Robert J. Wilcock

Water quality of a lowland stream in a New Zealand dairy farming catchment

Abstract A small stream in a predominantly dairying catchment in the Waikato region of New Zealand was monitored for 2 years at three sites. Total nitrogen (TN) concentrations were up to 7.09 g m‐3 in winter, with the bulk comprising nitrate nitrogen (NO‐ 3‐N). During summer NO‐ 3‐N was near zero and TN mostly comprised organic nitrogen. Maximum concentrations of total phosphorus (TP) and dissolved reactive phosphorus (DRP) were 1.64 and 0.555 g m‐3, respectively, and peaks coincided with spring and autumn applications of phosphorus fertiliser. Ammoniacal nitrogen concentrations exceeded 1 g m‐3 on several occasions and mean concentrations at the three sites were 0.165–0.272 g m‐3. Faecal coliform and enterococci bacteria concentrations were 64–26000 and 7–23000 cfu per 100 ml, respectively. Specific yields of TN and NO‐ 3‐N (35.3 and 30.7 kg ha yr‐1, respectively) were much greater than any previously reported for New Zealand pasture catchments, whereas TP and DRP yields (1.16 and 0.54 kg ha yr‐1, respec...

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.

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.

Riparian protection and on-farm best management practices for restoration of a lowland stream in an intensive dairy farming catchment: a case study.

Abstract Poor water quality (high concentrations of nitrogen (N), phosphorus (P), suspended solids (SS), and faecal bacteria) in Waiokura Stream, southern Taranaki, New Zealand, is attributed to diffuse and point source (PS) inputs from dairy farming. Trend analysis of concentration time‐series data (2001–2008) and annual yields (i.e., stream load divided by catchment area) showed that significant improvements occurring since 2001 may be attributed to changes in farming practices and riparian management. Yields of filterable reactive P, total P and SS declined by 25–40% as a result of increased riparian protection, a reduction in dairy shed effluent (DSE) pond discharges from 8 to 6 with conversion to land irrigation, and a 25% reduction in the average application rate of P fertiliser. Median annual Escherichia coli concentrations declined at a rate of 116 per 100 ml per year, as a result of fewer PS discharges and improved riparian management. Thus, improvements in stream water quality were attributed to adoption of on‐farm best management practices, fewer DSE discharges and riparian management involving permanent livestock exclusion from stream banks and riparian planting to mitigate runoff from pasture. During 2001–06, N fertiliser use increased by 30% and, with a 130% increase in supplementary cattle feed during 2003–08, led to an increase in average milk solids production 1021 to 1262 kg ha−1 during 2001–06 with the increased production likely associated with increased N leaching losses. Total N and nitrate‐N concentrations and yields increased during 2001–07 as a result of the intensification in land use and increased N cycling. Stream invertebrate surveys using the macroinvertebrate community index (MCI) metric showed little improvement in MCI during 2002–07, probably because of the relatively short timeframe of this study and because water temperatures were not a limiting factor for invertebrate communities. The absence of native forest streams in the proximity of Waiokura Stream that might act as sources of sensitive species to recolonise the restored stream should also be considered as a constraint to improvements in biological community structure.

WATER QUALITY IN A POLLUTED LOWLAND STREAM WITH CHRONICALLY DEPRESSED DISSOLVED OXYGEN : CAUSES AND EFFECTS

Abstract The Whangamaire Stream (North Island, New Zealand) has high concentrations of nitrate nitrogen (NO− 3‐N), biochemical oxygen demand (BOD5), and Kjeldahl nitrogen (TKN) as a result of catchment land use practices. The lower reaches of the stream drain intensively farmed land and have dissolved oxygen (DO) levels of 10–50% saturation. The dominant riparian vegetation, Apium nodiflorum, provides a large organic loading by intercepting nutrients in run‐off and then decaying in the stream channel. Water quality and reaeration aspects of the stream were studied in order to explain the observed low DO levels. Measurements of the reaeration coefficient at 20°C, K2 20, using methyl chloride (CH3Cl) as a gas tracer, yielded values of 1.1–3.0 d−1 for the upper part of the study reach and 15.5–16.2 d−1 for the lower reach (overall average 12.5 ± 2.5 d−1). These were in agreement with values inferred from single‐station diurnal curve analysis, which also showed that respiration was dominant in the lower reach...

Controlled release experiments to determine the effects of shade and plants on nutrient retention in a lowland stream

Understanding nutrient uptake and retention in streams remains an important challenge for lotic scientists. In this study a series of pulse and continuous releases of dissolved nutrients were made to shaded and unshaded (reference) reaches of a small lowland stream to determine whether suppression of macrophyte growth by riparian shade impaired nutrient retention. The nutrients were dissolved reactive phosphorus (DRP), total ammoniacal nitrogen (NH4–N) and nitrate nitrogen (NO3–N). Nutrient reductions ranged from 100% of DRP when stream water was anoxic, to 5–10% for NH4–N and NO3–N in the reference reach. Nutrient removals were affected by travel times in each reach. Percentage removals of NH4–N (46 ± 10) and NO3–N (52 ± 14) were higher in the shaded reach than in the swifter moving reference reach (15 ± 8 and 16 ± 10, respectively). DRP (%) removals were 75± 7 and 57 ± 12 for the shaded and reference reaches, respectively. The presence of emergent marginal macrophytes (Persicaria hydropiper) increased stream velocity in the reference reach by reducing the effective channel cross-section area. Shading reduced plant biomass, increased the channel cross-section and lowered velocity in the experimental reach, effecting dramatic reductions in nutrient concentrations over short distances. The opposite effect is more typical for larger, swifter streams having dense stands of submerged macrophytes, where lowering channel plant biomass will cause increased velocities and lower relative nutrient losses. Riparian shade does not necessarily impair nutrient uptake from small streams. Where invasive marginal species such as P. hydropiper dominate headwater streams shade may be beneficial to the protection of downstream waters from eutrophication. Where reduction of nutrient fluxes from small streams is a key objective for protection of downstream waters, active management of streams should seek to increase travel times, allowing greater potential for nutrient uptake. This will need to be weighed against the need for effective drainage in pastoral areas where reduced travel times are usually sought.

