Epixylic biofilm and invertebrate colonization on ... - CiteSeerX

Arch. Hydrobiol.

165

4

515–536

Stuttgart, April 2006

Epixylic biofilm and invertebrate colonization on submerged pine branches in a regulated lowland stream Bernd Spänhoff1, 2, *, Christian Reuter1 and Elisabeth I. Meyer1 With 3 figures, 3 tables and 1 appendix

Abstract: Epixylic biofilm and invertebrate assemblages on pine branches (Pinus sylvestris) experimentally submerged in October 2000 were investigated, after an initial colonization period of 3 months, by two-monthly sampling intervals for 13 months in a nutrient-rich sandy lowland stream subjected to flow disturbances caused by infrequent impoundment openings. After 12 weeks of exposure an epixylic biofilm with 0.58 ± 0.25 µg/cm2 chlorophyll-a (mean ± 1 SE) and 0.23 ± 0.04 µg/cm2 ATP, as well as an abundant invertebrate community (14326 ± 2532 Ind/m2; biomass: 974.6 ± 360.1 mg/m2) became established on the branch surfaces. During the subsequent sampling dates invertebrate numbers decreased significantly during periods of high discharge and simultaneously chlorophyll-a values of epixylic biofilms increased, likely due to reduced feeding by invertebrate grazers. During periods with low discharge fluctuations and mainly low flow conditions, the epixylic biofilm, especially algal growth, was negatively correlated with invertebrate grazer and shredder numbers. After the initial growth period of 12 weeks, ATP values of the epixylic biofilm showed a strongly negative response to sand deposited on the wood surfaces. The present study displays the influence of seasonal discharge fluctuations and sand deposition on the wood surfaces on epixylic biofilms and invertebrate assemblages on experimentally submerged pine branches, but also indicate interactions between food sources (algae and fine particulate organic matter) and invertebrates (grazers and collector/gatherers). Key words: Epixylic biofilm, flow disturbance, aquatic invertebrates, functional feeding groups, stream regulation, woody debris.

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Authors’ addresses: Department of Limnology, Institute for Animal Evolution and Ecology, University of Muenster, Germany. 2 Research Group for Limnology, Institute of Ecology, University of Jena, Carl-ZeissPromenade 10, D – 07745 Jena, Germany. * Author for correspondence; email: [email protected] DOI: 10.1127/0003-9136/2006/0165-0515

0003-9136/06/0165-0515 $ 5.50

 2006 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

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Bernd Spänhoff et al.

Introduction Sandy lowland streams that are subjected to channelization or straightening are often characterized by low habitat heterogeneity. In these stream systems, submerged woody debris is often the only naturally occurring stable substrate, representing a very important habitat for invertebrate species diversity (e. g. Hoffmann & Hering 2000, Benke & Wallace 2003) and secondary production (Benke et al. 1984). Additionally, it creates stable patches for drift resistance against spates (Hax & Golladay 1998) and provides a surface for biofilm development, which is an important food and energy resource for many members of the food web (Power & Dietrich 2002), especially in hard substrate poor low-order streams. Studies on freshly exposed wood substrates in streams have shown rapid development of epixylic biofilms (Golladay & Sinsabaugh 1991, Sinsabaugh et al. 1991, Sabater et al. 1998). Nevertheless, the current knowledge of epixylic biofilm development and invertebrate colonization of submerged wood in Central European lowland streams is poor (Hoffmann & Hering 2000). However, even in streams with coarse sediments, the importance of woody debris for the invertebrate community has been shown (Hernandez et al 2005). Little information on the interaction of epixylic biofilms and wood-inhabiting grazing invertebrates is available, despite many studies having been performed on the grazing pressure of invertebrates on epilithic biofilms (e. g. Feminella & Hawkins 1995). In Central Europe most streams are regulated by weirs to control the discharge of channelized or straightened streams. This impairment of natural stream morphology and the subsequent alteration of flow conditions has severe consequences for the whole stream ecosystem (Bunn & Arthington 2002, Collier 2002). In particular, streams with unstable sediments, like sandy lowland streams, can suffer heavily from flow regulation leading to an impoverishment of invertebrate species diversity (e. g. Grzybkowska et al. 1996, Verdonschot 2000). Sandy sediments are very susceptible to increasing sheer stress caused by enhanced flow during high discharge (Waters 1995). Consequently, bed-movement in sandy streams takes place at comparably low flow velocity (Mangelsdorf et al. 1990). Increased transport of fine sediments downstream is associated with sediment erosion of sandy stream beds during high discharge, resulting in sand-blasting effects on surfaces of solid substrates and enhanced deposition of sand after recovery of base flow velocities (Waters 1995). Such infrequent but episodic sedimentation can cause enhanced interspecific competition among invertebrates for patches with the highest habitat quality and result in increased mortality of species with lower competitiveness (Strand & Merritt 1997, Wood et al. 2005). We conducted this field experiment to investigate the seasonal variability of aquatic invertebrate assemblages inhabiting submerged branches and to

Biofilm and invertebrates on submerged wood

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characterize the interaction of epixylic biofilms with invertebrates and abiotic factors like discharge fluctuations in a regulated and nutrient rich lowland stream. The present study contributes to the knowledge on wood inhabiting invertebrates and epixylic biofilms in Central Europe, a region in which streams have been subjected to anthropogenic stress and disturbances since several centuries (Meybeck 2003). Studies on wood in streams from this region are still scarce likely due to the practice of wood removal for management purposes leading to an impoverishment of in-stream quantities of wood in streams (Hering et al. 2000).

