6 Recommendations for short-term management measures and better systems understanding
In a number of cases, there is enough information available to start investigating management options now, although in most cases there will need to be engineering evaluations and design. In other cases, there is not enough information available to support short-term management options and recommendations are made for urgent monitoring and investigations to fill critical information gaps.
Based on the detailed discussions and information provided to the author, there are a number of short-term and medium-term (one to four years) management options that can be implemented, some within one to two years and some, which will depend on further vital information in two to four years. There are also some cosmetic management measures that can be implemented if further blue-green blooms occur.
Most of the recommended management measures support the ideas and directions that are already being pursued by EBOP and RDC. Hopefully, this report will help prioritise and programme these management measures and any new ones that are recommended. It should also be noted that a number of management measures will require formal resource consents and that these processes will lengthen the time in which they can be implemented.
6.1 A whole systems approach
Appendix 3 is a foldout diagram which shows Lakes Rotorua and Rotoiti in cross section and an idealised catchment flowing into Lake Rotorua. Processes by which nutrients flow from the catchment into the lake are shown as well as the processes by which the nutrients cycle within the lakes and become available for macrophyte and phytoplankton growth.
It is important to consider and understand all of these processes in so far as they lead to the most cost-effective and efficient management measures. Building understanding and implementing management measures should be adaptive, so as sufficient understanding is gained, management measures are trialled and then implemented.
Each stage is a learning process where understanding and management is improved. Management of complex natural systems can never be left with the expectation that it will work 'forever'. Even when successful and resilient management is implemented, it must be monitored and fine-tuned if needed. If significant changes occur in the natural system, through climate or other influencing processes, larger changes or alterations to management may be needed.
Taking a whole-systems approach also helps all of the stakeholders and partners see their part, when they need to be involved and why certain things are being done at different times. In particular, the various stages of moving to management, including when studies need to be done to improve understanding, can be set into a 'project management' and time sequence.
This helps all stakeholders understand that some things can be done relatively quickly but that other things will take a while because they are more complex or difficult. The more organised amongst us can prepare flow charts and Gant charts to show the sequencing and timing of the different steps to reach agreed goals, outputs, targets and outcomes.
It is this context into which the recommendations for short-term understanding and management fit. The recommendations are divided up into understanding and managing the catchments, and understanding and managing the lakes.
6.2 Understanding the catchments
Understanding the nutrient sources
As discussed earlier, considerable work in the form of monitoring and investigations has been done on Lakes Rotorua and Rotoiti over the last 35 years. Even so, there is still an incomplete understanding of some key processes, including a more detailed understanding of particular nutrient sources.
It is clear that there are a number of springs that contribute relatively high levels of phosphorus and/or nitrogen from various land uses and natural sources. At least three springs contribute significant loads of phosphorus, namely Waingaehe, Hamurana and Awahou. As well, there are geothermal inputs that contribute significant amounts of nutrients, such as the Tikitere geothermal field.
There will be other specific sources of nutrients such as leakage from the sewage spray irrigation area, septic tanks and dairy sheds. All of these nutrient sources should be characterised so that those that are 'point sources' are identified. These will be the easiest to tackle as opposed to the more diffuse sources such as fertiliser leakage and nutrients coming from grazing stock.
A general rule in catchment management is to tackle the easiest and highest level nutrient sources first.
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.
Monitoring and modelling the catchments
Catchment monitoring can be expensive, but it is generally much less expensive than implementation of management measures. It is therefore important to have monitoring systems in place that show with statistical validity and good confidence levels whether the management measures are working or not, especially in the catchments and sub-catchments that are contributing the most nutrients. These type of monitoring systems also allow shorter-term targets to be set that communities can see are being achieved and which can then be celebrated. Such a monitoring system has been developed by the Western Australian Department of the Environment in partnership with the US Geological Survey and should be tested for its suitability in the Lake Rotorua catchments.
Recommendation 4: EBOP should work with the Western Australian Department of the Environment to investigate the use of its catchment monitoring system in the Lake Rotorua Catchments to measure the efficacy of catchment management measures.
Catchment modelling can be useful in transferring understanding of catchment processes between similar land uses and land types, and in calculating nutrient losses from various land uses where the processes are well understood and the nutrient losses have been accurately calibrated. They can be used at different scales from a single farm to sub-catchments to full catchments, and if used well, can save considerable resources and time.
