Lake Rotorua Research Articles

Natural and anthropogenic lead in sediments of the Rotorua lakes, New Zealand

Global atmospheric sources of lead have increased more than 100-fold over the past century as a result of deforestation, coal combustion, ore smelting and leaded petroleum. Lead compounds generally accumulate in depositional areas across the globe where, due to low solubility and relative freedom from microbial degradation, the history of their inputs is preserved. In lakes there is rapid deposition and often little bioturbation of lead, resulting in an excellent depositional history of changes in both natural and anthropogenic sources. The objective of this study was to use sediments from a regionally bounded set of lakes to provide an indication of the rates of environmental inputs of lead whilst taking into account differences of trophic state and lead exposure between lakes. Intact sediment gravity cores were collected from 13 Rotorua lakes in North Island of New Zealand between March 2006 and January 2007. Cores penetrated sediments to a depth of 16–30 cm and contained volcanic tephra from the 1886 AD Tarawera eruption. The upper depth of the Tarawera tephra enabled prescription of a date for the associated depth in the core (120 years). Each core showed a sub-surface peak in lead concentration above the Tarawera tephra which was contemporaneous with the peak use of lead alkyl as a petroleum additive in New Zealand. An 8 m piston core was taken in the largest of the lakes, Lake Rotorua, in March 2007. The lake is antipodal to the pre-industrial sources of atmospheric lead but still shows increasing lead concentrations from <2 up to 3.5 μg g − 1 between the Whakatane eruption (5530 ± 60 cal. yr BP) and the Tarawera eruption. Peaks in lead concentration in Lake Rotorua are associated with volcanic tephras, but are small compared with those arising from recent anthropogenic-derived lead deposition. Our results show that diagenetic processes associated with iron, manganese and sulfate oxidation-reduction, and sulfide precipitation, act to smooth distributions of lead from anthropogenic sources in the lake sediments. The extent of this smoothing can be related to changes in sulfate availability and reduction in sulfide driven by differences in trophic status amongst the lakes. Greatest lead mobilisation occurs in mesotrophic lakes during seasonal anoxia as iron and manganese are released to the porewater, allowing upward migration of lead towards the sediment–water interface. This lead mobilisation can only occur if sulfides are not present. The sub-surface peak in lead concentrations in lake sediments ascribed to lead alkyl in petroleum persists despite the diagenetic processes acting to disperse lead within the sediments and into the overlying water.

Estimation of total cloud cover from solar radiation observations at Lake Rotorua, New Zealand

The DYRESM-CAEDYM model is a valuable tool for simulating water temperature for biochemical studies in aquatic ecosystem. The model requires inputs of surface short-wave radiation and long-wave radiation or total cloud cover fraction ( TC). Long-wave radiation is often not measured directly so a method to determine TC from commonly measured short-wave solar irradiance ( E 0) and theoretical short-wave solar irradiance under a clear sky ( E c) has broad application. A more than 17-year (15 November 1991 to 20 February 2009) hourly solar irradiance data set was used to estimate the peak solar irradiance for each ordinal date over one year, which was assumed to be representative of solar irradiance in the absence of cloud. Comparison between these daily observed values and the modelled clear-sky solar radiation over one year was in close agreement (Pearson correlation coefficient, r = 0.995 and root mean squared error, RMSE = 12.54 W m −2). The downloaded hourly cloudiness measurements from 15 November 1991 to 20 February 2009 was used to calculate the daily values for this period and then the calculated daily values over the 17 years were used to calculate the average values for each ordinal date over one year. A regression equation between (1 − E 0/ E c) and TC produced a correlation coefficient value of 0.99 ( p > 0.01, n = 71). The validation of this cloud cover estimation model was conducted with observed short-wave solar radiation and TC at two sites. Values of TC derived from the model at the Lake Rotorua site gave a reasonable prediction of the observed values (RMSE = 0.10, r = 0.86, p > 0.01, n = 61). The model was also tested at Queenstown (South Island of New Zealand) and it provided satisfactory results compared to the measurements (RMSE = 0.16, r = 0.67, p > 0.01, n = 61). Therefore the model’s good performance and broad applicability will contribute to the DYRESM-CAEDYM accuracy of water temperature simulation when long-wave radiation is not available.

