Use of Colonization Baskets for the Investigation of Disturbance Phenomena in Streams Under Model Conditions

Limnologica 29 (1999) 120-127 LIMNOLOGICA http://www.urbanfischer.de/journals/limno © by Urban & FischerVerlag

Institute of Ecology, Friedrich-Schiller-University, Jena, Germany

Use of Colonization Baskets for the Investigation of Disturbance Phenomena in Streams under Model Conditions

PAUL ELSER

With 2 Figures and 5 Tables

Key words: Stream benthos, disturbance, recolonization, resilience, seminatural substrate, colonization basket

Abstract

Seminatural substrates were used to mimic conditions of a denuded Hence, many investigators choose the experimental ap- stream bed after a substrate-moving spate. For this purpose gravel- proach to simulate the effects of natural disturbances on an filled wire baskets were placed on the sediment surface of the Ilm experimental scale (e.g. BOULTON et al. 1988; DOES et al. river (Thuringia, Germany). Invertebrates colonized these structures 1989; LAKE & SCHREIBER 1991; MATTHAEI1996; MATHOOKO very rapidly. The results indicate that resilience of the stream ben- 1996). One possibility is the use of artificial colonization thos after physical disturbances, such as spates is high. Colonization units as models for freshly disturbed substratum patches (e.g. patterns of numerous taxa differed significantly between summer and winter exposures. Seasonally varying colonization rates indicate REICE 1985; ROBINSON & MINSHALL 1986; ROSSER & PEAR- that the disturbance response of taxa varies throughout the year and SON 1995). throughout the life cycle of lotic invertebrates. If these changes are In the present study gravel-filled wire-mesh baskets were ex- correlated with seasonal discharge dynamics, they can be interpreted posed on the sediment surface of the Ilm river (Thuringia, as an adjustment to the disturbance regime. Findings are discussed Germany). The aim was to mimic conditions of a partly de- in regard to the practicability of small-scale experiments for the in- nuded stream bed following physical disturbance and to vestigation of disturbance phenomena in streams. launch a process similar to the recolonization of disturbed substratum patches after a bedload-transporting spate. Atten- tion was focussed on taxon specific patterns of colonization. Introduction Furthermore it was examined whether these patterns differ throughout the year. Findings are discussed in regard to the Natural disturbances are increasingly regarded as important practicability of small-scale experiments for the investiga- factors which influence the structure and dynamics of many tion of disturbance phenomena in streams. ecological communities (SouSA 1984; STRONG et al. 1984; PICKETT & WHITE 1985; WIENS 1984). Disturbance by spates is considered to be a dominant organizing factor in lotic ecosystems (POFF 1992; YOUNT & NIEMI 1990). Stream com- Materials and Methods munities frequently disturbed by variable discharge hardly ever achieve ecological saturation and tight species packing Colonization substrates which should characterize a resource-limited and niche-con- trolled structure (RESH et al. 1988; TOWNSEND 1989). The colonization substrates were made of wire-mesh baskets (length Investigations of natural disturbance phenomena in x width x depth = 20 x 20 × 15 cm; mesh 1.05 cm) that were filled with a sterile gravel mixture. The filling consisted of the same min- streams are connected with large technical and methodical eral components as the natural sediment in the study reach and pro- difficulties. A major problem is that the time when a spate vided a heterogenous substrate with interstitial spaces similar to ad- will occur can not be foreseen, and therefore, a detailed work jacent substrate conditions. 60% of the filling was composed of schedule is difficult to be put up. In addition, extreme condi- gravel with a mean diameter of 12 mm (range 8-16 mm) and 40% tions during high discharge make operating in the stream gravel with a mean diameter of 24 mm (range 16-32 ram). The grav- quite risky. el was purchased close to the study area.

