C S I R O P U B L I S H I N G

Volume 48, 1997© CSIRO Australia 1997

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Introduction

Seasonal and diel variation in both abundance andcomposition are well known features of macroinvertebratedrift in streams (Schreiber 1995). In particular, the frequent,well-marked diel cycles in number, identity and size ofdrifting organisms have attracted considerable attention(Müller 1974; Brittain and Eikeland 1988). Phenology andpopulation dynamics of different species appear largelyresponsible for the seasonal changes, whereas behaviouralmechanisms may account for the daily shifts (Waters 1972;Benke et al. 1986; Brittain and Eikeland 1988). However, the

relative importance of each type of temporal variation (andthe mechanisms associated with each) in determining overalldrift composition and abundance, or the relationshipsbetween the two scales of temporal change, have been lessfrequently studied. This paper addresses this question andassesses the relative magnitude of seasonal and dailyvariation (by means of multivariate analyses) inmacroinvertebrate drift composition in a stream in northernSpain.

Published studies of invertebrate drift in streams in theIberian Peninsula are relatively scarce; most focus on a

Mar. Freshwater Res., 1997, 48, 455–64

10.1071/MF97037 1323-1650/97/050455$05.00

Temporal patterns in macroinvertebrate drift in a northern Spanish stream

Pedro A. RincónA and Javier Lobón-Cerviá

Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, c/ José Gutiérrez Abascal 2,28006 Madrid, Spain

Ae-mail: mcnar3c@fresno.csic.es

Abstract. Drift in the River Negro (Asturias, Spain) was dominated by terrestrial invertebrates,chironomid larvae, simuliid larvae, water mites and the mayfly Baetis rhodani. Trichopterans andLeuctra spp. (plecopterans) occasionally exhibited increased abundances related to events in their lifecycle (hatching, emergence). Total numbers of drifting macroinvertebrates increased after sunset, and onsome dates a second, pre-dawn peak appeared. Baetis spp., plecopterans, adult aquatic Coleoptera andsimuliid larvae drifted mostly at night. Terrestrial invertebrates and water mites were more abundantduring the day, whereas trichopterans, chironomid larvae and naidid oligochaetes showed no consistentdiel pattern. Drift composition changed seasonally and daily. Seasonal changes appeared to bedetermined mainly by seasonal shifts in the assemblage of benthic macroinvertebrates. The combineduse of UPGMA cluster analysis and detrended correspondence analysis showed that seasonal variationwas the major factor influencing overall drift composition, but that diel changes were only slightly lessimportant. The degree of diel variation also changed seasonally. In January and March, day and nightsamples from a given date were quite similar. In contrast, in May and October, diel variation was greaterand the dissimilarity between day and night samples was comparable to that between samples fromdifferent dates.

Resumen. Los invertebrados terrestres, larvas de quironómidos y simúlidos, ácaros acuáticos y laefémera Baetis rhodani dominaron la deriva en el Río Negro (Asturias, España). Tricópteros yplecópteros (Leuctra spp.) aumentaron puntualmente su abundancia en relación, aparentemente, coneventos en sus ciclos de vida (eclosión, emergencia). El total de macroinvertebrados en la deriva creciótras el ocaso y, en ocasiones, antes del amancer. Baetis spp., los plecópteros, coleópteros acuáticosadultos y simúlidos derivaron principalmente durante la noche. Los invertebrados terrestres y ácarosacuáticos lo hicieron durante el día, mientras que tricópteros, quironómidos y naídidos no mostraron unpatrón consistente. La composición de la deriva cambió estacional y diariamente. Los cambiosestacionales parecieron relacionarse con la variación de la comunidad de macroinvertebradosbentónicos. El uso del análisis de agrupamiento UPGMA y del detrended correspondence analysisidentificó la estacionalidad como el principal determinante de la composición de la deriva, pero loscambios diarios fueron sólo ligeramente menos importantes. La magnitud de éstos también varióestacionalmente. En enero y marzo las muestras de día y noche de una misma fecha fueron muyparecidas. En cambio, en mayo y octubre la diferencia fue marcada y comparable a la existente entremuestras de fechas distintas.

Extra keywords: drift composition, diel cycles, seasonal variation, multivariate techniques

© CSIRO Australia 1997

Pedro A. Rincón and Javier Lobón-Cerviá456

few taxonomic groups or are of limited temporal scope(Rieradevall and Prat 1986; Vilchez-Quero and Lavandier1986; Valente and Heland 1990). To our knowledge thepresent paper is the first to deal with all macroinvertebratesthroughout a complete year cycle in an Iberian stream.

