SEV Mangemangeroa

Overall our stream reaches had a high level of ecological health. The SEV values depicted below are typical of mature regenerating native forest in the Auckland region. The hydraulic function describes the structure of the stream, being the floodplain effectiveness, connectivity for fish migration, the flow regime and the connectivity to groundwater. These are all within the highest possible range for streams in this region. Our streams therefore have overall good structure and are able to provide the best possible habitat structurally.

The biogeochemical function refers to processes that are occurring within the stream. These refer to water temperature, dissolved oxygen levels, organic matter inputs, particle retention and the decontamination of pollutants. Again, these are all within the highest percentile for streams of this region. This is to be expected as the mature regenerating forest plays a significant role in filtering out contaminants and providing nutrients for instream processes. Upstream of each of these streams is predominantly residential or rural land, with some paved areas. Some contaminants such as heavy metals are to be expected in these streams, but riparian vegetation plays a significant role in mitigating these contaminants.

Habitat provisioning refers to the ability of the stream to provide habitat for instream invertebrate and fish species. Detritus from plant material as well as rocks are essential for stream dwelling species. Although this metric is reasonably high in all 3 streams, recent heavy rain events are likely to have reduced the presence of detritus in these stream reaches.

The lowest function for all three stream reaches was biodiversity. Although the previous three functions were quite high, we did not encounter a particularly high amount of invertebrate species in stream reaches. This may have been due to recent rainfall events, timing or simply because these streams are within regenerating native forest. Given the value of the physical elements of these streams, it is likely that the biodiversity function can only increase with time as more species colonise these areas. It is also important to note that although we observed fish species such as longfin eel, freshwater bully and banded kokopu, these were not included in the SEV calculations.

Overall, the stream reaches have a great level of health. This can be largely attributed to the excellent native planting and predator control work that is carried out in each of these areas. Riparian vegetation plays a significant role in improving stream health.

By Samantha van Ryn