J. Christopher Rutherford

Land use effects on habitat, water quality, periphyton, and benthic invertebrates in Waikato, New Zealand, hill‐country streams

Abstract Water quality, habitat, and biota were compared during spring amongst c. 100 m reaches on 11 streams draining pasture, native (podocarp‐broadleaf) forest, and exotic pine forest established on pasture 15 years previously. Differences were greatest between the pasture and native forest streams. Only 1–3% of incident light reached native and pine forest streams whereas 30% reached pasture streams. Pasture streams had 2.2°C higher mean temperature than the native streams, and 5‐fold higher nitrate, 30‐fold higher algal biomass, and 11‐fold higher gross photosynthesis. Native streams were 60% wider than pasture, with pine streams intermediate. Pine and pasture streams had 3‐fold higher suspended solids and fine sediment stored in the streambed than native streams. Woody debris volume was 17‐fold greater in pine than pasture streams, with native streams intermediate. Invertebrate taxa richness did not differ between land uses. Community composition differed most between pasture and native forest, with...

Predicting the effects of shade on water temperature in small streams

Abstract A computer model for stream water temperature was developed, and tested in a small pasture stream near Hamilton, New Zealand. The model quantifies shading by riparian vegetation, hillsides, and stream banks using three coefficients: canopy angle, topography angle, and canopy shade factor. Shade was measured directly and found to vary significantly along the channel. Using the maximum measured shade, a close match was achieved between observed and predicted daily maximum and minimum water and bed sediment temperature. Model predictions of incoming and outgoing long‐wave radiation flux closely matched measurements, but there were unexplained discrepancies in short‐wave radiation flux. Model predictions indicate that moderate shade levels (c. 70%) may be sufficient in temperate climates to restore headwater pasture stream temperatures to 20°C, an estimate of the thermal tolerance for sensitive invertebrates.

A Monitoring and Classification System for New Zealand Lakes and Reservoirs

Five variables gave strong indications of change in the trophic state of New Zealand lakes, namely; concentrations of chlorophyll a, total phosphorus and total nitrogen, as well as Secchi depth and...

Thermal tolerances of two stream invertebrates exposed to diumally varying temperature

Abstract For two key New Zealand freshwater invertebrates (the mayfly Deleatidium autumnale and the snail Potamopyrgus antipodarum) the upper thermal tolerances were measured in the laboratory under both constant and diumally varying temperatures. At constant temperature 50% mortality in 96 h (constLT50) occurred at 24.2 ± 0.9°C for mayflies and 31.0 ± 0.6°C for snails: values similar to previously published estimates (Quinn et al. 1994). For diurnally varying temperatures (daily amplitude 10°C) 50% mortality occurred when the daily mean temperature was 21.9 ± 0.7°C (mayflies) and 28.6 ± 0.4°C (snails) which is c. 10% (2.5 ± 1.3°C) lower than the constLT50. Conversely, 50% mortality occurred when the daily maximum temperature was 26.9 ± 0.7°C (mayflies) and 33.6 ± 0.4°C (snails) which is c. 10% (2.5 ± 1.3°C) higher than the constLT50. Many published temperature limits for stream organisms are derived from constant temperature experiments. Our results indicate that such limits should be applied to a temperature midway between the daily average and the daily maximum of a diurnal profile.

Modelling the time course of shade, temperature, and wood recovery in streams with riparian forest restoration

Abstract Action is increasingly being taken in New Zealand and elsewhere to restore ecological function to streams through planting of riparian zones. We used simulation modelling to explore the relative performance of three strategies to restore the riparian zone of a pastoral stream to native forest by: (1) passive regeneration; (2) planting then abandonment of a Pinus radiata plantation; and (3) active restoration by planting selected native trees. We linked the forest model LINKNZ with a shade and temperature model (sWAIORA), and a wood model (OSU_STREAMWOOD) to simulate recovery trajectories for key forest stream attributes in hypothetical streams (1.3–14.0 m channel width) in the central North Island, New Zealand. Both active restoration strategies outperformed passive regeneration in shade, temperature and stream wood volume for most of the simulation time (800 years). Although the abandoned pine plantation provided greatest shade initially (<100 years), active native planting provided the greatest benefits overall. In general, recovery of stream shade (and temperature) is expected within decades, is accelerated by deliberate planting, and is fastest in small streams in which thermal stress from sunlight exposure is greatest. However, full recovery of stream and riparian function may take centuries, being dependent on large trees providing wood to structure the channel.