Study areaField work was conducted in the River Negro, a 22 km long coastal

stream in Northern Spain (Valdés Council, Asturias). The sampling site, atÅ10 m above sea level, 3 km upstream from the town of Luarca (where theRiver Negro flows into the Cantabrian Sea), was a long run (Å 200 m) withshort riffles at both the up- and the downstream ends (Å20 and 10 m longrespectively). This macrohabitat structure (dominance of runs and scarcityof riffles or pools) is typical of the lower reaches of the River Negro. Waterdepth rarely exceeded 80 cm (average Å50 cm) and channel width rangedbetween 8 m and 12 m. Cobbles and, secondarily, gravel dominated thebottom substratum; larger particles, sand and silt were uncommon. Riparianvegetation, basically a strip of variable width of European alder Alnusglutinosa Gaertner, was abundant and formed a dense canopy. Brown troutSalmo trutta L. and European eel Anguilla anguilla (L.) were the only fishspecies present. Overall, these conditions are representative of many coastalstreams in northern Spain (Rincón, unpublished). The sampling site wasabout 200 m upstream of the site studied by Rincón and Lobón-Cerviá(1993); see that paper and Rincón (1993) for further details.

Materials and methodsField sampling

Drift samples were obtained on 2 July, 29 August and 1 October 1986,and on 10 January, 23 March and 27 May 1987; they were collected every3 h (beginning within 30 min around sunrise) until a 24-h cycle (eightsamples) was completed. Each sample was categorized as ‘night’ or ‘day’according to prevailing light conditions. A net (30 ´ 30 cm at the mouth, 1m long, 0·25-mm mesh) installed about 20 m below the upstream end of therun operated for periods of 0·5–1·0 h (the exact time was recorded on eachoccasion); these durations should yield reasonably representative samples(Culp et al. 1994). The net was set in such a way that it could capture itemsdrifting at the water surface so that the abundance of terrestrial invertebratesand winged adults of aquatic larvae could be assessed (Garman 1991). Thenet was placed in the same position for each sample (both within and amongsampling dates), with ad hoc permanent markers on the river bank as areference, in order to facilitate the comparison of drift rates at differenttimes of day (Waters 1972; Allan and Russek 1985). Unfortunately, watervelocity could not be measured at the mouth of the net. Drift samples werepreserved in 4% formaldehyde.

The protocol described above yielded eight drift samples per samplingdate except in July when, because of logistical problems, the 7th samplewas not obtained. An accidental fire in the laboratory resulted in the loss ofthe 2nd drift sample from July. Hence, 46 drift samples were analysed(Table 1).

A 30 ´ 30 cm Surber sampler (0·25-mm mesh size) was used to collect4–8 replicate samples of benthic macroinvertebrates on each date so thatdrift composition could be compared with that of the benthic assemblage.

Sample analysis

Organisms in drift and benthos samples were identified and countedunder a binocular microscope. Ephemeropterans (except very small Baetislarvae, collectively designated Baetis spp.), plecopterans, and most of thecommon aquatic coleopterans (except larvae of the genus Haliplus andadults of Hydraena) were identified to species. Some trichopteran larvaecould be identified to genus only. In the case of the genus Hydropsyche,most individuals found in drift samples were very small (first or second

instars) and we were unable to ascribe them to a particular species as wasdone with larger specimens. Dipteran larvae were identified to family, andthe remaining groups (aquatic collembola, water mites, crustaceans andnon-arthropods) to either order or family. All terrestrial invertebrates andterrestrial stages of aquatic forms were grouped under ‘terrestrial items’.

Drift rates were expressed as number of organisms captured per hour toassess fluctuations in drift abundance over the 24-h cycle (Waters 1972;Brittain and Eikeland 1988). It was assumed that water velocity at themouth of the net was similar in each of the eight diel samples of a particularsampling date or, at least, that possible diel variations in velocity wereminor compared with diel changes in macroinvertebrate numbers in thedrift. Continuous limnigraph records from a gauging weir Å1 kmdownstream from the sampling site measured no diel variation in dischargeduring the sampling periods (Anon. 1993 and Comisaría de Aguas del Nortepersonal communication). Hence, as the net was carefully installed in thesame position for every sample, we feel that the assumption is justified.This lack of diel flow variation is typical for the River Negro except duringfreshets (Anon. 1993).

To compare the number of drifting invertebrates on different samplingdates the average velocity of the water column, estimated by Rincón andLobón-Cerviá (1993) during simultaneous microhabitat use studies, wasused as a standardizing factor. This was considered to be the velocity at themouth of the net. At the site, average velocity of the water column wasstrongly dependent on total discharge (Rincón 1993; Rincón and Lobón-Cerviá 1993). Hence, the procedure was roughly comparable to the use oftotal discharge as a standardizing factor as suggested by Waters (1972).Estimates of drift abundance that have the same units (number ofindividuals per unit volume) as drift density (Allan and Russek 1985) arepresented. However, these values are not true density estimates (the truevelocities at the net are unknown) but represent an attempt to provide anidea of relative differences in drift abundance among sampling dates.