120 0075-9511/99/29/02-120 $12.00/0 The baskets were arranged on the stream bed in a systematic grid Also, accumulation of silt or particulate organic matter has not been in rows of 3 baskets. Nylon rope was used to nestle the baskets to recorded, following the findings of WINrERBOURN (1978) and iron rods that where driven deeply into the sediment. In order to WILLIAMS (1980) that correlations of the content of organic matter avoid interference, baskets were 2 m apart within a row and were and animal colonization are unlikely to be detected. 3 m from the next row. Ambient conditions, in particular the nature of the stream bed and the current, were considered to be homoge- Study area nous throughout the study reach. The experiments were designed as time series with exposure The studies were carried out in the Ilm river, a nutrient-rich hardwa- times ranging from 6 hours up to 30 days. In order to evaluate the inter stream (stream order 4; STRAHLER classification) in Thuringia, fluence of season, the first experiment was conducted from August Germany. Water quality can be classified as ~-mesosaprobic with 15, 1996 to September 12, 1996, the second experiment took place nutrient concentrations amounting to 0.32 mg 1-1 Total-P and from January 28, 1997 to February 5, 1997. During the second ex- 7.31 mg 1-~ Total-N (mean values for 1996 measured at Niedertrebra; periment, attention was focussed on short exposure times to investi- Thtiringer Landesanstalt for Umwelt 1998). The study site was lo- gate the initial stages of colonization. The winter experiment had to cated in the hyporhitric zone about 2 km upstream of the village of be ceased after day 8 because a spate made further working in the Buchfart (11°20 ' E, 50055 ' N, 255 m a.s.1.). The slope was about stream impossible. 0.5%; stream width varied between 8-10 m. The substrate was In summer, a total of 15 baskets was sampled on 3 occasions (5, mainly mixed gravel with sand and flat cobbles up to a maximum 14 and 30 days after exposition), in winter 30 baskets were exposed length of 20 cm embedded near the surface. Further description of to investigate 6 colonization periods (0.25, 0.5, 1, 2, 4, and 8 days). the stream is given by KREY (1995). The study reach is partly shaded On every sampling occasion 5 randomly chosen baskets were taken by a canopy of Fraxinus excelsior, Alnus glutinosa and other trees. out beginning at the downstream baskets. While retrieving the bas- During the colder months the stretch is heavily colonized by diatoms kets, a sampling net with a square opening (net mesh 0.25 ram) was and by the filamentous algae Cladophora sp. in the warmer months set up fight downstream. Then the rope was cut and the basket sur- (SCHONBORN 1996). rounded by the net was lifted out of the water. Any invertebrates dis- A hydrograph station is located in the village of Mellingen, lodged from the baskets were washed into the net and animal losses about 5 km downstream of the study reach with a 70-year mean dis- could be reduced to a minimum. Then the baskets were opened and charge amounting to 4.2 m 3 s ~; mean discharge in summer 1996 the fillings were rinsed through a series of sieves (2.0, 1.0, 0.25 mm) was 3.27 m 3 s-1, mean discharge in winter 1997 amounted to 5.73 using an engine driven water pump. These sieve fractions were used m 3 s 1 (Staatliches Umweltamt Erfurt, unpubl.). Between the study as a simple measure to estimate the size distribution of some inverte- reach and the hydrograph station there is one tributary contributing brate taxa. about 0.1-0.2 m 3 s-1 (W. N~RB, Institute of Geography, Friedrich- After the baskets had been removed, 5 benthos samples from the Schiller-University, Jena; pers. commun.). Discharge data during stream bottom were collected using a modified HEss-sampler (area the experimental periods are given in Table 1. At the beginning and 0.043 m 2, mesh size 0.25 mm). Sampling sites were chosen at ran- at the end of each exposure period current velocity was measured dom within the study reach. All samples were fixed with 70% upstream and downstream of the baskets at a point 10 cm above the ethanol and processed in the laboratory. Invertebrates were counted streambed and at 60% depth using an electromagnetic water and identified to the lowest possible taxonomic level using a dissect- flowmeter. Due to the higher discharge current velocity in winter ing microscope (ZEISS x 50). was higher than in summer (Table 1). Before the experiments and It was assumed that the retrieved baskets contained numbers and on each sampling occasion dissolved oxygen, electrolytic conduc- varieties of animals that reflected the recolonization activity of ben- tivity and acidity were measured using portable digital instruments. thic invertebrates after a physical disturbance. No estimate of the Physico-chemical parameters showed little variation during the distance travelled before reaching the substrates has been attempted. sampling periods (Table 2).

Table 1. Discharge of the Ilm river at hydrograph station Mellingen (a) and flow conditions at the sampling sites near Buchfart (b). Discharge data from Staatliches Umweltamt Erfurt (unpubl.). Mean values _+ standard deviation.

