Ecohydrological characterisation of Otakairangi wetland, Northland
Douglas, C. (2019). Ecohydrological characterisation of Otakairangi wetland, Northland (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12686
Permanent Research Commons link: https://hdl.handle.net/10289/12686
Otakairangi wetland is a 2.6 km² remnant wetland located on the western edge of the Wairua River floodplain, around 15 km north-west from Whangarei. Otakairangi has experienced over 100 years of degradation with long term hydrological modifications aimed at enabling agricultural land use. This includes a large central drain that has effectively divided the wetland into two sections, removed the natural diffuse flows of the Otakairangi stream and lowered water tables. This central drain, coupled with border drainage and frequent fire events caused the peat within the wetland to become severely degraded and decreased biodiversity. The ecohydrology of representative areas of the wetland was studied to assess the relationship between the anthropogenic disturbances and the wetland ecology and water level regime. The primary goal of this research was to characterise the wetland condition, which would aid in the development of future management goals and planning through measurement of wetland degradation and natural recovery. The study focused on five key areas of the wetland, with several transect lines established to determine the extent of degradation and recovery. This includes two transects across the central drain and three extending from the wetland margins. Peat physical and chemical characteristics were measured in the field and laboratory, and then classified through statistical ordination techniques. Vegetation compositions and patterns were assessed along with foliage chemistry, and then compared to peat characteristics. Hydrological data were retrieved and analysed from eight automatic water level sites along a transect perpendicular to the central drain, while meteorological data was collected at a nearby Northland Regional Council rain gauge. Peat surface oscillation in the restiad bog and water chemistry were also examined. Nutrients, isotopes (δ¹⁵N), heavy metals, and physical soil characteristics (such as bulk density and water pH) were highest in close proximity to the upper central drain and the northern border drain. A gradient was observed from the entrance of the central drain into the central north-eastern section of the wetland, with high fertility substrate and swamp conditions transitioning to low fertility bog-like conditions. This was also shown by the vegetation composition, with larger swamp species such as flax (Phormium tenax) transitioning to sedges (Machaerina teretifolia) and ferns (Gleichenia dicarpa) and then to the restiad bog species Empodisma robustum. Low nutrient conditions were present elsewhere in the wetland, such as the southern section by the southern border drain and southern central drain. Both exhibited low fertility, the area around the southern central drain was dominated by large patches of M. teretifolia and small patches of E. robustum (fen-bog transitional), while the area by the southern drain was predominantly G. dicarpa and Leptospermum scoparium (fen). A natural spread of E. robustum is restoring the wetland, in terms of natural vegetation composition and peat substrate functionality, as indicated by analysis of peat cores from beneath sites in which it dominated. The water levels in central areas of the wetland (100 - 290 m from the central drain) were relatively stable, with fluctuations around large rainfall events. These areas had rainfall as the primary water input, as they were independent from any surficial flows originating from either the central drainage channel or border drainage. However, several flood events impacted the marginal wetland water tables (February 2018, May-July 2018) with the drainage channel flooding and inundating between 50-100 m of the wetland area north-east of the central drain. In contrast, water table drawdown extended less than 20 m from the central drain, with the 20 m site experiencing mean water tables 1.5 m above that of the mean central drain water level. The major risk to the wetland is continued flood inundation from the nutrient and sediment-rich waters from the surrounding catchment. Recommendations for management include improving the water quality from the headwaters to the wetland entrance through onsite farm management, engineering structures such as constructed wetlands or buffer zones to reduce flood inundation of the wetland. Internal restoration includes spreading the main peat forming species, Empodisma robustum, to areas in which it is currently absent, and a feasibility study for the re-introduction of the locally extinct Sporadanthus ferrugineus to the bog area.
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