Maurice J. Duncan

Velocity and Sediment Disturbance of Periphyton in Headwater Streams: Biomass and Metabolism

Disturbance by floods is believed to be 1 of the fundamental controllers of temporal and spatial patterns in stream periphyton. However, the exact causes of biomass losses are still poorly understood and discharge measures of disturbance often only explain limited variance in periphyton development. We investigated the effects of 2 of the main mechanisms of flood disturbance to periphyton--frequency of high-velocity events and frequency of bed sediment movement--in an effort to better understand disturbance processes and improve the quantification of flood disturbance regimes for studies of stream periphyton. Three sites were selected in headwater streams in each of 4 groups according to a 2-way factorial design of frequency of high-velocity events and sediment stability, giving a total of 12 sites. Periphyton were sampled monthly for 15 mo and analyzed for chlorophyll a. Maximum photosynthetic rates (Pmax), chlorophyll-specific Pmax, community respiration (CR), and Pmax:CR ratios were determined seasonally. Nutrient concentrations were generally low and did not vary as a function of disturbance regime. Peaks in chlorophyll a were usually low reflecting the low nutrients. Chlorophyll was 2-10× higher where bed sediments moved <15×/y and with seasonal maxima most often in autumn. Frequency of bed movement, soluble reactive P, and the frequency of velocity perturbations were significant predictors of mean monthly chlorophyll a (r2 = 0.88). Chlorophyll a and water temperature were major correlates of Pmax, specific Pmax, and CR, and thus the metabolic variables partly reflected changes in biomass among the disturbance regimes. With chlorophyll and temperature removed as covariates, the main factor influencing all metabolic parameters was season. Pmax was 7× higher in summer than in spring when minima occurred, chlorophyll-specific Pmax was 10× higher in summer than in spring, and CR was 4× higher in autumn than in spring. Pmax:CR ratios indicated that the communities were generally autotrophic at times of maximum photosynthesis with the highest ratios in summer (3× higher than winter). The frequency of velocity perturbations also had a significant effect on Pmax:CR ratios with highest ratios at sites where there was a low frequency of high-velocity events. Our results suggest that sediment instability greatly increases disturbance intensity for periphyton. It is therefore essential to assess not just the frequency of floods, but also the degree of bed movement when quantifying disturbance regimes for periphyton in headwater streams.

Flow variability in New Zealand rivers and its relationship to in‐stream habitat and biota

Abstract How variability indices were determined for 130 sites on New Zealand rivers and the sites were divided into groups based on these indices. Univariate and discriminant analyses were used to identify the catchment characteristics which contributed to flow variability. Climate, as determined by topography, geographic location, and the composition of the regolith (especially water storage capacity and transmissivity characteristics), accounted for a broad regional distribution of groups. Flow variability decreased with catchment size and area of lake and, to a lesser degree, with catchment slope. Relationships were found between flow variability, and morphological and hydraulic characteristics. The longitudinal variability of water depth and velocity increased with flow variability, indicating a more pronounced pool/riffle structure in rivers with high flow variability. Mean water velocity at mean annual low, median, and mean flow was higher in rivers of low flow variability than in rivers of high fl...

Ecological characterisation, classification, and modelling of New Zealand rivers: An introduction and synthesis

Abstract A programme of research to characterise, classify, and model New Zealand rivers according to hydrological, water quality, and biological properties is introduced. The results are detailed in the accompanying eight research papers. These studies provide the first national perspective on water quality and biology in New Zealands rivers using a consistent methodology. They are also the first step toward providing managers with robust models for predicting the effects on aquatic biota of changes in flow regimes and catchment land use. A synthesis of the results is given in this paper together with recommendations for riverine ecoregions in New Zealand.

Measurement and analysis of alluvial bed roughness

Abstract Techniques for measurement and analysis of the physical roughness of alluvial beds are introduced and evaluated. Precise digital elevation models (DEMs) of bed surfaces are developed using laser-scanning technology. Hydraulic roughness is represented by the standard deviation of local bed surface heights, which can be measured by physical profiling, analysis of DEMs or by a simple sand-spreading technique. Results from grain size analyses are compared with these different surface roughness determination techniques. At low relative depths the blocking of flow area by roughness elements is shown to be related to the standard deviation of local bed surface heights. A directional standard deviation is suggested for beds with structural polarization and an inclination index is introduced to identify bed imbrication.

