Introduction gives background information and explains the purpose of the investigation.

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Diurnal variation in the drift of macro-invertebrates, Dubbs Beck Cumbria.

Introduction

Most macro-invertebrates occurring in running water have adaptations to maintain their position on the substrate. Despite this many enter the water current and drift downstream. Diurnal variation in the drift of macro-invertebrates has been well studied phenomenon (for reviews see Hynes, 1979; Moss, 1988). Invertebrates may enter the draft to aid dispersal, to avoid predation or competition, or simply by chance. The diurnal variation in the number of invertebrates suggests that there is a strong behavioural element. In this study the drift of macro-invertebrates in a small Lake District stream was studied. The aim has been to relate the changes in the number of macro-invertebrates in the drift to the changes in the physio-chemical variables over a 15 hour period.

Methods

Site description

Dubbs Beck was sampled 20m below Borrans Reservoir (see map 1). The stream is small and shallow with a mixed substrate of large and small stones. The stream is overhung by alder trees (Alnus glutinosa).

Measurement of physio-chemical variables.

Selected physio-chemical variables where monitored continuously using a Philip Harris DL-plus system. This apparatus allows up to four probes to be used. The data collected is store by the apparatus and can be down loaded into an IBM compatible computer using appropriate software later. The four probes selected for the experiment were light, temperature, oxygen and pH. Measurement of the physio-chemical variables was started at 7.45 and stopped at 23.45 on 26.5.94.

Macro-invertebrates.

A standard FBA pond net was placed in the stream, with its handle held in position by an overhanging tree. The net was emptied every two hours and the invertebrates identified to family level and counted.

Results

Physio-chemical variables

The results for light and temperature are shown in fig. 1 and those for oxygen and pH in fig. 2. Unfortunately the battery for the oxygen probe ran out after four hours. The results for light temperature and pH show a close correspondence. Temperature follows the trend for light with the expected time lag. The sharp fall in light levels after 10.49 hours may be due to the light meter being unable to register very low light levels. This fall is mirrored by the decline in temperature at this point. The changed in pH also follow those for light. The rise in pH during the day will result from carbon dioxide being used up in photosynthesis. When the light levels fall at dusk the pH also starts to decline again.

Macro invertebrates.

The results of the invertebrate sampling are given in appendix 1. Fig. 3 shows the changes in the numbers of invertebrates. Drift throughout the day is very low, rising after the light levels fall. However, the numbers of invertebrates caught is still low compared with those caught in similar experiments (Williams, 1989). This may be due to the low flow rates in the stream at the time of sampling. The location of the sampling site close to the Borrans Reservoir dam may also have an effect, giving a relatively short stretch of stream bed for the invertebrates to begin their drift from.

Discussion

The results from this experiment concur well with those from other studies. Hynes (1979) points out that the numbers of Baetis and Gammarus show a strong diurnal pattern, with the numbers increasing after dusk. These two taxa are the dominant ones in the drift in this study, with their numbers increasing once the light levels fall. If invertebrates need to drift to aid dispersal, or avoid competition, then drifting at dusk and during the night may help to avoid predators. Brown trout (Salmo trutta) will often hold their position in a stream and feed on food items being brought to them by the flow. Their location of food items may be mainly visual, and so it may be an advantage for invetrebrates to drift at times of low light intensity.

References.

Hynes (1979). The ecology of running water. Liverpool University Press.

Moss (1988). Ecology of fresh waters. Blackwell. Oxford.

Williams (1989). Diurnal variation in macro-invertebrate drift in the River Wye, Welsh Bicknor. Unpublished M.Sc. data.

Appendix 1. Macro-invertebrates caught in a drift net. Dubbs Beck, High Borrans.