Material and methods Study area The Ladberger Muehlenbach (LMB) is a nutrient rich, sandy, lowland stream typical of the region of the Westfälische Bucht, in the northern part of the county NorthrhineWestphalia, Germany. Catchment area of the stream is 352 km2 and mean annual precipitation ranges between 700 and 750 mm (Aschemeier 1996). The predominant substrate of the streambed is sand with a mean grain size of 250 µm (Cleven 1999). Some sections of the stream bed in the upstream course are vertically limited by a hard iron pan that prevents an exchange of water between the stream and the groundwater (Cleven & Meyer 2003). Parts of the headwater brooks and the midreach courses are bound by a riparian zone consisting of native tree and shrub species, mainly black alder (Alnus glutinosa), common oak (Quercus robur) and willows (Salix spp.), but the stream water is distinctly enriched by nutrients from adjacent corn fields. The study site was located in a 3rd order midreach section with near-to-natural riparian woodland vegetation consisting mainly of black alder, common oak and scots pine (Pinus sylvestris). Mean stream width at the study site was ca. 4 m and mean water depth at base flow ca. 20 cm. Stream regulation by an impoundment downstream from the headwater brooks, leads to a flashy discharge regime in the midreach sections caused by infrequent openings of the impoundment (Spänhoff et al. 2004).

Discharge, water temperature, physical parameters and water chemistry A pressure transducer water-level recorder was installed under a bridge near the study site to continuously record the water level and temperature. Discharge:water-level rating curves were obtained at irregular intervals (the discharge-water-level relationship followed a power regression with, r2 = 0.96, P < 0.001). Influence of the discharge regime on epixylic invertebrates and biofilms were estimated by calculating mean discharge over a 7-day-period before each sampling date. Singer et al. (2005) reported that influence of average discharge of a certain time period can explain temporal and spatial fluctuations of periphyton better than instantaneous flow velocity. The discharge regime at the study site was characterised by high fluctuations from October to

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Fig. 1. Discharge and temperature regime at the Ladberger Muehlenbach study site from January 2001 to February 2002.

March and sometimes May, and a summer period with relatively stable base flow conditions (Fig. 1). Impoundment openings resulted in a sharp increase of the discharge within a few hours and an abrupt decrease to base flow after closing of the impoundment (Spänhoff et al. 2004). The stream channel is incised and the morphology corresponds to a U-shaped profile (Cleven & Meyer 2003, Spänhoff et al. 2006). Increasing discharge results in increasing flow velocity due to the incised channel morphology of the stream. Mean flow velocity in 40 % depth from the water surface was between 10 cm/s (near the banks) and 25 cm/s (main current) at base flow discharge (ca. 250 l/s). During bank full discharge (ca. 1200 l/s) the flow velocity can increase up to 65 cm/s (main current). The LMB is a summer warm stream with a maximum daily mean temperature of 17˚C in summer and a minimum temperature of 0.7˚C in January (Fig. 1). Specific conductivity, pH, oxygen saturation, and concentrations of dissolved oxygen were all measured using transportable WTW field probes (WTW Weilheim, Germany). Concentrations of dissolved nutrients (nitrate, nitrite, ammonium, and ortho-phosphate) were analysed in the laboratory using common photometric analyses (Cleven & Meyer 2003). Data on physical parameters and water chemistry for each sampling date are given in Table 1.

Preparation of experimental substrata and sampling procedure Pine branches (Pinus sylvestris) of approximately the same surface area (400 cm2, mean length 28.6 cm, mean diameter 4.6 cm) were placed in the stream on 18 October 2000. Pine was chosen because it is a common tree species in the stream catchment and a clear-cutting of a forest section close to the study site provided sufficient amounts of equally sized branches of the same physical condition for the experiment. A single wood species was used to provide uniform starting conditions, even though


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Table 1. Chemical and physical properties of water samples collected on each sampling date in the Ladberger Muehlenbach from 09 Jan 01 to 17 Feb 02. Discharge are mean values of a 7 day period before each sampling date. Sampling date 9 Jan Discharge (l/s) 339 pH 7.59 Conductivity (µS/cm) 589 Oxygen (mg/l) 11.7 Temperature (˚C) 5.8 0.13 PO4-P (mg/l) NH4-N (mg/l) 0.11 NO3-N (mg/l) 9.93 0.06 NO2-N (mg/l)

6 Mar

8 May

25 Jun

3 Sep

12 Nov

17 Feb 02

360 7.83 594 13.1 2.9 0.09 0.14 8.19 0.04

473 7.71 571 11.1 10.8 0.09 0.01 1.35 0.04

174 8.03 586 8.7 14.7 0.13 0.07 7.32 0.11

104 7.57 574 9.1 14.2 0.13 0.09 6.00 0.06

303 8.01 593 11.1 6.5 0.11 0.06 7.35 0.04

370 7.88 552 10.2 4.1 0.12 0.11 8.20 0.03

the tree species might be of minor importance for the wood-colonizing invertebrate fauna in the study stream (Spänhoff et al. 2000). Six branches were fastened with polyester threads to a metal rod. The wood bearing metal rod was fixed by a laboratory clamp to another metal rod forming a T-shaped construction. Overall, 10 constructions were driven into the sandy sediment until the tied branches almost lay on the sediment surface (Spänhoff & Meyer 2004). Samples were taken from January to November 2001 at two monthly intervals, together with a final sample in February 2002. Several branches were lost during spate events reducing the study period to 7 sampling dates. Five branches were collected at random from different fixing devices on each sampling date by cutting the fastening thread and collecting the branch in a sampling net (mesh size 63 µm), which was held behind the branch to catch drifting invertebrates and fine particulate matter washed from the branch surface. Each branch and the content of the sampling net were collected in a single pvc freeze bag filled with stream water and taken to the laboratory.

