Macroinvertebrates and diatoms are involved in the provision of many ecosystem services and are frequently used for monitoring ecological quality; indices being primarily based on community structure. Macroinvertebrate-induced changes in the structure of diatom assemblages have the potential to result in erroneous assessments of ecological quality by altering the value of biotic indices (e.g. Trophic Diatom Index, TDI) in the absence of change in environmental quality. The first aim of this thesis was therefore to determine how macroinvertebrate grazers, with different feeding modes, influence diatom assemblages. This was investigated in laboratory studies with artificial streams and via a field manipulation experiment. Mayfly grazers consistently decreased the relative abundance of high-profile diatoms, but the effect this had on the TDI was dependent on the relative sensitivity of the diatom species in the assemblage. Grazing induced changes in TDI, which were most pronounced when there was a significant difference in the average sensitivity of lowprofile versus high-profile diatoms, has the potential to change the ecological quality assessment of a site, resulting in a possible misclassification. The second aim was to explore the relationships between biomass, diversity and composition of the diatom and macroinvertebrate assemblages in minimally impacted sites. This was investigated by surveying minimally impacted streams and assessing the response of macroinvertebrate and diatom assemblages to a major disturbance event (flood). No correlation was found between the diversity or ecological quality indices of the two groups. A higher biomass of periphyton (as Chlorophyll a) was associated with greater macroinvertebrate abundance and high-profile diatoms in the assemblage were positively correlated with the abundance of mayfly grazers, indicating some trophic links. Macroinvertebrate diversity and abundance decreased due to the flood but there was no consistent response from diatoms. Seasonality appeared to be more important in determining changes in macroinvertebrate community than a one off flood event. In conclusion, grazing macroinvertebrates (in particular mayflies) influence diatom assemblage structure by decreasing the relative abundance of high-profile diatoms, which can influence the TDI. Diatom and macroinvertebrate assemblages respond to different abiotic factors, as well as each other, and associations between them are not always detectable in the field (i.e. diversity). Assessments of ecological quality based on diatoms and macroinvertebrates were not concordant, meaning that one cannot be predicted from the other for monitoring purposes. Monitoring using both groups should provide better protection for the environment, as the lowest value can be taken, reducing the chance of false positives and provide greater understanding of how the system is functioning.