Glenys F. Croker

The influence of aquatic macrophytes on the hydraulic and physico-chemical properties of a New Zealand lowland stream

The effects of macrophytes on hydraulic and physico-chemical variables were examined by conducting tracer experiments with SF6, CH3Cl and rhodamine WT in a stream before and after complete removal of plants from a 180 m reach. Whakapipi Stream has high average biomasses (up to 370 g dw m-2) of macrophytes (predominantly Egeria densa) that, on average, cause summer velocities to be lowered by 30% and depths increased by 40%, compared to a plant-free channel. Mannings roughness coefficent was consistently higher by 0.13 and longitudinal dispersion coefficients were more variable (CV = 52%, cf. 20% when plants removed), when macrophytes were present. Stream dissolved oxygen (DO) and temperatures were unevenly distributed, possibly as a result of transient storage zones attributable to plant biomass. Surface water in macrophyte patches was 1-5 °C warmer than water in channels or beneath the plants near the bed of the stream, and DO was 2-28% of saturation higher at the top of the plants than in channel water and up to 7% higher than in bottom water. Effects of increased small-scale turbulence on the reaeration coefficient, K2(20), were cancelled by increased stream depth and reduced velocity so that it varied little with flow. Application of a single-station diurnal curve model, DOFLO (Dissolved Oxygen at Low Flow), to continuous monitoring data gave values of K2(20) in broad agreement with those measured by the gas tracer method and showed that rates of gross photosynthetic production in daylight (10-27 g m-2 d-1) and respiration at 20 °C (19-37 g m3 d-1) were high by comparison with other rural streams. Streams with smaller K2(20) values than Whakapipi Stream but with similar levels of productivity and community respiration would show more pronounced diurnal variations in DO and even be anoxic at times.

Human perception of water appearance

Human perception of two important visual aspects of the appearance of river and lake water, clarity and colour, was investigated from the perspective of bathing and aesthetics in New Zealand. Bankside interviews were conducted at the same time as measurements were made for clarity and colour. We found that water appearance, perceived suitability of the water for bathing, and bathing activity are closely linked, as is perception of water clarity and colour. Water is perceived as just suitable for bathing at a horizontal black disc visibility of 1.2 m (equivalent Secchi depth c. 1.5 m) with a 90 percentile of 2.2 m (equivalent Secchi depth c. 2.75 m), confirming earlier work. As might be expected, blue waters are preferred to yellow waters, and water is perceived as just suitable for bathing if the Munsell colour is greater than around 30 units (i.e., at the low end of the green-yellow range). For bathing waters, consideration of personal safety is very important; for aesthetics, surroundings are important. Peoples perception of visual aspects (i.e., clarity and colour) appears to have a strong influence on their rating of overall site suitability. M 94050 Received 27 September 1994; accepted 9 January 1995

Patch- and reach-scale dynamics of a macrophyte-invertebrate system in a New Zealand lowland stream

Abundant growths of macrophytes are a common feature of streams in open lowland areas of New Zealand during summer, but the values of these to aquatic biota are poorly understood. We studied the temporal dynamics of, and associations amongst, elements of a macrophyte-invertebrate system to provide an improved information base for lowland stream management. The biomass of macrophytes increased significantly over the four quarterly sampling occasions from 43.8 g m-2 in June to 370.8 g m-2 in March; biomass was dominated by Egeria densa on all dates, except in December when Potamogeton crispus was dominant. We did not detect strong associations between epiphyton biomass and invertebrate abundance in our study, but this may reflect the fact that we sampled loosely-adhering epiphyton on young, surface-reaching shoots whereas invertebrates were collected from macrophytes growing through the water column. Density of some invertebrate species per gram dry weight of plant material varied by macrophyte type, with the chironomids Tanytarsus vespertinus and Naonella forsythi displaying positive correlations with Egeria and Potamogeton biomass, respectively. The shrimp Paratya curvirostris accounted for 50% of phytophilous invertebrate biomass, with Chironomidae the only other group to comprise more than 9%. Abundance of total phytophilous invertebrates displayed a positive linear relationship with macrophyte biomass in a sample (0.1 m2), and a humped relationship with species richness, such that highest numbers of taxa occurred at macrophyte biomass levels around 400 g dw m-2. Our study suggests that intermediate macrophyte biomass levels are likely to enhance macroinvertebrate biodiversity in sandy-bottomed lowland streams.

