The impact of heavy metals on benthic macroinvertebrate communities in Christchurch's urban waterways
Thesis DisciplineWater Resource Management
Degree GrantorUniversity of Canterbury
Degree NameMaster of Water Resource Management
The urbanisation of a catchment results in substantial changes to associated waterways. These effects, known as the “urban stream syndrome” can include flashier hydrographs due to stormwater inflows, altered geomorphology, and increased inputs of sediment, nutrients, and toxicants. Metal pollution of rivers and streams is an area of significant concern for management of freshwaters, and urban runoff is recognised as an increasingly relevant source of metals. Heavy metals can to be toxic to aquatic invertebrates, and can impact community structure and abundance. To investigate the influence of heavy metals on macroinvertebrate community composition, I compared invertebrate community composition over a gradient of heavy metal pollution within Christchurch City’s urban waterways. I also investigated the survival of three taxa, the mayfly Deleatidium spp., the caddisfly Pycnocentria spp., and the snail Potamopyrgus antipodarum in short-term in situ mesocosm experiments in six streams of varying metal contamination. CCA analysis identified that sediment bound metals, dissolved metals, and impervious surface area were the three most significant environmental factors explaining invertebrate community structure. Stepwise regression analysis of invertebrate community metrics and indices identified metals bound to the sediment to be among the prevailing factors in explaining invertebrate community composition across my study sites. The results of my mesocosm experiments suggest that heavy metal contamination could be rendering more impacted streams uninhabitable to relatively sensitive taxa (such as Deleatidium and Pycnocentria). However, over the seven day time frame of my mesocosm experiment, conditions in moderately polluted streams did not appear to directly affect survival of Deleatidium significantly more than conditions in streams containing natural populations of the mayfly. Knowledge of relevant stressors is key to the management and rehabilitation of urban streams. My results suggest that heavy metals are likely a key stressor on many invertebrate communities in Christchurch’s urban waterways. While rehabilitation of streams in Christchurch’s heavily urbanised areas can improve attractiveness and societal value, unless stormwater inputs and associated pollutants are mitigated an improvement in biological communities seems unlikely.