Invertebrate Drift in the Piedmont Part of the Kedrovaya River (Primorsky Krai, Russia) in Warm Season

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1 ISSN , Inland Water Biology, 14, Vol. 7, No. 1, pp Pleiades Publishing, Ltd., 14. Original Russian Text M.V. Astakhov, 14, published in Biologiya Vnutrennikh Vod, 14, No. 1, pp ZOOPLANKTON, ZOOBENTHOS, AND ZOOPERIPHYTON Invertebrate Drift in the Piedmont Part of the Kedrovaya River (Primorsky Krai, Russia) in Warm Season M. V. Astakhov Institute of Biology and Soil Sciences, Far Eastern Branch, Russian Academy of Sciences, pr. 1-letiya Vladivostoka 159, Vladivostok, 69 Russia mvastakhov@mail.ru Received May 3, 1 Abstract Diurnal dynamics of invertebrate drift in the Kedrovaya River (Primorsky Krai, Russia), which flows in the zone of influence of the monsoon climate, has been studied. It has been shown that drifting invertebrates tend to shift from a predominantly daytime drift pattern to a distinguished nocturnal drift pattern during the frost-free period. The ratio of the total number of nighttime migrants to the total number of daytime migrants increased with every subsequent month. It has been proposed that the degree of light contrast between day and night can be significant in the regulation of nocturnal drift intensity. Keywords: drifting invertebrates, Diptera, Ephemeroptera, stream, diurnal dynamics DOI: /S INTRODUCTION The study of the diurnal dynamics of drift (downstream transport of organisms in streams) is important in investigating the diet of rheophilous fish and in scheduling river-water intake operations [13, 16, 17]. Depending on the time of day when various aquatic organisms migrate, daytime and nocturnal drifters [, 1] and, accordingly, daytime and nocturnal drift patterns are distinguished. The overall set of drifting organisms is called sirtone [6, 19]. Sirtone consists of autochthonous (aquatic organisms) and allochthonous (terrestrial invertebrates) fractions. The role of the terrestrial component increases in the warm season []. A massive rise of benthic invertebrates into the thickness of the current can occur within a short period of time and is often confined to the evening twilight and the first hours of night [3]. The purpose of this study was to estimate the seasonal characteristics of the diurnal dynamics of drift in a typical small river in the Far East of Russia. MATERIALS AND METHODS The material was collected in the Kedrovaya River, a salmon-rich river of the southern part of the Russian Far East. The basin of the river is located in Primorsky Krai in the zone of influence of the monsoon climate. In the warm season there is high rainfall in this area, with little snow in winter. In spring and early summer, a cold sea fog often penetrates the Kedrovaya River valley. Up to 5% of annual precipitation falls here in the second half of summer and in early autumn [5]. Rain floods are short-term, whereas low-water periods are longer. The total length of the river is 1 km, the total length of its tributaries is 46 km, and the total catchment area is 45 km. Investigations were carried out in one of the shoals in the middle reach of the river within the Kedrovaya Pad State Nature Biosphere Reserve. The soil in the area of observation consists of gravel and pebbles; the depth and width of the flow in the time of material collection did not exceed.6 and 1 m, respectively. A total of six round-the-clock collection series were performed: on June 5 6, July 5 6, August 3 4, September 3, October 1, 6, and on May 15 16, 7. Consistently clear weather was observed only during the series performed in July. When material was collected in June, August, and May, it sometimes rained. Water temperature ranged from to 1 С in June, 11. to 14.6 С in July, 17 to 11. C in September, 7.5 to 11.4 С in October, and 7. to 11 С in May. In August, when the amplitude was С, the highest average daily temperature of water for the entire study period was recorded (15.1 С). The depth of the average flow velocity at the point of observation was.5.7 m/s in June September,.4 m/s in October, and. m/s in May. Drifting invertebrates were caught with a sampler of our own design [1] equipped with three stacked drift dip nets. The entry width of each dip net was.5 m; height, was.1 m, filtering cone length was.6 m, and mesh size was µm. During catching, the sampler was adjusted so that the total height of its nets (.3 m) was equal to the depth of the flow. The 4

