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Executive summary

Purpose and methodology

Dr Bruce Hamilton was engaged to evaluate the range of current, proposed and potential actions to manage the short-term symptoms and causes of water quality problems in Lakes Rotorua and Rotoiti and provide a written report to the Secretary for the Environment. This work follows considerable concern about large blooms of blue-green algae, particularly in Lake Rotoiti over the summer of 2002-2003.

The work was undertaken by working closely with management, technical and scientific staff in Environment Bay of Plenty (EBOP), Rotorua District Council (RDC), the University of Waikato, the National Institute of Water and Atmospheric Research and officers from relevant central government agencies. In addition, a series of meetings with local community and industry groups was organised by Paul Dell from EBOP (Group Manager Regulation and Resource Management and Lakes Project Manager), and these provided valuable advice and information.

Most of the recommendations for both vital short-term investigations and short-term management measures are set out in detail in section 7, while all of the recommendations have been summarised together for ease of reference after this Executive Summary. The numbers of two recommendations that arise before section 7 are noted in the Executive Summary below while the remainder are noted against a summary of the discussion from section 7 in the last part of the Executive Summary.

Sources of information and context for the recommendations

Nearly all of the information on which this report is based was obtained directly from EBOP, and people and organisations working for EBOP. In the former case, the staff and management of EBOP were extraordinarily generous in providing information and assisting the author. In the latter case, special thanks are due to a number of talented scientists from the National Institute for Water and Atmospheric Research and Professor David Hamilton from the University of Waikato.

EBOP has done and is doing a large amount of work on the Rotorua lakes, including Lakes Rotorua and Rotoiti, and it was a privilege for the author to work with the very professional team from EBOP. In this context, a number of the recommendations in this report are already underway, or have begun to be investigated, or have been implemented since the report was written.

Accordingly, it is difficult to separate those recommendations that are entirely new, so the author's intention is to support work that is already being done by EBOP and provide an 'adaptive management' and 'systems' context to assist EBOP in its further management initiatives and working with the community.

EBOP is to be commended on its leadership and overall approach to managing the Rotorua lakes and this excellent approach is summarised in the body of the report. EBOP is also to be commended for the way it has partnered with the Māori people at the highest levels and with the RDC, which has an important role to play in the management of the Rotorua lakes.

Significance of the Rotorua lakes

Rotorua is the most visited tourist destination in New Zealand and the lakes are a focal point for tourists and locals for recreation and trout fishing. Lakes Rotorua and Rotoiti are especially important being the two biggest lakes with the most settlement around them. Lake Rotoiti is famous for the large 'trophy' trout that are caught and is a favourite place for holiday homes.

Thus, the blue-green algal blooms, which closed Lake Rotoiti for several weeks in early 2003, were seen as a real indication that the lake was in trouble and many local people called for an urgent response from the 'authorities'. There is no doubt that these lakes are very important for New Zealand and are of national significance.

Leadership and responsibility for lakes management - Environment Bay of Plenty and Rotorua District Council

Environment Bay of Plenty (EBOP) is one of 16 regional councils in New Zealand charged with integrated management of land and water resources under the Resource Management Act 1991. Rotorua District Council (RDC) is the local government body for the Rotorua district where the lakes are situated. These two bodies together have the responsibility for management of the lakes with EBOP providing overall leadership and co-ordination.

EBOP has taken a strategic approach to its leadership role, establishing a series of policies and plans which set out the basis for protection and management of the Rotorua lakes. These are briefly summarised below and set out in more detail in the body of the report.

1) The Proposed Regional Water and Land Plan sets out how EBOP will collaboratively manage the land and water resources of the region. It addresses issues relating to the adverse environmental effects of the use and development of land, water and geothermal resources that are within the scope of EBOP's functions and responsibilities under the Resource Management Act 1991.

Considerable attention is paid to the strong relationship that the Māori people have with the land and the concepts of kaitiakitanga (guardianship) and mauri (life force or spirit). This is important because the Māori people need to be fully involved in all stages of the investigations and implementation of any management measures.

