Current Freshwater State

The highest scoring attributes in this FMU are Ammonia and Nitrate, which are in the A bands for all sites and both years. The main changes in attribute bands are the E. Coli 2023 results with both improvement and decline, as well as one individual decline each from B to C band for APSM and DRP attributes. The poorest scoring attribute state in this FMU is E. Coli, particularly as the Linkwater Stream declined to an E band at the 2023 report.

E. coli concentrations in natural conditions are typically low, with occasional spikes observed during rainfall runoff events. Streams in high rainfall areas in the north of the region experience more surface runoff events, resulting in higher E. coli concentrations and consequently lower NPS-FM states. This effect is particularly noticeable for some of the Marlborough Sounds Streams, such as the Graham, which has an NPS-FM state in the D-band despite a large area of native vegetation in the catchment. Linkwater Stream is one of two streams regionally exhibiting the highest E. coli concentrations, leading to its placement in the NPS-FM state within the E-band.

Generally, there is also an increase in E. coli concentrations with higher pasture cover. However, the patterns become more evident when waterways in high-rainfall areas are separated from those in areas with comparatively little rainfall. Regionally, four of the monitored streams show significantly higher E. coli concentrations than can be attributed solely to pasture cover. One of these is the Waitohi River, where urban sources from the Picton township are the primary reason for this discrepancy. In the case of the other three, particularly Linkwater Stream, potential causes include poor pastoral management practices or malfunctioning private sewage treatment systems.

Waterways with increasing DRP concentrations include the Kenepuru River. Although Dissolved Reactive Phosphorus (DRP) concentrations tend to be elevated in catchments with a substantial proportion of pastoral land cover, Linkwater Stream and Cullen Creek exhibit somewhat higher concentrations. A contributing factor to this is the naturally higher background levels of DRP. Catchment studies conducted in the Waitohi River and Linkwater area have revealed some of the highest DRP concentrations originating from native bush catchments within the region. This finding points to phosphorus-rich geological features in these parts of the Marlborough Sounds, which also explain the comparatively elevated DRP levels in the Graham River. Comparing DRP concentrations with those for the rest of New Zealand highlights that concentrations observed within Marlborough's waterways generally fall within the lower end of the spectrum.

Overall, all recent WQI measures are either Fair or Marginal in this FMU. General influencing factors include climate, vegetation cover, and the land uses in a catchment area.

One of the most significant factors influencing water quality is the alteration of natural land cover. Prior to human settlement, New Zealand was predominantly cloaked in forests. Since human arrival, extensive deforestation has occurred, transforming most of our waterways from pristine to altered states. Almost 30% of the region has been converted into pasture, primarily for sheep and beef farming, although dairy farming has also gained a foothold, including in the Linkwater areas. Production forests, mainly consisting of Pinus radiata, dominate lower hill regions of Wairau River tributaries, the Rai/Te Hoiere area, and parts of the Marlborough Sounds.

Dissolved Inorganic Nitrogen (DIN) constitutes the soluble nitrogen content within the water column, readily available for uptake by algae thriving on the river and stream beds. Elevated DIN concentrations can trigger excessive algae growth, posing detrimental impacts on both aquatic ecosystems and the visual appeal of water bodies. Naturally, DIN concentrations are very low. This can be seen for rivers with a high percentage of native vegetation cover within their catchment such as the Goulter River, Branch River and Black Birch Stream. Waterways with elevated DIN concentrations are found in catchments with significant areas of pasture and higher rainfall, such as the Rai, Ronga and Kaituna River as well as Cullen Creek and Linkwater Stream.

Catchments dominated by native vegetation and groundwater-fed streams tend to have the clearest water. Turbidity serves as a surrogate measure for the amount of sediment suspended within the water column. Sediment can have detrimental effects on sensitive downstream environments, such as estuaries, leading to the smothering of benthic habitats. Significant reductions in turbidity are evident in several waterways regionally, including the Linkwater Stream. Most of these sites also exhibit significant decreasing trends in Total Phosphorus concentrations, highlighting the close relationship between phosphorus and sediment.

On smaller streams, the presence of shading riparian vegetation can significantly affect water temperature. An example is Cullen Creek and neighbouring Linkwater Stream. Cullen Creek benefits from a comparatively intact riparian buffer composed of mature trees and shrubs along most of its length, while the lower reaches of Linkwater Stream lack adequate shading. Consequently, Cullen Creek consistently maintains lower and less variable water temperatures.