Periphyton growth in the Waipara River, North Canterbury

Periphyton was monitored monthly at four sites on the Waipara River from July 1999 to January 2002. Interactions with river flows, nutrients and invertebrates were examined to determine how these factors controlled periphyton development. Comparison of the Waipara River to other New Zealand streams indicated that periphyton biomass at the uppermost site (Site 1) was generally low to moderate. Further downstream, moderate to high biomass occurred at sites 2 and 4. Biomass at Site 3 was generally low, although some very high values occurred on occasions. Periphyton biomass at sites 2 and 4 exceeded periphyton guidelines for the protection of aesthetic/recreational values at least once during each full year monitored. In contrast, the guidelines were rarely exceeded at Site 1. Dissolved inorganic nutrients were generally poor indicators of the nutrient status of the river because of plant uptake. Cellular N and P values indicated nutrient enrichment at sites 2 and 4, which correspondingly had the highest biomass values. Conductivity tended to positively correlate with temporal and spatial patterns in periphyton biomass and was useful as a surrogate indicator of nutrient supply regimes. It correlated negatively with river flows, indicating higher nutrient concentrations may occur during reduced flows. Notable differences occurred in biomass development between periods of contrasting flow regimes. In particular, annual mean and maximum biomass at the three downstream sites was considerably higher during a period of low stable flows compared to a period of higher base flows. However, at the uppermost site, differences in biomass between these periods were much less pronounced. Invertebrate densities increased significantly with increasing periphyton biomass at the three downstream sites. There was little indication that invertebrates had any major control on periphyton biomass at these sites. However, at the uppermost site, although the invertebrate densities were generally much lower than at the other sites, they are more likely to have a controlling influence on periphyton biomass. Overall, the nutrient supply regime of the Waipara River is the primary controller on biomass development. Flow regimes (both frequency of disturbance and extent of low flows) operate as secondary controls of biomass