The Water and Land Plan sets Trophic Level Index (TLI) targets for each lake, which are a useful measure of the trophic status (level of nutrient enrichment and plant growth) for the lakes. Each TLI target provides a good goal for the community to work together and aim for. The current TLI levels and targets for each lake are shown in Appendix 2.

2) The next level of policy and planning is for the Rotorua lakes as a whole and is contained in the Strategy for the Lakes of the Rotorua district -'Te Kaupapa mo Nga Taonga o Rotorua'(Protecting the Jewels in the Crown of the Lakes of the Rotorua district). This important document, which was developed and adopted by EBOP, RDC and the Te Arawa Māori Trust Board, sets out a vision, goals and priorities for protection and restoration of the Rotorua lakes.

The vision for the lakes is:

The lakes of the Rotorua district and their catchments are preserved and protected for the use and enjoyment of present and future generations, while recognising and providing for the traditional relationship of Te Arawa with their ancestral lakes.

Fourteen key goals are set under the headings of Protection, Use, Enjoyment and Management Goals, and the Strategy contains a 'Schedule of Tasks' for each goal.

Overall co-ordination and direction for the Strategy is vested in the Lakes Strategy Joint Committee comprising the Chair of the Te Arawa Māori Trust Board, the Chair of Environment Bay of Plenty and the Mayor of RDC, plus a further representative from each of the three organisations. This key body ensures all of the work is done cost effectively and collaboratively. Paul Dell from EBOP has been appointed as the Co-ordinator.

3) Implementation is being co-ordinated through the Rotorua Lakes Restoration and Protection Programme, which is shown in Figure 2 in the report. This brings together the large number of elements that are being undertaken, from ongoing research to education and communication. Currently, it is a 'work in progress' as work is added and changed, but it would be useful if the Rotorua Lakes Restoration and Protection Programme was documented to show the practical side of EBOP's overall approach and how the community, the research organisations and government are working together. (Recommendation 1)

4) Lake Action Plans are the next step where the current TLI exceeds the target TLI. These Action Plans bring the community together to identify the preferred management options. Each Action Plan defines the existing catchment nutrient budget, determines what level of nutrient inputs are sustainable, identifies agreed nutrient reduction targets and determines actions to achieve the agreed targets.

Lakes Rotorua and Rotoiti will have a single Action Plan because of the way they are inextricably linked by the Ohau Channel.

5) Tracking the health of the Rotorua lakes is through EBOP's 'Water QualityMonitoring Programme', which started in 1990 and builds on previous monitoring and studies that began in the late 1960s. The results of the programme to 2002 are summarised in a valuable document, Rotorua Lakes Water Quality 2002 (Ref 3), which shows the different parameters that have been measured, including oxygen levels, total phosphorus and nitrogen, chlorophyll a, algal species in the lakes, and nutrient loads and concentrations from the catchments. Lake Quality Indicators, including the TLI and oxygen levels, are used to report on the health of the lakes each year, but additional water quality indicators that are more closely linked to the processes causing algal blooms would be useful to complement the longer-term health indicators. (Recommendation 2)

The condition of Lakes Rotorua and Rotoiti

Appendix 2 shows that the five worst lakes according to their TLI are Okaro, Rotorua, Rotoehu, Rotoiti and Okareka and these are EBOP's priority for action. Professor David Hamilton, who holds the EBOP chair of Lakes Management and Restoration at Waikato University, has analysed all of the historical data and classified the lakes into four categories based on mixing regimes and dissolved oxygen as shown in Table 2 in the report.

This analysis has shown that there has been a steady decline over the last 30 to 40 years in a number of the lakes as shown by the depletion of oxygen in the bottom layers. Lakes Okaro and Rotoiti are the worst but Lake Rotorua is also in poor condition, and because it is shallower, it contributes significant inputs of nutrients to Lake Rotoiti during periods of stratification and destratification over summer.

Thus it is not surprising that both Lakes Rotorua and Rotoiti are suffering problems and that Lake Rotoiti is showing the greatest signs of stress.