Statistical procedures

A hierarchical cluster analysis was performed on the 46 samples ´ 82groups matrix to detect patterns in the composition of River Negro drift.The UPGMA (unweighted pair-group method using averages) algorithm

Table 1. Time of collection (hours, GMT) and duration (min) foreach River Negro drift sample

D, day; N, night

Sample MonthJuly August October January March May

1 0500 (D) 0530 (D) 0530 (N) 0530 (N) 0630 (D) 0430(N)45 30 60 60 60 30

2 — 0830 (D) 0830 (D) 0830 (D) 0930 (D) 0730 (D)30 60 60 60 30

3 1100 (D) 1130 (D) 1130 (D) 1130 (D) 1230 (D) 1030 (D)60 30 60 60 60 35

4 1400 (D) 1430 (D) 1430 (D) 1430 (D) 1530 (D) 1330 (D)60 30 60 60 60 30

5 1700 (D) 1730 (D) 1730 (D) 1730 (D) 1830 (D) 1630 (D)60 30 60 40 60 30

6 2000 (N) 2030 (N) 2030 (N) 2030 (N) 2130 (N) 1930 (D)30 30 60 30 60 30

7 — 2330 (N) 2330 (N) 2330 (N) 2430 (N) 2230 (N)30 60 30 60 45

8 2430 (N) 0230 (N) 0230 (N) 0230 (N) 0330 (N) 0130 (N)45 30 60 30 60 45

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(James and McCulloch 1990) was used together with Horn’s (1966)modification of Morisita’s (1959) index as a similarity measurementbetween each pair of samples. The formula of this index (M) is

2 · p1i p2i / · (p1i )2 + · (p2i )

2,

where pji is the relative frequency of category i in the j sample. M has beenwidely used (Zaret and Rand 1971; Angradi and Griffith 1990) and itsvalues range between 0 (completely dissimilar samples) and 1 (identicalsamples). See Wolda (1980) for further details on its properties.

The data matrix was also subjected to a detrended correspondenceanalysis (DCA) to obtain an ordination of drift samples along gradients (e.g.seasonal or diel variation) in their composition. The graphicalrepresentation of DCAs allows visual identification of those groups ofsamples with similar composition, their relative positions and the organismsthat characterize them (Hill and Gauch 1980; Gauch 1982). It also allowsexamination of the placement of the groups previously identified in thecluster analysis along the DCA axes. Baltanás and Rincón (1992) provide adiscussion on the combination of cluster and DCA analyses.

Despite some controversy (Wartenberg et al. 1987; Peet et al. 1988;Jackson and Somers 1991), DCA appears to be an ordination techniqueparticularly well suited for data sets including discrete data or variablesnon-normally distributed or with non-linear relationships between them(Gauch 1982) and it has been applied to the analysis of seasonal driftsamples (Schreiber 1995). The eigenvalues of the ordination axes yieldedby the procedure range from 0 to 1, and those axes reflecting the structureof the original data matrix can usually be distinguished from those that onlyrepresent uninterpretable variation by a clear gap between their eigenvalues(Persat and Chessel 1989; Copp 1990). With this procedure, the aim was tocomplement the information provided by the clustering technique on thefactors influencing the composition of drift in the River Negro.

Results

Appendix 1 lists the 82 different categories (5776individuals) in drift samples and 110 categories (25342individuals) in benthos samples (119 categories for bothsample types combined).

Temporal variation in drift composition

Most of the groups of organisms that were recognized indrift samples were scarce or infrequent; only 33 out of the 82categories attained relative abundances higher than 1% ofeither the day, night or total samples on any of the sixsampling dates. Furthermore, only terrestrial invertebrates,chironomid larvae, simuliid larvae, water mites and Baetisrhodani presented relative abundances over 1% on all sixdates. With the exception of October night samples (wherethey represented 88%) these 33 categories always comprisedover 90%, and frequently over 95% (13 out of 18 cases), ofall organisms captured (Table 2).

There were obvious seasonal fluctuations in the relativeabundance of some of the organisms present in the drift.Terrestrial invertebrates were scarcer in October and January(Table 2). In the case of aquatic invertebrates, some of thesechanges, such as the increases in abundance of chironomidlarvae in January and March or of adults of the elmid beetleOulimnius troglodytes in October, seemed to reflect seasonalvariations in the benthic community (Table 3). Other

variations seemed to relate to the phenology of species: e.g.the trichopterans Micrasema moestum and Hydropsyche spp.appeared only as very small, apparently recently hatched,larvae, present only during the summer. Also, the individualsof the three Leuctra species (Plecoptera) captured duringtheir peak in abundance in October (particularly marked forL. aurita) were all large nymphs that appeared to be close toemergence (Table 2).

Diel variation in drift composition was also clear on allsampling dates. Ephemeropterans, plecopterans, and simuliidlarvae were more abundant during the night, and Elmidaeand Hydraena adults were almost completely absent fromday samples (Table 2). In contrast, water mites, chironomidlarvae and pupae, and terrestrial organisms, representedlarger portions of the total drift during daylight hours.

These two sources (diel and seasonal) of temporalvariation appeared to be the main determinants of thepatterns revealed by the cluster analysis. Eight groups ofthree or more samples, all of them formed at similarity levelsof 0·6–0·9, were identified, but six samples were ascribed tono group (Fig. 1a). Groups tended to be formed by samplesfrom the same date and period of the day, but whereas nightsamples from January and March (Groups V and VI,respectively) were most similar to those from the daytime ofthe same months (Group IV), night samples from Augustand May (Group VII) and October (Group VIII) showedhigher similarity among themselves than with theirrespective daytime samples (Group II for May, Group III forOctober).