Summer Winter (08/15-09/12/1996) (1/28-02/05/1997) a) Hydrograph station Mellingen Discharge [m3 s-1] mean 1.58 + 0.31 2.89 + 0.23 (daily mean values) rain 1.16 2.53 max 2.24 3.39 b) Sampling sites Water depth [m] mean 0.35 _+ 0.06 0.51 + 0.07 Current velocity [m s-~] above the baskets (at 0.6 x depth) 0.25 + 0.10 0.57 + 0.07 upstream from baskets 0.10 + 0.05 0.19 + 0.08 downstream from baskets 0.06 _+ 0.04 0.04 + 0.04

Limnologica 29 (1999) 2 121 Table 2. Physico-chemicat parameters of the Ilm river near Buch- Table 4. Kendalls Rank Correlation Coefficients for the compari- fart on the sampling occasions. Mean values of all measures + stan- son of the natural benthic invertebrate assemblages and the assem- dard deviation. blages on the colonization baskets (2-tailed test at P < 0.1). "c = Kendalls tan. Summer Winter (08/15-09/12/1996) (1/28-02/05/1997) Length of exposure time (days)

Water temperature [°C] 13.0 + 2.1 2.8 + 0.4 Summer Winter Conductivity [gS cm-1] 775 + 97 529 +_ 146 pH 8.1 _+0.1 8.2+0.1 Days 5 14 30 0.25 0.5 1 2 4 8 02 [mg 1-1] 10.4 + 1.2 13.2 + 1.4 z 0.45 0.51 0.46 0.45 0.38 0.50 0.57 0.53 0.58

Statistics Kolomogorov-Smirnov-tests revealed that counting data of most in- tionally long and nymphs are present in the benthos only for vertebrate taxa were distributed contagiously. Due to the high vari- a short time before emergence (ULFSTRAND 1968). ance and in order to provide an identical statistical treatment, data were analyzed by employing nonparametric statistics. Comparisons between sampling occasions were made using two-sided Mann- Taxon richness Whitney U-tests. Significance levels were all P < 0.05 except where noted. Comparisons of the natural benthos composition and the as- After short time taxonomic composition on the colonization semblages on the baskets were performed by estimating Kendalls baskets corresponded quite closely to the one recorded in Rank Correlation Coefficients using 2-sided tests at P < 0.1. On the natural benthos (Table 4). With the exception of some slow colonization curves of abundant taxa regression analyses were car- colonizing taxa like the mollusc Ancylus fluviatilis MULLER ried out using either linear or logarithmic models, all dominant benthos taxa were present on the baskets within short time. Taxon richness on the baskets increased most rapidly within the first 4-5 days the baskets were in the Results stream. In summer there still was an increment between day 14 and 30 whereas in winter taxon richness had levelled off Natural benthos even after day 4. At the end of both experiments an average of 12-14 taxa per basket was recorded (Fig. la). The faunal composition of natural benthos (based on HESS- samples) is given in Table 3. Total benthic density was significantly higher in summer. In regard of dominant taxa the Total invertebrate density taxonomic composition did not differ very much between Total invertebrate densities rised very steeply within the first summer and winter. Nevertheless, some species, e.g. the days and reached high values within short times in both ex- mayfly Serratella (= Ephemerella) ignita PODA were only periments (Fig. lb). As shown in the winter experiment, present in summer. Egg dormancy of this species is excep- many taxa were present on the baskets even after 6 hours. Significant seasonal differences became obvious: in sum- Table 3. Abundances of common taxa in benthos samples (HEss- mer, the total number of invertebrates levelled off after day samples) collected in the Ilm river near Buchfart. Taxa that con- 14 at approximately 1300-1400 individuals per basket. In tributed less than 1% to the total benthic densities are listed together winter, approximately 800 organisms were counted after under 'Others'. Mean values (n = 5) + 95% confidence intervals. 8 days. Although colonization was slower in winter, it showed a constant increase that did not level off within the Summer Winter experimental period. (09/12/1996) (02/08/1997) Taxon Ind. m -2 Ind. m -2 Colonization patterns of single invertebrate Gammaruspulex L. 4579 + 899 612 + 479 taxa Chironomidae 1775 + 432 1969 + 696 Baetis rhodani PICT. 146 + 67 1620 _-. 722 The colonization patterns of single taxa also revealed signifi- Hydropsyche sp. PICT. 774 +_ 301 127 _+ 70 cant differences between summer and winter exposures. In Ancylusfluviatilis MOLLER 520 _+ 130 380 _+ 139 summer, the amphipod Gammarus pulex L. was the most Serratella ignita PODA 87 + 61 - abundant colonizer. Density reached about 1200 individuals Others 219 + 28 243 _+ 35 per basket after 14 days and then somewhat declined (Fig. lc). However, the decay from day 14 to 30 was not signifi- Total 8100 + 831 4950 _+ 448 cant. In winter, density was significantly lower (comparing

