Executive Summary
A powerful state of environment analysis is able to report spatial patterns in both state, relative to guidelines, and temporal trends. Because patterns are scale dependent, state of environment analysis should also characterise conditions at a variety of spatial scales. The River Environment Classification (REC) is a framework that assists detection of spatial patterns in water quality and biological variables at multiple scales.
In this study, the REC was used to examine state and trend characteristics of a selection of water quality and biological variables collected at long term river monitoring sites. REC classifies parts of rivers according to a hierarchy of four 'factors', which characterise the upstream catchment's climate, topography, geology and land cover. The National Rivers Water Quality Network (NRWQN), and the Environment Southland Water Quality Network (ESWQN) provided data of appropriate coverage, consistency and quality to examine patterns in state and trends for New Zealand's rivers from national to regional scales. The objectives of this study were:
- To characterise patterns in the state of rivers in New Zealand at varying spatial scales within the spatial framework provided by the REC.
- To detect patterns in trends in water chemistry and biological variables at varying spatial scales within the spatial framework provided by the REC.
The analysis of state indicated that climate is a dominant factor determining patterns in many water quality and biological variables. Water clarity, nutrients (i.e. SIN, SRP), Total Ammonia, BOD5 and E. coli concentrations vary among the REC climatic classes with water quality tending to increase as rainfall increases. The biological variables showed a similar but less consistent relationship with climate. Macroinvertebrate Community Index (MCI) scores generally decreased as rainfall decreased and periphyton cover generally increased as rainfall decreased.
Classifying river monitoring sites at the Source of Flow level of the REC further increased the discrimination of patterns in water quality and biological variables. Rivers with Dry Climate categories and Low Elevation Source of Flow categories had higher concentrations of SIN, SRP, Total Ammonia, BOD5 and E. coli in general than rivers with Wet and Extremely Wet climate categories and Hill and Mountain Source of Flow categories.
The regional scale analysis of state discriminated patterns in water quality that were associated with geology and land cover. Water quality was consistently poorer where pastoral land cover dominated in the catchment. Significant differences in water quality variables among REC classes could also be associated with geology at the regional scale. Mean clarity was lower and SRP concentrations were generally higher in sites with a Cool Dry Low Elevation Soft Sedimentary class (CD/L/SS) compared with sites with Hard Sedimentary geology (CD/L/HS). This difference was attributable to higher phosphorus content, as well as erosion and weathering rates associated with Soft Sedimentary geology.
Sun ray diagrams were used to summarise water quality state at the national scale. At the Source of Flow level, SIN and to some extent SRP, guidelines were often exceeded, indicating widespread nutrient enrichment. BOD5 and Total Ammonia were never exceeded. Clarity guidelines were exceeded in Low Elevation Source of Flow categories. At the regional scale, rivers may exceed guidelines at a Land Cover level, whereas higher-level aggregations of rivers meet the criteria. For example, Cool Dry Hill rivers (CD/H) are within the clarity and SRP guidelines at the national scale. However, Cool Dry Hill rivers with catchments dominated by pastoral land cover (i.e., CD/H/HS/P) in Southland exceeded the guidelines. This result indicates that poorer water quality was associated with pastoral land cover. Microbiological contamination was also associated with Pastoral Land Cover classes. E. coli guidelines were exceeded in Low Elevation and some Hill Source of Flow classes in Southland that had Pastoral land cover categories.
The exceedance of nutrient guidelines was the most consistent pattern nationally. SIN and SRP promote the growth of periphyton. Excessive biomass may be detrimental to aquatic animals and reduce cultural, natural character and recreation values. Many classes exceed the SIN guideline. However, in some classes SRP levels are lower than the guideline. We note that the trend analysis has indicated that SRP, in particular, is showing upward trends nationally and specifically in Cool Wet and Warm Wet Low Elevation Source of Flow categories (CW/L and WW/L). This suggests that there is a risk of more widespread eutrophication if this trend were to continue beyond the period of analysis (i.e. 1989-2001).
We predicted that groups of rivers with similar environmental attributes (i.e. REC classes) would exhibit similar trends for selected variables. Hence, our aim was to describe differences in trends among REC classes, particularly at the Land Cover level, to provide more powerful assessments of spatial extent of trends than is possible using individual sites.
Nation-wide (i.e. using aggregated data for all sites), SRP showed an increasing trend in the period 1989 to 2001. Aggregating sites by Source of Flow classes showed that the trend in SRP was most pronounced in three Low Elevation categories; Cool Wet Low Elevation (CW/L), Warm Wet Low Elevation (WW/L) and Warm Extremely Wet Low Elevation (WX/L). Increasing nutrient concentrations are consistent with the known effects of intensification of land use (e.g. Parkyn 2002), which tends to occur at the highest rate in lower elevation areas. Thus, the observed trend in SRP is indicative of effects of intensification of agricultural production or changing land management. However, we found that BOD5 has decreased in rivers nation-wide, which is difficult to explain in terms of human induced changes. Reduced BOD5 is consistent with improved management of organic contaminants loads to rivers. However, aggregating sites at the Source of Flow level, showed that BOD5 decreased in river classes that were least likely to have been subject to organic contaminant discharges being: Cool Extremely Wet Mountain (CX/M), Cool Wet Mountain (CW/M), Cool Extremely Wet Lake (CX/Lk) and Cool Wet Lake (CW/Lk). Thus, it is unlikely that the BOD5 trends can be attributed solely to improved water quality management.
