Understanding the hydrological and physicochemical drivers of Phormidium proliferation in the Hutt and Waipoua Rivers

The Hutt and Waipoua rivers are affected annually by proliferations of the potentially toxic benthic cyanobacteria - Phormidium. Ingestion of these mats has resulted in numerous dog deaths and is therefore a risk to human health. This has resulted in the establishment of warning signs at many recreational sites on these rivers during summer months. Recent research has concluded that river flow and water column nutrients are two of the primary factors regulating Phormidium growth. Proliferations tend to form in rivers where there is slightly elevated water column dissolved inorganic nitrogen, low dissolved reactive phosphorus, and during periods of stable flow. It has been hypothesised that fine sediment may provide a source of phosphorus for Phormidium. These mats ‘capture’ fine sediment suspended in the water column, which becomes incorporated into the mat matrices when motile Phormidium filaments move over the sediment. Diffusive boundary layers at the surface of the mats limit the flow of nutrients and gases between mat and bulk river water, creating conditions (for example, lower dissolved oxygen, elevated pH) conducive to the release of phosphorus from sediment.  The aim of this project was to identify why Phormidium proliferates in certain parts of the Hutt and Waipoua rivers, as well as investigate the relationship between fine sediment and mat growth. Monitoring of river data was carried out in the Hutt and Waipoua Rivers between November 2014 and May 2015. Over this period, physicochemical and hydrological data was monitored to identify the influencing factors of Phormidium abundance. During February 2015, sediment traps were deployed to determine the sedimentation rates in parallel to Phormidium cover at each site. The collected fine sediment was fractionated and analysed for biologically available phosphorus. Finally, a manipulative study using stream channel mesocosms was undertaken to provide causative evidence that fine sediment deposition influences Phormidium growth. In this three-week study, four mesocosms were deployed containing different fine sediment treatments. Biomass samples were collected at regular intervals to determine total photosynthetic biomass and Phormidium specifically.  Phormidium cover during 2014-15 was influenced by water column nitrate-nitrite nitrogen concentrations and sediment deposition. Phormidium cover was considerably lower compared to previous years, with a maximum cover of 20.7% occurring in the Hutt River during February 2015. Analysis of historical flow and nutrient data suggests that the annual variation in Phormidium proliferation over the summer months was site specific and not generally driven by flow or nutrient concentrations.  It is likely that fine sediment plays a role in providing Phormidium mats with phosphorus in the Hutt and Waipoua River. This is shown through phosphorus concentrations within Phormidium mat water, which were 200-fold higher than the bulk water column. Maximum values of sedimentation, 272.0 g/m²/day, and biologically available phosphorus (bound to sediment) 1.4 mg P g⁻¹, occurred at sites with the highest Phormidium cover, which further confirms this correlation. Furthermore, mesocosm experiments showed that Phormidium biomass increased significantly (p=0.015) with an increased amount of sediment. However, the maximum biomass of 64.75 mg/m² did not occur in the mesocosm channel with the most sediment added to it. This suggests that a deposition threshold exists due to the attenuation of light.  Findings from this research provide some insights in to management options which may help to mitigate Phormidium proliferations in the future. The data indicates that reducing sediment inputs, or resuspension of fine sediment during flood remediation works, would reduce Phormidium proliferations. Riparian planting as well as the collaboration with local councils is needed to help reduce diffuse and remaining point sources of sediment and river bed disturbance during flood protection activities. Using a combination of observational and experimental studies, this research has shown that multiple factors influence Phormidium proliferation, and has highlighted the key role that fine sediment plays. Suggestions for future studies include in-river experiments to further explore the role of fine sediment and the optimisation of mesocosms, which may also help to investigate finer scale data on causative factors such as sediment thresholds.