The DCA confirmed these patterns and provided someadditional details. We recognized two significant axes. Thefirst (eigenvalue 0.39) ordered drift samples according toseasonal succession. Winter samples (Groups IV, V and VIin the cluster analysis) showed the lowest values, withspring (Groups II, VII), summer (Group I) and autumn(Groups III, VIII) samples successively exhibiting highervalues along Axis I (Fig. 1b). Axis II (eigenvalue 0.30)seemed to reflect the differences in drift compositionbetween day and night. Day samples had higher values thannight samples and the distance between both groupsincreased along the winter-to-autumn gradient defined byAxis I, being maximal for October samples (Fig. 1b).

Winter samples were generally characterized by anincrease in the proportion of chironomid larvae (particularlyfor daytime samples) and of naidids. Night samples fromJanuary and March were associated with a higher abundanceof small Baetis spp. larvae (Fig. 1b). May daytime samplesseemed largely intermediate in composition, with nodistinguishing category associated with them, but nightsamples from May were characterized by simuliid larvaeand Baetis rhodani. Summer samples displayed an increasedfrequency of small larvae of Hydropsyche spp. Octoberdaytime samples showed higher numbers of water mites,whereas night samples from the same month reflected a

Macroinvertebrate drift in a northern Spanish stream

Pedro A. Rincón and Javier Lobón-Cerviá458

greater importance of Leuctra aurita (Fig. 1b). This patternis largely coincident with changes in the composition of thebenthic community (Table 3, and Rincón 1993).

Temporal variation in drift numbers

The seasonal changes in the abundance of total, aquaticand terrestrial drifting organisms exhibited an almostidentical pattern, although the numbers of terrestrialinvertebrates tended to be higher in daytime sampleswhereas the converse was true for aquatic and totalinvertebrates (Fig. 2). In all cases, however, abundance wasquite high in July, progressively decreased in August andOctober, and reached a minimum in January. Numbers oforganisms increased notably in March, a tendency thatcontinued throughout the spring, resulting in maximumabundance values in May. This cycle closely paralleled thatof the abundance of benthic invertebrates in Surber samples(Fig. 2). However, because there were shortcomings in theprocedure used to compare drift numbers on different dates(see Methods), these results should be viewed only as a

rough approximation.The number of organisms captured in drift samples

exhibited clear changes along the diel cycle. Aquatic formsclearly dominated the drift (Table 2), and consequently thetotal number of organisms and the number of aquaticorganisms displayed similar patterns (Fig. 3). Typically, bothexhibited a peak after sunset (less marked in the summermonths) and there were hints of a late-night to pre-dawn peakin August, October and March. Except for the summermonths, the peak after sunset represented the absolute dailymaximum in abundance. In contrast, terrestrial organismsdrifted more during the daytime, generally showingmaximum abundances in the middle of the day (Fig. 3).

These diel changes in the abundance of total and aquaticorganisms seemed to be the result of the summation of theindividual cycles of a number of taxa that had similar patterns.Thus, the species of the genus Baetis, the plecopteransProtonemura beatensis and the species of the genus Leuctra,the adults of aquatic Coleoptera such as Dupophilus brevis,O. troglodytes and Hydraena sp. (Fig. 4) and simuliid larvae

Fig. 1. (a) Similarity dendrogram of drift samples from the River Negro. Groups indicated by numbered bars; samples belonging to the same group havethe same symbol. Asterisks denote samples not included in any group. Open and shaded areas of a bar indicate day and night samples respectively. (b)Position of samples and representative categories of drifting organisms in the plane defined by the two significant DCA axes. LEUa, L. aurita; OUad, O.troglodytes adults; MICm, M. moestum; OXlv, Oxyethira sp. larvae; WM, water mites; HPsp, Hydropsyche spp.; BAEsp, Baetis spp.; BAEr, B. rhodani;SIMlv, simuliid larvae; CHlv, chironomid larvae; TERR, terrestrial organisms; NAID, naidid oligochaetes.

459

(Fig. 5) generally conformed to the pattern of increasedabundance after sunset despite occasional deviations (theplecopterans in May, simuliid larvae in January).

Other aquatic taxa also displayed changes in theirnumbers during the day, but these did not follow anydiscernible pattern or were not consistent among thedifferent sampling dates. Such was the case in chironomidlarvae (Fig. 4), in the trichopterans M. moestum, Oxyethirasp. and Hydropsyche spp. (in those months when theyappeared in drift samples), and in naidid oligochaetes(Fig. 5). Some of these taxa occasionally showed quite cleardaytime increases in abundance: chironomids in August andMarch, trichopterans in July and May (mostly Oxyethira sp.and Hydropsyche spp., respectively) and naidids in Januaryand March. Water mites, however, were the only group ofaquatic invertebrates that exhibited a consistent tendency to

drift more during the daytime (Fig. 5).