NIWA has developed a good model for nutrient loss management at the farm level for the Rotorua lakes, and has developed sub-catchment and catchment models for other situations. NIWA (Ref 5)is also involved in a project to develop 'a new catchment model for use by non-specialists'.
Recommendation 5: EBOP should consider developing a simple catchment model that can be co-operatively used by landowners and community groups to see how nutrients move through their catchments as a aid to understanding how they can be managed.
Another very important issue for catchment management in the Lake Rotorua catchments is the increasing levels of nitrate in most of the streams coming from deeper groundwater that is 50 to 70 years old. It is not clear, with more recent changes in land use, whether these concentrations will continue to increase or reach a plateau. Equally, if changes are made to reduce nitrate losses, it is not clear how quickly stream concentrations will decrease and what the new equilibrium concentrations will be.
While consideration of this aspect is not strictly to do with short-term management, it is seen as so important that some comment should be made. While EBOP has started work on aging younger groundwater in streams it would be useful to build on work that is being done for Lake Taupo to develop a 'mid-level catchment model'. This 'model' should be relatively simple and would enable managers to estimate how land-use changes in the past have affected stream nutrient concentrations (a reality check) and how proposed land-use changes and/or mitigation measures will affect them in the future.
A parallel benefit would be that some hydrogeological work would need to be done to check the age and chemistry of the deeper groundwater to calibrate the model. This would be of considerable benefit in its own right as it would throw light on the issue of nitrate concentrations (and other nutrient concentrations) in the younger groundwater.
Recommendation 6: EBOP should consider carrying out hydrogeological investigations to determine the chemistry of the younger and deeper groundwater and use this and other information to develop a 'mid-level catchment model', to enable land managers to estimate how proposed land-use changes and/or mitigation measures will modify nutrient concentrations in the deeper groundwater.
Storm-flows
While storm-flows do not carry as much of the nutrient load to the lakes as the stream base-flows they can carry significant quantities of nutrients over short periods. Work in Western Australia has shown that the heaviest nutrient and carbon loads are typically found at the beginning of a stormflow and during summer and autumn periods when there is little rainfall and flow in streams and drains. These summer-autumn 'events' often cause algal blooms, with the severity depending on the exact nature of the event.
No clear patterns of nutrient inputs have been determined for storm-flows into Lake Rotorua although there have been several studies that have considered storm-flows. Therefore, it is not clear if storm-flows, especially in late spring, summer and autumn cause algal blooms, but they may play a role along with internal loads of nutrients, in causing particular blooms. It may be important to clarify whether storm-flows during these times have any causative role in initiating or prolonging the blue-green blooms.
If this is the case, there are a number of well-proven management measures, such as stormflow diversions and wetland detention basins, that may be used to manage these events, and EBOP and RDC have considerable experience with these types of measures.
Recommendation 7: EBOP and RDC should carry out more detailed monitoring of storm-flows into Lake Rotorua, especially in the late spring, summer, autumn period to determine if these 'events' are initiating or prolonging blue-green blooms.
6.3 Short-term catchment management options
Tools for nutrient management - Phoslock and other 'benign' nutrient stripping materials
There are a number of tools for reducing nutrients that will have application in the catchments and in the lakes. These include the use of alum, calcium carbonate, and various minerals to absorb or modify nutrients. EBOP is trialling a number of these substances. This is to be applauded and should be continued to add to the number of tools in the 'nutrient management toolbox'.
Another useful material that has been developed in a partnership between the Western Australian Department of the Environment and the CSIRO is Phoslock, a patented, modified clay that irreversibly absorbs phosphorus in virtually all natural situations, and requires relatively small quantities to be effective. EBOP has already obtained small quantities of Phoslock and is proposing to trial it when the Environmental Risk Management Authority (ERMA) approval has been granted and resource consents obtained.
Trials in Western Australia in freshwater systems have proved promising and when used in conjunction with oxygenation, Phoslock has changed some N limited systems to P limited and eliminated blue-green blooms. It therefore seems appropriate for EBOP to form a closer partnership with the Western Australian Department of Environment to conduct joint trials and exchange information and ideas. Phoslock in various forms could be used to cost-effectively strip phosphorus in stream base-flows, springs and artificial and enhanced wetlands.
Recommendation 8: That EBOP approaches the Western Australian Department of the Environment to form a partnership to jointly trial and develop Phoslock for nutrient stripping in catchments and water bodies, separately and in conjunction with other nutrient management methods.
