0.30 0.879 6.47 0.000 5.73 0.000 2.53 0.044 3.61 0.008 4.65 0.002 4.50 0.002 0.17 0.952 1.92 0.111

2*3 2 2.194 0.116 14.278 0.000 19.60 0.000 11.30 0.000 1.58 0.210 5.71 0.004 12.53 0.000 0.25 0.779 0.12 0.888 13.33 0.000 30.76 0.000

1*2*3 8 5.01 0.000 2.14 0.037 3.30 0.002 5.37 0.000 5.52 0.000 5.22 0.000 3.45 0.001 4.39 0.000 2.06 0.045 1.82 0.080 4.79 0.000

taxa, such as Ephemeroptera and Trichoptera (except Hydropsychidae and Psychomyidae) which were replaced by dipterans: Chironomidae and Simuliidae (Tszydel et al. 2009, Szczerkowska et al. 2010). Also, this pattern is consistent with data from other dammed rivers (Bredenhand & Samways 2009, Głowacki et al. 2011). On the basis of the current study, we may conclude that the importance of the highest studied spate for the whole macrobenthos assemblages is clear (obvious decreases), but the reaction of a given taxon to stress may vary depending on its mode of life (species traits) and the habitat parameters. This is congruent with findings of Palmer et al. (1995), who noticed that species of riverine assemblages that are able to cling to the substrate, or are rapid burrowers, or good swimmers, may exhibit high resistance to spates. Obviously the sixteen-fold increase in the discharge observed in the Drzewiczka caused major changes both in the abiotic and biotic parameters, but also the washing out of numerous taxa (decimation of the invertebrate community). As it follows from our analyzes, the population of Micropsectra notescens (Chironomidae, Tanytarsini) as well as those of Prodiamesinae (Diptera, Chironomidae) and Oligochaeta are not modified by discharge changes. According to Robinson (2012), an increase in the discharge causes the washing out of many taxa, except those of Trichoptera. Robinson et

412 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska

al. 2003 reported a strong decline in the density of invertebrates of high body weight (Gammaridae); in further investigations, they also recorded a low resistance of Chironomidae, Simulidae and Baetidae ephemeropterans, whose losses in density ranged from 20% to 70%. In general, floods reduce the density of benthofauna (Maier 2001, Jowett et al. 2008). In the study summarizing the long-term research based on the analysis of 22 floods, Robinson (2012) reported that the resistance of an invertebrate community is negatively correlated with the flood size. The sixteenfold increase in the Spöl River caused an increase in the density by 75%, while lower increases in the discharge did not cause such large changes. A similar situation was observed in the Drzewiczka. Nevertheless, our investigations indicated a different reaction of zoobenthos in each of the studied habitats. Less animals were washed out from the macrophyte and stagnant habitats, while most animals were washed out from the riffle habitat. This response is different to that described in Robinson et al. (2004), where the highest mortality was observed in a pool habitat, and the lowest in a bedrock habitat. Also Lytle (2000) reported that pools are heavily scoured during disturbance. It appears from our investigations that both the three-fold and the fivefold increase in the discharge induced a minor change in the investigated assemblage, although animals reacted differently in each of the five studied habitats. The study of Robinson et al. (2004) showed that after a low magnitude flood, the density of macroinvertebrates increased, while in the Drzewiczka in September, when the discharge increased three times in relation to base-flow, a decrease in the density was observed in the pool and an increase in the riffle habitats; meanwhile no changes were recorded in the stagnant, macrophyte and bank habitats. In the Drzewiczka River, there was only one deep pool habitat (PH), and it appeared to have the same number or even more dwellers after the spate events than before the floods. This means that many organisms, mainly Oligochaeta, were displaced to this habitat during spates. Thus, the conclusion is simple: only investigations that include species-specific reactions to disturbances in different habitats are useful. And a partial refugium may be composed of qualitatively different habitats. However, regardless of the total chironomid density, all the habitats are more similar to each other after high discharge perturbations than before.