Whole systems, understanding and adaptive management

Appendix 3 is a foldout diagram, which attempts to portray a whole systems approach by showing the way the inputs of nutrients and other materials from a catchment interact in Lake Rotorua and are carried through the Ohau Channel to Lake Rotoiti. While this report is primarily about short-term management measures to tackle the blue-green algal blooms, it is important to first look at how the whole system works and whether there is enough understanding of the various interactions to move to management.

Appendix 3 shows that two levels of understanding are needed; an understanding of the processes in the catchments that cause the nutrient inputs to the lakes and an understanding of the processes in the lakes that mobilise the nutrients and cause the algal blooms. This does not mean that every process has to be understood in great detail, but rather that there is enough understanding to give confidence that management measures can be trialled and tested.

This process of adaptive management isa powerful way to work and move forward. In the case of Lakes Rotorua and Rotoiti, there are aspects where the understanding is good enough to begin trialling management options, while in other cases there are critical gaps in understanding where short-term investigations are needed before it is safe to begin management.

The discussion and recommendations in section 7 cover both aspects and clearly identify where urgent studies are required and where short-term management measures can begin.

A short history of monitoring and investigations

There have been studies and monitoring of Lakes Rotorua and Rotoiti since the late 1960s when problems of algal growth first emerged. All of this work has been elegantly brought together in a paper being prepared for EBOP by Dr Kit Rutherford from NIWA, on 'Lake Rotorua Nutrient Targets' (Ref 4). The author is grateful to Dr Rutherford and EBOP for making the draft report available as it helped identify many of the key management issues.

Section 6 in this report sets out a summary of the investigations from the late 1960s to the present, including the historical catchment target loads that have been set for total phosphorus and total nitrogen. The latter are set out in Table 3 and more fully in Appendix 4, which is a detailed foldout table of the 'Rotorua Nutrient Inputs and Water Quality - Loads and Targets' adapted from two tables in Dr Rutherford's draft report.

From this work a number of 'key issues for management' are identified and these are detailed in section 6 and briefly listed below:

  • determining whether the Rotorua Land Treatment Site (RLTS), where the treated sewage is sprayed, can continue to strip nutrients so they do not leak into Lake Rotorua
  • support for connecting a number of smaller urban areas around the lakes to reticulated sewage where septic tanks are leaking nutrients into the lakes
  • continuing the work begun by EBOP to define a suite of water quality indicators that can be used to measure the impact of management measures on lakes water quality, and the frequency and severity of algal blooms
  • the need to more accurately characterise the nutrient inputs in the Lake Rotorua catchment streams to identify possible 'point' sources that could be reduced by appropriate management measures in the short to medium term
  • the need to better understand the relationship between deeper groundwater, which emerges as springs, and the land uses in the recharge areas. A concerning trend is that while riparian retirement and catchment revegetation has reduced particulate nutrients, soluble nitrogen in the form of nitrate in deeper older groundwater (50 to 70 years old) has increased, negating the gains made by diverting the treated sewage to the RLTS. Work has begun to test younger groundwater to determine if the upward nitrate trend is continuing
  • clarification of whether storm-flows, which only contribute 10% of the nutrients, cause algal blooms by introducing relatively large 'slugs' of nutrients at times of the year which favour undesirable algal blooms
  • further work on the total nutrient loads and concentrations in the lakes when blooms are likely to occur to determine the N:P ratio, because if this is less than about 22:1, blue-green nitrogen fixing and blue-green non-nitrogen fixing blooms will be favoured
  • further investigations to determine the size and role of internal nutrient loads in both lakes, which could be at least as high as half the external nutrient loads. This is particularly important at those times of the year that the blue-green algal blooms occur
  • further urgent investigations to better understand where the Ohau Channel flows in Lake Rotoiti, as it has both a beneficial role in supplying oxygenated water and an adverse role in bringing large amounts of nutrients into Lake Rotoiti, especially into the surface waters in summer when blue-green algal blooms are favoured. This work will be critical for any management measures
  • a better understanding of the dynamics and succession of algal blooms to aid in the introduction of algal risk prediction and short-term cosmetic management measures.