Managing the lakes of the Rotorua District, New Zealand

In 2005, Burns, McIntosh and Scholes described strategies to manage the Rotorua Lakes using lake monitoring together with designated baseline Trophic Level Index values established for each lake. Continued monitoring has revealed that 9 of the 12 Rotorua Lakes have Trophic Level Index values in excess of their baseline values. Action Plans have been drawn up for the remediation of these damaged lakes that specify the excess nutrient loading to each lake and propose actions for the decrease of these loadings. Nutrient loading to various lakes has been decreased by upgrading waste treatment facilities, dosing tributary streams with alum, diverting an enriched tributary flow directly into the outflow channel of a lake, precipitating in-lake phosphorus with Phoslock™ and zeolite additions, and removal of macrophyte biomass from a lake and planting an artificial wetland at the entry point of a tributary to a lake. Where data are available, the results of these actions are explored. The similarities between the management system for the Rotorua lakes with the management systems used for two American and European lakes are described.

Sedimentology and allostratigraphy of post-240 ka to pre-26.5 ka lacustrine terraces at intracaldera Lake Rotorua, Taupo Volcanic Zone, New Zealand

Lake Rotorua partially occupies a nearly circular 20 km diameter volcano-tectonic depression formed at c. 240 ka by eruption of the voluminous Mamaku Ignimbrite. Three distinct lacustrine littoral terraces, defined on the basis of contrasting geomorphology and field relations, and separated by tephrostratigraphically dateable unconformities and basin-floor disconformities, fringe much of the lake basin. They are here correlated with former high-stands of the lake which resulted from the blockage and re-establishment of a number of alternative outlets due to tectonic activity and volcanism at both the host and adjacent volcanic centres. The unconformities allow division of the deposits into three allostratigraphic units, each of which is then characterised by elevation and sediment provenance. The < 240 ka, post-Mamaku alloformation comprises the highest terrace (up to 415 mASL), and represents the high-stand of an intracaldera lake accumulated in the newly created basin after the eruption of the Mamaku Ignimbrite. Considerable uncertainty surrounds the initial direction of overflow from this level, but the lake may have drained southwards for a period through the Hemo Gorge, through the Ngakuru Graben/Kapenga Caldera area and into the Waikato River catchment. The second alloformation, consisting of volcaniclastic sediments forming shoreline and littoral terraces at c. 380 m elevation developed after the eruption of the 60 ka Rotoiti/Earthquake Flat pyroclastic flows from the neighbouring Okataina Volcanic Centre blocked northern and southern routes out of the lake basin. A northeasterly outlet subsequently became established at a lower level through tectonic subsidence of the Tikitere Graben, creating a drainage path into the Haroharo caldera from where it flowed into the Bay of Plenty via the Kawerau Canyon. The post-36 ka Hauparu alloformation forms the third shoreline terrace at elevations up to 349 mASL. It is the product of a temporary high-stand from blockage of the Tikitere Graben drainage path by pyroclastic debris from the voluminous 36 ka Hauparu eruption. Subsequently, episodic growth of the Haroharo resurgent dome complex between 25 and 9 ka in the adjacent Okataina Volcanic Centre forced Lake Rotorua to rise above its post-Hauparu lowstand level to an elevation where it could overtop a drainage divide on the northern rim of Lake Rotoiti and gain access to the catchment of the Kaituna River, hence establishing the current outlet channel.

Icon Lakes in New Zealand: Managing the Tension Between Land Development and Water Resource Protection

The increasing intensity of land development in New Zealand has accelerated the decline in the environmental quality of the nation's lakes. The declining water quality of iconic lakes that have helped to define the nation's cultural identity is a cause for concern. The causal link between agricultural development and water quality decline raises concerns about the role that another component of the nation's identity—farming—should play in the country's future. New Zealand, like many other countries, faces some difficult trade-offs between the economic benefits of continuing to intensify land use practices versus the adverse environmental costs to the land and water environment. Reversing the decline in lake water quality requires partnerships among government, landowners, and the community. At New Zealand's iconic Lake Rotorua a partnership approach is being established that may signal a new direction for the sustainable development of natural resources.

