The response of macroinvertebrate communities to sediment clearance in a small canalised stream

1. The response of macroinvertebrate communities to sediment clearance in a small canalised stream Lisa King - BSc Biological Conservation & Physical Geography Abstract Benthic macroinvertebrate communities from a short, urban stretch of Dallington Brook, Northampton, were sampled in November 2004 and again in November 2005 in order to assess the impact that the removal of silt from the stream may have on water quality. For each year, abiotic data was collected and the Biological Monitoring Working Party (BMWP) score and average score per taxon (ASPT) biotic indices were calculated. Analysis of the results showed that the removal of the silt, leaving a substrate dominated by concrete, did not appear to have an adverse effect on the water quality, as the BMWP score and ASPT value increased in 2005. However, differences in abiotic factors such as electrical conductivity, current velocity and air and water temperature may also have had some bearing on this result. Methods Discussion Upon visiting the site in November 2005, we were initially struck by the visual alteration that had occurred following the removal of a large quantity of sandy sediment from the sampling site (Figures 3 and 4). Studies have suggested that there may be a correlation between the amount and type of sediment in a stream and the effect that it has on the composition of the benthic macroinvertebrate communities present (Grumiaux et al., 1998; Kaller and Hartman, 2004). Accumulations of sediment in the substrate of a stream have a great capacity to absorb pollutants such as metals which may be washed into streams from agricultural, industrial or domestic activities (Grumiaux et al. 1998). Many macroinvertebrates live in or on sediments, and consequently may be particularly susceptible to accumulations of these pollutants. As the sampling site lies downstream from an industrial estate, it is possible that the accumulated sediment contained high concentrations of pollutants which may have had a deleterious effect on the macroinvertebrate communities present at the site. Indeed, the improvement in the BMWP score and consequently the higher ASPT value between the samples taken in 2004 and 2005, suggest that the water quality in Dallington Brook has improved following the removal of the sediment from the stream. However, the improvement may also have been affected by the changes in certain abiotic factors identified in the results; further tests would be required to verify this. Use of the BMWP score and ASPT value in isolation may not provide an entirely accurate interpretation of the results, as the index values used differ according to the physical features found at different sites (Clarke et al, 2002); for example, the removal of the sediment at Dallington Brook has changed the physical attributes of the site. In order to address this limitation, the River InVertebrate Prediction And Classification System (RIVPACS) site classification computer model has been developed. Based on 614 reference sites found in Great Britain, this model uses environmental data collected at the sample site to provide a prediction of the fauna that could be expected to be found in the absence of major environmental stress (Clarke et al. 2002), thus allowing a more objective basis for comparisons to be made when assessing the water quality of a stream. A 20m stretch of the stream was selected and the following environmental measurements were recorded: stream width; water depth; substratum characteristics; electrical conductivity; current velocity; water and air temperature. After recording the environmental data, a sample of macroinvertebrates was collected by utilising a standardised ‘3+1’ kick sampling technique. Over a period of 3 minutes and using a standard kick net, samples were taken from a number of locations within the stretch that were considered to be representative of the habitats present. At each location, the net was placed in the water with the bottom touching the substrate and the area immediately in front of the net was vigorously agitated for 10-20 seconds, thus dislodging any invertebrates that were present in the substrate. The contents of the net were emptied into a container and removed to the laboratory for identification to family level. A further minute was spent searching the 20m stretch by hand, paying particular attention to microhabitats not sampled by the kick sampling. Introduction Following the implementation of The Water Framework Directive in December 2000, there is a growing requirement for the quality of water in inland and coastal waterways in the United Kingdom and European Community member states to be regularly monitored and assessed (Clarke et al., 2003). Assessments may take the form of either chemical analysis or biological assessment methods. In isolation, chemical analysis is not able to elucidate on the effects of pollution and, therefore, it is becoming increasingly important to use biological assessment methods to identify the ways in which pollution affects benthic macroinvertebrate communities (Clarke et al., 2003). In this two year study of Dallington Brook, Northampton, we used the Biological Monitoring Working Party (BMWP) score system, adopted in 1978 as the national standard for biological monitoring of rivers in the United Kingdom (Walley and Fontama, 1998), and the average score per taxon (ASPT) to determine the water quality of this stretch of urban stream. Figure 3 Dallington Brook, November 2004 Figure 4 Dallington Brook, November 2005 Figure 2 Kick sampling Results Following the removal of sediment from Dallington Brook, the substrate characteristics were altered from predominantly sand (65%) in 2004 to predominantly concrete (70%) in 2005. Analysis of the abiotic data collected for 2004 and 2005 revealed that there had been a slight increase in the electrical conductivity from 0.73μS cm-1 to 0.89μS cm-1 and that the current velocity had increased from 0.44cm/s to 0.95cm/s. It was also noted that although the data for 2005 was collected at the same time of year as the 2004 data, the air temperature was lower (4.1°C compared to 11.7°C) and the water temperature was also lower (7.4°C compared to 10°C). The benthic macroinvertebrates that were collected at the sampling site are shown in Table 1, together with the associated Biological Monitoring Working Party (BMWP) scores and final average score per taxon value for each sampling year. Following the removal of the sandy substrate from the sample site, the ASPT value shows an increase from 3 in 2004 to 4.3 in 2005. Site Description The source of Dallington Brook is located in Brices Spinney, 5.3 km upstream from the sampling site (Figure 1). From Brices Spinney the stream meanders through agricultural land before reaching the outskirts of New Duston, where it follows the boundary of the Lodge Farm Industrial Estate. The stream has been canalised from where it borders the edge of the industrial estate to beyond the area of the sampling site, although this was not immediately apparent in 2004 when the initial site visit was conducted. The sampling site is a 20m stretch of urban brook which lies downstream from the industrial estate. There is a housing estate and allotments to the south-west side of the stream and King’s Heath, an area of open heath land, lies to the north-east. The banks of the stream are dominated by grasses, nettles (Urtica dioica) and ivy (Hedera helix). Mature alder (Alnus glutinosa) and willow (Salix) are also present on both banks. Table 1 Benthic macroinvertebrates found at Dallington Brook with associated BMWP score and ASPT value (x=not present) References Clarke, R.T., Furse, M.T., Gunn, R.J.M., Winder, J.M. and Wright, J.F. (2002) Sampling variation in macroinvertebrate data and implications for river quality indices. Freshwater Biology 47: 1735-1751. Clarke, R.T., Wright, F. and Furse, M.T. (2003) RIVPACS models for predicting the expected macroinvertebrate fauna and assessing the ecological quality of rivers. Ecological Modelling 160: 219-233. Grumiaux, F., Leprêtre, A. and Dhainaut-Courtois, N. (1998) Effects of sediment quality on benthic macroinvertebrate communities in streams in the north of France. Hydrobiologia 385: 33-46. Kaller, M.D. and Hartman, K.J. (2004) Evidence of a threshold level of fine sediment accumulation for altering benthis macroinvertebrate communities. Hydrobiologia 518: 95-104. Walley, W.J. and Fontama, V.N. (1998) Neural network predictors of average score per taxon and number of families at unpolluted river sites in Great Britain. Water Research 32: 613-622. Figure 1 The course of Dallington Brook from its source at Brices Spinney near Harpole to the sampling site