Recommendations for short-term management measures and vital investigations

Detailed discussion and recommendations for short-term management and vital investigations to support short-term management are set out in section 7. A full summary of the recommendations follows this Executive Summary. A summary of the key issues that lead to the recommendations and the recommendation number is set out below.

The recommendations are split into short-term understanding and then short-term management, firstly for the catchments and then for the lakes. Finally there is a sub-section on 'other management issues and options', which do not readily fall into the earlier sub-sections.

Understanding the catchments

EBOP and RDC accept that there are key aspects to better understand catchment processes to move to the short-term management options that are summarised below.

1) Quantifying the nutrient sources in the catchments and relating them as far as possible to land uses and point sources. Some of the inputs are 'natural point sources' like some of the springs, which have relatively high phosphorus levels and at least one geothermal input, with a high soluble nitrogen level. Identifying these inputs will lead to short-term management measures that can be applied using existing technologies and methods. (Recommendation 3)

2) Monitoring the catchments to quantify nutrient inputs from different land uses and land types, and to measure the results of often expensive management measures. Using the monitoring results and other information to build simple catchment models that can be used to work with landowners to show how they contribute to nutrient reductions across the catchment and how their management efforts are working. (Recommendations 4 and 5)

3) Quantifying the levels of nutrients in the deeper groundwater, especially nitrate levels, that appear to be rapidly increasing, is an important step for management. Monitoring of the deeper groundwater, where it emerges as springs, has shown it is aged between 50 to 70 years, so that the high nitrate concentrations are due to land uses and land-use changes that occurred 50 to 70 years ago. EBOP has begun work to measure nutrient levels in younger groundwater and it is important to continue this work as the deeper groundwater could be the most significant source of nutrients for the foreseeable future. (Recommendation 6)

4) Understanding the role that storm flows have in promoting 'event driven' algal blooms as they may trigger undesirable blooms at certain times of the year. If this is the case, short-term management measures could be put in place relatively quickly. (Recommendation 7)

Short-term catchment management options

5) EBOP has been investigating materials like alum for stripping nutrients from catchment inputs and has imported a quantity of a new efficient material for stripping phosphorus, PhoslockTM, from Western Australia. EBOP should continue this work and consider forming a closer working relationship with the Western Australian Department for the Environment, which is conducting similar trials. A number of local soils may also be useful for nutrient stripping and as a substrate for the rare earth used in Phoslock. (Recommendations 8 and 9)

6) Several springs that enter streams in the Lake Rotorua catchments have relatively high concentrations of phosphorus and these are essentially natural point sources of nutrients. Hence, there is an opportunity to manage them in the short-term to strip phosphorus using the type of materials discussed for the previous recommendations. (Recommendation 10)

7) At least one geothermal input has a relatively high level of soluble nitrogen by the time it enters the Waiohewa stream and EBOP and RDC are considering diversion to the reticulated sewerage when it is extended. This is good short-term management option and should be pursued as a priority. (Recommendation 11)

8) Diversion of the treated sewage from Rotorua to the Rotorua Land Treatment Site in 1991 was initially successful in reducing nutrients in Lake Rotorua, but intensive spraying subsequently caused leakage of nutrients via the Puarenga Stream. Changes to the spraying regime appear to have initially been effective in stopping the leakage but resilience needs to be built into the system to cope with future increases in loads. Also, as this is a relatively cost-effective method of removing nutrients from Lake Rotorua, the load targets for phosphorus and nitrogen should be reduced to zero. (Recommendations 12, 13, 14 and 15)

9) Previous studies and monitoring have shown that 90% of the catchment flows come in stream base-flows, bringing with them the majority of the nutrients. EBOP has characterised the high nutrient input streams so that short- and medium-term opportunities for wetland stripping of nutrients can be identified. (Recommendation 16)

10) Once point sources of nutrients have been identified and prioritized, 'Best Management Practices' (BMPs) can be identified and applied to particular point sources like dairy sheds. EBOP is already applying BMPs and with better identification of point sources, they can be prioritised for short-term management. (Recommendation 17)