Nutrient Trading in Lake Rotorua: Overview of a Prototype System

Water quality in Lake Rotorua has been declining for at least the last 30 years as increased levels of nutrients have entered the lake. Despite significant effort and expenditure, the level of nutrients entering the lake still exceeds sustainable levels. A nutrient trading system would help the catchment achieve this goal at least cost. Nutrient sources would bear the cost of their impact on water quality and hence take these costs into account in their decision-making. This paper presents a prototype nutrient trading system for achieving cost effective nutrient loss reductions for the Lake Rotorua catchment.

Nutrient Trading in Lake Rotorua: Determining Net Nutrient Inputs

Lake Rotorua is experiencing increasing nutrient-related water quality problems. This paper is one in a series that explores the idea of creating a nutrient trading system as part of the ongoing policy response to this problem.1 Most of the current nutrient flows to the Lake come from non-point rural sources - measuring these emissions is challenging. We find that it is possible to monitor/model nutrient loss from a wide range of activities in the Rotorua catchment. The model OVERSEER combined with ROTAN and some other models for forestry, urban and geothermal activities and horticulture already exist. They are currently in a process of enhancement - a particular area of current weakness is knowledge of the groundwater lags from specific locations in the catchment. The land-based models need to be used in a specific form that relies on initialisation with verifiable data and uses easily collated and verified data on an annual basis. The form of the model should be fixed for each regulatory year to minimise uncertainty for landowners and regulators. The models need to be updated to reflect new science. The process for doing this needs to be strategic and credible (this will be discussed in a later paper on governance processes). Once changes are recommended they need to be implemented in a way that is perceived to be fair.

Nutrient Trading in Lake Rotorua: Choosing the Scope of a Nutrient Trading System

A nutrient trading system is one mechanism that is currently being considered to control and reduce nutrient loss into Lake Rotorua. However this may not be the best mechanism for controlling nutrient loss from all sources. A more comprehensive system improves efficiency and decreases market power opportunities, but it can also bring increased compliance and administrative costs. This paper discusses which sources should be included in a nutrient trading system for Lake Rotorua including. It examines existing systems and presents an empirical analysis to estimate the impact of including different nutrient sources.

Preface: Soils &amp; Society 2006 Conference—A short report and an introduction to special issue

The soil resource affects all aspects of New Zealand life, and is recognised as a living substrate that supports growth and filters waste. The resource needs to be protected and nourished so that it will continue to look after us long into the future. The NewZealand Society of Soil Science (NZSSS) organised a conference with the conference theme “Soils & Society”. This encompassed the economic, environmental, social, and cultural aspects of soil science. Rotorua was chosen as the conference venue and is an ideal location due to the growing local emphasis on water quality stemming from soil management issues. The water quality of Rotorua Lakes has become one of the important national environmental issues in New Zealand. Local and central government agencies are looking for solutions to this problem. It was expected that conference delegates from a wide range of soil-related disciplines in New Zealand – including universities, research institutes, Government departments, analytical services, the fertiliser industry and regional councils, as well as representatives from agricultural, horticultural and forestry businesses – would look at water quality issues from different angles (soil management, waste management, farm nutrients, policy issues, etc.). In addition, the conference provided a forum for discussion of a wide range of current soil research topics. The conference was held in Rydges Hotel, Rotorua, from 27 to 30 November 2006. This is the first time ever that the New Zealand Society of Soil Science had organised the conference in a hotel, and to some extent, it helped to raise the profile of our Society. The conference organising committee was formed in mid 2005, to plan, organise and execute a memorable conference. The conference managers, Innovatek Ltd, played a key role in the running of the conference.

Modelling the relative importance of internal and external nutrient loads on water column nutrient concentrations and phytoplankton biomass in a shallow polymictic lake