A number of allophanic soils in the Rotorua area are similar to the bentonite clays used as the substrate for Phoslock and have strong phosphorus retention characteristics. These soils should be tested for use in wetlands and other situations (along with other benign materials for phosphorus stripping) tested for their suitability as a substrate for the rare earth used in Phoslock.
Care should be taken with the use of such materials that release of nutrients does not occur under anoxic conditions. EBOP is already testing these types of materials and recommendation 10 supports continuation of this work as a key measure for short- and longer-term management.
Recommendation 9: EBOP should investigate the use of allophanic soils and related clays for use in stripping phosphorus and as a substrate for preparation of Phoslock. Work should continue on the use of other benign materials such as alum.
High phosphorus springs
As noted earlier, a number of deep groundwater springs contain elevated levels of phosphorus, which enter Lake Rotorua in a number of the main streams. These are, in effect, 'natural point sources' and an earlier recommendation was that all of the deep groundwater springs should be characterised as to their nutrient inputs.
Three streams in particular contribute higher levels of phosphorus from the springs that enter them: Waingaehe, Hamurana and Awahou. It is understood that EBOP is considering phosphorus stripping from these streams and this should be treated as a high priority short-term management measure.
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.
Geothermal nutrient inputs
Geothermal waters enter both the catchments and the waters of Lakes Rotorua and Rotoiti and could add significant amounts of nutrients. An earlier recommendation was that the catchment geothermal flows be characterised to quantify their nutrient inputs. As noted earlier, the Tikitere geothermal field injects about 30 tonnes of nitrogen, mainly as nitrate, into Lake Rotorua via the Waiohewa stream. It is understood that EBOP and RDC intend to divert this flow into the reticulated sewerage when it is extended. This will quickly and easily reduce a significant source of nitrogen and should be implemented as a high priority.
Recommendation 11: That EBOP and RDC divert the flow from the Tikitere geothermal field to the Rotorua District Sewage Treatment Plant if practicable, or to some other suitable location such as the Kaituna River if acceptable.
Rotorua Land (Sewage) Treatment Site (RLTS)
As noted earlier, changes to the spray irrigation of treated sewage onto pines in the Whakarewarewa Forest appear to have initially been effective in reducing nutrient inputs to Lake Rotorua via the Puarenga Stream. However, it is not known what effect increased volumes of effluent from increasing populations and extension of the reticulated sewerage scheme will have.
Previous NIWA work at the Rotorua Land Treatment Site (RLTS) under the higher spray regime showed springs formed and the short soil-water contact times and preferential pathways through the wetlands markedly reduced the nutrient stripping capacity, especially removal via denitrification. NIWA has also done work (Refs 6 and 7) to show how the RLTS and other wetlands could be made more efficient at nutrient stripping simply and at relatively low cost.
The RLTS is a good way to reduce nutrients entering Lake Rotorua and, given the likely relatively low cost of nutrient removal, load targets of zero should be set for nitrogen (N) and phosphorus (P), rather than the existing targets of three tonnes of total P per year and 30 tonnes of total N per year. Repeat studies should be done of the nutrient stripping processes under the new spray regime and, if necessary, the wetlands within and downstream of the RLTS should be modified to increase their nutrient stripping capacity.
Recommendation 12: That EBOP and RDC establish a zero target for nutrient inputs from the RLTS to Lake Rotorua.
Recommendation 13: Repeat studies of the nutrient stripping processes and pathways at the Rotorua Land Treatment Site should be undertaken under the new spray regime to determine the nutrient stripping effectiveness.
Recommendation 14: Wetlands within and below the Rotorua Land Treatment Site should be modified if necessary to strip any residual nutrients 'leaking' from the spray irrigation areas.
Recommendation 15: The RDC should continue to monitor nutrient levels above and below the Rotorua Land Treatment Site to measure its efficiency at stripping nutrients, particularly if spray volumes are increased to cope with increasing population or inputs from newly reticulated areas.
Wetlands and stream base-flows
Previous work, as discussed earlier, has shown that stream base-flows carry about 90% of the nutrients into Lake Rotorua. NIWA, in work for EBOP, has studied the use of constructed and 'enhanced' natural wetlands to strip nutrients (see, for example, Ref 8) and has developed methodologies for using these wetlands in the catchments of the Rotorua lakes. An earlier recommendation was that all of the catchment nutrient inputs be characterised and prioritised for management.
All opportunities to use wetland stripping in the Lake Rotorua catchments should be identified, especially where there are streams with high nutrient base-flows. As EBOP has identified the streams entering Lake Rotorua that have high levels of nutrients, a further step for short- and medium-term management is to identify the best opportunities for wetland stripping.