Water clarity criteria for bathing waters based on user perception

Perception of bathing water quality has been studied by water-side surveying of bathers and/or by-standers. Bathing water quality assessment was strongly related to visual cues and, in particular, water clarity. A minimum water clarity (as measured by horizontal black disc sighting range y BD ) of 1·2 m (which corresponds to a Secchi disc depth of 1·5m) is required before a water is perceived, on the average, as suitable for bathing. If, for management purposes, it is required that 90% of people perceive a water clarity as suitable for bathing, then y BD needs to exceed about 2·2 m, corresponding to a Secchi disc depth of approximately 2·75 m. However, perception of water clearness appears to markedly affect a sites overall suitability for bathing only at low water clarities. In this study, perception of colour seems to be a minor factor, in part because the colour of the waters studied was very difficult to assess. Data for other factors, for example facilities present and litter, were also collected but could not be analysed in detail.

Integrated catchment management effects on flow, habitat, instream vegetation and macroinvertebrates in Waikato, New Zealand, hill‐country streams

Abstract We investigated the short‐term effects on stream now, habitat, water temperature, periphyton biomass, and macroinvertebrate communities of applying integrated catchment management (ICM) to improve the poor economic and environmental performance of a hill farm used for sheep and cattle grazing. Stream responses to a range of changes in land use and riparian management were compared at four reaches in sub‐catchments within a pastoral hill farm area and nearby pasture and native forest reference reaches over 6 years before and 6 years after ICM implementation. At the whole catchment scale, annual runoff from the pasture treatment catchment before ICM was 7% higher than from the native forest reference catchment, and 7‐day low‐flow was 11% lower, but runoff and low now both declined by c. 6%/yr following afforestation (mainly Pinus radiata) of 62% of the pasture catchment. At sub‐catchment level, pine afforestation with 10 m wide riparian setbacks (site A) was associated with declining channel width, water width, water temperature and periphyton biomass, and increasing periphyton organic content and changes in macroinvertebrate community indices and composition indicating recovery towards reference native forest stream conditions. Site M, involving a mix of partial pine afforestation (36% of catchment) and riparian cattle exclusion with riparian Populus deltoides planting, was associated with generally similar trends to site A. Riparian reforestation with natives and exclusion of all livestock was associated with more rapid reduction in water temperature and streambed cover by fines and macrophytes in a reach with a1m channel (site RS) than a 2 m wide channel (site RM). Reaches with active riparian tree planting had increased streambed cover by small wood and roots. In general, the macroinvertebrates indicated a “rubber band” response to ICM actions, with relatively rapid recovery towards native forest stream macroinvertebrate metric values and faunal composition, although restoration of composition was incomplete. Water temperature reduction was identified as a key factor in this improvement along with the 1–2 km proximity of colonists in undisturbed native forest streams.

Biodiversity of stream invertebrate faunas in a Waikato hill-country catchment in relation to land use

Stream invertebrates were collected from 24 sites surrounded by a mixture of native forest and pasture in the Mangaotama catchment near Hamilton, Waikato, between 1992 and 1999. Sampling methods used included Surber sampling for benthic invertebrates, pump sampling, freeze coring and colonisation baskets for hyporheic invertebrates, and light, sticky and Malaise trapping for winged adults.A total of 188 stream invertebrate taxa were identified during the course of this work, the greatest number reported for any catchment in New Zealand. Of these, 85% were insects comprising largely Trichoptera (68 taxa or 36%), followed by Diptera (40 taxa, 21%) and then Ephemeroptera (23 taxa, 12%). Light-trapping in summer on four occasions coupled with benthic sampling once in summer yielded 81% of total recorded taxa. Streams that drained catchments entirely in pasture or native forest had similar percentages of total invertebrate taxa, and of the Coleoptera, Diptera and Ephemeroptera faunas. However, native forest sites had lower percentages of Mollusca and “other” taxa, and more Trichoptera, Plecoptera and Crustacea taxa than pasture sites. Stream invertebrate distributional patterns in the Mangaotama catchment suggest changes in community composition and loss of some species adapted to forest cover in catchments converted entirely to pasture.