Sampling of invertebrates and deposited fine particulate matter on wood surfaces The first sampling date was 10 weeks after exposure to let the biofilm and invertebrate assemblage reach an equilibrium state. Some branches collected on the penultimate sampling date were partially buried in sand, and all branches collected on the last sampling date were completely covered by sand. No data on deposited fine particulate matter were obtained from these branches. Additionally, data from the branches completely buried in the sediment were not used for statistical analyses of discharge-biofilm-invertebrate interactions, because complete sand cover obviously impaired both biofilm and invertebrate colonization. Each branch was carefully washed several times under a forced water jet into a sieve (63 µm mesh size) to collect wood-inhabiting invertebrates and deposited fine particulate matter. Large invertebrate specimens were picked from the sieve and preserved in 70 % ethanol. The remaining fine particulate matter was preserved in 98 % ethanol, and smaller invertebrates were sorted from it under a stereomicroscope at 16 ×

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to 40 × magnification. Bark pieces and larger fragments of leaf litter were removed from the samples and discarded in order to separate coarse particulate organic matter (CPOM) from the finer fraction. Total fine particulate matter (FPM < 2 mm) was dried for 24 h at 60 ˚C in aluminium pans, after removal of invertebrates from the samples. The samples were stored for 24 h in a desiccator and organic matter (FPOM) was calculated by combustion loss after burning the samples for 6 h at 560 ˚C in a muffle furnace. The remaining fraction was defined as fine particulate inorganic matter (FPIM).

Biomass and functional feeding groups of invertebrates Preserved invertebrates were identified to the lowest possible identification level, dried for 24 h at 60 ˚C, weighed after cooling to room temperature in a desiccator and subsequently burned at 560 ˚C in a muffle furnace for calculation of ash free dry matter (AFDM). Mass loss of invertebrates caused by ethanol preservation was assumed to be 25 % of fresh dry weight (Mackay & Kalff 1969) and a correction factor (1.33) was used to calculate fresh dry weight of invertebrates. Invertebrates were classified into functional feeding groups (FFG) according to Schmedtje & Colling (1996) (see Appendix).

Sampling procedure for biofilm components Three replicates for chlorophyll-a and ATP analyses were taken per branch. Chlorophyll-a was used to indicate algal biomass of the epixylic biofilm, whereas ATP has been used to indicate total biomass of microbial communities in surface biofilms (Golladay & Sinsabaugh 1991, Sinsabaugh et al. 1991). Each biofilm subsample was scraped with a scalpel from an area of 2 cm2 from the surface of the branches. Extraction of chlorophyll-a was performed by the hot ethanolic extraction method (Nusch 1999). ATP analyses were performed with ATP test kits from ConCell (Nettetal, Germany) after suspending the scraped biofilm material in double distilled water and brief sonication. The solution was transferred to a sterile falcon tube, diluted with double distilled water to a volume of 15 ml, homogenized thoroughly on a vortex, and a subsample of 100 µl was taken for ATP analysis. ATP-content of each sample was measured against a standard by adding 25 µl ATP-standard solution (ConCell, Nettetal). This procedure was used to minimize interference in measuring the ATP content of the samples due to quenching caused by turbidity. Fungal colonization of the branches estimated by ergosterol analyses, and breakdown rates of the branches have been already published (Spänhoff & Gessner 2004), and both parameters showed no correlations to invertebrate numbers, biomass, and biofilm characteristics.

Statistical analyses Data on invertebrate numbers and biomass, as well as ATP, FPOM and FPIM were square-root transformed, whereas chlorophyll-a data were log10 transformed to obtain normal distributions. Differences in invertebrate abundance, chlorophyll-a, ATP, deposited FPOM and FPIM among sampling dates were analysed by one way ANOVA followed by a Student-Newman-Keuls (SNK) post-hoc test. Pearson correlations were


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used to reveal relationships among biofilm parameters, invertebrate numbers, biomass, FFGs and deposited material, as well as discharge fluctuations (based on the mean of a 7 day period before sampling). Probability level for significant differences of the multiple correlation analysis was set to P < 0.01 according to Holm (1979). All statistical analyses were performed using SPSS 11.5 software (SPSS Inc., Chicago, Illinois).