Lake Rotorua is a large (area 79 km 2), shallow (mean depth 10.8 m), polymictic lake in central North Island, New Zealand. The lake is eutrophic, with a mean external aerial load of 18.5 mg m −2 d −1 for total nitrogen and 1.2 mg m −2 d −1 for total phosphorus. Blooms of cyanobacteria and occasional anoxia of bottom waters occur during summer (December–March). We used a vertically resolved water quality model, DYRESM–CAEDYM, to examine the relative importance of internal and external nutrient inputs on water column nutrient concentrations and phytoplankton biomass, with particular emphasis on cyanobacteria. External nutrient loads associated with nine major inflows to the lake and three additional inflows representing smaller geothermal and coldwater flows and residual flows, were represented as inputs to the model. Other forcing inputs to the model included local meteorological data, discharge from the only outflow, the Ohau Channel, and measured rates of sediment nutrient release obtained from benthic chamber deployments which were used to prescribe ranges of sediment nutrient release that were simulated dynamically within the model. Profiles of water column nutrient concentrations, surface chlorophyll a concentrations and continuous temperature and dissolved oxygen measurements were used to validate the model. Simulated water column temperature and soluble reactive phosphorus (SRP) and ammonium (NH 4) concentrations closely matched field measurements, and captured the timing and duration of stratification events as well as subsequent changes in bottom water nutrient concentrations. Surface water concentrations of chlorophyll a were also similar between simulated and observed data. Model simulations indicate that reductions in sediment nutrient fluxes would be more effective in reducing cyanobacterial biomass than similar proportional reductions in catchment fluxes, due to the coincidence of large sediment nutrient release events with high cyanobacterial biomass. This finding indicates that only a significant and prolonged reduction in external loads, which in turn reduces internal loads, will ultimately reduce cyanobacterial biomass in Lake Rotorua.

Nutrient Trading in Lake Rotorua: Goals and Trading Caps

For a nutrient trading system to achieve the desired environmental outcome, or goal, this outcome needs to be translated into nutrient flows and allowances. To connect the nutrient loss provided for under the allowances with the environmental goal, a number of decisions need to be made. These decisions will shape the nutrient trading system. This paper looks at the information and analysis needed to ultimately define allowances and set trading caps for a nutrient trading system.

Nutrient Trading in Lake Rotorua: Where Are We Now?

A number of decisions need to be made when setting up a nutrient trading system including defining a target, allocating allowances and setting up a monitoring system. To ensure that the nutrient trading system implemented operates in harmony with existing regulation, existing work and institutions need to be used to guide this decision making process. This paper briefly explores each of the decisions required to implement a nutrient trading system and to what extent they have been addressed so far. This will provide context for following papers which will examine each issue in more depth.

Towards Design for a Nutrient Trading Programme to Improve Water Quality in Lake Rotorua

This paper explores how to enhance the role for academic research (natural sciences, economics and their integration; and stakeholder management) within the development and implementation of water quality policy in New Zealand. Our focus is on the use of market based instruments and particularly nutrient trading programmes, which are one important part of the potential tool kit to address these issues. We discuss why nutrient trading might be an appropriate instrument for the Lake Rotorua catchment. We survey the existing literature and then outline the outstanding scientific, economic and governance questions that need to be addressed to design an effective trading programme. Finally we discuss how to design a process to address these questions drawing on both technical and practical knowledge through a learning process.

Phytoplankton nutrient limitation in a polymictic eutrophic lake: community versus species-specific responses

Phytoplankton nutrient limitation was examined in polymictic, eutrophic Lake Rotorua. New Zealand using three in situ incubation experiments of 4 to 6 days' duration in summer 2004. Two of the incubations were conducted during stratification, and one immediately after breakdown of stratification. Samples were enriched with ammonium (1 mg NH 4 -N L -1 ), phosphate (0.1 mg PO 4 -P L -1 ) or with both nutrients. A control containing no added nutrients was used for comparison. Phytoplankton responses to nutrient additions were determined at a species level from cell counts and at a community level from changes in chlorophyll-a concentration. Phytoplankton biomass generally responded to N plus P additions to a greater extent than with single nutrient additions, though results were often not statistically significant. Increase in community biomass was greater for P than N, and nutrient demand decreased after breakdown of stratification. Individual species responded differently to N and P additions, suggesting co-limitation within the phytoplankton species assemblage. A simple, dynamic phytoplankton growth limitation model, with time-varying responses to hourly inputs of temperature, nutrients and light, was applied to consider the interacting effects of phosphorus (P), nitrogen (N) and light. Model results indicate that light plays a major role in regulating phytoplankton biomass, although there were periods of P limitation. For the management of water quality in Lake Rotorua it is suggested that simultaneous reductions in external inputs of both N and P is mostly likely to reduce phytoplankton biomass, including cyanobacterial populations.