Land-use impacts and water quality targets in the intensive dairying catchment of the Toenepi Stream, New Zealand.

Abstract Water quality monitoring in Toenepi Stream, New Zealand, started in 1995 in a study of dairy farming influences on lowland stream quality and has continued since then with brief interruptions. Surveys have provided information about changes in farm and soil management practices as they relate to environmental sustainability. Although average water quality in Toenepi Stream has changed little during 1995–2004, there have been some notable improvements. Water clarity measured by black disc has improved from 0.6m to 1.5m, and median ammonia‐N and nitrate‐N concentrations have declined by 70% and 57%, respectively. The frequency and magnitude of extreme concentrations have declined—most notably for nitrogen (N) forms, which also had decreased mean values. Specific yields for suspended solids (SS) and phosphorus (P) forms in 2002–04 were 47–67% of 1995–97 values, mainly because of lower water yields. Reduced specific yields for N forms in 2002–04 (34–37% of 1995–97 yields) were also attributable to lower mean concentrations in stream water. Faecal bacteria concentrations have not abated and are on average 2–3 times recommended guideline values for contact recreation. Fewer dairy farms and an increased proportion irrigating dairyshed effluent to land, rather than discharging it to the stream via two‐pond systems, were likely causes of improvement in water quality. Water quality targets were developed for Toenepi Stream to achieve contact recreation criteria for the Piako River (downstream) and for intrinsic habitat values for Toenepi Stream. A range of mitigation measures has been formulated to meet these targets, but substantial uptake of sustainable farming practices is needed to improve water quality in Toenepi Stream.

Physical characterisation of microform bed cluster refugia in 12 headwater streams, New Zealand

Abstract Recent sediment transport research has demonstrated that microform bed clusters (MBC) are particularly resistant to entrainment during floods and preliminary biological surveys have shown that such structures could be providing important refugia for benthos in streams. We therefore surveyed MBC in a selection of headwater streams, South Island, New Zealand to determine how common such structures are and then related their occurrence to flow variability, relative armouring, reach gradient, and sediment geology/ geometry to obtain a greater understanding of hydrological and hydraulic stream‐scale factors affecting their density and composition. MBC were present in all streams and ranged in density from 0.067 to 0.279 m−2. They occupied up to 4.4% of the surface area of the survey reaches, generally had 2–3 particles (a maximum of 7), and the average size for the largest particles ranged from 18.5 to 42.8 cm. MBC density and percentage site cover was significantly related to the relative armouring, ...

Rolling Stones and Mosses: Effect of Substrate Stability on Bryophyte Communities in Streams

Effects of 3 disturbance parameters (% of the streambed in motion at bankfull discharge [P_BF], catchment-specific discharge [SPECQ_BF], and frequency of bankfull floods [N_BF] on aquatic bryophyte richness and community composition were assessed. Relationships between taxonomic richness and P_BF and SPECQ_BF were strongly curvilinear, such that richness was low in both stable and highly unstable streams, and in streams of both low and high SPECQ_BF. Richness was highest in streams with moderate P_BF and SPECQ_BF. Interspecific competition may have reduced species richness at stable sites. In contrast, only a few bryophyte taxa were able to grow at unstable sites. Low richness at sites with a low SPECQ_BF most likely reflects the intolerance of some species to desiccation. Low richness at sites with high SPECQ_BF may reflect differences in the resistance of different taxa to high discharges. Community composition (expressed as ordination scores), and presence/absence of individual bryophyte species and life forms also differed among streams of different P_BF and SPECQ_BF. Thalloid or weft liverworts were common in streams with low P_BF and high SPECQ_BF, whereas cushion-forming mosses were common in streams of high P_BF and low SPECQ_BF. A conceptual habitat template model for bryophyte occurrences in streams is presented, which shows how communities changed predictably along the axes of P_BF and SPECQ_BF.