Results Fine material deposited on the branch surfaces (FPIM and FPOM) and biofilm development

Fine particulate matter deposited on the branch surfaces displayed no successional pattern depending on the exposure time of the branches (Table 2). The highest amount of FPOM deposited on the branch surfaces was 0.88 mg/cm2, whereas the highest value of FPIM was 9.31 mg/cm2. Generally, FPOM contributed approximately 10 % to the total amount of fines deposited on the branches (Table 2). Similar to invertebrate data, FPOM and FPIM varied among samples taken on the same date, but the only significant difference could be found between FPOM values from the 2nd and 3rd sampling date (Table 2). Initial growth of epixylic biofilms resulted in 0.58 µg/cm2 chlorophyll-a and 0.23 µg/cm2 ATP after 12 weeks of incubation. Thereafter, the two biofilm parameters showed different temporal fluctuations and the ANOVA statistics displayed significant differences among chlorophyll-a values (d. f. = 6, F = 3.28; P = 0.013) as well as among ATP values (d. f. = 6, F = 9.35; P < 0.001) during the study period (Table 2). Chlorophyll-a showed a significant negative correlation with the number of grazing (P = 0.006) invertebrates and a trend to be also negatively correlated to shredders (P = 0.020). ATP values exhibited only non-significant trends towards a positive correlation with graTable 2. Biofilm parameters (n = 3 per branch, n = 15 per sampling date) and deposited fine particulate organic (FPOM) and inorganic matter (FPIM) (n = 5) on submerged pine branch surfaces during the study period (means ± 1 SE). Different letters indicate significant differences among dates. FPOM and FPIM were not measured on the last sampling date (n. a.) due to the burying of branches in the sediment. Date

Incubation Chlorophyll-a ATP time (d) (µg/cm2) (µg/cm2)

9 Jan 01 6 Mar 8 May 25 Jun 3 Sep 12 Nov 17 Feb 02

83 139 202 250 320 390 487

0.58 ± 0.25 ab 1.09 ± 0.26 a 1.45 ± 0.47 a 0.59 ± 0.08 ab 0.78 ± 0.31 ab 0.44 ± 0.14 ab 0.18 ± 0.05 b

0.23 ± 0.04 a 0.21 ± 0.04 a 0.17 ± 0.01 ab 0.08 ± 0.02 ab 0.11 ± 0.02 ab 0.04 ± 0.01 b 0.05 ± 0.01 b

FPOM (mg/cm2)

FPIM (mg/cm2)

0.61 ± 0.17 ab 0.26 ± 0.08 a 0.88 ± 0.07 b 0.81 ± 0.24 ab 0.75 ± 0.18 ab 0.60 ± 0.14 ab n. a.

3.64 ± 1.15 a 3.54 ± 2.13 a 8.45 ± 2.16 a 8.58 ± 3.39 a 7.29 ± 3.58 a 9.31 ± 2.41 a n. a.

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zers (P = 0.015) and shredders (P = 0.016). Additionally, ATP content of the epixylic biofilms was strongly correlated with the biomass of collectors (P = 0.002) and grazers (P = 0.005). Invertebrate density and biomass

The wood-inhabiting invertebrate assemblage was characterised by high fluctuation in numbers and biomass during the study period (Fig. 2). From exposure of the branches in October 2000 until the first sampling date in January 2001 an established invertebrate community developed with a mean density of 14326 ± 2532 individuals per m2 and a mean biomass of 974.6 ± 360.1mg/m2. In March and May invertebrate numbers and biomass were much lower but in June they were comparable to values found in January (14788 ± 3652 ind/m2 and 1012.5 ± 371.1mg/m2). Densities and biomass remained high in September but were low in November and February when branches were partially or almost completely buried in sediment. Statistically significant differences were found between invertebrate numbers in spring (March and May) and summer (June and September), but due to the high intersample variability biomass did not differ significantly between these two seasons (Fig. 2). The dominant invertebrate colonizers of exposed branches were Chironomidae, mainly Orthocladiinae and Chironominae (Appendix). Towards the end of the study period, Oligochaeta also reached high numbers on branches with increased amount of deposited FPIM. Large specimens of Gammarus pulex, late instars of Heptageniidae (Heptagenia flava and H. sulphurea) and Hydropsychidae (mainly Hydropsyche angustipennis), as well as rarely occurring

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Fig. 2. Temporal patterns of invertebrate colonization of submerged branches, indicated by numbers and biomass (mean ± 1 SE). Different letters indicate statistically significant differences between sampling dates (P < 0.05).


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large Limnephilidae (Halesus radiatus and Potamophylax rotundipennis) contributed mainly to total invertebrate biomass (Appendix). Invertebrate colonization of branches collected on the last sampling date were characterised by very small species, such as chironomids and Oligochaeta (Fig. 2). Total invertebrate numbers showed a significant negative correlation with discharge from January to November (P = 0.013), but the correlation was not significant for invertebrate biomass (P = 0.083). In contrast, invertebrate biomass was positively correlated with FPOM deposited on the branches (P = 0.032). Functional feeding groups (FFGs)

The dominant feeding group in numbers throughout the study was grazers (Fig. 3 a) followed by collector/gatherer and filter-feeders, which had similar grazer collector/gatherer filterer predator shredder

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Fig. 3. Temporal variability (mean ± 1 SE) of invertebrate functional feeding groups on submerged branches (A) Invertebrate numbers, (B) Invertebrate biomass.