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.
Best management practices for point sources
An earlier recommendation covered the need to characterise the higher nutrient sources in the catchments, especially 'point sources'. Another recommendation was the use of constructed and enhanced natural wetlands to strip nutrients from the baseflow in streams with high nutrient loads. There are a number of 'Best Management Practices' (BMPs) for stripping nutrients from other point sources such as dairy sheds and piggeries. In the longer term, such BMPs will need to be used for diffuse sources such as pasture and stock and EBOP is already using a number of BMPs such as riparian retirement.
In the short-term useful reductions of nutrients may be achieved by the use of proven BMPs for smaller point sources. A particular BMP that is widely used in Australia for sites that are difficult or too costly to provide sewage reticulation are Alternative Treatment Units (ATUs) to replace septic tanks and leach drains. Where nutrients are a problem, nutrient stripping modules are added to the ATU.
Compliance is guaranteed by certification and registration of particular ATU systems by state authorities, and approvals and management contracts through local government authorities for installation and maintenance. These ATUs should not be seen as replacement for reticulated sewage in the areas already identified by EBOP and RDC but they could be considered in sensitive areas where reticulated sewage is too costly or too difficult.
Recommendation 17: EBOP and RDC could consider the use of certified and contractually managed Alternative Treatment Units with nutrient stripping capacity in nutrient sensitive areas where reticulated sewerage is too costly or too difficult.
6.4 Understanding the links between the catchments and the lakes - nitrogen and phosphorus loads and the N:P ratio
Nitrogen and phosphorus loads and targets, and the concept of resilience
Work by NIWA for EBOP by Rutherford (Ref 4) has summarised previous studies and produced target loads for nitrogen and phosphorus from the catchments of Lake Rotorua. Two of Rutherford's tables have been combined in Table 5 in this report and this table is reproduced as a foldout in Appendix 4 for ease of reference. In summary, Rutherford estimates that the 2002 annual catchment input is 35 tonnes of phosphorus (compared to a target of 37 tonnes) and 692 tonnes of nitrogen (compared to a target of 435 tonnes).
However, this does not include raw sewage, presumably from septic tanks and internal loads in Lake Rotorua, which Professor Hamilton estimates could be at least half of the catchment loads. It also assumes that the inevitable growth around the lakes will not contribute any more nutrients so that little or no resilience (or capacity for the systems to cope with change) is built into the targets. Further, it does not take into account the processes and internal loads in Lake Rotoiti which add to the loads from Lake Rotorua to produce problems like the blue-green blooms.
It must be said however, that Dr Rutherford's paper that has kindly been made available for this report, is a draft and his conclusions and recommendations could change. Nevertheless, further work should be done to improve the understanding of the relationship of the Lake Rotorua catchment and internal nutrient loads to Lake Rotoiti's nutrient concentrations over the year, but especially over the late spring, summer and autumn period when blue-green blooms appear. This is discussed further in the next sections and specific recommendations are made.
Even without this fuller understanding of the processes that cause algal blooms in the lakes the approach in the short-term should be to reduce both phosphorus and nitrogen, so that interim catchment targets below those in Table 5 should be set. It is hard without the necessary further studies that are recommended in this report to set firm figures, but a 25% reduction for both nutrients should not cause any problems in the short-term.
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 19: That EBOP focuses in the short-term on investigations that will set clear targets for:
- an annual total catchment load for Lake Rotorua for phosphorus and nitrogen that builds in capacity for future development around the lake
- annual catchment phosphorus and nitrogen load targets for each of the main Lake Rotorua catchments
- internal lake phosphorus and nitrogen load targets for Lakes Rotorua and Rotoiti
- annual average stream phosphorus and nitrogen concentration targets for each of the nine Lake Rotorua catchments
- annual average lake water quality phosphorus and nitrogen concentration targets for Lakes Rotorua and Rotoiti.
The nitrogen to phosphorus ratio
As discussed earlier, the ratio of nitrogen to phosphorus (N:P ratio) is critical in determining the types of algae that will bloom in most types of water bodies, from oceans to estuaries, rivers and freshwater lakes. Current thinking is that the N:P ratio should be above about 22:1 to prevent blue-green booms.
This is because below this ratio the system becomes 'nitrogen limited' and phytoplankton, like blue-green algae that can fix nitrogen from the air, are therefore favoured. Furthermore, most species of blue-greens are more efficient at utilising nitrogen, hence even non nitrogen-fixers such as Microcystis are favoured at low N:P ratios.
