Spatial and temporal patterns in phytobenthic biomass and community composition at

Despite hypothesising that there would be differences in in biomass between the hydro and non-hydro side of the channel as a result of differences in flow velocity (Chapter 6) there was limited difference in chlorophyll-a concentration across both sides of the channel (Figure 6.7). Regardless of the fact that measured flow velocities were consistently higher on the hydro side of the channel there was only one occasion when chlorophyll-a concentration was significantly higher on the non-hydro side of the channel. This was during the August 2015 sampling occasion and even though there was a difference between both sides of the channel this was small in comparison to the difference seen temporally across surveys. In addition while there was evidence to suggest that there were hydraulic zones of impact below the scheme similar to those at river confluences, there was also minimal difference in chlorophyll- a concentration across each zone. This suggests that something other than spatial variations

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in flow velocity were controlling growth for example nutrient concentrations, grazer density or light intensity.

Furthermore it must be noted that all spatial surveys across Chapter 6 were conducted following periods of low stable flow when both abstraction levels and weir levels were low. As such this study did not capture spatial patterns following periods of high abstraction and this could be the reason as to why no differences were observed in phytobenthic biomass between the hydro and non-hydro side of the channel. Yet Anderson et al., (2015b) measured longitudinal velocities below the outlet of a low head ‘mill leat’ scheme during periods of high flow/high abstraction and found similar patterns in relation to invertebrate communities. Even though this study was focused on invertebrate communities this pattern could also be echoed across the phytobenthic community and in this case it seems like low head ‘on weir’ hydro schemes are having minimal effect and that current mitigation measures are sufficient. This pattern was also reflected across the riffle surveys (Chapter 5). Despite the hypothesis that there would be differences in biomass between the riffle below the scheme and riffles upstream and downstream of the scheme as a result of increased flow velocities extending from the outlet there were minimal differences in chlorophyll-a concentration between sites. There was only one occasion when chlorophyll-a concentration differed spatially across sampling locations. This was during the May 2015 sampling occasion when chlorophyll-a concentration was significantly lower below the scheme in comparison to riffles both upstream of the scheme (Figure 5.12). Given that the phytobenthic biofilm can vary spatially as a result of a number of disturbance factors including flow velocity, substrate instability and grazing as well as limitations in resource factors including nutrients and light (Biggs, 2000) it is difficult to understand which is the controlling factor. Yet with the area below the scheme having similar velocities to the areas upstream and downstream of the scheme across all sampling locations it seems unlikely that, on this occasion, the difference was a result of spatial differences in velocities created by the scheme as hypothesised. While this was not expected it again shows how current mitigation measures appear sufficient. The hydro scheme in question, Stockport Hydro, does not appear to be causing any negative effects on the phytobenthic biofilm.

Although the riffle investigated was located approximately 20meters downstream from the outlet. It is plausible that the riffle was too far from the outlet to experience any effects created by the scheme. This is consistent with findings from Mould et al., (2015a) and

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Anderson et al., (2015b). Mould et al., (2015a) modelled the changes in velocities below a low head ‘on weir’ scheme in the River Thames and found that changes in velocities were limited to within 20meters of the weir. Anderson et al., (2015b) measured flow velocities below the outlet of a low head ‘mill leat’ scheme and found that while there were increased velocities immediately below the outlet velocities had already decreased 10meters away from the outlet. Moreover when comparing this to the results from Chapter 6, looking at the finer scale patterns in flow, it is apparent that while there where areas of increased flow velocity below the scheme these were often confined to areas of constriction and did not extend to the riffle were the measurements were collected. As such future studies should attempt to collect measurements closer to the outlet. Although this could be difficult in the field due to high water depths and high levels of turbulence.

While temporal patterns were more obvious the main focus of this research was to see whether the hydro scheme was causing an affect spatially so it is unclear as to what the consequence of this pattern was. Although on some occasions, in particular the April 2014 and April 2015 sampling occasions, there were very high levels of chlorophyll-a. Moreover these levels surpassed the concentration commonly used to indicate nuisance growths (Dodds et al., 1997) and as such could cause consequences for ecosystem functioning. Therefore future studies, along the River Goyt, should concentrate on establishing factors that control growth over time.

Similar to biomass there were minimal differences in community composition across the hydro and non-hydro side of the channel (Figure 6.8). It was hypothesised that community composition would differ between the hydro and non-hydro side of the channel as a result of flow velocity but this did not appear to be the case (Figure 6.8). In addition there were minimal differences in community composition across riffles surveys (Chapter 5). Across all sampling occasion there was no difference in community composition between the riffle below the weir and riffles upstream and downstream of the weir (Figure 5.8, Figure 5.9, Figure 5.13, and Figure 5.14). This suggests that any changes created by the scheme were not strong enough to influence community composition and can be related to the fact that communities can tolerate a range of environmental conditions before they alter. For example stalked and short filamentous diatoms can dominate in moderate flow velocities between 0.3-0.7m/s were as mucilaginous diatoms can dominate at high velocities above 0.7m/s (Biggs et al., 1998c). Essentially the hydro scheme investigated appears to be having little effect on phytobenthic biomass or community composition across a range of seasons. While this could mean that the

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current mitigation strategies are sufficient hydro developers could argue that they are too strict and that they should be allowed to divert more flow to the turbines to produce more power. As such this study would recommend an investigation where Stockport Hydro is allowed to divert more flow to the turbines over time. This investigation should continue to monitor a range of physiochemical and biotic river conditions but should also think about extending the investigation beyond the phytobenthic biofilm. While this thesis identified that the low head weirs and low head ‘on weir’ hydropower have limited impact on the phytobenthic biofilm there is no evidence to suggest that this conclusion can be extended to overall ecosystem health and other aquatic communities such as macrophytes, invertebrates or fish.