2 INVERTEBRATE DRIFT IN THE PIEDMONT PART OF THE KEDROVAYA RIVER 49 flow velocity was measured with a GR-55 flow wheel at three levels corresponding to the position of dip nets in the river. The duration of the night was defined as the time interval between the end of the evening and the beginning of morning twilight. In the dark period, the material was collected every hour; during the daytime it was collected every h; the exposure time was 5 min. Samples were fixed in 4% formaldehyde. The number of invertebrates that drifted in 1 h through an individual horizon section (1..1 m) was determined taking into account the volume of water filtered through each net during the exposure time [1, 4]. Summarizing the characteristics of individual horizons, the integrated value of the invertebrate drift through the test section (1..3 m) in 1 h was determined. The biomass was evaluated similarly. The overall values of the drift intensity (ind./day or mg/day) were determined as the sum of the corresponding indices of each hour. The invertebrates that accounted for 15% of the overall values were regarded as dominant, and the remaining drifters were categorized as other. RESULTS Among the invertebrates drifting in the Kedrovaya River, the larvae of Diptera (chironomids (Chironomidae) and black flies (Simuliidae)) and the larvae of mayflies (Ephemeroptera, primarily Baetidae) were most active. The larvae of dipterans dominated in June August and in May; the larvae of mayflies dominated in September and October. Among other invertebrates, only water mites (Hydracarina) dominated in October. The invertebrates that prevailed in the sirtone biomass in June were terrestrial invertebrates, in August October they were Amphipoda, and in October, they were caddisflies (Trichoptera). The larvae of stoneflies (Plecoptera) and beetles (Coleoptera), as well as springtails (Collembola), Oligochaeta, and roundworms (Nematoda), played a less important role in sirtone. The least abundant groups were represented by Oribatida, Ostracoda, Copepoda, Daphniiformes, Heteroptera, Gordiacea, and the larvae of butterflies of the family Nymphulinae (table). The terrestrial invertebrates that most frequently occurred in the river flow were Hymenoptera, Homoptera, and Aranei. The maximum overall values of drift intensity were recorded in May; the lowest were found in October (Fig. 1). In June 6, the largest number of drifting invertebrates was recorded during the daytime; however, the biomass of the daytime sirtone was 4.6 times lower than that of the nocturnal sirtone (Figs. a, 3a). During the daytime, the abundance of drifting dipterans was approximately two times greater, whereas the abundance of mayflies was two times smaller than in the dark period. The biomass of dipterans during the thous ind./day 1 VI VII VIII daytime was slightly greater than in the night, whereas the biomass of mayflies was 9 times lower. In addition to the mayflies, nocturnal drifting invertebrates were represented primarily by water beetles, stoneflies, caddisflies, amphipods, and oligochaetes. The highest total abundance was detected at 4 p.m., when the daily maximum number of dipterans and the second peak of activity of mayflies were observed. The total activity of migrants in the first and second halves of the night was approximately the same. In the light period, invertebrates preferred the morning migrations. In July most organisms were involved in the nocturnal drift: the biomass of nocturnal sirtone was 3.7 times higher than that of the daylight sirtone (Figs. b, 3b). In the first half of the night, the total number of drifters was twice as high as in the second. The abundance of dipterans involved in the nocturnal drift exceeded the corresponding daylight value only by 1%, whereas their biomass was more than.5 times greater. The abundance and biomass of mayflies transferred at night 5.7 and 9.6 times, respectively, exceeded the respective values recorded in the daytime. The maximum total abundance was recorded at 11 p.m., when the peaks of activity of dipterans, mayflies, amphipods, water mites, nematodes, stoneflies, and caddisflies were