Discharge (l/s) – 0.366 Chlorophyll-a (µg/cm2) – 0.358 ATP (µg/cm2) 0.317 FPOM (mg/cm2) 0.432 – 0.272 FPIM (mg/cm2) Significant at * P < 0.01

– 0.402 – 0.492 * 0.455 0.406 – 0.412

Collectors Grazers – 0.379 – 0.422 0.450 0.330 – 0.382

– 0.674 * – 0.185 0.084 0.448 – 0.238

Shredders Filterers

Invertebrate numbers (Ind/m2) – 0.618 * – 0.228 0.168 0.542 * – 0.353

Predators 0.097 – 0.250 0.549 * 0.388 – 0.343

0.030 – 0.088 0.516 * 0.186 – 0.380

Collectors Grazers

– 0.323 – 0.218 0.273 0.455 – 0.363

– 0.414 – 0.198 0.223 0.395 – 0.262

Shredders Filterers

Invertebrate biomass (mg/m2) – 0.257 – 0.197 0.185 0.394 – 0.254

Predators

Table 3. Correlations between macroinvertebrate numbers and the biomass of different functional feeding groups and biofilm parameters (ATP and chlorophyll-a) and selected abiotic factors. Data from the last sampling date (17 Feb 02) were not considered because branches were buried in sediment.

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densities in most samples, except in November when high abundances of Oligochaeta (collector/gatherer) were recorded. Predators and shredders both reached their highest abundances during summer. Water mites (Hydrachnidia) and Tanypodinae (Chironomidae) were the main predators, whereas Gammarus pulex was the main shredder. Overall, predators and shredders contributed little to the number of wood-colonizing invertebrates. In contrast to their low numbers, shredders generally had the highest biomass (Fig. 3 b). The biomass peak for shredders (616.6 ± 269.3 mg/m2) was in June and coincided with the highest abundance of G. pulex. Despite their high abundance, grazers contributed distinctly less than shredders to the total biomass. In September the highest biomass values were contributed by filterer species (large Hydropsychiidae larvae) (Fig. 3 b). In summary, both total numbers and biomass of invertebrates varied seasonally with a first maximum in January and another maxima in summer. Summer is the main oviposition time for many insects and large numbers of early instar chironomids, ephemeropterans, and trichopterans were present. The high biomass values found during the summer months reflected the presence of G. pulex as well as late instar hydropsychiid and heptageniid larvae. Abundances of all FFGs were negatively correlated with discharge (Table 3). Additionally, biomass of filterers was also negatively correlated to discharge (P = 0.023). Number of invertebrates displayed a positive correlation to the amount of FPOM deposited on the branch surfaces, except shredders, but shredding invertebrates biomass was negatively correlated to FPIM deposition. In contrast to the positive correlations between invertebrate numbers and FPOM, grazers (numbers: P = 0.024; biomass: P = 0.038) and shredders (numbers: P = 0.037; biomass: P = 0.049) had significantly negative correlations with the amount of sand (FPIM) deposited on the branches (Table 3).

Discussion Invertebrate colonization and fine matter deposition on branch surfaces

Invertebrate numbers and biomass found in the present study were among the findings of other studies using natural branches (McKie & Cranston 2001, Benke & Wallace 2003, Collier et al. 2004) although some studies reported much higher invertebrate numbers (up to 102,000 per m2 after 6 weeks of submergence, Nilsen & Larimore 1973). Invertebrate assemblages on the pine branches were dominated mostly by chironomids, with Orthocladiinae and Chironominae contributing in nearly equal numbers to the high abundances. Chironomid larvae were not identified to genus, but a laboratory emergence study (Spänhoff et al. 2004) showed the presence of mainly non-

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xylophagous grazing Orthocladiinae (Corynoneura spp., Rheocricotopus fuscipes), while Chironomini were the predominant surface collector/gatherers (Polypedilum spp.) and Tanytarsini the most abundant filterers (Rheotanytarsus curtistylus). The life cycles of species play an important role when submerged woody debris is colonized, and must be considered especially in studies with few sampling dates over a one year period to avoid misleading interpretations of disturbance effects on this community (Spänhoff et al. 2000). Some species contributed remarkable abundances in different seasons to the wood-inhabiting invertebrate assemblage, including the shredding amphipod Gammarus pulex (that prefer leaf litter as food resource after leaf fall in autumn), the predatory larvae of Orectochilus villosus (Coleoptera) (leaving the stream for pupation in the moist soil of stream banks in summer), early instars of the trichopterans Lype spp. and Hydropsyche spp. (peak emergence of adults and subsequent oviposition during summer) and water mites. These species were abundant especially during summer, coinciding with oviposition of adults and subsequent hatching of insect larvae and the lack of leaf litter, which might have resulted in a migration of shredder species onto submerged wood (e. g. G. pulex). Additionally, invertebrate communities in regulated streams are subjected temporarily to a variety of disturbances, making it difficult to estimate the effect of a single factor (Bunn & Arthington 2002). Fine particulate organic matter (FPOM) is an important food resource for a range of invertebrates (Malmqvist et al. 1978). Invertebrate numbers in our study were related to the amount of deposited FPOM, although only predators showed a significant correlation very likely caused indirectly by the presence of prey organisms. Accumulation of FPIM (sand) on the branch surfaces displayed a negative effect on invertebrate numbers and biomass, but not in a statistical significant way. Transported fine particulate material was trapped by the scaly bark of the pine branches, which likely served as microhabitat for small invertebrates like chironomids or early larval instars of mayflies, stoneflies and caddisflies. Collector/gathering species like nemourid stoneflies, leptophlebiid mayflies and Chironomini benefit from the FPOM which is trapped on the branch surface, but with increasing amount of sand they might leave this habitat because it no longer delivers suitable habitat conditions (Wood et al. 2005). The effect of burying could be observed in the last sample in which all branches were almost completely covered by sand resulting in low invertebrate numbers, mainly Oligochaeta and Chironomini, typical colonizers of sandy habitats. The impact of sediment deposition on invertebrates at a microscale has not been studied so far, but reach-scale studies have shown distinctly negative effects of increased sediment deposition on invertebrates (Strand & Merritt 1997, Spänhoff et al. 2000, Verdonschot 2000, Collier 2002, Wood et al. 2005).