If the ratio is greater than 22:1 there is more than adequate nitrogen and the system becomes phosphorus limited. This favours phytoplankton that are less efficient at utilising nitrogen, typically diatoms and green algae. Some of these algae can also cause severe problems, so just shifting a system from nitrogen limited to phosphorus limited is not always the best solution, although it must initially be a priority in Lakes Rotorua and Rotoiti.
The situation in many water bodies becomes more complex because they can 'flip' from being nitrogen limited to phosphorus limited during a year, so any management measures need to be tailored to both situations. In Lakes Rotorua and Rotoiti it is known that the N:P ratio varies between 5 to 15:1 although it has gone as high as 19:1. The current target for the N:P ratio is 15:1 (John McIntosh - pers comm), which, according to Professor Hamilton, may be too low to prevent blue-green blooms.
There is not enough information to say whether the lakes flip from being nitrogen limited to phosphorus limited during the year, but the situation in both lakes puts them in a category where either nitrogen or phosphorus could be limiting. This subtle balance makes it difficult to say whether one or the other nutrient is truly limiting without bioassay studies.
Thus, it is most important to reduce excess levels of both phosphorus and nitrogen, and manage the N:P ratio to prevent blooms of phytoplankton that cause problems. In the case of Lakes Rotorua and Rotoiti, the priority should be on reducing or eliminating blue-green blooms, so the N:P ratio should be about 20-22:1 or greater, especially over the late spring, summer and autumn period.
While such 'numerical' targets are important, the overall water quality and state of the lakes must not be forgotten through 'lake health' indicators like the TLI, which cover water clarity as well as nitrogen and phosphorus levels.
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.
6.5 Understanding the lakes
Internal loads from sediment resuspension events
Many of those who have studied the lakes have emphasised the importance of the internal nutrient loads that are released when the lakes stratify and destratify. Recent work by Professor Hamilton and a PhD student is showing that these loads could be quite high, at least half of the catchment loads. Dr Rutherford in his most recent work concludes that the internal loads have always been, and will remain, a feature of the lake. However, if external loads can be reduced then internal loads can be expected to become less frequent and smaller, although it could take at least 20 years for such reductions to become apparent in Lake Rotorua based on known release coefficients and modelling.
The situation is complicated with Lake Rotoiti because it gets a lot of its nutrients from Lake Rotorua through the Ohau Channel as well as nutrients from internal loadings during stratification, which are recycled during destratification. Lake Rotoiti also has geothermal inputs, which are discussed below.
Another important factor in Lake Rotorua is resuspension of the sediments by stronger winds, which can then be transported through the Ohau Channel especially with the dominant summer westerly winds. There needs to be a better understanding of both the frequency of stratification-destratification and resuspension events in Lake Rotorua to focus short-term management measures. This can best be achieved by further monitoring and hydrodynamic modelling of both the deoxygenation-nutrient release and sediment resuspension events for a range of meteorological conditions.
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.
Geothermal inputs
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. These 'hot' inputs need to be better understood as part of developing a better understanding of the hydrodynamic processes that move nutrients and cause algal and blue-green blooms in Lake Rotoiti.
Recommendation 23: That EBOP initiates studies to characterise the geothermal inputs to Lake Rotoiti, especially to determine if they contribute nutrients and how they interact with other hydrodynamic processes to cause algal blooms.
Baseline monitoring and phytoplankton succession
Very useful monitoring has been carried out over many years in Lakes Rotorua and Rotoiti. Since 1990, this has been done systematically by EBOP and much valuable data has been collected as shown in the Rotorua Lakes Water Quality Report 2002 (Ref 3). But a more complete understanding of a complex interconnected system like Lakes Rotorua and Rotoiti will need more frequent monitoring for a number of parameters. This will provide a firm foundation on which to build more intensive studies to quantify particular processes.
The current EBOP monitoring regime for both lakes consists of monthly profiles and samples taken at three to four stations on each lake. Profiles are taken with a Seabird CTD probe, which gives cm-resolution of conductivity, temperature, dissolved oxygen and chlorophyll fluorescence. These data are then post-processed to give one-metre depth resolution of these parameters.
The other monthly data include an integrated surface mixed layer sample and a near- bottom sample of nutrients (phosphate, nitrate, ammonium, total nitrogen and total phosphorus). Samples for phytoplankton identification and enumeration are taken only intermittently as part of the routine monitoring. These data are pooled over the course of a year, to produce an annual 'snapshot' of phytoplankton species composition.