detected. The larvae of the latter did not participate in the daytime drift. Copepods and cladoceran, however, drifted only during the daytime. A general enhancement of the daytime drift was observed at 3 p.m. In August the night type of activity became more pronounced (Figs. c, 4). In this case the biomass of the nocturnal sirtone was 5.7 times greater than that of the daytime sirtone. The most intensive drift of invertebrates was recorded in the first half of the dark period. Dipterans drifted primarily during the day; however, the IX 1 X V g/day 1 9 month Fig. 1. Dynamics of invertebrates drifting through a test section in the Kedrovaya River flow: (1) abundance, ind./day; () biomass, g/day. INLAND WATER BIOLOGY Vol. 7 No. 1 14

3 5 ASTAKHOV Relative abundance (above the line) and biomass (below the line) in the diurnal drift in the Kedrovaya River, % Group VI 5 6.VII 3 4.VIII 3.IX 1.X V Aquatic invertebrates Diptera Ephemeroptera Trichoptera Plecoptera Coleoptera Heteroptera.1 Nymphulinae.4.1 Collembola Oligochaeta Nematoda..1. Gordiacea Amphipoda Copepoda Ostracoda Daphniiformes Hydracarina Oribatida.1.1 Other Terrestrial invertebrates INLAND WATER BIOLOGY Vol. 7 No. 1 14

4 INVERTEBRATE DRIFT IN THE PIEDMONT PART OF THE KEDROVAYA RIVER 51 Thous ind./h (a) (d) 1 3 (b) (e) (c) (f) h Fig.. Diurnal dynamics of abundance of invertebrates drifting through a test section of the Kedrovaya River flow: (1) Diptera, () Ephemeroptera, and (3) other invertebrates. Dates: (a) June 5 6, 6; (b) July 5 6, 6; (c) August 3 4, 6; (d) September 3, 6; (f) October 1, 6; and (g) May 15 16, 7. Arrows indicate the time of sunset and sunrise. transfer of the biomass of this group at night was.5 times more than in the daytime (Figs. c, 3c). The abundance and biomass of mayfly larvae transferred at night was 4. and 15.6 times, respectively, higher than in the daytime. Other drifters (except for ostracods, oligochaetes, and water mites) also preferred nocturnal migrations. The peaks of the abundance of the daytime and nocturnal drifters were detected at 1 and 3 p.m., respectively. In September the most intense drift of all invertebrates was observed in the first half of the night. The density and biomass of the nocturnal drifters was and 7 times, respectively, greater than that of the daytime drifter (Figs. d, 3d). The ratio of dipterans involved in the daytime and nocturnal drift was close to unity, and the corresponding ratio of their biomass was 1 :. The number and biomass of drifting mayflies at night by 5.7 and 15 times, respectively, exceeded the respective daytime values. Other invertebrates migrated primarily in the dark period. The most numerous organisms in this category were the caddisfly larvae, water mites, and amphipods. The maximum total nocturnal and daytime drift was observed at 1 and 1 p.m., respectively. Most organisms of the October series rose to the water column in the first half of the night. The total INLAND WATER BIOLOGY Vol. 7 No. 1 14

5 5 ASTAKHOV g/h (a) (d) 1 3 (b) (e) 4 1 (c) (f) h Fig. 3. Diurnal dynamics of the biomass of invertebrates drifting through a test section of the Kedrovaya River. For designations, see the legend to Fig.. intensity of drift during the daytime was.4 and.7 times smaller in abundance and biomass, respectively, than at night (Figs. d, 3d). The abundance and biomass of dipterans drifting at night was and 6. times, respectively, greater than that during the daytime. The intensity of the nocturnal drift of mayflies was 3.4 and 7. times greater in terms of abundance and biomass, respectively. The abundance of larvae transferred in the first half of the night was almost 3 times greater than in the second half. The most abundant representatives of the group other were water mites, caddisfly larvae, and amphipods; the latter were involved in the drift only in the dark period. The maximum abundance of migrants was recorded at 11 p.m. In May 7, the greatest abundance of invertebrates was recorded in the daytime; however, the biomass of the nocturnal sirtone was 3 times greater than that of the daytime sirtone (Figs. f, 3f). Night drifters were more active in the second half of the dark period; however, the maximum total abundance was recorded at 11 p.m. (Fig. e). The abundance of dipterans drifting in the daytime was 1. times higher, whereas their biomass was almost twice as low as at night. The nocturnal drift of mayflies exceeded the daytime one.3 and 11 times in terms of abundance and biomass, respectively. In the daytime, the maximum abundance of migrants was detected at 3 p.m. At this time, the peaks of activity of dipterans and invertebrates from the group other were recorded. In May, the latter INLAND WATER BIOLOGY Vol. 7 No. 1 14

