The functionality of riparian zones in flat sandy catchments

Intermittent streams and flat sandy catchments are common throughout the world. In south Western Australia, Ellen Brook provides a model landscape for the study of water quality issues associated with managing flat sandy coastal catchments, where the highly permeable sands have limited capacity to intercept and store nutrients arising from agricultural activity. Revegetating riparian zones is universally used as a best-management practice to intercept nutrients and improve stream condition. However, the riparian zone can only be effective in nutrient removal if there is interaction between runoff and riparian vegetation. Key elements driving this interaction are slope, providing flow through the riparian zone and a mechanism for uptake and storage of nutrients (e.g. reactive soils). In addition, riparian vegetation is known to provide a suite of environmental benefits, however, there is little information on the influence of riparian vegetation on in-stream biota of intermittent streams. The questions posed in this thesis are 1) can vegetated riparian zones effectively ameliorate nutrient inputs in flat sandy catchments (no slope and unreactive soils) and 2) do riparian zones on intermittent streams in flat sandy catchments influence in-stream biological communities? This thesis compares hydrology and nutrient dynamics between an intermittent stream in a flat sandy landscape (Bingham Creek) and a perennial stream in a nearby landscape with slope and reactive soils (Lennard Brook). Nutrient stores within groundwater, soil and vegetation were analysed. A column experiment was undertaken to determine how the vertical rise and fall of groundwater affected nutrient dynamics in paddock and riparian soils and compared the effect of nutrient addition from fertiliser and cow manure. The thesis also examined the ecological condition of riparian vegetation and its influence on in-stream biota amongst a wider range of intermittent and perennial streams in this catchment. In flat sandy catchments, the lack of slope and unreactive soils reduced the capacity for riparian vegetation to intercept and store nutrients. The dominant flow path was the vertical rise and fall of groundwater in response to rainfall, which limited the interaction of groundwater with the active root zone of riparian soils, reducing the potential for nutrient assimilation. However, slow (0.23-0.3 m/day) horizontal groundwater flow through riparian zone, results in extended residence times (106-141 days), allowed nutrient transformations to occur. Riparian and paddock soils at Bingham Creek had a poor nutrient holding capacity, resulting in nutrients being stored in the groundwater (e.g. filterable reactive phosphorus (FRP) 48-600 μg/L-1) and not the soil (e.g. 4-510 mg.P/kg). Whereas at Lennard Brook the improved soil characteristics (increased iron oxides and clay) provided a greater opportunity for nutrient interception and storage by the soils (e.g. 29-1037 mg.P/kg) and groundwater flow to the stream was low in phosphate (e.g. FRP 10-74 μg/L-1). The riparian vegetation had a similar community structure at both sites and contributed comparable amounts of litterfall (369 and 404 g/m2) and nutrients per unit area (0.8-1.5 and 0.5-1 mg.P/m2) at Bingham Creek and Lennard Brook, respectively. This enhanced the organic matter content of underlying riparian soils that in turn slowed the rate of water movement (paddock 0.08-0.7 L/hr, riparian 0.04-0.24 L/hr), improved the phosphorus binding capacity of the soil and promoted loss of mineral nitrogen, probably by denitrification. However, the increased carbon content also supported a greater microbial community and the input of extra nutrients (including labile carbon) increased microbial respiration, leading to release of iron-bound phosphorus into groundwater. In the Ellen Brook catchment, flow regime (Global R = 0.444, P < 0.001) and the presence/absence of riparian vegetation (Global R = 0.407, P < 0.001) were the most influential factors associated with invertebrate assemblage composition. On average there were more invertebrate families in vegetated (30 ± 1.4) streams than unvegetated (27 ± 3.2) streams. Riparian vegetation helped shape and improve in-stream biological communities through the provision of organic matter for food and habitat and by limiting algal growth in stream water through shading. This was illustrated by a higher proportion of algal grazers (Chironominae and Physidae) in unvegetated streams and more detrital feeders (Leptoceridae, Gripopterygidae, Ceinidae) in vegetated streams. A conceptual model of hydrology and nutrient dynamics for riparian zones in flat sandy catchments was developed, which describes the key points of difference to the hydrological and nutrient dynamics in sloped catchments with reactive soils. Riparian vegetation appears to be acting similarly between catchment types but the capacity for riparian zones to remove nutrients in flat sandy catchments is limited due to soil type and hydrology. However, in these catchments, riparian zones are also the only sink contributing to a reduction in nutrient export from the catchment and so still have value in regulating nutrient dynamics. Clearly, riparian vegetation alone across the catchment alone cannot stem the flow of nutrients in flat sandy catchments. Instead, a multi-pronged approach is required to reduce nutrient export and improve the nutrient holding capacity of soils. Furthermore, riparian vegetation helps maintain and improve in-stream biological communities in intermittent and perennial streams in sandy catchments with low relief. Overall, riparian vegetation increased the retention of nutrients, improved soils and in-stream biodiversity, warranting the protection and rehabilitation of riparian zones in flat sandy catchments.