Benthic community dynamics during summer low‐flows in two rivers of contrasting enrichment 2. Invertebrates

Abstract We examined the effects of summer low‐flow on invertebrate communities in two New Zealand rivers of contrasting enrichment and associated periphyton development. Quantitative benthic samples were collected and hydraulic measurements made from three runs in each river over a 6‐week period. Although hydraulic conditions and time since last flood disturbance were similar in both rivers, invertebrate communities were dissimilar. This dissimilarity reflected differences in periphyton communities within each river. The invertebrate community in the low enrichment river (Okuku River) was dominated by high numbers of “clean‐water” insect taxa, and this community changed little over time. Periphyton biomass was low here, and the community was composed of diatoms. Major changes occurred to the invertebrate community in the high enrichment river (Waipara River) over the summer, where the relative abundance of dominant invertebrate taxa changed from insects and snails to ostracods. Periphyton biomass was high in this river, which was originally dominated by diatoms and cyanobacteria but which became dominated by filamentous green algae. Changes to invertebrate communities were linked to this increase in filamentous green algae. Multiple regression analysis showed that changes to the invertebrate community in the enriched river were strongly correlated with the number of days at low flow, suggesting that successional processes occurred during the low‐flow period. Such changes may occur naturally during summer low‐flows in enriched rivers or could be exacerbated if velocities are reduced by abstraction. Our results suggest that a rivers degree of enrichment should be considered when assessing minimum flows for consents or for the development of flow management guidelines.

Assessment of Streambed Stability in Steep, Bouldery Streams: Development of a New Analytical Technique

Substrate stability in streams is fundamental to benthic communities, many of which decline in biomass and taxon richness with decreased stability. Although of obvious importance in describing a streams disturbance regime, substrate movement is often difficult to quantify accurately. Traditional techniques rely either on subjective scoring systems (e.g., Pfankuch stability score), or require repeated site visits to follow downstream displacement of painted stones. Newburys Instability Index measures substrate movement based on calculations of a streams tractive force at bankfull discharge, but the assumptions on which this method is based limit its use to lowland streams. We have developed a new method to assess substrate movement suitable for shallow, steep mountain streams with a high bed roughness, based on an extensive review of existing hydraulic literature. We calculated the % of the streambed in motion at bankfull discharge (PBF) in 48 New Zealand headwater streams using our method, the Instability Index, and Pfankuch score. Concordance analysis showed that the 3 methods of assessing substrate stability ranked the 48 streams differently. We next related the 3 stability measures to aquatic bryophyte cover because these plants are good biological indicators of substrate stability. We observed a highly significant correlation (p < 0.001) between PBF and bryophyte cover, a weaker correlation between the Pfankuch score and bryophyte cover (p = 0.023), but no significant relationship between cover and the Instability Index. These results suggested that our method is a useful and potentially powerful tool to accurately quantify substrate movement in steep headwater streams.

21 Contemporary morphological change in braided gravel-bed rivers: new developments from field and laboratory studies, with particular reference to the influence of riparian vegetation

Abstract Contemporary (event to decadal-scale) morphological changes in two large braided rivers in Canterbury, New Zealand, are described, along with laboratory studies that support the field observations. In the process, some new developments in field and laboratory methods for investigating morphological change in braided rivers are presented, and Paolas (2001) hypothesis that braiding tendency should be influenced by a rivers ability to turn over its bed within the characteristic time for riparian vegetation to establish and grow to a mature, scour-resistant state is examined. The lower Waitaki River has been regulated for hydropower since 1935, and since then vegetation has encroached over the riverbed and braiding intensity for a given discharge has reduced. Measurements of vegetation removal by floods indicate that floods are not able to turn over the bed fast enough to contain vegetation encroachment, and the present braiding state is held by virtue of a regular spraying programme. In contrast, on the unregulated and sparsely vegetated Waimakariri River, remotely sensed high-resolution topographic surveys using LiDAR and digital photogrammetry have shown that even sub-annual floods turn over large proportions of the braidplain. The laboratory studies show that a braided river will evolve into a single-thread channel when its bed is invaded by vegetation and floods are too infrequent to contain the vegetation growth. Collectively, the field and laboratory evidence confirms that Paolas (2001) dimensionless time-scale parameter is a reasonable first-order predictor of whether floods or vegetation will achieve ascendancy, driving a river towards either braided or single-thread end-points, respectively.