122 Limnologica 29 (1999) 2 day 5 and 14 in summer to day 8 in winter) and did not ex- summer curves looked alike, regression analyses did not proceed 206 + 89 individuals per basket. The number of gam- vide significant results, and coefficients of determination marids stabilized after 4 days and did not increase signifi- were low. cantly from day 4 to day 8. The winter colonization curve of Comparing the size distribution of Gammarus pulex Gammarus pulex could most adequately be described by a based on sieve-fraction data, even more seasonal differences logarithmic model (Table 5). This pattern was quite similar became obvious. The summer population mainly consisted to the curve of the total invertebrate density. Although the of little individuals, whereras in winter only few freshly emerged specimens were found (Fig. 2). The caddis larvae Hydropsyche sp. P~CT. colonized the Summer Winter baskets following approximately linear kinetics in both ex- a) Taxon richness periments (Fig. ld). Like Gammarus pulex, the population in summer mostly consisted of little individuals and fewer but bigger specimens in winter• The steeper slope of the summer regression line (Table 5) was caused by high numbers of instar-I larvae in summer. 0 10 20 30 0 2 4 6 8 Other insect taxa, e.g. the mayfly Baetis rhodani PICT. and the blackflies (Simuliidae) showed converse colonization b) Total invertebrate density patterns. Baetis rhodani was the most abundant colonizer in winter, whereas the number in summer stayed low (Fig. le). These findings closely correspond to the results for the natural benthos (Table 3). In summer, mainly late instar larvae

• were found, whereas in winter no age class dominated. The 0 10 20 30 0 2 4 6 8 density on the baskets reached some saturation after 4 days

c) Gammarus pulex

Table 5. Regression analyses of invertebrate colonization kinetics on wire-mesh baskets• Only results with coefficients of determination (r 2) greater than 0.50 are shown• * = P < 0.05; ** = P < 0.01; , { • ** = P < 0.001; n.s. = not significant• 0 10 20 30 0 2 4 6 8 Equation r: Signi- ficance d) Hydropsyche sp 1601

Summer Total invertebrate 267.04 log (x) +522.40 0.66 n.s. 0 density 0 10 20 30 0 4 6 8 Gammarus pulex 294.07 log (x) + 186.42 0.60 n.s. Hydropsyche sp. 29.11 log (x) + 11.88 0.72 n.s. 2.32 × + 48.20 0.90 n.s. e) Baetis rhodani Chironomidae -58.43 log (x) + 248.18 0.92 n.s. -3.72 x + 160.00 0.74 n.s. ~'~ 250 t Simuliidae -1.27 log (x) + 6.79 0.72 n.s. °1 -0.10 x + 5.20 0.91 n.s. o1-- f--~ : Winter 0 10 20 30 0 2 4 6 8 Total invertebrate 210.15 log (x) + 275.51 0.95 * * density 91.70 x + 107.63 0.94 * * f) Chironomidae Gammarus pulex 54.59 log (x) + 89.30 0.99 * * * 21.71 x + 51.22 0.82 * '°°t L. Baetis rhodani 106.88 log (x) + 120.83 0.92 * * 100 46.47 x + 35.88 0.91 ** Hydropsyche sp. 1.96 log (x) + 3.58 0.65 n.s. Or ...... 1.01 x + 1.61 0.90 * * 0 10 20 30 0 2 4 6 8 Chironomidae 25.91 log (x) + 36.72 0.63 n.s. Days of colonization Days of colonization 12.51 x + 12.85 0.77 * Simuliidae 12.67 log (x) + 13.46 0.76 * Fig. 1. Colonization of gravel-filled wire-mesh baskets by benthic 6.23 x + 1.50 0.96 * * invertebrates. Error barsrepresent standard deviation (n = 5).