6 INVERTEBRATE DRIFT IN THE PIEDMONT PART OF THE KEDROVAYA RIVER 53 were represented in sirtone primarily by water mites and oligochaetes. DISCUSSION With more than 6 years of research of invertebrates inhabiting the Kedrovaya River [, 11, 14], the first information on their drift appeared relatively recently [1]. Studies in the cold period of the year showed that, after the formation of ice on the river, high activity of aquatic organisms in the drift could be observed both at night and in the daytime; however, the abundance of the daytime migrants was usually greater [1, 4]. According to the results of this study, in the warm season, invertebrates inhabiting the river switched from a predominantly daytime drift pattern (June) to a pronounced nocturnal drift pattern (August October). The ratio between the total abundance of the nocturnal and daytime drifters increased in each subsequent month (Fig. 4). Importantly, at the end of spring, most invertebrates were involved primarily in the daytime drift. A comparison of these results with the previously published data [4] suggests that the pattern of the diurnal activity of invertebrates drifting in the Kedrovaya River changes during the year: the nocturnal drift pattern is characteristic of most invertebrates from mid-summer to late autumn, whereas the daytime pattern prevails from the beginning of the freeze-up period until the beginning of summer. In spring and summer, an increased invertebrate drift during the daylight hours was observed earlier in the rivers of Kamchatka. This phenomenon was explained as the result of the numerical dominance of dipterans (namely, chironomids) with a daytime migration pattern [15]. The same explanation was given to the predominantly daytime migration of drifters in May in one Canadian stream []. In this regard, it should be noted that, in May and June, the sirtone of the Kedrovaya River was dominated by chironomids, among which the species with the daytime pattern of activity could prevail. Dipterans, in general, primarily showed the daytime drift pattern in the study period. In July the predominantly nocturnal drift pattern of dipterans was determined by the predominance of larvae of black flies in this group, which, according to our data, usually migrate at night. The susceptibility of larvae of black flies to nocturnal migrations was also indicated by other authors [1,, 1]. Throughout the observation period, the larvae of mayflies were drifting in the Kedrovaya River mostly at night. According to the results of the study performed during the warm season in other streams, mayflies are primarily nocturnal migrants because of their negative phototaxis [, 1]. However, the mature mayfly nymphs ready to emerge can also exhibit a high migratory activity at moonlit nights, which is probably due to the necessity of spatial orientation of winged individuals in the air [9]. It should be noted that, in the Kedrovaya River, mayflies retained overnight activity % (a) (b) (c) (d) (e) (f) Fig. 4. Proportion (%) of (1) daytime and () nocturnal drift of invertebrates according to the abundance in the overall diurnal drift. The abscissa axis shows (1) Diptera, () Ephemeroptera, (3) Others, and (4) all invertebrates. The other symbols are the same as in Fig.. under freeze-up conditions [4]. Despite the fact that most mayflies registered in January drifted in the daytime, their migration was confined to the weakly illuminated near-bottom layer of the water column. Therefore, negative phototaxis, a characteristic of this group, was also manifested in the Kedrovaya River. Among the other permanent drift members, a primarily nocturnal drift pattern was shown by amphipods, stoneflies, caddisflies and water bugs, whereas aquatic mites, springtails, and ostracods drifted mainly in the daytime. These observations are consistent with the data of other authors [16, 1, ]. In