Many authors do not record any or only slight changes in invertebrate assemblages. Benthic invertebrate communities in rivers change temporally and spatially mostly due to changes in environmental factors (Hynes 1970, Corkum 1990). Among parameters that mostly affect the community structure are discharge, substrate, riparian vegetation and temperature (Hynes 1970). Many of them change seasonally, although few studies have demonstrated seasonal changes in the assemblage structure (Giller & Twomey 1993). Life cycles of different taxa, for example, may affect the community structure (Hynes 1970, Reece & Richardson 1998), while the cycles are correlated with temperature, the amount and quality of food resources, and discharge (Poff & Ward 1989). In a Mediterranean stream investigated by Dolédec (1989), seasonal changes in benthofauna composition were related to the occurrence of severe floods in winter and high temperature in summer. Similarly, Mathuriau et al. (2008) demonstrated that the highest invertebrate density occurred between January and mid March, and between July and mid October. Additionally, many organisms reach a peak in their density in spring (before thaws) and in autumn (during high flows and high temperature), and in late winter (before aerial stages) (Bothwell & Culp 1993), i.e. in the same periods of the year when the Drzewiczka was studied. Assemblages display uniform seasonal patterns despite changing hydrological regimes (Boulton et al. 1992). Our study did not attempt to analyze the seasonality, although the increased water levels of the river flow occurred in some seasons (one should remember that intervals between the floods were 6 and 11 months, respectively). We focused on the impact of discharge increases of varied extent and on the reaction of invertebrates to these events. As it was demonstrated in the present study, a fast recovery of invertebrate assemblages followed a several-fold increase in the discharge below the dam and in the whitewater canoeing track. Only several months after the return of the discharge to normal values, the structure of benthofauna was similar to that before the event (Tszydel et al. 2009, Szczerkowska-Majchrzak et al. 2010). On the other hand, after the catastrophic flood and the following return to the natural discharge, the taxa composition of zoobenthos became similar to benthofauna of the natural river section (Dukowska et al. 2007).

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Response of chironomid assemblages to spate Chironomidae are abundant and taxonomically diverse (Ferrington 2008) in water bodies, because they display resistance to environment changes, including both spate and low discharge events, as a result of their behavioral, physiological and lifehistory adaptations (McLachlan 1983, Armitage et al. 1995, Stubbington et al. 2009). On the one hand, according to some authors who analyzed the behavior of Chironomidae as one group, these insects, together with Simuliidae, are known to enter and/or to be washed away into the water column in response to flood disturbances (Jakob et al. 2003, Bredenhand & Samways 2009). On the other hand, Chironomidae are able to recolonize the riverbed very quickly after each catastrophic event like spate (Jakob et al. 2003), drought and heavy pollution (Brittain & Eikeland 1988) owing to their adult winged female stages (although adult chironomids are classified as poorly flying dispersers; Heino 2013) or via aquatic stages (drift of eggs and larvae), depending on the season (Wallace 1990). Usually numerous first and second instars in water column, independently of the life mode of older instars (which is the case irrespectively of the taxonomic affiliation) are elements of distributional drift, which enables them to seek suitable habitats for further development (Waters 1965, Grzybkowska et al. 1996). However, the behavior of older instars differs in the group, i.e. pelophilous Chironomini (Chironominae) larvae show lower mobility compared to Orthocladiinae (Miyake et al. 2005). It should be emphasized that disturbances of the highest magnitude in the Drzewiczka River discharge caused the depletion of both mobile Orthocladiinae (larvae prone to drift) and less mobile inbenthic Chironomini. Smaller floods in some river systems showed that macroinvertebrate assemblages were more resistant to flow disturbances than those affected by large floods (Robinson 2012); this pattern occurs also in the benthos of riffles in the Drzewiczka River. Their higher water velocity and the coarse elements of the inorganic substrate (gravel and pebbles) offer many microspaces for invertebrates; additionally, their large surface favors the development of periphyton (chlorophyll α). Thus, this kind of habitat may support higher densities of benthic fauna (Bournaud et al. 1998), including Orthocladiinae (Cricotopus spp., periphyton scrapers), as in the Drzewiczka River. Consequently, riffles may be classified as the most www.oandhs.org

productive habitats, with a large number of taxa (Lindegaard 1989, Brooks et al. 2005, Runck 2007). However, high disturbances in the Drzewiczka River in February 2002 were able to obstruct the habitat specific production (about 90% loss of organisms). Some larvae of Chironomini continued to thrive at the bank and macrophyte habitats after high flow events, but at lower levels than previously. The most obvious change in the bank habitat was the redeposition of fine mineral inorganic particles. Therefore, some organisms of Chironomini were rather covered up but not detached. In the Drzewiczka River, the colmatation lasted for a long time after the reservoir was emptied in the spring of 2002 and caused changes in the macrobenthic assemblages (Tszydel et al. 2009, SzczerkowskaMajchrzak et al. 2010). Although the bank habitat in large rivers usually covered a small area in relation to the remaining part of the river channel, the production and retention of organic matter were much higher in the “littoral” biota than in the midchannel (Bournaud et al. 1998, Głowacki et al. 2011). On the other hand, in the medium-sized Drzewiczka River, the abundance of benthic organisms in the bank habitat may be compared with those occurring on the bottom covered by submersed macrophytes (Grzybkowska & Witczak 1990). Several physical characteristics within macrophyte beds, such as reduced water velocity, increased sand and FPOM intrusion, may create sub-optimal conditions for benthic organisms (especially gathering collectors), provide excellent surfaces for epiphytic fauna and its food, establish refugia from predators for both macroinvertebrates and fish, and create heterogeneous substrates for the co-existence of invertebrates (Grzybkowska et al. 2003, Franklin et al. 2008, Kleeberg et al. 2010, Ibáñez et al. 2012, Dukowska et al. 2013). The stems and leaves of hydrophytes in the Drzewiczka River were destroyed together with their fauna during the flood, but this process did not concern their roots in deeper sediments. Thus, some pelophilous Chironomini larvae (M. chloris) might have survived among the roots. The macrophyte habitat may have thereby acted as a refugium during a spate only for this inbenthic species, while other taxa of epiphytic fauna (other Chironomini, Orthocladiinae and Tanytarsini) were completely destroyed. Within the range of the investigated river, the stagnant and pool habitats were affected by changes in the flow to a different extent than the other abovementioned habitats, and each of the two patches had