There is a separate blue-green algal monitoring programme, which involves collection of samples, mostly around the foreshores and bays, for identification and enumeration of blue-green cell counts at intervals corresponding to public health standards.
Discussions have been held with scientists at NIWA and Professor Hamilton, and the consensus is that monitoring needs to be stepped up to at least two weekly and possibly even weekly, given the importance of the lakes and the potential costs of management. One to two additional deep water samples could be required for Lake Rotoiti. More frequent monitoring should include all algal species that are present to gain a better understanding of the succession of algal blooms as an aid to determining the best management measures. More intensive studies would need even more intensive monitoring tailored to meet the needs of the study.
It would be wrong to be prescriptive on the detail of more frequent monitoring as this should be determined by EBOP working with NIWA and Professor Hamilton to determine the best regime to suit the agreed aims and objectives, and data needs for modelling and management. More frequent monitoring of blue-green blooms will also provide better information for the public and how the blooms progress along the lakes and intensify in bays. The latter understanding will aid cosmetic management measures which are suggested below.
Recommendation 24: EBOP should work with NIWA and Professor David Hamilton to establish a more frequent monitoring regime for Lakes Rotorua and Rotoiti to gain a better understanding of the processes that cause algal blooms, especially blue-green blooms as a basis for management measures that will best eliminate or reduce them.
The Ohau Channel and hydrodynamic processes
The Ohau Channel flow has been, and will be, a major focus for management of algal blooms in Lake Rotoiti given the large amount of nutrients and other material it transports into the lake. As discussed earlier in this report, there is good evidence that the channel flow falls into the deeper parts of Lake Rotoiti as an 'underflow' in winter and into the western arm of the lake and down the Kaituna River as an 'overflow' some of the time in summer.
Recent analysis of a range of data has led to thoughts that other intermediate flows or 'interflows' could occur under various conditions (Burns, Scholes and Gibbs - pers comm). These 'interflows' may not always flow into the Kaituna River and could be entering the surface waters in the western end of Lake Rotoiti and be a contributing cause of algal blooms (see the foldout diagram in Appendix 3).
Longer-term climate information shows that the Rotorua district has been warmer than average for about 20 years and that, as a consequence, there has been an average 1 degree Celsius rise in the average water temperature of the Rotorua lakes. This may also be having an effect on the flow from Lake Rotorua to Lake Rotoiti.
For all of these reasons, it is vital that there is a better understanding of the hydrodynamics of the flow of the Ohau Channel water into Lake Rotoiti and its fate once it enters the lake. This will be important for the whole lake, but will be especially important in managing areas like Okawa Bay and other parts of the lake's western end that have experienced bad blue-green blooms and are quite degraded. This understanding will be critical for any management measures that might be tried, including diversion of the Ohau Channel for all or part of the year, using part of the flow to flush Okawa Bay, or building groynes to channel the water from the deeper parts of Lake Rotorua into the channel.
From discussions, it is clear that appropriate hydrodynamic modelling could be done relatively quickly with simpler models being available in less than six months and a more sophisticated model available in 12 months. Some additional monitoring would be needed, such as a temperature sensor in the Ohau Channel and a thermistor chain in Lake Rotoiti. This modelling would provide a number of benefits, including:
- simulations of the flow path or mixing of Ohau Channel water in Lake Rotoiti and circulation patterns induced by the inflow
- residence times in various bays and lake areas that are subject to algal blooms
- assessment of the sensitivity of Ohau Channel underflow/interflow/overflow to air-temperature changes or channel diversions
- assessment of the impact of full or partial diversion of the Ohau Channel on circulation in the western bays of Lake Rotoiti
- simulation of the effects of engineering structures on flows and therefore clarification of the best engineering structures to trial management options
- simulation of the path of algal blooms blown or carried from specific areas
- assessment of typical wind-driven circulation patterns and connectivity between the western and eastern ends of Lake Rotoiti.
EBOP is aware that this work is urgently needed and is moving to have it commenced. Thus the recommendation below is strong reinforcement of an action that is already underway. Both NIWA and Professor Hamilton have strong modelling capabilities and it may be advantageous to have cross collaboration for this important work (see also Recommendation 26).
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.