Rivers in three Source of Flow classes: Cool Dry Low Elevation (CD/L), Cool Dry Hill (CD/H) and Cool Wet Lake (CW/Lk), showed decreasing trends in periphyton cover over the 1989 to 2001 period. However, these classes also experienced an increasing trend in SRP, which would be expected to lead to increased periphyton. Thus, it is difficult to attribute the trend in periphyton to river management. It is possible that changes in climate, in particular an increasing frequency of floods, may be the cause of the periphyton trend, although we have not tested this hypothesis.
The observed trends in water quality and biological variables may be partly attributable to climatic variation. We decreased the scale of analysis to the regional scale to minimise the potential variation associated with large-scale climate patterns, and at the same time find sites that shared the same class with respect to Climate, Source of Flow and Geology but which differed with respect to Land Cover. Our analysis detected one trend at the Land Cover level. Nitrate concentrations increased in rivers in the Cool Wet Hill Hard Sedimentary Pastoral (CW/H/HS/P) class in the 1995 to 2001 period. This result was consistent the national trend in nitrate in the Cool Wet Hill (CW/H) Source of Flow class detected in the NRWQN data.
When NRWQN sites were grouped on the basis of land cover we detected significant differences in trends derived by simple regressions of mean annual concentrations between Indigenous and Pastoral land cover categories. These results also provide evidence of decreasing water quality in the period 1989 to 2001 associated with catchments that are dominated by pastoral land use. Even with flow-adjusted NRWQN data submitted to the Seasonal Kendall test, there were statistically significant differences in SRP and Total Ammonia between Indigenous and Pastoral categories with the net difference between the two categories indicating that degradation of water quality was associated with catchments that are dominated by pastoral land use.
Analyses of ESWQN data, grouped by land cover, mirrored the NRWQN trends to a large extent. Trends in nitrate and SRP concentrations differed significantly between Pastoral and Indigenous land cover categories. Thus, there is evidence of increasing degradation of water quality in the period 1995 to 2001 associated with pastoral land use, both within the Southland Region and Nationally.
These water quality trends need to be considered alongside the state analysis. We found that the Cool Wet Hill class, which is a large class comprising 21% of New Zealand's main stem rivers, exceed the SIN guideline, but comply with the SRP guideline. The analysis of trends over the period 1995 to 2001 indicated that SRP was trending upwards in the Cool Wet Hill (CW/H) class both nationally and within Southland. Thus, the increasing trend in phosphorus in this class has the potential to create conditions favourable to the growth of periphyton, possibly leading to nuisance algal blooms.
There is strong evidence for relationships between water quality variables, including SRP, and measures of climatic state such as the Southern Oscillation Index (SOI). Furthermore, different parts of New Zealand exhibit quite marked differences in hydrological response to climatic variation such as the SOI. This spatial variation in climate variability has implications for interpretation of results from trend analyses. For example, our results indicate decreasing trends in water quality associated with Pastoral land cover categories. However, the Pastoral sites are heavily biased by Low Elevation Source of Flow catchments. Therefore, we cannot exclude the possibility that trends in Pastoral and Indigenous categories, observed in this report are influenced, at least partially, by temporal variability in climate.
The study has highlighted the very heterogeneous nature of New Zealand's rivers and the difficulty this creates for effective environmental monitoring. Each of the four factors that define the top levels of the REC is implicated in differences in water quality and biological variables. This makes it difficult to attribute differences in state, or changes in time, to land use or management. To be certain that differences in state or trends are attributable to human activities, data from sites that are similar with respect to Climate, Source of Flow and Geology but which differ with respect to land use or management, must be compared. Monitoring networks must be either very targeted or must have a large number of sites that cover a wide range of REC classes. Greater coverage of different types of rivers involves increased monitoring effort. Thought could be given to achieving greater spatial coverage by monitoring more integrative variables (e.g. biological variables) with decreased sampling frequency.
Consideration needs to be given to the effect of climatic variability on water quality, as it may confound attempts to determine the causes of observed water quality patterns. One approach for controlling climate effects is the use of paired sites; impact and control. The net difference between data at the sites could be used in trend analyses. However, this solution does not provide replicate sites that would allow confidence that the pattern in trends can be interpolated across similar REC sites. An alternative approach, which would allow pattern detection, would involve increased replication, particularly of control sites. Trend detection would look at the net difference in changes through time between sets of aggregated control and impact or treatment sites. This could potentially require significant changes the structure of some monitoring programs.
Further work should include more data from regional council monitoring networks to increase replication. This may enable statistical tests of significance of differences in state variables among REC classes. Furthermore, greater replication may provide increased certainty that spatial patterns in trends are associated with differences in land use.
