Abstract
The global expansion of road networks has significant ecological impacts on freshwater
catchments which are inherently linear and susceptible to fragmentation. The construction of
road networks necessitates the use of road crossings to navigate stream and river systems.
Engineers often resort to using concreted piped culverts with built banks to redirect stream
flow underneath roads, a technique that channels stream paths and alters natural hydrological
process and flood regimes. This can have a negative impact on aquatic taxa that are sensitive
to changes or fluctuations in environmental variables.
Chapter 2 of this thesis aimed to assess the impact of road crossings on fine sediment
deposition in forested streams. Using the Quorer Resuspension Method, sediment samples
were collected upstream and downstream of 10 road crossings. The findings indicated that
unsealed road crossings exhibit higher Suspendable inorganic sediment (SIS) concentrations
upstream compared to downstream, with sealed road crossings exhibiting highly variable
results. Suspendable organic sediment (SOS) levels were elevated upstream of both sealed
and unsealed road crossings. Differences in SIS were not statistically significant, likely due to
high variance between upstream and downstream differences in SIS. It appears that road
crossings are causing differences in SIS accumulation upstream and downstream of a road
crossing, however the cause is not clear from this study.
Chapter 3 explores the impact of road crossings on the biodiversity and abundance of benthic
macroinvertebrate communities. Kick sampling and full-count macroinvertebrate
identification was used to investigate the impact of road crossings on benthic communities at
10 road crossings. Despite higher total abundances upstream in both sealed and unsealed
sites, no significant differences were found. Taxa richness was found to be higher upstream in
unsealed sites, while being lower upstream of sealed sites, though analysis found no
significant results. Non-Metric Multidimensional Scaling (NMDS) analysis was used to
analyse the Simpson’s Diversity index scores attributed to the benthic macroinvertebrate
communities upstream and downstream of each site. No consistent directional trend was
observed, however there were marked differences between upstream and downstream
communities at some sites. This suggests that road crossings may be driving benthic
macroinvertebrate community composition outside of the parameters measured in this study.
Permutational Multivariate ANOVA (PERMANOVA) analysis of the Simpson’s Diversity Index against a samples location to a road crossing, road paving type, log10 SIS, log10 SOS,
water temperature (°C), DO and flow rate (m/s), returning no statistically significant results.
This thesis underscores the importance of considering regional and stream-specific variations
when assessing the ecological impacts of road crossings. I suggest exploring how road
crossings are changing the sedimentation dynamics, morphology, and flood path of
intersecting streams. This would allow for a better understand into what mechanisms are
driving accumulation of SIS and changes of benthic macroinvertebrate communities at road
crossings.