Skip to main content.

4. Discussion

Periphyton cover, mainly as filamentous algae, was sufficiently high to impact on river recreation and aesthetic values (as indicated by exceeding New Zealand guideline values) at about a quarter of the NRWQN sites at some time during an average year between 1990-2006. This suggests that nuisance periphyton may be a fairly widespread problem in New Zealand rivers. Note, however, that extrapolation of the NRWQN findings to all New Zealand rivers is not possible because the NRWQN sites were not selected randomly and are biased towards larger than average rivers.

The annual maximum filamentous algal cover exceeded the MfE guideline for protection of recreational and aesthetic values more commonly at nominal impact sites (28%) than baseline sites (6%), indicating that human activities in catchments have increased the frequency of nuisance periphyton occurrence. The two baseline sites where annual maximum filamentous cover exceeded 30% (TU2, Tongariro, and DN10, Monowai) were below dams that reduce flow variability and therefore probably contributed to periphyton development at these sites (Clausen and Biggs 1997). The higher proportion of pastoral land use at the impact sites (mean = 36%) than baseline sites (mean = 11%) is a likely contributor to the higher occurrence of nuisance levels at impact sites. Ballantine and Davies-Colley (2009) found nutrient concentrations were strongly correlated with percentage pasture amongst these NRWQN sites and that trends of increasing nitrogen and phosphorus were correlated with percentage pasture in the catchment. This was confirmed by the strong relationships between annual mean and maximum filamentous algal cover and percentage pasture in the catchment upstream (Fig. 6). Periphyton cover was also typically higher at downstream than upstream sites along the same river, indicating that periphyton is responding to human pressures that typically increase in downstream along rivers.

Figure 6: Relationships between mean annual mean and mean annual maximum filamentous algal cover and the percentage of pasture in the catchment of 73 NRWQN sites (1990-2006)

View maps | View text description
Figure 6: Relationships between mean annual mean and mean annual maximum filamentous algal cover and the percentage of pasture in the catchment of 73 NRWQN sites (1990-2006)

Note: that the two outliers are DN10 and TU2, both downstream of dams.

The MfE aesthetic/recreation use guideline for mat cover (<60% cover) was exceeded less frequently than the lower filamentous algal cover guideline (<30% cover), and annual mean and maximum mat covers at sites were not correlated significantly with percentage pastoral land use in the catchment upstream of the sites. The higher mean mat cover at impact than baseline sites did indicate some influence of human activities on this component of the periphyton, but that was weaker than for filamentous algae.

It is unlikely that differences in riparian shading, that can control periphyton biomass (Davies-Colley & Quin 1998; Quin et al 1997), had a significant influence on these patterns of higher periphyton cover and more frequent exceedence of nuisance guideline levels in impacted sites than baseline site and in a downstream direction because, none of the relatively wide NRWQN sites were heavily shaded (Appendix 1).

Our trend analysis found more declines than increases in the occurrence of obvious periphyton growths in the NRWQN rivers over the 17 years of monitoring since 1990. This encouraging finding was not expected given the increasing agricultural intensification and associated increase in nitrogen inputs over this period (Ministry for the Environment 2007) and general trends of increasing nutrient levels at the NRWQN sites between 1989 and 2007 (Ballantine & Davies-Colley 2009). This suggests that other factors, such as flow variability, grazing by macroinvertebrates or changes in periphyton community composition, may be influencing the responses of periphyton cover to increased nutrient concentrations at the NRWQN sites over the study period.

Some of the trends of declining cover along river systems may be associated with improvements in point source effluent management (e.g., Mataura and Manawatu Rivers). Previous studies have shown increases in periphyton biomass are commonly associated with nutrient enrichment from wastewater discharges (Welch et al 1992) and concentrations of Biochemical Oxygen Demand (BOD) and ammonium, that are associated with wastewater discharge, declined between 1989 and 2005 at the NRWQN sites (Scarsbrook 2006). Phosphorus concentration also declined at the lower Manawatu site (WA9, Opiki) over 1989-2007, suggesting a possible nutrient link to trend of reduced difference in periphyton cover between the sites upstream and downstream of the effluent discharges near Palmerston North. However, phosphorus and nitrogen have increased in the lower Mataura at DN5 (Ballantine & Davies-Colley 2009) indicating that factors other than mean nutrient levels caused the decline in periphyton cover.

Water clarity can limit periphyton development by reducing the amount of light reaching the riverbed (Davies-Colley et al 1992) and this is the likely cause of the downstream decrease in periphyton cover along the Wanganui River (between TU1 and WA4). However, clarity generally improved at the NRWQN sites over 1989-2007 (Ballantine & Davies-Colley 2009), so this cannot explain the trends that we found of more common occurrence of declining than increasing periphyton cover.

This initial analysis of the NRWQN periphyton database from 1990-2006 has provided a broad description of the state and trends in cover of potential nuisance periphyton cover and its association with general human pressures encapsulated in the classification of the sites as baseline, pseudo-baseline and impact. Better definition of the reasons for the patterns between sites and site-specific trends will require further analysis to consider factors that can control periphyton cover, including nutrients, substrate size and stability, flow variability, temperature and abundance of invertebrate grazers at the sites. This more detailed analysis is beyond the scope of the present report, but is likely to provide valuable insights to support targeted management of pressures to reduce the occurrence of nuisance periphyton levels in our rivers.

 

|