Discussion

Terrestrial invertebrates, chironomid, simuliid and baetidlarvae (mostly B. rhodani) and water mites dominated thedrift in the River Negro. A predominance ofephemeropterans, particularly baetids, and simuliids in driftsamples has been frequently reported (see references inBrittain and Eikeland 1988) and has been noted in streamsnear the River Negro (López 1981) and other rivers in Iberia(Valente and Heland 1990), Europe (Cellot 1989) andelsewhere (O’Hop and Wallace 1983; Benke et al 1986;Mathooko and Mavuti 1992; Brewin and Ormerod 1994;Schreiber 1995). Chironomids have also been known tocomprise large proportions of the drift, both in the IberianPeninsula (Rieradevall and Prat 1986) and elsewhere

Macroinvertebrate drift in a northern Spanish stream

July August October

T D N T D N T D N

Naididae 1·9 2·1 1·4 – – – 4·9 5·0 4·8Hydracarina 13·5 13·7 13·0 8·3 14·0 2·3 18·5 41·3 8·1Cladocera 2·0 2·5 0·7 0·0 – – 0·3 1·1 –Cyclopoidea 1·6 2·1 0·7 0·4 0·7 – 1·2 2·7 0·5Collembola 0·4 0·3 0·7 – – – 0·4 1·1 0·1Baetis fuscatus 0·2 0·3 – 1·9 2·2 1·6 3·8 0·3 5·4Baetis melanonyx 2·3 3·2 0·4 – – – – – –Baetis rhodani 4·1 3·0 6·5 6·8 – 13·9 3·4 – 4·9Baetis spp. – – – – – – – – –Ecdyonurus forcypula – – – 8·7 5·2 12·4 0·3 – 0·5Drunella paradinasi – – – – – – – – –Habrophlebia fusca 1·2 0·8 2·2 – – – 1·6 0·3 2·2Chloroperla tripunctata – – – – – – – – –Protonemura beatensis – – – – – – 3·7 0·3 5·3Leuctra aurita 0·2 – 0·7 0·4 – 0·8 7·9 1·6 10·8Leuctra leptogaster – – – 2·3 – 4·7 1·5 – 2·2Leuctra major – – – – – – 1·6 – 2·3Haliplus sp. 0·1 0·2 – – – – – – –Dupophilus brevis (a) 0·9 – 2·9 0·8 – 1·6 0·7 – 1·0Oulimnius troglodytes (a) 2·0 0·2 6·1 5·7 – 11·6 7·3 – 10·6Oulimnius troglodytes – – – – – – 2·2 2·7 1·9Hydraena sp. 1·0 – 3·3 0·4 – 0·8 0·7 – 1·0Oxyethira sp. 14·9 16·5 11·2 5·7 7·4 3·9 3·1 2·7 1·9Wormaldia sp. 0·1 0·2 – 0·4 0·7 – 0·8 0·3 1·1Hydropsyche spp. 5·8 6·5 4·3 – – – 0·8 1·1 0·7Psychomyia pusilla 0·7 0·5 1·1 0·4 – 0·8 2·1 5·0 0·7Micrasema moestum 8·2 8·4 7·9 1·1 1·5 0·8 2·2 4·8 1·1Beraea sp. – – – 0·4 0·7 – 0·2 0·3 0·1Simuliidae 6·7 5·8 8·7 21·1 14·0 28·7 9·4 6·6 10·6Chironomidae 5·8 4·4 9·0 11·3 18·4 3·9 1·9 – 2·8Chironomidae (p) 1·5 1·7 1·1 1·5 2·9 – 0·1 – 0·1Ceratopogonidae – – – 1·9 3·7 – – – –Terrestrial 23·1 26·8 14·8 15·9 23·5 7·8 7·4 16·7 3·1

Total (%) 98·2 98·9 96·7 95·1 94·9 95·4 90·7 96·6 88·0No. samples 6 4 2 8 5 3 8 4 4Total no. individuals 753 583 170 265 136 129 1206 378 828

January March May

T D N T D N T D N

20.3 14·8 23·6 3·3 4·7 1·3 – – –3·0 6·6 1·0 4·0 4·0 3·9 7·3 13·3 2·20·3 – 0·5 0·2 0·3 – – – –0·6 0·8 0·5 0·4 0·5 0·1 0·9 1·1 0·70·8 1·6 0·2 0·5 0·2 1·1 0·1 0·3 –0·3 – 0·5 0·5 0·2 0·9 1·6 0·3 2·70·6 – 1·0 0·3 0·1 0·5 1·6 – 2·95·9 2·5 8·0 7·8 0·1 19·7 5·3 0·3 9·6– – 4·4 3·9 5·1 3·8 4·9 2·90·2 – 0·2 0·1 0·1 – 0·1 – 0·1– – 0·6 0·7 0·5 2·3 0·8 3·60·6 – 1·0 0·7 0·9 0·5 1·9 1·6 2·10·8 0·8 0·7 0·6 0·5 0·9 1·2 – 2·10·9 0·4 1·2 1·1 0·3 2·3 0·5 0·9 0·12·9 0·8 4·1 0·2 0·3 – 0·2 0·3 0·10·9 – 1·5 0·3 0·4 – 0·1 0·3 –– – 0·1 – 0·1 0·5 – 0·90·3 0·8 – 2·3 2·2 2·4 – – –0·3 – 0·5 0·1 – 0·1 0·5 – 0·9– – 0·5 – 1·3 2·8 – 5·1– – – – – – – –1·5 – 2·4 0·1 – 0·3 0·5 – 0·90·3 – 0·5 0·1 0·1 – 0·6 0·8 0·51·4 0·4 1·9 – – – 0·6 0·3 0·9– – 0·1 – 0·1 5·6 8·4 3·3– – 0·1 – 0·1 0·1 – 0·1– – – – – – – –1·7 – 2·7 0·1 0·1 0·1 – – –8·5 7·4 9·2 12·1 12·2 12·0 33·0 23·2 41·2