Trends in water quality of five dairy farming streams in response to adoption of best practice and benefits of long-term monitoring at the catchment scale

Five streams in catchments with pastoral dairy farming as the dominant land use were monitored for periods of 7–16 years to detect changes in response to adoption of best management practices (BMPs). Stream water quality was degraded at the start with respect to N, P, suspended solids (SS) and E. coli concentrations, and was typical of catchments with intensive pastoral agriculture land use. Trend analysis showed a decrease in SS concentration for all streams, generally increasing water clarity, and lower E. coli concentrations in three of the streams. These are attributed to improved stream fencing (cattle exclusion) and greater use of irrigation for treated effluent disposal with less reliance on pond systems discharging to streams. Linkages between water quality and farm actions based on survey data were used to develop BMPs that were discussed at stakeholder workshops. Generic and specific BMPs were developed for the five catchments. The 3–7 year periodicity of major climate cycles, as well as market forces and a slow rate of farmer adoption of simple BMPs mean that monitoring programs in New Zealand need to be much longer than 10 years to detect changes caused by farmer actions. Long-term monitoring is also needed to detect responses to newly legislated requirements for improved water quality.

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.

Inputs of Nutrients and Fecal Bacteria to Freshwaters from Irrigated Agriculture: Case Studies in Australia and New Zealand

Increasing demand for global food production is leading to greater use of irrigation to supplement rainfall and enable more intensive use of land. Minimizing adverse impacts of this intensification on surface water and groundwater resources is of critical importance for the achievement of sustainable land use. In this paper we examine the linkages between irrigation runoff and resulting changes in quality of receiving surface waters and groundwaters in Australia and New Zealand. Case studies are used to illustrate impacts under different irrigation techniques (notably flood and sprinkler systems) and land uses, particularly where irrigation has led to intensification of land use. For flood irrigation, changes in surface water contaminant concentrations are directly influenced by the amount of runoff, and the intensity and kind of land use. Mitigation for flood irrigation is best achieved by optimizing irrigation efficiency. For sprinkler irrigation, leaching to groundwater is the main transport path for contaminants, notably nitrate. Mitigation measures for sprinkler irrigation should take into account irrigation efficiency and the proximity of intensive land uses to sensitive waters. Relating contaminant concentrations in receiving groundwaters to their dominant causes is often complicated by uncertainty about the subsurface flow paths and the possible pollutant sources, viz. drainage from irrigated land. This highlights the need for identification of the patterns and dynamics of surface and subsurface waters to identify such sources of contaminants and minimize their impacts on the receiving environments.

Climate Change Mitigation for Agriculture: Water Quality Benefits and Costs

New Zealand is unique in that half of its national greenhouse gas (GHG) inventory derives from agriculture--predominantly as methane (CH4) and nitrous oxide (N2O), in a 2:1 ratio. The remaining GHG emissions predominantly comprise carbon dioxide (CO2) deriving from energy and industry sources. Proposed strategies to mitigate emissions of CH4 and N2O from pastoral agriculture in New Zealand are: (1) utilising extensive and riparian afforestation of pasture to achieve CO2 uptake (carbon sequestration); (2) management of nitrogen through budgeting and/or the use of nitrification inhibitors, and minimizing soil anoxia to reduce N2O emissions; and (3) utilisation of alternative waste treatment technologies to minimise emissions of CH4. These mitigation measures have associated co-benefits and co-costs (disadvantages) for rivers, streams and lakes because they affect land use, runoff loads, and receiving water and habitat quality. Extensive afforestation results in lower specific yields (exports) of nitrogen (N), phosphorus (P), suspended sediment (SS) and faecal matter and also has benefits for stream habitat quality by improving stream temperature, dissolved oxygen and pH regimes through greater shading, and the supply of woody debris and terrestrial food resources. Riparian afforestation does not achieve the same reductions in exports as extensive afforestation but can achieve reductions in concentrations of N, P, SS and faecal organisms. Extensive afforestation of pasture leads to reduced water yields and stream flows. Both afforestation measures produce intermittent disturbances to waterways during forestry operations (logging and thinning), resulting in sediment release from channel re-stabilisation and localised flooding, including formation of debris dams at culverts. Soil and fertiliser management benefits aquatic ecosystems by reducing N exports but the use of nitrification inhibitors, viz. dicyandiamide (DCD), to achieve this may under some circumstances impair wetland function to intercept and remove nitrate from drainage water, or even add to the overall N loading to waterways. DCD is water soluble and degrades rapidly in warm soil conditions. The recommended application rate of 10 kg DCD/ha corresponds to 6 kg N/ha and may be exceeded in warm climates. Of the N2O produced by agricultural systems, approximately 30% is emitted from indirect sources, which are waterways draining agriculture. It is important therefore to focus strategies for managing N inputs to agricultural systems generally to reduce inputs to wetlands and streams where these might be reduced to N2O. Waste management options include utilizing the CH4 resource produced in farm waste treatment ponds as a source of energy, with conversion to CO2 via combustion achieving a 21-fold reduction in GHG emissions. Both of these have co-benefits for waterways as a result of reduced loadings. A conceptual model derived showing the linkages between key land management practices for greenhouse gas mitigation and key waterway values and ecosystem attributes is derived to aid resource managers making decisions affecting waterways and atmospheric GHG emissions.