414 | Eliza Szczerkowska-Majchrzak, Joanna Lik, Joanna Leszczyńska

a relatively distinct assemblage of macroinvertebrates with different susceptibilities to flow disturbances. The stagnant habitat, with emergent macrophytes and high amounts of BFPOM, did not show any resistance to the most extreme spate. The three-fold increase in the discharge caused no perturbations in the macrobenthic assemblages (except in some Orthocladiinae), but the higher rises in the flow depleted the benthic fauna, including Chironomini. The pool is rather known as the habitat that shows a low resistance to floods, resulting from the less stable substrata (Robinson et al. 2004). However, the deep pool area of the Drzewiczka River was characterized by small or no loss of invertebrates after the high discharge as compared with the preceding period. Thus, some organisms were probably carried to this habitat and sank during such events. When dredging Lake Drzewieckie and during the highest artificial spate, a decrease in the habitat mosaic diversity was not observed at the investigated site. However, during the following recovery to natural discharge without the daily pulsating flow fluctuations (similar to those occurring over the recent 20 years), over a period shorter than a year after the highest accidental flood, the habitats and their biota did exhibit any changes (SzczerkowskaMajchrzak et al. 2010). During this natural discharge regime with lower water velocity and higher amounts of benthic organic matter, gathering collectors represented by Chironomini (including both smalland large-sized specimens) clearly benefited, while the density of Orthocladiinae (periphyton scrapers, rather small organisms), which typically inhabited rivers with more varying flows, were depleted or remained at the same level, depending on the habitat. Consequently, although the Orthocladiinae/ Chironomini density ratio decreased, it did not reach low values typical of the natural reaches of the Drzewiczka River studied by Dukowska et al. 2007. Thus, our results are not consistent with the trend observed by Robinson and Uehlinger (2008) in the Spöl River, where the average individual size of organisms decreased by almost half after a series of artificial floods. Another response pattern of colonization was documented in other reaches of comparable orders (Grzybkowska & Witczak 1990, Grzybkowska et al. 1996). In summer, after several successive natural floods in the Grabia River, the abundance and diversity of chironomids sharply decreased. As a consequence of inorganic sediment redistribution, the whole riverbed was covered with sand. After recovery to its natural discharge regime,

small psammophilous Polypedilum begun to dominate; the increase in the number of this species was very fast owing to females, which laid large numbers of eggs. While analyzing the Orthocladiinae/ Chironomini density ratio, we have to be aware that it is determined by various factors. For example, its high values in the Drzewiczka were caused by a high abundance of eurytopic C. sylvestris. This species may be widespread along almost the entire river course (Lindegaard & Brodersen 1995). In conclusion, biota in the Drzewiczka River have adapted their life history to the long-term moderate flow disturbance, but the largest flood was a physical forcing event that mobilized bed sediment and reduced the abundance of sensitive macroinvertebrates of Ephemeroptera, Trichoptera and Simuliidae. For the dominant benthic group, i.e. Chironomidae, diverse resistance patterns were observed, depending on their mode of life (species traits) and patch occupancy. Orthocladiinae, common in the riffle and on submersed macrophytes, and less abundant at the bank, were detached. On the other hand, the refugia of inbenthic M. chloris (Meigen) (Chironomini) were very stable thick sediment layers between the macrophyte roots or at the bank. However, during the recovery of this river to the natural discharge regime, the density and diversity of benthic assemblages began to modify themselves toward those in the undisturbed section of the river (Szczerkowska-Majchrzak et al. 2010). Three years after this study (in 2005), WCT began to function again and the Drzewiczka River returned to daily moderate flow fluctuations. Thus we are going to observe a shift in chironomids and other macroinvertebrates during a very long period, which started in 2000 and will long continue, and which covers disturbances of varying magnitudes and frequencies. ACKNOWLEDGEMENTS This work was partly supported by grants of the State Committee for Scientific Research No. 6 PO4F 047 19. The authors are also greatly indebted to M. Grzybkowska, M Dukowska, and students for assistance with the field and/or lab work, and Ł. Głowacki for revising the English. The authors thank the Department of Hydrography and Morphology of River Beds, the Institute of Meteorology and Water Management, Poland, for granting them a license to use the Electronic Map of Poland’s Hydrographic Partitions.

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