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Factors affecting epixylic biofilms

Chlorophyll-a concentrations from the present study correspond to recent findings on epixlic biofilms (Sabater et al. 1998, Collier et al. 2004), but Tank & Dodds (2003) showed a high variability of epixylic algal biofilms by comparing 9 streams (range from approximately 0.12 to 3 µg/cm2) depending on the light irradiance and concentrations of dissolved nutrients. Studies on epixylic biofilm development have reported different patterns of chlorophyll-a and ATP dynamics. Golladay & Sinsabaugh (1991) reported increasing concentrations of chlorophyll-a and ATP on white birch (Betula papyrifera) ice-cream sticks during the first 4 and 10 weeks of submergence, respectively. Both parameters declined slightly after reaching their maxima and remained almost constant until the end of their study. Similar patterns were reported by Sinsabaugh et al. (1991) for epixylic biofilm development on small white pine (Pinus strobus) plates, but their chlorophyll-a and ATP concentrations were lower than those obtained in our study. Other studies have reported maximum chlorophyll-a concentrations rapidly after submersion of wood into streams (Sabater et al.1998), or different growth patterns depending on the season in which the experiments were started (Couch & Meyer 1992, Scholz & Boon 1993). Biofilm growth in our study was not limited by concentrations of dissolved nutrients, due to frequent nutrient enrichment by drainage run-off from arable fields adjacent to the stream (Cleven & Meyer 2003, Spänhoff & Gessner 2004). Chlorophyll-a concentrations on the submerged branches were mainly lower than those reported for wood substrates exposed in several streams studied by Tank & Dodds (2003), who reported distinctly lower nutrient concentrations for most of their study streams. High concentrations of chlorophyll-a in our study coincided with low concentrations of PO4-P and NO2-N (March and May), which could be explained by the effect of increased discharge, that diluted the concentrations of dissolved nutrients but lowered the grazing pressure on epixlic algae due to the reduction of wood-inhabiting invertebrates by flow disturbances. This patterns could be not ascertained for ATP concentrations, which were not correlated with discharge or nutrient concentrations. However, ATP is a sum-parameter for all living cells and therefore may not lend itself to displaying general patterns. Interactions between periphyton and grazing invertebrates have been reviewed by Feminella & Hawkins (1995), who showed that most studies had been performed on epilithic periphyton. Only few studies reported correlations between biofilm parameters and invertebrates on wood substrates (Hax & Golladay 1993, Collier et al. 2004). In the present study, algal biomass was negatively correlated to numbers of shredding and grazing invertebrates possibly due to the impact of invertebrate feeding on algal growth by grazing

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pressure. A similar pattern was reported by Collier et al. (2004) who found a decrease in algal biofilm with increasing invertebrate density on pine and native woods over a period of two years in a New Zealand stream. They also measured higher chlorophyll-a concentrations on wood surfaces subjected to higher flow velocity. Chlorophyll-a values in the present study were not correlated with grazer or shredder biomass in contrast to their abundance, which contradicts the findings of Anderson et al. (1999) who reported correlations between chlorophyll-a concentration and grazer biomass on stones. Bourassa & Cattaneo (1998) described indirectly the control of periphyton by invertebrate grazing activities with a correlation of dissolved nutrient concentrations to grazer biomass. We found a correlation between PO4-P concentrations and invertebrate biomass but not for grazers and there were much stronger correlations between nutrients and invertebrate numbers (this could be again an indirect effect of discharge, because both, PO4-P and invertebrate numbers were low during periods of high discharge and high during low flow conditions). Thus, we believe that in the LMB epixylic algal biofilm growth was controlled mainly by numbers of invertebrate grazers and shredders. Gammarids, as the main shredders on the branch surfaces, might use the softened surface of moist wood for feeding simultaneously ingesting the epixylic biofilm (Tank & Winterbourn 1995). The overall low chlorophyll-a values in our study could be attributed to the shading of the canopy during the summer and the high turbidity of the stream water during periods of increased discharge, although the dissolved nutrient concentrations in the stream were high. Nevertheless, the significant negative correlation between invertebrate abundance and chlorophylla might indicate an influence of invertebrate feeding on algal biofilm growth (Rosemond et al. 2000). Fluctuations of ATP content in the biofilm samples differed from the chlorophyll-a pattern. In general there was a tendency of low ATP values being correlated with high amounts of deposited FPIM. Other studies on epixylic biofilm development using ATP as a measure to estimate the total microbial biomass have reported various ATP:chlorophyll-a correlations. Sinsabaugh et al. (1991) and Golladay & Sinsabaugh (1991) found no correlation between the two parameters, but they provided few (Sinsabaugh et al. 1991) or no (Golladay & Sinsabaugh 1991) data on invertebrates colonizing the wood substrates. Hax & Golladay (1993) reported significant positive correlations between ATP and invertebrate densities consistent with data from the present study, but they also found a positive correlation between chlorophyll-a and invertebrate densities. In the present study the trends towards positive correlations of ATP and invertebrates might be due to the analyses of the whole material scraped from the branch surfaces. We cannot exclude that larger protozoans, like ciliates, or meiobenthic fauna, like nematodes contributed to the ATP values measured in the present study, thus the increasing