6.6 Short-term lake management options
Groynes in Lake Rotorua
It is clear that one of the main sources of nutrients, sediment and possibly algae in Lake Rotoiti is the resuspension and transport through the Ohau Channel of sediments from the shallows of Lake Rotorua. Given that much of the movement of this suspended material is from the shallows on each side of the entrance of the Ohau Channel, a quick way to reduce the amounts is to build groynes on each side of the entrance.
This should trap the majority of the suspended material, which may have to be removed from time to time. Initially the groynes should not be built all the way to the deeper water as not enough is known about the importance of the flows of deeper Lake Rotorua water into Lake Rotoiti. The groynes could be extended later when the modelling work recommended earlier provides sufficient understanding of the best way to manage the deeper water flows.
It would be prudent to build temporary groynes first until the best configuration to minimise transport of suspended material is determined. Engineering investigations and design will be needed to determine the best length and position for the groynes but work should be able to be commenced relatively quickly.
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.
Ohau Channel
As stated earlier, diversion of the Ohau Channel for all or part of the year has been considered a number of times and is still being assessed, but as discussed in section 7.5 this should not be trialled until the flow regime into Lake Rotoiti is better understood as recommended above. The monitoring and modelling needed to provide this understanding should not take long, 12 months at most.
Engineering designs for temporary structures to trial favoured diversions of the Ohau Channel, whether into the Kaituna River or to flush Okawa Bay, should be commenced as soon as possible in an interactive process as modelling results become available. Trials can then begin quickly and the models can be tested and tuned against real world data. In time, more permanent structures can be built, but because of the dynamic nature of the Ohau Channel flow and the hydrodynamic processes in Lake Rotoiti, ongoing monitoring and adjustments may be needed.
In addition, there are water level control gates at the head of the Okere Falls on the Kaituna River and any management measures that are trialled for the Ohau Channel should take these gates into account as some manipulation of water levels and flows at the gates may be beneficial.
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.
Oxygenation and nutrient stripping in the lakes
Oxygenation has been used successfully in several large waterways (for example, the Thames River) and smaller-scale trials have been successful in improving oxygen levels and reducing nutrients in several smaller waterways in the south west of Western Australia. When used in combination with Phoslock in smaller freshwater systems in Western Australia (Ref 9), dominated by blue-green algal blooms, similar nutrient reductions were obtained and, in at least one case, the blue-green blooms were eliminated and replaced by less dense, green phytoplankton blooms.
EBOP is trialling a number of chemical and mineral additives to strip nutrients (for example, alum in Lake Okaro) and is investigating the use of oxygen to improve oxygen levels in several stressed lakes. EBOP is also aware of the benefits of Phoslock and is in the process of obtaining ERMA approval for its use in New Zealand. An earlier recommendation was for EBOP to form a partnership with the Department for the Environment in Western Australia to jointly trial Phoslock and this would also apply to the joint use of oxygenation and Phoslock.
It is likely that direct oxygenation of Lake 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. Clearly, smaller oxygenation trials (for example in Lake Okaro or parts of Lake Rotoiti) and investigations of the best way to carry out oxygenation of the larger lakes would be needed before any large-scale work could begin.
Phoslock could be used in conjunction with oxygenation where it would be useful in stripping phosphorus from the water column and locking it up in the sediments. Trials in Western Australia indicate an amount the equivalent of 1 mm depth coverage of the sediment will achieve this (Ref 9). If no new sediment is being introduced this could be a cost-effective method of modifying at least some of the conditions that cause algal blooms.
Another bonus is that direct oxygenation and nutrient stripping with Phoslock or other materials, could be used in combination to manipulate the N:P ratio and thus eliminate or significantly reduce blue-green blooms.
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 25 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.
Trapping and removing algal blooms
Blue-green blooms are buoyant and in light-wind conditions float to the surface and can be blown into bays or inlets. This certainly occurred in Lake Rotoiti last summer and high densities were measured in a number of locations. Oil pollution booms have been used in other places to contain similar blooms and even concentrate them further.
Suction trucks using hoses with skimmer heads can then be used to remove the worst accumulations. While this is mainly a cosmetic management measure it can help to allay public concern and remove the worst concentrations.
Given the observation that most of the blooms started in the western end of Lake Rotoiti and sometimes in bays and inlets, oil pollution booms may be able to be used to contain the spread of some blooms. The Department of the Environment in Western Australia have had experience with these techniques and could be contacted for advice.
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.
Use of herbicides - a wild card
Herbicides are rarely used in natural water bodies to kill algae because of the danger of adverse effects on aquatic life. However, in some situations, targeted spraying, even in narrow waterways has proved effective.