Limnologica 29 (1999) 2 123 Summer Winter "toO % lOO %

[] 0.25-1.0 mm

50 % [] 1 .(D-2.0 mm

• > 2.0 mm

/o 0% -- i " 5 14 28 0.25 0.5 1 2 4 8 Days of colonization Days of colonization

Fig. 2. Size distribution of GammaruspulexL. on wire-mesh colonization baskets (based on sieving data).

amounting to about 300 individuals per basket. The increase day 8 in winter. These findings are in good accordance with from day 4 to day 8 was not significant. Regression analyses the results from classical colonization studies in streams: e.g. showed that the kinetics could adequately be described either KHALAF & TACHET (1977) reported that colonization of by a logarithmic or by a linear model. stones in trays was complete by 16 days; ULFSTRANDet al. Chironomid midges exhibited typical features of early (1974) stated that colonization of the same type of substra- colonizers in both experiments (Fig. lf). In summer, their tum was completed after 15-32 days. In contrast to own number rose very fast within the first period of exposition findings, ULFSTRANDet al. found that the colonization rate and declined after day 5. The winter density of Chironomids on clean substrates was comparatively low during the first also showed an early peak followed by a decrease, but there days of exposure due to lacking detritus. was a second rise that did not level off after 8 days. Benthic But perhaps all these 'equilibrium numbers' are tempo- density had remained roughly the same (Table 3). rary. Several workers investigating the colonization dynamics of benthic invertebrates have noted that artificial substrates are modified when placed in streams (SHELDON 1977; Discussion MINSHALL& MINSHALL1977; ULFSTRANDet al. 1974). Prob- ably, a longer duration of the exposure time would have al- Quite a high proportion of the literature on disturbance phe- tered the numbers of individuals and taxa either due to nomena refers to tropical, subtropical or desert ecosystems changes of substrate or due to nutrition conditions. Further- (e.g. BOULTON et al. 1988; COVlCH et al. 1996; DUDGEON more, predation or inter- and intraspecific competition are 1993; DOEG et al. 1989; FISHER & GRIMM 1988; GRIMM & known factors leading to changes in community structure on FISHER 1989; LAKE & SCHREIBER 1991; MATHOOKO 1996; artificial substrates (PECKARSKY 1985). In general, it seems ROSSER & PEARSON 1995). Investigations in temperate re- problematic to describe the completeness of colonization ei- gions still are comparatively rare (but see e.g. MATTHAEI ther by taxon richness or by total benthic density; and it is 1996; MATTHAEI et al. 1996, 1997; SCULLION & SINTON quite ambiguous at all to decide whether or not colonization 1983). In the present study it was assessed whether coloniza- is completed. Probably, an appropriate parameter to measure tion studies can be used as an appropriate method to mimic the successional stage of stream benthic communities does benthic recolonization patterns in a central European stream. not exist yet. The experiments provided insights to the colonization dy- Beyond this, many stream-ecologists have come to realize namics of some benthic invertebrate taxa under model condi- that succession in streams exceptionally seldom is attaining a tions. Numbers of individuals and taxa seemed to stabilize climax-like state. TOWNSEND (1989) postulated that lotic after 1--4 weeks and the density of most abundant taxa had communities are disturbed so frequently that biotic interac- approached some kind of saturation within short time. Using tions become unimportant, especially in streams with flashy the number of taxa as a criterion for completeness of colo- discharge regime. Recent hypotheses assume that stream nization, colonization was completed after 5 days in summer communities disturbed by variable discharge permanently and after 4 days in winter, respectively. Using the total inver- are kept in a nonequilibrium state and never achieve ecologi- tebrate density as a criterion, colonization was completed cal saturation (REsH et al. 1988; TOWNSEND 1989; MACKAY after 30 days in summer but had not stopped developing until 1992).