Natural water temperature variations in the lower Waikato River, New Zealand

Abstract Natural temperature in a 30 km reach of the Waikato River during autumn 1984 varied synchronously across the channel and for practical purposes can be assumed transversely uniform (within 0.03°C). Significant differences between sites were found in the amplitude and phase of diurnal temperature cycles which resulted in instantaneous temperature differences along the channel of up to 0.5°C. These were caused by a reduction in mean depth and a change in channel orientation along the study reach. The observed lag between peak radiation and maximum water temperature yielded preliminary estimates of the surface heat exchange coefficient K = 155–670 W m−2 s−1 °C−1 but there is some doubt about the accuracy of these estimates. A heat budget model was used to predict longitudinal temperature changes. Model predictions were accurate only to within ± 0.2°C (RMS error) probably because meteorological data came from sites 40–50 km away from the study reach. Uncertainties in natural temperature of about 0.5°C...

Predicting the effects of time‐varying temperatures on stream invertebrate mortality

Abstract Stream managers often need to predict the impacts of high and time‐varying temperature on key stream invertebrates. A simplified model has been developed and calibrated using mortality observations made at constant temperature for two important New Zealand stream invertebrates: the snail Potamopyrgus antipodarum and the mayfly Deleatidium autumnale. A close fit was obtained for Deleatidium but Potamopyrgus showed evidence of acclimation during the 96‐h tests. The model successfully predicted the mortality observed when temperature varied diurnally by ±5°C in seven experiments but over‐estimated mortality in the eighth. The experiments used to calibrate and test the model were performed on test animals collected in winter (April‐June) and acclimated at 16–17°C. The model was less successful at predicting mortality on Deleatidium collected in summer (December‐January) and/or acclimated at 21–22°C. The likely reason is that the model, calibrated using winter data, does not account for summer acclimation.

Transverse mixing and surface heat exchange in the Waikato River: A comparison of two models

Abstract Two mathematical models of different complexity were used to study transverse dispersion and surface heat transfer in the lower Waikato River. A simple analytical streamtube model (HPLUME) gave adequate temperature predictions in a reach where the channel was fairly regular but performed poorly where there were extensive shallows. In the latter reach, a two‐dimensional numerical model (SYSTEM21) gave good temperature and flow predictions once properly calibrated. Model calibration proved to be difficult in the Waikato River because the natural river temperature varied significantly along the channel. A search method was developed to estimate both the transverse dispersion and surface heat exchange coefficients from measured plume temperature profiles based on the observation that transverse variations in natural temperature were small. This method was used to calibrate SYSTEM21 in two separate reaches. Coefficient estimates were sensitive to measurement errors and slight departures in homogeneity...

Eutrophication in Lake Rotorua. 1. Using OVERSEER to estimate historic nitrogen loads

ABSTRACT Nitrogen concentrations are increasing and causing eutrophication in many New Zealand waterways, including Lake Rotorua. Groundwater (mean age 35 to 170 years) dominates lake inflows, and nitrogen losses from farmland may take several decades to reach the lake. This study collates published agricultural statistics and estimates farm losses from 1900–2015 using the OVERSEER model. Changes in survey methods, reporting only at district scale, and uncertainty in OVERSEER mean that uncertainty in losses averages ±50%. Nevertheless, there is a clear trend of increasing nitrogen losses arising from the intensification of farming which mimics the trend in measured stream loads. From 1958 to 2015 farm losses doubled and now comprise c. 80% of lake load. OVERSEER provides a quantitative index of the changes in nitrogen losses, which in a companion paper are routed to the lake accounting for groundwater lags and attenuation.

Eutrophication In Lake Rotorua. 2. Using ROTAN and OVERSEER to model historic, present and future nitrogen loads

ABSTRACT OVERSEER is used in New Zealand to estimate nutrient losses from farmland, but does not quantify subsequent movement through the catchment, or attenuation. This paper uses the ROTAN model, based on the Scandinavian HBV-N model, to route nitrogen losses from 1900–2015 to Lake Rotorua where groundwater age ranges from 14 to 170 years. ROTAN conceptualises three delivery pathways (quickflow, groundwater and streamflow) with different attenuation. When calibrated to measured stream and groundwater concentrations, several combinations of attenuation gave equally good fits largely because of sparse and uncertain input and calibration data. Nevertheless, lake N loads were predicted for current land use (754 ± 39 t y−1) and with proposed N loss reductions (431 ± 26 t y−1). Probabilities were also calculated that the reductions are more (12%–18%) or less (82%–88%) than required to meet the target lake N load (405 t y−1). ROTAN shows promise for calculating nitrogen movement in catchments dominated by groundwater where there is limited data.