the Kedrovaya River, no aquatic organisms migrating only at night or only during the daytime have been identified. For example, the amphipod Gammarus koreanus Ueno in autumn occurred only among the nocturnal drifters, whereas in summer and spring it also drifted in the daytime. The most significant (1-fold) difference between the maximum and minimum intensity of the total drift was recorded in July, in clear weather (i.e., under conditions of the strongest light fluctuations throughout the day). It is known that the daily changes in the abundance of sirtone can be more significant and reach 17 [9], [4], and 4 times []. It is believed that the factor that initiates the nocturnal drift is the sharp change in light immediately after sunset [1]. However, according to our unpublished data and [16], the abundance of drifters often peaks only a few hours after the onset of the dark phase. For example, a study in the Syl va River in the Urals showed a stable confinement of the drift intensity maxima to the middle of the night ( a.m.), regardless of the length of the day [9]. However, in the rivers of Altai, as the duration of the light phase increased, the peak of abundance shifted from 1 p.m. in early May to a.m. at the end of June. A subsequent reduction in the length of the day resulted in shifting the maximum drift intensity to INLAND WATER BIOLOGY Vol. 7 No. 1 14

7 54 ASTAKHOV an earlier period (to midnight) [1]. Similar results, but with less divergence of peaks in time, were obtained for one of the rivers in the Amur River basin [7]. The number of such maxima depended on the duration of the day (night) in some streams [5] and did not depend in others [], similarly to the Kedrovaya River. In some cases the synchronization of peaks of abundance of drifting invertebrates of some taxa can be observed [7, 1]. For example, in the Chemal River (Altai), two coinciding peaks of the nocturnal activity of mayflies and black flies at midnight and 4 a.m. were observed during the three summer months [1]. In the Kedrovaya River, the mass-scale rising of some taxonomic groups of invertebrates in the water column were also synchronized within certain diurnal series (Fig. ). However, the peaks of the total abundance did not coincide from series to series, and the dependence of the time of their occurrence on the length of the day was not established. Nevertheless, the significance of the light factor as a regulator of drift intensity also manifested itself. In July, when the light conditions during the day rapidly change, the abundance of drifters in the first half of the night was twice as high as in the second half and the period of high nocturnal activity was shorter than in other series, which proceeded in cloudy weather. This finding suggests that, for initiating a more intensive drift, not only the onset of darkness as such at the end of evening twilight but also the degree of light contrast between day and night periods is important. This assumption is partly supported by the facts of a poorly pronounced diurnal drift dynamics under conditions of a polar day [1, 3]. CONCLUSIONS Representatives of 17 groups of aquatic invertebrates were identified in the sirtone inhabiting the Kedrovaya River. The larvae of amphibiotic insects (mayflies and dipterans) were most abundant in terms of quantitative parameters. Dipterans prevailed in the late spring and summer; the larvae of mayflies prevailed in September and October. Drifting terrestrial invertebrates were often represented by Hymenoptera, Homoptera, and spiders. The highest total drift intensity was observed in May; the lowest was revealed in autumn. The nocturnal pattern of activity prevailed from July to October. In other months of migration, most invertebrates were confined to the daylight hours. Under conditions of drastic diurnal changes in illumination, the largest amplitude of fluctuations in the total abundance of drifters was observed. Under these conditions, high nighttime activity was shorter than in the cloudy weather. The number of peaks in the abundance of drifting organisms and the timing of their occurrence did not depend on the duration of the day. ACKNOWLEDGMENTS This study was partially supported by project no. 1-1-P3-1. REFERENCES 1. 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