Understanding the links between the catchments and the lakes - nitrogen and phosphorous loads and the N:P ratio

11) Current estimates of the annual catchment nutrient loads to Lake Rotorua are 35 tonnes of total phosphorus (compared to a target of 37 tonnes) and 692 tonnes of total nitrogen (compared to a target of 435 tonnes). These figures do not include septic tank inputs and internal loadings in the lakes, and as stated earlier, the N:P ratio in the lakes is also important in determining what type of blooms occur. Therefore, it is believed more work needs to be done to clarify the relationships between catchment nutrient loads and lake nutrient dynamics, to set a range of targets for nutrients entering the lakes and in the lake waters. The aim for these targets is to minimise unwanted algal blooms and focus both short-term and longer-term management measures. (Recommendations 18 and 19)

12) As discussed earlier, the nitrogen to phosphorus ratio (N:P ratio) is critical in determining the types of algae that will bloom. The priority for Lakes Rotorua and Rotoiti, at least for the next few years, will be to eliminate toxic blue-green algal blooms and therefore the in-lake N:P ratio should be kept above 22:1 if possible, especially over late spring, summer and autumn. Therefore, short-term investigations should focus on ways to manipulate the N:P ratio, by both reductions in catchment nutrient inputs and in-lake management to change the ratio to greater than 22:1. (Recommendation 20)

Understanding the lakes

13) A major issue for short-term management is to determine the role of internal lake nutrient loads in promoting algal blooms. In Rotorua, these loads could be at least half the catchment loads and they occur at the worst time of the year for blue-green algal blooms, caused by stratification and destratification events during summer. Equally important is a better understanding of how soluble and particulate nutrients are mobilised in Lake Rotorua and transported through the Ohau Channel into Lake Rotoiti. Information from short-term intensive monitoring to build on work already done could then be used for hydrodynamic modelling to quantify the transport and fate of these nutrients. This work is critical to underpin management options involving diversion of the Ohau Channel. (Recommendations 21 and 22)

14) It is well known that there are a number of geothermal inputs into Lake Rotoiti but their contributions of nutrients and energy and how they modify transport processes and residence times are not well known. A better understanding of these 'hot' inputs is vital for short-term management measures involving diversion of the Ohau Channel or in-lake management measures like oxygenation. (Recommendation 23)

15) While algae samples have been collected monthly and there is a more regular sampling programme for blue-green algae, little is known about the succession and cycling of phytoplankton in Lakes Rotorua and Rotoiti. In other systems this knowledge has often been vital in testing management options, and data to develop the necessary understanding is collected by monitoring at weekly or fortnightly intervals. It would be wrong to be prescriptive for Rotorua and Rotoiti, but it is important that EBOP considers more frequent monitoring, especially during periods when blue-green algal blooms are expected. (Recommendation 24)

16) A critical issue that has already been mentioned is the role of the Ohau Channel in transporting nutrients into Lake Rotoiti. One of the main management options for Lake Rotoiti could be whole or partial diversion of the Ohau Channel, but unfortunately there is not enough information on the hydrodynamics and circulation patterns in Lake Rotoiti to confidently make management recommendations. Therefore, short-term investigations and hydrodynamic modelling are vital and discussions indicate suitable models can be developed in 6 to 12 months. (Recommendation 25)

Short-term lake management options

17) A significant source of nutrients that flow into Lake Rotoiti through the Ohau Channel come from resuspended sediments, especially when the wind is in the western quarter. An earlier recommendation was to carry out investigations and modelling to better understand how these resuspension events interact with internal nutrient loadings from stratification and destratification, but this should not stop short-term management to trap the resuspended material. EBOP and RDC are investigating the construction of groynes each side of the entrance to the Ohau Channel and this work should be strongly supported. Initially, the groynes should not be built out to the deeper water in Lake Rotorua until the flow of water (and nutrients) from the deeper water is better understood, but this could be done at a later date. (Recommendation 26)

