Aquatic macrophytes provide important functions in fresh waters. However, excessive growth in small lowland agricultural streams, particularly of introduced macrophytes, can have negative impacts. These include accumulating sediment and causing flooding of farmland. Many New Zealand farmers see drainage as the primary function of small agricultural streams, and they are often referred to as “drains”. These drains are perceived to be primarily for removing floodwaters and high flows as efficiently as possible and are considered to have little ecological value, despite studies showing the contrary.

During summer, agricultural drains can become choked with macrophytes requiring local water management agencies to control their growth. Conventional macrophyte control techniques, including mechanical clearance and chemical sprays, have a range of adverse effects, such as over-steepening of banks, damaging to in-stream habitat, causing death of native fishes, spreading weed fragments and hindering aquatic ecosystem function. My thesis investigated some of the factors that influence macrophyte diversity, abundance and biomass in agricultural streams in Canterbury and evaluated the effectiveness of alternative practical macrophyte control options for small farm waterways.

To investigate the factors that influence macrophyte species diversity and growth (percent cover), I undertook a region-wide field survey of 28 small streams (<5 m wetted width) across the Canterbury region, South Island, New Zealand. Sites were surveyed at both the stream (i.e. 50 m) and patch scale (i.e. 1 m). Overall, macrophyte diversity was very low with only thirteen species in total found. Streams were dominated by the introduced Erythranthe guttata (monkey musk) and Nasturtium microphyllum (watercress). Physical and chemical conditions and macrophyte cover varied greatly both between streams and at the patch scale. At the stream scale, I found a significant positive relationship between macrophyte and sediment cover and significant negative relationships between macrophyte cover and both water temperature and dissolved oxygen saturation. At the patch scale, significant positive relationships were recorded between macrophyte and sediment cover, sediment depth and distance to nearest plants and significant negative relationships between macrophyte cover and stream shade and water velocity. I present a conceptual model of factors influencing macrophyte distribution and growth operating at the stream and patch scale. At the stream scale, my work indicates that disturbance regime is the key factor limiting macrophyte growth, compared to shading at the patch scale. Improving our understanding of these factors which influence macrophyte abundance and success is helpful in terms of informing alternative management regimes to manage excessive growth in lowland streams.

Given the known adverse effects of conventional macrophyte control techniques, I undertook several small- (2 m and < 5 m), reach- (50 m) and large-scale trials (up to 400 m), to evaluate the effectiveness of alternative macrophyte control tools. Alternative control techniques tested included: hand weeding; herbicide spray; weed mat; flower and seed removal; shading; physical disturbance; and sediment removal. At a small-scale, hand weeding, weed matting and herbicide spraying were effective at reducing macrophyte cover to <5 %. Hand weeding and weed mat immediately reduced cover, while dieback from herbicide took two months. Weed mat was a novel and effective control mechanism, particularly for sprawling emergent macrophytes which are rooted in stream banks. In contrast, macrophyte growth was enhanced under a partially shaded channel; whereas, in a subsequent more intensive trial with full shading (80 % light reduction), cover was reduced from almost 100 % to 17 % within six months. In the reach- to large-scale trials, both artificial shading and weed mat also proved to be very effective macrophyte control techniques. Furthermore, although large-scale intensive hand weeding provided short-term control, it proved not to be an effective long-term control option. The combination of weed mat and shading provides effective short- and long-term macrophyte control. Weed mat is practical and effective to suppress macrophyte growth while new riparian plantings grow and establish to provide the necessary shade that ensures continued macrophyte control. There is clearly some value in considering alternative tools to effectively control macrophytes in agricultural streams.

Identifying “drains” as “streams” and recognising that they provide important contributions to overall fresh water ecosystem health, further promotes the case for widespread implementation of alternative control methods.