529

ATP values displayed likely increasing abundances of meiofauna. Filtration of the scraped surface material would had separated larger organisms (ciliates, rotatoria, nematodes) from the bacteria making more precise analyses of microbial biofilms indicated by ATP values possible. Effect of discharge and sediment deposition on epixylic biofilms

Discharge showed no significant effects on biofilm growth as indicated by chlorophyll-a and ATP concentrations. Bourassa & Cattaneo (1998) found a significant decrease of chlorophyll-a on rock surfaces with increasing current velocity, but a small direct effect of flow velocity on periphyton variation among several stream sites. The frequency of bed-movement events may have been the main influence on periphyton variations, whereas high-velocity events might have only minor influences on periphyton growth on stones (Biggs et al. 1999). Most studies on periphyton and factors influencing its growth patterns have been performed in streams with coarse sediments. Studies in streams with unstable sediments are scarce and investigations on the influence of deposited sediments on epixylic biofilms in sandy streams are lacking. Spates or even moderately increased discharge events can cause larger disturbances to the stream biocoenosis in streams with unstable sandy sediments, due to the rapid erosion of sand with increasing discharge and high sedimentation rates of sand during recovery to normal flow conditions (Waters 1995). The most prominent effect of enhanced sedimentation on submerged woody debris during the present study was the burial of the substrates into the stream bed leading to a reduction of numbers and biomass of colonizing invertebrate and epixylic biofilms. Increased discharge was significantly correlated to decreased numbers of wood-inhabiting invertebrate, especially during periods of high discharge. This decrease of invertebrate numbers likely promoted algal growth due to the lowered grazing pressure. Bond & Downes (2003) concluded that flow disturbance had a higher effect on benthic invertebrates than deposited fine sediment in an upland stream with predominantly stable bed substrata and low amounts of fine sediments. They tested the impact of relatively low discharges (maximum 12.5 l/s) and fine sediments of comparatively large grain size (500–1000 µm) on invertebrates in artificial channels exposed into the stream. The impact of transported sand on benthic invertebrates and their habitats during peak discharges in the present study might be much higher compared to the experimental conditions in the study of Bond & Downes (2003), due to much higher flow velocity and larger amount of fine sediments transported with the current (“sand-blasting effect” on exposed surfaces) and deposited during recovery to base flow. This study focused on a single stretch in one stream making general conclusions on the interaction of wood-inhabiting invertebrates and epixylic bio-

530


films, as well as the impact of abiotic factors like discharge fluctuations and sand deposition in streams difficult. Nevertheless, the results indicated a relationship between invertebrates and biofilms, especially algal biomass. Additionally, flow regulation might impact the invertebrate assemblage on woody debris in streams by altering the natural discharge regime and possibly leading to unpredictable stream bed movements. These influences can disturb the natural processing of woody debris in sandy stream systems, and would explain the lack of truly xylophagous invertebrate species on the exposed pine branches. Acknowledgements Many thanks to Gudrun Schulze for analyses of chlorophyll-a and water chemistry. We are especially grateful to two anonymous referees for their very thoughtful reviews.

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Appendix. List of the wood-inhabiting fauna on submerged pine branches. Presented are % of the total community and % of the total community biomass in parentheses. Biomass values below 0.01 % were indicated by ( – ). For some taxa total biomass values at a higher taxonomic level are presented only (e. g. Oligochaeta and Chironomidae). FFG = functional feeding group: c = collector/gatherer, g = grazer, s = shredder, f = filterer, p = predator, par = parasite; bold type = main feeding type; juv = juvenile larvae; I = Imago. Days submerged Taxon

FFG 83

Annelida Oligochaeta Naididae Tubificidae Enchytraeidae Lumbriculidae Hirudinea Helobdella stagnalis

c/g c c c

Glossiphonia complanata Mollusca Bivalvia Pisidium henslowanum Gastropoda Ancylus fluviatilis

139

(–) 0.03 0.03

p

202

250

320

390

487

(1.5) 7.4 3.1 1.2 1.7

(0.8) 4.0 2.6 0.1 0.08

(0.2) 3.3 1.5 0.1 0.2

(3.2) 19.1 0.6

(14.3) 11.9 0.7

5.4

2.0

0.2 (1.8) 0.7 (10.7)

p

0.2 (5.8)

f

0.05 (0.2)

g

Crustacea Gammarus pulex

s/c

Asellus aquaticus

s/c

1.3 (31.9)

1.4 (0.2)

0.2 (4.5)

0.05 (–)

0.05 (–)

5.7 (41.4)

5.2 (60.9) 0.03

6.2 0.3 (33.5) (0.03)

0.7 (1.3)

1.4 (0.3)

2.7 (1.1)

3.8 (0.2) 0.2 (1.0)

5.0 (1.1) 1.8 (–) 0.3 (–) 0.3 (0.8)

3.4 (4.2)

4.6 (0.8) 1.9 (2.9)

7.6 (1.1) 1.0 (–) 0.5 (–) 0.3 (0.5)

0.6

1.0

0.3 (0.2) 0.6

0.03 (6.4) 0.5 (0.03)