124 Limnologica 29 (1999) 2 Studies of overall community patterns can offer useful in- often patchily distributed. But while some authors found evi- sights, even though explanations of those patterns must be dence that food is a causal agent for colonization others have sought at individual species level (WILLIAMS 1980). Colo- failed to show correlations between the amount of organic nization curves for individual taxa revealed both taxonspe- matter and the number of detritivores. WINTERBOU~e~(1978) cific and seasonal patterns. Discontinuities in the curves sug- collected almost identical numbers and kinds of insects on gested seasonal variation in mobility, and this may be associ- empty mesh bags as on those that contained dead leaves. ated with particular stages in life cycle as was proposed by WILLIAMS (1980) found no correlation between numbers of WILLIAMS (1980). This particularly became evident looking colonizing animals on implanted substrates with amounts ei- at the results for Gammarus pulex and Baetis rhodani. Dur- ther FPOM or CPOM settling in the substrates. ULFSTRAND ing the winter exposure relatively low numbers of gam- et al. (1974) showed that Baetis rhodani clearly found fresh- marids were moving onto the baskets compared with high ly implanted substrates more suitable to live on than later. numbers in the summer experiment. This could simply be Accordingly, MATTHAEI et al. (1997) reported that Baetis due to the higher benthic density of Gammarus in summer spp. recovered faster from experimental disturbance than and/or because different life stages of Gammarus pulex dis- their food resource, the benthic algae. On the other hand played different patterns of dispersal. SCHMITT (1996) found HILDEBRAND (1974), BOHLE (1978), McAULIFFE (1983) and that juvenile Gamrnarus pulex mainly moved by drift where- PECKARSKY (1980) did obtain responses to manipulated den- as upstream movement was dominated by adult males. NILS- sities of periphyton or added detritus in colonization cages, SON & SJOSTROM (1977) stated that small gammarids colo- respectively. nize more rapidly than large ones. They concluded that the Own findings indicate that food was not a causal agent to initial (1-2 d) colonization of a substrate by Gammarus pulex influence colonization patterns strongly. At the beginning of is a random process during which individuals colonize in the experiments the substrates did not contain any food sup- proportion to their representation in the drift. M(?LLER(1966) ply such as particulate organic matter or benthic algae. Nev- showed that an increase in temperature always increased the ertheless, they were spontaneously colonized by numerous drifting of Gammarus pulex, particularly during its summer taxa. This suggests that food availability was not a limiting breeding season. In contrast to own results, ULFSTRANDet al. factor and that benthic invertebrates in the nutrient-rich river (1974) found an approximately even colonization rate over a Ilm rather compete for empty spaces than for food. In this re- 32 day period. gard natural disturbances such as spates would contribute to Baetis rhodani revealed a converse recolonization pattern the coexistence of competing taxa in the sense of CONNELLS with relatively low densities in summer and higher numbers 'intermediate disturbance hypothesis' (CoNN~LL 1978) by in winter. One reason could be, that a high proportion of supplying empty habitats (see also McAuLIFFE 1983, 1984; mayflies had left the water for reproduction in summer and DENSLOW 1985; TOWNSEND& SCARSBROOK1997). hence, the number of colonists remained low. On the other The substratum-filled baskets permitted replication to an hand it is well known that dispersal of Baetid larvae is pri- extent that is quite impractical in larger systems. However, marily by downstream drift (BOULE 1978; BRITTAIN& EIKE- the crucial point of the experiments is the extent to which LAND 1988). Therefore, a higher colonization rate in winter they could simulate recolonization after natural spates. It could partly be a function of the higher discharge (see Table particularly has to be considered that the baskets represented 1). MATTHAEI et al. (1997) found that recolonization of small disturbed patches amongst an undisturbed matrix. The Baetis rhodani mostly originated from the drift, particularly spatial scales of the experiment and of a natural disturbance by young larvae. Accordingly, WILLIAMS (1980) reported differed by orders of magnitude. After a real spate the pro- that mass movements ofBaetis rhodani were particularly ev- portion of disturbed and undisturbed patches might be con- ident in first instar larvae. MACKAY (1992) suggested that verse. Hence, it is likely that the colonization process was colonizing larvae actively enter drift for dispersal from their somewhat accelerated compared to situations when more of crowded hatching sites. Mt)LLER (1973) termed this phe- the stream bed is affected. Nevertheless, MACKAY (1992) nomenon 'distributional drift'. Other workers have support- emphasized that even small-scale studies are reasonable ed this theory (e.g. TOWNSEND & HILDREW 1976; WILLIAMS simulations of the colonizing events that might follow minor & HYNES 1977). spates. LAKE & DOEG (1985) concluded as a generalization All the same, seasonally varying colonization rates indi- that the time needed for colonization is a direct function of cate that the disturbance response of certain taxa varies the size of patch to be colonized and the source of coloniza- throughout the year and throughout the life cycle of inverte- tion. MINSHALL(1988) stated that experiments conducted at brates, respectively. If these changes are correlated with the small spatial scales provide valuable information on the re- seasonal discharge dynamics, they can be regarded as an ad- covery dynamics of small patches of substratum although he justment to the disturbance regime like it was postulated by suspected this method to be unsuitable for studying the im- BUTLER (1984) and NCHOWALTER(1985). pact of spates on a whole stream. In general, a reliable evalu- Especially for detritivores and herbivores it would seem ation of the representativeness of the colonization basket advantageous to aggregate on their food supply, which is technique would require a direct comparison of recoloniza-

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