Water Quality Monitoring
NZFM carry out a regular water quality monitoring programme within Lake Taupo Forest. The main objective of the programme is to assess the effect of forest harvesting on water quality in the Waimarino River and the Mangakowhitiwhiti Stream. The potential impacts of harvesting are determined by measuring suspended sediment, nitrogen and phosphorus concentrations and water clarity. Water quality assessments are undertaken at 3 sites along each watercourse. The location of these sites were carefully chosen in each catchment to ensure that water quality measurements were taken from upstream, within and downstream of the plantation forest boundary.
Water Quality Monitoring – Key points for 2010
- Harvesting commenced in the Waimarino catchmentin 1993 and continued through to 2006. Planting continued into 2007. Since then, there have been no harvesting, roading or planting activities in the Waimarino catchment. Harvested area remains at 85% of the plantation forest areas, and 30% of the total catchment area.
- The total rainfall in 2010 was 1471mm, the maximum daily rainfall was 77mm and the number of rain days was 150. Mean daily flow at Kepa Road was 3430 l/s, similar to the long-term avearge. the 40 flood days that occurred in September. Three flood events were sampled in 2010. Mean annual baseflows have declined significantly since the study began, and were the lowest on record in 2010.
- Water clarity remains consistently higher at Te Pohatu (the upper indigneous site) in 2010 compared with the two downstream sites and declined significantly with increasing flow at all three sites. Mean annula baseflow water clarity was also higher at Te Pohatu and baseflow water clarity shows a significant decline down the main stem of the Waimarino River catchment.
- Suspended sediment (SS) concentrations were higher in 2010 than in previous years, influenced by the higher number of floods sampled. SS concentrations were positively correlated with flow and negatively correlated with water clarity. Mean annual baseflow SS concentrations in 2010 were similar to previous years. Baseflow SS concentrations increased down the catchment, with concentrations at S.H.1 significantly higher than the two upstream sites.
- Total nitrogen (TN) concentrations were elevated in 2010 partly reflecting the three floods sampled in that year. TN concentrations were positively correlated with flow rate and SS concentration at all three sites, and the response was strongly influenced by the Total Kjedahl Nitrogen (TKN) component of TN. Baseflow TN concentrations remain significantly lower at Te Pohatu compared with the two downstream sites. Mean annual baseflow TN concentrations remain higher than normal at the two upper sites but are within normal range at S.H.1. Annual TN yields increased in 2010 and were highest on record at Te Pohatu (range 3.8-6.8 TN/ha/yr).
- Total phosphorus (TP) concentrations were also elevated in 2010 mainly because of the high number of flood events sampled this year. TP showed a stronger positivie relationshop with SS concentrations than with flow rate. This is most likely due to the adsorption of phosphorus onto the allophanic fraction of the volcanic soils. Baseflow TP concentrations increased significantly down the river system. Mean annual baseflow TP concentration in 2010 were similar to previous years.
- TN:TP ratios have been predominantly N-limiting for phytoplankton growth at the two upper sites and entirely N-limiting at the lower site. this trend has continued in 2010.
- There were no identifiable temporal trends in baseflow water clarity, SS and TP in the Waimarino catchment which could be attributed to harvesting. However, the two downstream sites continue to show a small but significant increase in baseflow TN over time.
- The Waimarino sub-catchment was harvested in 1997 and 1998 and re-planted in 1998.
- Mean annual flow and SS concentrations were higher in the pre-harvest, harvest and immediate post-harvest period compared with the subsequent nine years to 2010. There was a considerable increase in TN from the sub-catchment during and immeidately after harvest, almost entirely from the mobile TOXN (total oxidised nitrogen) component of TN. TP response to harvesting was muted, most likely reflecting the adsorption of phosphorus onto the allophanic fractions of volcanic soisl in the area.