34·8 43·0 29·9 37·8 46·4 24·7 5·6 6·7 4·81·2 2·1 0·8 1·9 2·2 1·5 0·8 1·3 0·3– – – – – – – –8·7 17·2 3·6 17·1 17·8 15·9 15·6 28·7 4·7

96·7 99·2 95·2 96·9 98·0 95·4 92·7 93·9 91·88 4 4 8 5 3 8 5 3

518 280 238 2021 1223 798 1013 307 706

Table 2. Total (T), day (D) and night (N) relative abundance (%) of organisms dominant (³1% in at least one period of the day on one samplingdate) in drift samples from the River Negro

In parenthesis: a, adults; p, pupae

Pedro A. Rincón and Javier Lobón-Cerviá460

(Neveu 1980; Scullion and Sinton 1983; Benke et al. 1986;Cellot 1989; Schreiber 1995). Water mites can alsooccasionally be quite abundant in drift samples (Dunbrack1992; Brewin and Ormerod 1994). Trichopterans werescarce in our samples and, as in other studies (O’Hop andWallace 1983; Benson and Pearson 1987), were mostlyindividuals in the initial instars. Although we recognized 82categories of drifting organisms, most were very scarcelyrepresented and the 33 most common categories typicallycomprised over 90% of all drifting invertebrates. Otherresearchers have similarly shown that a limited number oftaxa contribute the great majority of drifting organisms(Cellot 1989; Schreiber 1995).

Both the UPGMA cluster analysis and DCA identifiedseasonal variation, largely dictated by the seasonal change inthe benthic macroinvertebrate community, as the main factordetermining drift composition in the River Negro, but, also,both techniques clearly showed that diel variation was onlyslightly less relevant (note the small difference in theeigenvalues of DCA Axes I and II). Moreover, both analysesindicated that there is an interaction between the two scalesof temporal variation: diel variation appears moderate inwinter months and far more marked in May and,particularly, in October; in those two months the magnitude

of changes in drift composition between day and night wassimilar to that of seasonal changes.

Multivariate methods have rarely been employed in thestudy of drift. However, Hemsworth and Brooker (1981)used average-linkage clustering to examine relationshipsamong drift samples from different sites, and Cellot (1989)provided an example of the use of principal componentsanalysis to elucidate the importance of different sources ofvariation in drift composition. Schreiber (1995) used DCAto examine seasonal variation in drift composition andconcluded that it was an effective way to detect seasonalpatterns for all drifting taxa. Our results indicate that thejoint use of UPGMA clustering and DCA can detect bothseasonal and diel patterns and can illustrate theirinterrelationship. Therefore, we suggest that multivariateordination techniques may be profitably applied in analysesof the taxonomic composition of drift.

The number of drifting organisms in the River Negroexhibited temporal changes. The pattern of seasonalvariation, with a winter minimum and a spring peak, closelymatched the seasonal changes in invertebrate abundance inbenthic samples and conforms well with cycles of generalproductivity and invertebrate abundance in temperatestreams (Hynes 1970; Bilby and Bisson 1992). Theshortcomings of our abundance estimates have already been

Table 3. Relative abundance (%) of organisms dominant (³1% in at least one period of the day on one sampling date)in benthos samples from the River Negro

In parentheses: a, adults; p, pupae

July August October January March May

Tricladida 0·88 0·52 4·95 0·00 0·06 0·14Ancyllus fluviatilis 7·98 6·42 12·32 0·00 0·39 3·07Naididae 1·09 5·09 2·588 0·15 1·52 0·37Hydracarina 5·97 5·96 8·58 0·00 1·23 8·78Collembola 0·03 0·00 0·05 2·25 0·13 0·04Baetis rhodani 6·69 0·26 3·79 16·79 15·86 7·87Epeorus sylvicola 1·18 0·44 1·16 0·61 1·49 3·60Ecdyonurus forcipula 0·68 0·30 1·00 0·30 0·55 0·54Drunella paradinasi 1·08 0·34 0·05 0·08 0·81 2·20Calliarcys humilis 0·00 1·19 5·74 0·11 0·23 0·00Protonemura beatensis 0·30 0·08 0·42 1·60 2·58 0·66Leuctra aurita 1·00 1·01 1·63 0·27 2·10 2·35Dupophilus brevis 1·55 2·41 2·32 0·04 0·55 0·23Oulimnius troglodytes (a) 0·04 0·82 4·42 0·19 0·03 0·27Wormaldia sp. 1·08 0·00 1·21 3·73 4·91 8·80Hydropsyche pellucidula 4·18 0·32 1·63 0·27 0·13 0·87Hysropsyche spp 3·04 1·63 0·26 0·00 0·00 8·10Psychomyia pusilla 4·12 3·80 0·53 0·00 0·23 0·12Micrasema moestum 1·33 1·29 5·11 0·34 0·06 0·27Thremma sp. 2·59 0·70 1·53 0·15 0·06 1·07Simuliidae 3·83 2·25 2·63 19·49 10·53 2·43Chironomidae 40·43 48·05 15·22 40·69 43·36 35·35Chironomidae (p) 1·53 1·11 0·37 3·12 1·62 1·17