Effects of hydraulic conditions and larval size on the microdistribution of Hydrobiosidae (Trichoptera) in two New Zealand rivers

Abstract Hydraulic conditions utilised by common taxa of the caddisfly family Hydrobiosidae were characterised for the Mangles River, South Island, and the Tongariro River, North Island, New Zealand. Costachorema spp. (predominantly C. xanthopterum and C. callistum) and Hydrobiosis parumbripennis larvae were collected from a wide range of water depths (10–155 cm) and velocities (8–178 cm s−1) and from predominantly small gravel to large cobble substrates. Overall densities of Costachorema spp. were significantly (P < 0.01) and positively correlated with water velocity, Froude number, boundary layer Reynolds number, and inferred shear velocity at both sites. Similar relationships were evident for total densities of H. parumbripennis in the Mangles River, whereas a significant inverse correlation was detected between density of this species and water depth and Reynolds number in the Tongariro River. Strongest correlations with densities of Costachorema spp. or H. parumbripennis were detected using water dep...

Ecoregional differences in macrophyte and macroinvertebrate communities between Westland and Waikato: Are all New Zealand lowland streams the same?

Abstract We characterised water chemistry, aquatic habitat, macrophytes, and invertebrate assemblages in eight lowland streams—five in Westland, South Island, and three in the Waikato, North Island, New Zealand. Factors influencing invertebrate community structure over large (between ecoregions) and small (within an ecoregion) spatial scales were investigated. The Westland sites had generally lower nutrient concentrations, conductivity, and water clarity, and coarser substrates than the Waikato sites, reflecting differences in geological history, hydrology, and land‐use intensity. The macrophyte communities in each region were very different in species composition and structure, but seasonal abundance patterns in both regions were fairly typical of New Zealand streams in general. Alien obligate submerged macrophyte species were absent from the Westland lowland stream sites, and may partly reflect the isolated nature of this region where colonisation by asexual macrophytes would be difficult. Waikato stream invertebrate faunas were dominated by molluscs (mainly Potamopyrgus) and crustaceans (mainly amphipods), whereas Epheme‐roptera, Trichoptera, and Coleoptera taxa dominated the Westland stream faunas. The overall structure of invertebrate assemblages appeared to be influenced by a combination of regional differences in substrate type, nutrient concentrations, water clarity, and macrophyte cover. Westland streams with more upstream pastoral development had higher proportions of molluscs than those with predominantly forested or scrub catchments. Our study suggests that large‐scale ecoregional differences may override smaller‐scale land‐use effects on lowland stream invertebrate communities, and that management strategies should be developed on an ecoregional basis for lowland stream ecosystems.

HUMAN PERCEPTION OF WATER APPEARANCE. II: COLOUR JUDGMENT, AND THE INFLUENCE OF PERCEPTUAL SET ON PERCEIVED WATER SUITABILITY FOR USE

Abstract Human perception of two important visual aspects of the appearance of river and lake water, clarity and colour, was investigated from the perspective of bathing and aesthetics in New Zealand. Bankside interviews were conducted at the same time as measurements were made for clarity and colour. We found that water appearance, perceived suitability of the water for bathing, and bathing activity are closely linked, as is perception of water clarity and colour. Water is perceived as just suitable for bathing at a horizontal black disc visibility of 1.2 m (equivalent Secchi depth c. 1.5 m) with a 90 percentile of 2.2 m (equivalent Secchi depth c. 2.75 m), confirming earlier work. As might be expected, blue waters are preferred to yellow waters, and water is perceived as just suitable for bathing if the Munsell colour is greater than around 30 units (i.e., at the low end of the green‐yellow range). For bathing waters, consideration of personal safety is very important; for aesthetics, surroundings are i...

Biodiversity values of small headwater streams in contrasting land uses in the Auckland region

The biodiversity values of 12 small headwater streams (catchments mostly <30 ha) were evaluated. Densities of aquatic macroinvertebrates were the same or higher in three headwater habitats (flowing water, isolated pools and wet mud) as in adjacent perennial streams. Alpha (per sample) richness of all aquatic macroinvertebrates and of Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa were as high in headwater flowing habitats and isolated pools as in perennial streams. Gamma (landscape-level) and beta (taxa turnover) richness appeared to be similar among isolated pools, mud habitats and perennial streams, but higher in headwater flowing habitats. Community composition was very similar among headwater flowing, isolated pool and perennial habitats. Some taxa in headwater habitats were headwater specialists, but none was identified as endemic to headwaters. Thus, macroinvertebrate evidence indicates that headwater streams should be given similar protection status to perennial reaches. Headwaters in pasture catchments had lower EPT richness than native forest headwaters. Riparian native woody vegetation appeared effective in maintaining EPT richness and community composition similar to the native forest condition.