- Since 2003, flow, SS, TN, TOXN and to a lesser extent TP have declined to levels similar to pre-harvest. This trend has continued in 2010 for TN and flow, with mean annual flow in 2010 the lowest on record. Conversely mean annual TP and SS concentrations increased in 2010 with mean annual SS concentrations the highest on record for the post-harvest period. TN levels however, show higher variability than in the pre-harvest period. It took 5 years for TN concentrations to decrease to pre-harvest levels and 7 years for TKN concentrations to again exceed TOXN concentration as in the pre-harvest situation.
- The increase and subsequent decrease in mean annual TN yields frm the sub-catchment coincided with the pre-harvest, harvest and post-harvest periods. TN yeilds had returned to pre-harvest levels by 2003, 6 years after harvesting commenced. TN yields in 2010 were 0.2 kg/TN/ha/yr.
- TN and the TOXN component of TN responded positively to increased flow whereas the TKN component of TN and TP showed stronger positive relationships with SS concentrations. In contrast to the main channel of the Waimarino River, SS concentrations in the sub-catchment showed a weaker but significant response to increased flow.
- TN:TP ratios in the Waimarino sub-catchment in the pre-harvest period were similar to the Waimarino River, mainly N-limiting, and change to P-limiting in the harvest and immediate post-harvest period. Since 2004, the trend has gradually reversed and in 2010 TN:TP ratios were entirely in the N-limiting range, similar to pre-harvest.
- Harvesting began in the Mangakowhitiwhiti catchment in 1998 and continued through to 2005 with replanting through to 2006. Since then, there has been no harvesting, roading or planting activities in the Mangakowhitiwhiti catchment. The harvest area remains at 60% of the plantation forest area and 45% of the catchment area.
- Total rainfall in the Mangakowhitiwhiti in 2010 was 1995 mm, the daily maximum rainfall was 60 mm and 177 rain days were recorded. Flow patterns and mean annual base flow in 2010 was similar to previous years and one flood even was sampled.
- Water clarity remains consistently higher at Kiko Road, the upper monitoring site in indigenous forest, and baseflow clarity has shown a slight but significant increase over time at this site. Mean annual baseflow clarity was lower in 2010 compared with previous years but still within normal range. Water clarity decreased significantly with increasing flow. There was a significant decline in baseflow clarity down the Mangakowhitiwhiti.
- There was an increase in SS concentrations in 2010 and mean annual SS concentrations were the highest on record at Kiko Road. SS concentrations were positively correslated with flow and negatively coresated with clarity. Baseflow SS concentrations increased significiantly down the river system with the largest differences between Kiko Road and the two downstream sites.
- TN concentrations have been generall higher at the 2 lower sites compared with Kiko Road since 2003 and this margin has continued in 2010. TN and the TKN and TOXN components of TN were positively correlated with flow and SS concentrations at all three sites. Mean annual baseflow TN is significantly lower at Kiko Road than at the two downstream sites. TN yields in 2010 were 1.0 and 1.21 Kg TN/ha/yr respectively at Kiko Road and Te Waaka Eruera Road.
- TP concentrations were elevated in 2010 and were generally higher at the two downstream sites compared with Kiko Road. Baseflow TP concentrations were significantly lower at Kiko Road, compared with the two downstream sites. Mean annual baseflow TP increased on 2009 but was still within the normal range. TP concentrations were postiivelty correlated with flow and SS concentrations.
- Over the monitoring period TN:TP ratios in the Mangakowhitiwhiti have been mainly in the 'neither N-nor P-limiting' range. For the past three years, including 2010, TN:TP ratios have remained in the P-limiting range at the two downstream sites, a result of the increase in TN at these two sites over the same time period.
- Although baseflow water clarity has increased over time at Kiko Road, there were no significant temporal trends at the two downstream sites. There were no identifiable temporal trends in baseflow SS in the Mangakowhitiwhiti catchment. Baseflow TN showed a slight but significant increase over time at Kiko Road and a significant increase at the two downstream sites. Baseflow TP concentrations show a slight but significant increase over time at Kiko Road but not at the two lower sites.