The introduction of excessive amounts of fine inorganic sediment into waterways is one major pathway by which anthropogenic activities exert negative pressure on freshwater ecosystems worldwide. Once in a waterway fine inorganic sediment particles either remain suspended (turbidity) or become deposited (sedimentation) depending on the water velocity and the particles size and shape.

A literature review revealed that both suspended and deposited fine inorganic sediment can negatively affected all trophic levels. Fish communities became dominated by sediment tolerant species. Moreover, these changes potentially reduce diversity and abundance in the top predators, thus diminishing food web length and decreasing ecosystem function. The combination of high levels of sedimentation and concomitant high turbidity, creates multiple stressors with the potential to work synergistically to further reduce diversity and abundance. Multiple fine inorganic sediment stressors imposed on key species in a trophic level potentially diminish energy flow to higher trophic levels, suggesting the most significant effects will be borne by the top predators.

I conducted a field survey of 52 stream and drains covering a sediment gradient, in the South Island of New Zealand. Results showed that several freshwater fish species were sensitive to deposited sediment particularly torrentfish, bluegill bully while several other species were not affected such as shortfin and longfin eels. Change-point analysis indicated marked declines in fish diversity and abundance between 5 - 25% deposited sediment cover.

Structural equation modelling indicated that both direct and indirect effects were responsible for sediment impacts on fish community composition. Sediment increases were tied to catchment land-use and fish diversity was linked to habitat size. The survey results support the hypothesis that fine inorganic sediment deposited on the bed of a waterway causes environmental stressors by homogenising habitat and reducing resource (i.e., invertebrates) quality and quantity.

To better understand if turbidity was responsible for changes in fish diversity or abundance, I investigated the effect of high turbidity on fish feeding response by conducting a series of 24 hour laboratory feeding trials. However the results did not fully support my hypothesis that high turbidity would reducing feeding success. Two (i.e., brown trout and shortfin eels) of the four species tested did not show a significant altered feeding response even when turbidity was as high as 200 NTU. Only the redfin bully (Gobiomorphous huttonii, Ogliby), showed any sensitivity to high turbidities. The Canterbury galaxias (Galaxias vulgaris, Stokell) data provided an unexpected significant result. As turbidity increased Canterbury galaxias feeding rate also increased. These results point to varied evolved tolerances to suspended sediment.

Overall, the results have provided new insights into the effects of fine inorganic sediment on New Zealand freshwater fish communities and provide some guidelines for waterway management.