Comparison of carbon dioxide emissions with fluid upflow, chemistry, and geologic structures at the Rotorua geothermal system, New Zealand

During 2002 and 2003, carbon dioxide fluxes were measured across the Rotorua geothermal system in the Taupo Volcanic Zone (TVZ), New Zealand. The results of a 956-measurement survey and of modeling studies show that CO 2 fluxes could be used to determine the main hot fluid upflow areas in Rotorua, and perhaps in undeveloped geothermal regions. Elevated degassing was observed along inferred fault traces and structures, lending confidence to their existence at depth. Degassing was also observed along lineaments that were consistent with the alignment of basement faulting in the TVZ. Areas where elevated degassing was spatially extensive typically overlapped with known regions of hot ground; however, elevated CO 2 fluxes were also observed in isolated patches of non-thermal ground. The total emission rate calculated from sequential Gaussian simulation modeling of CO 2 fluxes across the geothermal system was 620 t d −1 from an 8.9-km 2 area. However, because approximately one-third of the geothermal system is known to extend beneath Lake Rotorua, we expect the emissions could be minimally on the order of 1000 t d −1. Comparing the emission rate with geochemical analyses of geothermal fluids and estimated upflows suggests that the majority of deep carbon reaches the surface in the form of carbon dioxide gas, and that less than one tenth of the CO 2 emissions is dissolved in, or released from, the fluids at depth. Thus, the geothermal reservoir exerts very little control on deep degassing of CO 2. Carbon isotopic analyses of soil gases suggest a primarily magmatic source for the origin of the CO 2. The total Rotorua emission rate is comparable to those from active volcanoes such as at White Island, New Zealand, and, when normalized by geothermal area, is comparable to other volcanic and hydrothermal regions worldwide.

Phosphorus leaching from pastures can be an environmental risk and even a significant fertiliser expense

While it is true that leaching is usually not a strong pathway for phosphorus (P) loss under many systems, is it true for all? Two studies reported in this paper sought to establish if significant phosphorus leaching can occur under normal pastoral production systems. Undisturbed-core lysimeters collected from a Wharekohe silt loam from Northland were treated with fertiliser P (reactive phosphate rock and superphosphate) then leached from August to November, 2005. In a second study, soil profiles under pasture for sheep/beef and dairy production in the catchments of the Rotorua lakes were sampled to depths of 1.5 m (28 sites), and soil Olsen P and P retention capacity index were determined down these profiles. Phosphorus losses from the Wharekohe soil to 25 cm depth were up to 33% of the P applied (superphosphate applications of 50 and 100 kg P/ha). Some Rotorua soils displayed enriched P concentrations at depth (to 1.5 m), often coupled with moderate or low P sorption capacities. If connectivity exists between leaching pathways and surface water bodies these observations indicate that alternative management strategies need to be developed and adopted for soils that leach significant quantities of P. The Wharekohe silt loam is one such soil. Keywords: phosphorus, leaching

Sediment phosphorus release during stratification in polymictic Lake Rotorua, New Zealand

Sediment phosphorus release during stratification in polymictic Lake Rotorua, New Zealand

Earthquake focal mechanisms in the central Taupo Volcanic Zone and their relation to faulting and deformation

The New Zealand Taupo Volcanic Zone (TVZ) is an area with considerable crustal seismicity, but the shallowness of the seismicity has made it difficult to determine good focal mechanisms from the permanent seismic stations. Within the central TVZ, the only focal mechanisms previously available were composite ones derived from earthquake swarms. These mechanisms were dominantly strike‐slip, rather than the normal mechanisms that might be expected from the dominant southeast‐northwest extension seen from deformation measurements and geology. The 1987 ML 6.3 Edgecumbe earthquake near the northern end of the zone had a normal mechanism consistent with the deformation, as did some large earthquakes just offshore. We present focal mechanisms obtained from a 6 month (Jan‐Jun 1995) seismometer deployment consisting of 64 short‐period and 23 broadband seismometers together with the 15 permanent stations within the TVZ. This close‐spaced network enabled us to get focal mechanisms for a considerable number of crustal earthquakes. We present here 23 TVZ mechanisms, mainly for events around the Rotorua lakes, and southwestwards towards Lake Taupo, together with one strike‐slip mechanism for an earthquake east of the TVZ. The focal mechanisms are mainly normal, consistent with the observed extension, although most mechanisms have a strike‐slip component. Three earthquakes appeared to have thrust mechanisms, but a non‐double‐couple solution is also a possibility for two of these events. Even within a single swarm, earthquakes with normal focal mechanisms had considerable variation in the strike direction. These variations suggest that the high pore pressure has produced a very low strength region in which slip can occur in a range of directions. These normal mechanisms make unnecessary the model of extension by conjugate strike‐slip events proposed by Smith & Webb in 1986. We also carried out a direct stress inversion from the first‐motions of a larger set of earthquakes. The principal stress directions obtained by this method agree with the average for the 19 normal earthquakes, and both support extension in a northwest‐southeast direction, in general agreement with the geodetic results.