18) Clearly diversion of the Ohau Channel will be a significant short-term management option, including the possibility of using part of the flow to flush Okawa Bay, but as stated earlier, some short-term investigations are vital to provide the level of understanding necessary to trial preferred diversions. But, this must not stop parallel engineering investigations into the best diversion techniques for trialling as soon as possible. Monitoring and investigations can then continue as the trials proceed to give the best ongoing management options, a truly adaptive approach. (Recommendations 27 and 28)

19) EBOP is planning to trial nutrient stripping with alum in Lake Okaro in the near future and is also considering oxygenation trials. Phoslock could also prove beneficial with, and without, oxygenation. It is likely that direct oxygenation of Rotorua over the short periods when it is stratified and longer-term oxygenation of Lake Rotoiti in combination with diversion of the Ohau Channel will prove beneficial for both lakes. Therefore, it is important that EBOP continues, and is supported in carrying out trials of nutrient stripping materials and oxygenation to develop these important management tools. (Recommendations 29 and 30)

20) Cosmetic measures have been used to trap and remove algal blooms in other parts of the world by using floating booms to concentrate the blooms and 'sucker' trucks to remove dense concentrations. Booms may also be able to contain a bloom in an embayment. While such measures do not stop blooms, they can be used to remove or trap dense accumulations and show the community that some action is being taken. (Recommendation 31)

21) Herbicides are already being used in the Rotorua lakes to control nuisance build-up of invasive aquatic 'weeds' and in other places to selectively control aquatic weed infestations. Herbicides have also been used to kill blue-green blooms in farm dams, but it is a big step to use herbicides to control blue-green blooms in natural systems. Nevertheless, herbicides as a control measure could be investigated, particularly 'Roundup' which has a short half life in water bodies. (Recommendation 32)

22) It is often valuable to predict when certain types of algal blooms may occur, to warn the public and to prepare any management measures that may be appropriate. From discussions with NIWA, it appears that a longer-term prediction - say three months ahead - could be made using NIWA's climate models and, shorter-term predictions - say 10 days or less - could be made based on more frequent monitoring and shorter-term weather forecasts. EBOP should investigate whether such a capacity would be worthwhile. (Recommendation 33)

Other management issues and options

In addition to the recommendations made above, there were a few issues that did not fit simply into catchment or lake management to tackle short-term management of algal blooms and these issues are summarised below together with recommendations.

23) It became apparent during meetings and discussions that there is a significant amount of work on management of lakes occurring in New Zealand. EBOP is aware of the extensive work on Lake Taupo and has established a working relationship with Environment Waikato. 'Cross regional co-operation and learning' should reduce the cost of investigations and the trialling of management options and should be pursued wherever possible. (Recommendation 34)

24) Another issue that became apparent in discussions is the apparent lack of consistent co-ordination of the research and science that underpins lakes management. This is not to belittle the very good work that EBOP is doing, but it is apparent that some of the science could be better co-ordinated towards focused management outcomes. It is not for the author to say how this should be done, but it would be useful for EBOP to consider this aspect as part of its overall leadership role. (Recommendation 35)

25) Trout fishing is a very important part of the tourist attractions of the Rotorua district and the economy. Unfortunately, the problems in Lake Rotoiti appear to be having an adverse effect on the trout fishery, with the average size of fish falling over the last few years. Fish and Game New Zealand are working with the University of Waikato and EBOP to investigate the reasons for the decline, including how the food webs may be changing. While this work is not directly applicable to this report it should continue to be supported, as it is one of the 'public faces' of the health of the lakes. (Recommendation 36)

26) Consideration of the broader ecosystem health of the lakes is also not directly part of this report, but it must be the ultimate aim of any management. Some very good work has been done on developing broader ecosystem health indicators, based on the health of native aquatic plants and the degree of impact by invasive weed species. EBOP has been supporting this work and the development of broader ecosystem indicators and targets complements the TLI and other water quality targets. (Recommendation 37)

Priority recommendations

All of the recommendations in this report should be considered by EBOP for short-term (1-2 years) and medium-term (2-4 years) management measures, recognising that many management options will require resource consent approvals that may delay their implementation.