0.05 0.9 (1.1) (17.6) 0.3 (4.7) 0.2 (0.02)

(0.03) Arachnida Hydrachnidia Insecta Ephemeroptera juv. Baetidae juv. Baetis vernus

p/par – g/c g/c

Ephemeridae Ephemera danica

c/f

Heptagenia spp.

g/c

Heptagenia sulphurea

g/c

Heptagenia flava

g/c

Leptophlebiidae juv. Ephemerellidae

0.2 (–)

–

0.7 (15.1)

1.0 (15.7)

0.4 0.4 (17.5)

0.03 (–)

0.2 (–)


535

Days submerged Taxon Ephemerella ignita

FFG 83

139

202

g/c

250

Plecoptera Perlodidae Isoperla juv.

p

Nemouridae juv.

s/c

Nemoura cinerea Heteroptera Corixidae Micronecta poweri (I)

s/g/c

c

0.06 (–)

Megaloptera Sialidae Sialis juv.

p

0.03 (0.01)

Coleoptera Gyrinidae Orectochilus villosus

p

Elmidae juv.

0.09 (0.5) 0.06 (0.8)

0.7 (2.7)

g

Elmis aenea (I)

g

Elmis sp.

g

Oulimnius tuberculatus (I)

g/c

Oulimnius sp.

g/c

320

390

0.1 (–) 1.2 (0.1)

0.3 (0.5) 0.6 (–)

0.5 (0.4)

0.5 (4.0)

1.0 (–)

1.0 (0.5) 0.2 (–)

3.3 (1.9)

0.9 (2.7)

0.7 (9.4)

0.2 (0.2)

0.7 (0.7)

0.03 (0.14)

0.8 (0.03) 0.03 (0.2) 1.0 (0.1)

0.06 (0.3) 1.2 (0.6) 0.1 (0.4)

0.9 (0.6)

0.6 (2.2)

0.3 (0.4) 0.3 (54.1)

1.0 (1.8)

0.1 (–)

Hydraenidae Hydraena riparia (I)

g/p

0.05 (0.06)

Scirtidae Elodes sp.

c/g/s

0.1 (–)

Dytiscidae Platambus maculatus

p

0.5 (0.7)

Trichoptera Hydropsyche spp.

f/g

Hydropsyche angustipennis

f/p

Hydropsyche saxonica

f/p/g

Hydropsyche siltalai

f/p

Limnephilidae juv.

487

0.06 (–)

0.4 (–) 0.3 (16.8)

s/g/c 0.03 (–)

1.3 (0.1) 0.2 (8.9)

0.3 (13.5)

0.2 (0.1)

0.2 (2.5) 0.2 (0.6)

2.6 (6.4) 1.2 (22.4) 0.7 (11.5)

0.3 (20.4)

536

B. Spänhoff et al.

Days submerged Taxon Halesus radiatus Potamophylax rotundipennis Leptoceridae juv.

FFG 83

139

s/g

0.2 (33.3)

0.03 (2.3) s/g/p 0.03 (3.6) –

Athripsodes sp.

s/c/p

Oecetis testacea

p

Polycentropodidae juv.

p

Polycentropus spp.

p

Polycentropus irroratus

p

Polycentropus flavomaculatus

p

Plectrocnemia conspersa

p

Psychomyiidae Lype spp. Lype phaeopa Lype reducta Diptera Ceratopogonidae Chironomidae Orthocladiinae Prodiamesinae Chironomini Tanytarsini Tanypodinae

250

0.5 (0.1)

f

Empididae

p –

Total number of invertebrates (Ind/m2) Total biomass of invertebrates (mg/m2)

487

0.7 (1.9)

0.8 (0.1) 0.3 (0.5) 0.06 (0.5) 0.1 (1.1) 0.2 (2.1)

0.03 (0.02) 0.03 (0.19)

0.1 (0.33)

0.03 (0.05) (21.3) c/g 59.9 c/f 0.03 c/g/f 18.2 c/f 14.4 p 1.1 (0.1)

Simuliidae juv.

390

0.06 (–) 0.05 (–)

0.2 (0.3) 0.7 (–) 3.2 (6.9) 0.5 (0.4)

1.6 (3.8)

1.2 (0.4) 0.5 (4.3)

c/p

p

320

0.3 (0.9)

g/w 0.4 (0.05) g/w 1.5 (3.4) g/f 0.09 (0.3)

Limoniidae Dicranota sp.

Brachycera

202

(22.6) 66.0

(13.0) 26.6

13.2 10.1

30.1 10.3 0.2 (–)

(14.5) 34.6 0.1 21.9 14.7 2.4 (0.2)

12.5 (1.4) 0.7 (0.8)

3.0 (2.1) 3.7 (11.0)

0.7 (3.3)

0.1 (–) (2.1) 24.8

0.3 (–) (4.7) 14.6

(41.3) 37.1

13.7 9.7 3.5 (0.2)

29.0 9.9 0.9 (–)

18.9 16.4 0.3 (–)

0.06 (–)

0.6 (–)

10254 ± 2950 895 ± 248

1831 ± 717 129 ± 86

0.4 (0.4) 0.7 (–) 0.2 (–) 0.4 (–) 14326 3064 ± 2698 ± ± 2532 755 422 975 348 ± 443 ± ± 360 131 198

0.06 (0.6)

14788 ± 3652 1013 ± 371

1171 ± 282 60 ± 5