Population dynamics of reintroduced forest birds on New Zealand islands

AimWe studied dynamics of four populations of New Zealand forest birds for 5–9 years after reintroduction to islands. We primarily aimed to predict whether these populations were viable, and what, if any, management was needed to maintain them. However, the small scale of these islands also provided an opportunity to study density‐dependent population growth over a short time frame.LocationWe studied New Zealand robin (toutouwai,Petroica australis) and stitchbird (hihi,Notiomystis cincta) populations reintroduced to Tiritiri Matangi, a 220‐ha offshore island near Auckland, and saddleback (tieke,Philesturnus carunculatus) and stitchbird populations reintroduced to Mokoia, a 135‐ha island in Lake Rotorua. These islands are free of mammalian predators, but have highly modified habitat following clearing and regeneration.MethodsWe closely monitored each population, individually marking all or most of the birds and in some cases experimentally manipulated population density or food supply. We used model selection procedures to understand factors affecting survival, fecundity and dispersal, and developed stochastic simulation models.ResultsThe Tiritiri Matangi robin and Mokoia saddleback populations grew without management and appear to be viable. Both showed strong evidence of density‐dependent growth, with fecundity (saddlebacks) and juvenile survival (both populations) declining with increasing density. Neither stitchbird population appears viable without management and supplementation experiments showed reproduction and/or survival to be limited by food supply. The Tiritiri Matangi population appears viable as long as supplementary feeding continues. However, the Mokoia population has a high mortality rate regardless of supplementary feeding, resulting in tenuous viability even with intensive management. Mokoia stitchbirds suffer from infection byAspergillus fumigatus, a pathogenic fungus that is prevalent in highly modified habitats and more abundant on Mokoia than Tiritiri Matangi.Main conclusionsSome forest birds can thrive in regenerating forest on islands and strong evidence of density dependence can be detected in such populations in as little as 5 years. This allows density‐dependent models to be developed, providing guidance when island populations are harvested for further translocations. Other species are limited by food supply in regenerating environments, a problem potentially overcome by management. However, prevalence ofA. fumigatusmay render highly modified environments uninhabitable by some species regardless of management.

Land use in the Taupo catchment, New Zealand

This paper describes current initiatives to develop a lake management plan for Lake Taupo, New Zealand. Recently there has been an indication that a shift from dry stock farming to large‐scale dairy farming will occur in the Taupo catchment. The local community has expressed an interest in this shift towards more intensive land use. There is public concern regarding the current state of Lake Taupo's water quality and the ability to maintain environmental quality in the face of increasing development pressures. Concerns about rising levels of nutrients in the lake are supported by trend analysis of lake water quality monitoring data. The Taupo District Council and the Waikato Regional Council have proposed development of a Lake Taupo Management Plan. A number of mechanisms for achieving this process could be explored. These include a plan largely developed by an inter‐agency management group or a more public participatory approach directly involving the community. A general lake planning framework is described and some of the limitations of the process explored. Comparison with experiences at Lake Rotorua, New Zealand, and Lake Tahoe, United States of America, supports a pre‐cautionary approach to the use of predictive ecological models in policy development. The inability of aquatic ecological theory to conform to quantitative approaches in other physical sciences needs to be acknowledged by scientists and conveyed to the public. Until the link between land‐use practice and water quality is better understood, a pre‐cautionary approach to catchment development at Lake Taupo should be encouraged. One cannot assume that common desires for both environmental protectionism, and progressive intensification of land use, will lead to sustainable resource management. The environmental effects of dairy farming indicate that encouraging this land‐use practice will not lead to the desired community expectation of water quality protection at Lake Taupo.