In this context, some recommendations are considered to be of a higher priority for either direct implementation of management measures or for urgent studies to improve understanding of the lakes that will lead to short-term management measures. These 'high priority' recommendations are reproduced below, in addition to being in the full Summary of Recommendations.

Recommendation 3: That as far as practicable, monitoring should be extended into the catchments and sub-catchments with the highest nutrient inputs to characterise any 'point' sources that may become a priority for management.

Recommendation 10: That EBOP urgently investigate options to strip phosphorus from streams that have high levels of phosphorus coming from springs and other sources, starting with the Waingaehe, Hamurana and Awahou streams.

Recommendation 11: That EBOP and RDC divert the flow from the Tikitere geothermal field to the Rotorua District Sewage Treatment Plant as a high priority.

Recommendation 12: That EBOP and RDC establish a zero target for nutrient inputs from the Rotorua Land Treatment Site to Lake Rotorua.

Recommendation 16: That EBOP identifies opportunities for constructed and enhanced natural wetlands to strip nutrients in the catchments and on the foreshores of Lake Rotorua and, as a high priority, construct or enhance such wetlands where there are opportunities to intercept high nutrient level base-flows.

Recommendation 18: That EBOP continues to support investigations that are aimed at clarifying the relationships between catchment and internal lake nutrient loads, lakes water quality and algal blooms.

Recommendation 20: That EBOP focuses as a priority in the short-term on studies and investigations leading to management measures that reduce lake total nitrogen and lake total phosphorus concentrations and produce a high N:P ratio of 20-22:1 or greater in Lakes Rotorua and Rotoiti, especially during the late spring, summer and autumn period.

Recommendation 21: That EBOP focuses in the short-term on monitoring of the stratification and destratification, and sediment resuspension events over the most common meteorological conditions in Lake Rotorua, including concurrent monitoring of the Ohau Channel to provide critical data for management.

Recommendation 22: That EBOP uses the data collected from implementation of Recommendation 21 to interactively carry out hydrodynamic modelling to quantify the quantities and timing of nutrients and phytoplankton transport through the Ohau Channel into Lake Rotoiti.

Recommendation 25: That EBOP continues to pursue as a high priority further monitoring and hydrodynamic modelling of the flow of the Ohau Channel into Lake Rotoiti as the basis for short-term management decisions on manipulation of this flow to improve the health of the lake and reduce or eliminate blue-green blooms.

Recommendation 26: That EBOP and RDC initiate engineering design and construction of temporary groynes on either side of the entrance of the Ohau Channel to minimise transportation of suspended material into Lake Rotoiti. Permanent structures can be constructed when the best configuration to minimise transport of suspended material is determined.

Recommendation 27: That EBOP and RDC begin engineering investigations and designs for trial structures to divert the Ohau Channel to the Kaituna and/or Okawa Bay as soon as possible, and interactively with the monitoring and modelling work that has been recommended in Recommendations 24 and 25, so that work on trial structures can begin as soon as resource consent approvals have been obtained.

Recommendation 28: That EBOP builds temporary structures to test the favoured options for diversion of the Ohau Channel to the Kaituna River and/or Okawa Bay when resource consent approvals have been obtained, and monitor the trials to further refine the hydrodynamic models for the channel flows and the western end of Lake Rotoiti before building any more permanent diversion structures.

Recommendation 29: That EBOP continues trials with nutrient stripping materials such as Alum and Phoslock and begins trials with direct oxygenation separately and in combination, to determine the best methods of using these methods to reduce nitrogen and phosphorus in the lakes and manipulate the N:P ratio.

Recommendation 30: That the use of oxygenation and nutrient stripping materials be built into the modelling proposed in Recommendation 27 so as to inform any trials and help predict the best options for using these management methods individually and together in Lakes Rotorua and Rotoiti.

Recommendation 31: That EBOP and RDC investigate the use of oil pollution booms to contain and concentrate the worst algal blooms, and the use of 'suction trucks' to remove the worst of the accumulations.