Total (%) 90·60 83·99 77·508 90·18 88·43 88·30No. of Surber samples 8 5 5 7 4 5Total no. individuals 7605 4970 1899 2627 3095 5146

461

mentioned. However, minimum values of drift abundance inwinter have frequently been reported for streams intemperate regions (Clifford 1972; Armitage 1977;Stoneburner and Smock 1979; Hemsworth and Brooker1981; O’Hop and Wallace 1983; Anderwald et al. 1991;Schreiber 1995). Similarly, the seasonal changes in theabundance of terrestrial invertebrates are in accordance withprevious reports (Mason and MacDonald 1982; O’Hop andWallace 1983; Garman 1991; Cloe and Garman 1996). Theclose similarity with the seasonal pattern of abundance in thebenthos lends some support to our results. Other researchershave also found a relationship between benthic densities anddrift abundance (Lehmkul and Anderson 1972; Stoneburnerand Smock 1979; Hemsworth and Brooker 1981; Allan1987; Mathooko and Mavuti 1992), although this has notalways been the case (Armitage 1977; O’Hop and Wallace1983; Fleituch 1985; Benson and Pearson 1987; Statzneret al. 1987).

The diel changes in drift abundance showed a peak after

sunset and, on some dates, a second peak immediatelybefore dawn as reported in numerous studies (reviews:Waters 1972; Müller 1974; Brittain and Eikeland 1988). Thegroups that contributed most to those nightly increases inabundance have also shown a tendency to drift more at nightin other lotic systems: simuliid larvae, Baetis, Leuctra andProtonemura species, and adults of aquatic Coleoptera haveall exhibited marked nocturnal peaks in abundance instreams in Iberia (Rieradevall and Prat 1986; Valente andHeland 1990) and elsewhere (Elliott 1973; Reisen 1977;Newman and Funk 1984; Fleituch 1985; Benke et al. 1986;Allan 1987; Cellot 1989; Flecker 1992; Schreiber 1995).

Similarly, other authors have found a propensity fordaytime drift in water mites (Hemsworth and Brooker 1981;O’Hop and Wallace 1983; Bergey and Ward 1989; Cellot1989; Brewin and Ormerod 1994). Hydropsyche spp. andother trichopterans, chironomid larvae and naididoligochaetes, groups that displayed no consistent diel patternor occasional daytime maxima in abundance in this study,have not shown a consistent pattern in other streams either(Brusven 1970; Armitage 1977; Neveu 1980; O’Hop andWallace 1983; Fleituch 1985; Rieradevall and Prat 1986;Cellot 1989; Valente and Heland 1990; Anderwald et al.1991). However, hydropsychids have been reported to showclear diel patterns with dusk and dawn maxima (Neveu 1980;

Macroinvertebrate drift in a northern Spanish stream

Fig. 2. Seasonal changes in (a) total number of organisms in drift andbenthos samples, (b) aquatic macroinvertebrates and (c) terrestrialorganisms in drift samples from the River Negro: (○) day and (Á) nightsamples; vertical lines mark 95% confidence limits.

Fig. 3. Diel changes in the abundance of aquatic macroinvertebrates andterrrestrial organisms in drift samples from the River Negro. Horizontal barindicates period of darkness on each sampling date.

Pedro A. Rincón and Javier Lobón-Cerviá462

Statzner et al. 1987; Bergey and Ward 1989; Cellot 1989;Schreiber 1995). Nevertheless, in one case where the differentsize classes were analysed separately, the very smallindividuals that dominated our samples did show a tendencyto drift more during the day (Cellot 1989). In the case ofchironomids, the low precision of our identification may haveprecluded the detection of possible diel cycles of individualspecies. However, researchers who have distinguished amongchironomid subfamilies (Cellot 1989) or even species(Schreiber 1995) have also failed to detect any diel driftperiodicity. Nevertheless, there are instances of increased driftat night for the group (Mundie 1970; Steine 1972).

Some aspects of the present findings on diel driftperiodicity are consistent with the claim that increasednocturnal drift is an antipredator evolutionary response thatminimizes exposure to visually foraging, drift-feeding fishes(Allan 1978; Flecker 1992). In the River Negro, B. rhodaniand L. aurita, which were night drifters, were also asignificant part of the diet of the brown trout populationinhabiting the study site, and selection estimates indicatedpositive selection for both groups. The trout showed nomarked preference for, or avoidance of, chironomid larvae.Water mites, the only group showing a clear drifting duringthe day, were always quite clearly rejected (Rincón 1993).

