A question of scale: the effects of catchment urbanisation and riparian vegetation on organic carbon processing in small streams

Urbanisation has severe impacts on stream hydrology, biotic integrity and water quality. To date however, few studies have assessed the effects on ecosystem processes such as organic matter dynamics. Organic matter represents a vital basal heterotrophic energy pathway in aquatic ecosystems. Subsequently, organic matter availability and its processing have implications for energy transfer through microbial communities and higher trophic levels of lotic food webs. Riparian vegetation regulates the interface between terrestrial and aquatic environments and is frequently used to ameliorate the negative impacts of catchment land use change on lotic ecosystems. However the effectiveness of riparian vegetation in urban and suburban catchments, where the extent and connectivity are reduced and stormwater is piped directly to streams, remains poorly understood. Despite this, few studies have assessed the effects of coupled catchment scale urbanisation and reach scale riparian vegetation cover on organic matter inputs, processing or storage. I selected several small streams located in Victoria, Australia, which varied in catchment scale urbanisation and reach scale riparian vegetation cover. I investigated the sources of organic matter contributing to detrital pools and microbial respiration using elemental ratios and stable isotopes of C and N. Long-term coarse particulate organic matter (CPOM) and small wood storage were determined using cores and direct counts respectively. Breakdown and retention of CPOM was assessed via the deployment of standardized cellulose substrates and release of marked Eucalyptus leaves, respectively. I also determined rates of whole-stream metabolism using the diurnal dissolved oxygen change technique. While catchment urbanisation had no clear effect on CPOM decomposition, retention distances or metabolic rates under base flow conditions, I did observe significant alterations to both the sources of organic matter dominating detrital pools and long-term CPOM and small wood storage. Urbanisation increased the contribution of labile autochthonous material to the suspended particulate organic matter detrital pool and significantly reduced long-term CPOM and small wood storage. Reach scale riparian vegetation cover at the extent observed, had no significant influence on organic matter sources, decomposition, retention distances or metabolism, but had a weak positive effect on terrestrial CPOM inputs, long-term CPOM storage and small wood abundance. The effects of catchment scale urbanisation on organic matter availability and lability were significantly stronger than those of reach scale riparian vegetation cover. While the lengths of riparian vegetation in the current study were constrained by historic clearing, my results demonstrate only weak effects at these commonly observed scales. While riparian vegetation provides critical organic matter inputs, these results showed a pervasive effect of urban catchment hydrology and associated alterations to the frequency and magnitude of high flows, nutrient concentrations and temperature on organic matter dynamics. This study has direct implications for the management of aquatic ecosystems in urban environments. My findings highlight the need for both catchment scale water sensitive urban design and the protection/restoration of riparian vegetation in order to maintain organic matter availability, lability and food web structure in these urban streams. However, this data also suggests that the restoration of riparian vegetation in the absence of catchment scale water sensitive urban design is unlikely to achieve this goal.