Acknowledgments

We benefitted from the facilities of the EstacionBiologica de Valdés of the Consejo Superior deInvestigaciones Científicas (CSIC) while carrying outfieldwork. Y. Bernat and J. Cubo helped in this task.Dr M.A. Puig reviewed an earlier draft of the manuscript.The study was funded by DIGICYT project PB 89-0040 anda cooperation protocol between Valdés Municipal Counciland the CSIC. During that time P.A.R. was financed by aPFPI grant from the Ministerio de Educación y Ciencia(MEC) and the manuscript was completed while P.A.R. wasat the Warnell School of Forest Resources of the Universityof Georgia funded by a postdoctoral MEC/Fulbrightscholarship.

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ANNELIDAOLIGOCHAETA

Lumbriculidae (1)Naididae (2)

ARTHROPODAARACHNIDAE

Acari (3)CRUSTACEA

Cladocera (4)Copepoda

Cyclopoidea (5)INSECTA

Collembola (6)Ephemeroptera

BaetidaeBaetis fuscatus (L.). (7)Baetis melanonyx Pictet. (8)Baetis rhodani Pictet. (9) Baetis spp. (10)Centroptilum luteolum Müller. (11)Cloeon dipterum (L.). (12)

HeptageniidaeEpeorus sylvicola (Pictet). (13)Ecdyonurus forcipula Pictet. (14)

EphemerellidaeDrunella paradinasi González del

Tánago (15)Caenidae

Caenis luctuosa (Burmeister). (16)Leptophlebiidae

Habrophlebia fusca (Curtis). (17)Habroleptoides modesta

(Hagen). (18)Calliarcys humilis Eaton. (19)

Odonata (20)Plecoptera

PerlodidaePerlodes sp. (21)

PerlidaePerla marginata Panzer. (22)

ChloroperlidaeChloroperla tripunctata

(Scopoli). (23)Nemouridae

Amphinemura sulcicollis(Stephens). (24)

Protonemura beatensis(Despax). (25)

CapniidaeCapnioneura mitis Despax. (26)

LeuctridaeLeuctra aurita Navas. (27)Leuctra leptogaster Aubert. (28)Leuctra major Brinck. (29)

HemipteraHydrometridae

Hydrometra stagnorum (L.). (30)Gerridae

Gerris sp. (31)Naucoridae

Aphelocheirus aestivalisFabricius (32)

ColeopteraHaliplidae

Haliplus sp. larvae (33)Gyrinidae

Orectochillus villosus Müller.larvae (34)

SpercheidaeSpercheus sp. larvae (35)

DytiscidaeStictonectes sp. adult (36)

and larvae (37)Hydraenidae

Hydraena sp. adult (38)Helodidae

Helodes sp. larvae (39)Elmidae

Dupophilus brevis Mulsant & Rey.adult (40) and larvae (41)

Elmis aenea Müller. adult (42) andlarvae (43)

Esolus parllelepìpedus Müller.adult (44) and larvae (45)

Normandia sp. adult (46) Oulimnius troglodytes (Gyllenhäl).

adult (47) and larvae (48)Trichoptera

RhyacophilidaeRhyacophila cf. munda

MacLachlan. (49)Rhyacophila cf. occidentalis

Navás. (50)Rhyacophila cf. tristis Pictet. (51)Rhyacophila spp. (52)

HydroptilidaeHydroptila sp. (53)Ithitrichia lamellaris Eaton. (54)Oxyethira sp. larvae (55) and

pupae (56)Philopotamidae

Wormaldia sp. (57)Hydropsychidae

Hydropsyche cf. contubernalisMacLachlan. (58)

Hydropsyche pellucidulaCurtis. (59)

Hydropsyche siltalai Döhler. (60)Hydropsyche spp. (61)

PolycentropodidaePolycentropus sp. (62)Plectronemia sp. (63)

PsychomyidaePsychomyia pusilla Fabricius. (64)Tinodes sp. (65)

BrachycentridaeMicrasema moestum Hagen. (66)Micrasema cf. difficile Mosely. (67)

UenoidaeThremma sp. (68)

LepidostomatidaeLepidostoma hirtum Fabricius. (69)

BeraeidaeBeraea sp. (70)

LeptoceridaeTriaenodes sp. (71)

DipteraNematocera

Psychodidae (72)Dixidae (73)Simuliidae larvae (74) and

pupae (75)Chironomidae larvae (76) and

pupae (77)Ceratopogonidae (78)

BrachyceraEmpididae (79)Dolichopodidae (80)Athericidae

Atherix sp. (81)

Terrestrial invertebrates and aerial or terrestrialstages of aquatic forms (82)

Manuscript received 2 April 1997; revised and accepted 11 June 1997

Appendix 1. Organisms found in drift and samples from the River NegroDifferent stages of the same species (larvae, pupae, adult) have been assigned different category numbers