There is currently treatment of large aquatic weeds with herbicide when they become a nuisance in some of the Rotorua lakes, including Lake Rotoiti. Thus there is experience with the use of herbicides and this could be extended to careful trials on blue-green blooms, but extreme care would need to be taken to minimise any adverse effects on aquatic life, especially the prized fish.
Recommendation 32: That EBOP and the RDC, in liaison with the appropriate authorities, investigate the potential use of herbicides to control blue-green algal blooms.
Algal bloom risk prediction
While not a direct short-term management measure, the ability to predict blooms would be beneficial to keep the public informed and allow EBOP to be prepared with short-term management measures, such as herbicide spraying and containment of blooms with booms, if these methods prove feasible and environmentally acceptable.
Blooms caused by buoyant algae can appear suddenly, usually associated with periods of calm weather in summer when a phytoplankton community circulating through the mixed layer accumulates on the surface and is blown into bays. Adverse publicity invariably follows, particularly when the blooms are those of toxic species.
Discussions with NIWA indicate that risk prediction could operate at two relevant time scales; up to three months ahead and up to 10 days ahead.
- The risk of a summer bloom up to three months ahead may be derived from current lake phytoplankton information, a knowledge of the previous lake history and the probabilities, derived from NIWA's climate models, of (say) an impending above average summer temperature and above average number of summer calm days.
- For shorter periods of 10 days or less, information gained from more frequent phytoplankton monitoring of the mixed layer (as recommended earlier) should show if the species composition is dominated by potentially buoyant algae. Monitoring frequencies can then be increased so that the phytoplankton monitoring can be linked to 10 day (or less) forecasts of calm weather.
NIWA could supply the longer-term weather forecasting and work with EBOP to combine this with the phytoplankton monitoring, to provide a high quality algal bloom risk prediction capacity.
Recommendation 33: That EBOP considers working with NIWA to develop an algal bloom risk prediction capacity for both longer periods (three months) and shorter periods (10 days or less).
6.7 Other management issues and options
Cross regional co-operation and learning
From discussions with NIWA, Professor David Hamilton and staff from the Ministry for the Environment it became clear that similar problems to those in the Rotorua lakes are occurring in other waterways. For example, considerable work has been done for Lake Taupo to identify the levels of nutrient reduction that are necessary to prevent the type of algal blooms that have occurred in Lake Rotoiti. Much of this work is being carried out by the same organisations that are working with EBOP on the Rotorua lakes.
In the case of Lake Taupo, the responsible authority is Environment Waikato and they were participants at the Rotorua Lakes 2003 Symposium. It is understood that EBOP and Environment Waikato are collaborating and it is important that this continue, because lake and other waterway systems are complex and there needs to be pooling of knowledge and experience to get the best and most cost-effective management outcomes.
In addition, various authorities can jointly contribute to studies and investigations that need to be done, that have applicability across a number of systems. This is true for both catchment management and in-water management.
Recommendation 34: That EBOP continues to work with Environment Waikato, and other organisations carrying out lake investigations, to share information and contribute to joint studies and management trials where appropriate.
Science co-ordination
While a number of good studies have been done on Lakes Rotorua and Rotoiti over the last 35 years, it is apparent that the science has not been co-ordinated as well as it might to produce a whole systems understanding that leads to the best management options. EBOP is now doing a good job in co-ordinating the various science and management investigations, but it is the author's view that this could be strengthened by bringing the scientists together to debate the science that is needed to best support the highest priority management options.
Clearly this is a matter for EBOP to determine, but similar approaches in other parts of the world have helped to optimise the final management measures. This is especially true of the critical investigations that have been referred to earlier, to support the short-term management measures that have been recommended.
Recommendation 35: That EBOP considers ways of better co-ordinating and focusing the scientific investigations that will underpin the preferred short- and longer-term management measures.
Trout fishing
Trout fishing is an important part of the tourist economy for the Rotorua district and Lake Rotoiti is prized for its large 'trophy trout'. Lake Rotorua is also an important lake for trout fishing. Fish and Game New Zealand have a sophisticated monitoring and release strategy to optimise larger trout, especially in Lake Rotoiti.
There is evidence that the average fish size in Lake Rotoiti is declining (Fish and Game New Zealand and Ted Boucher - pers comm) and this is concerning for the tourist industry and the economy of the district. Recent studies by the University of Waikato are showing that marked habitat changes have occurred in Lake Rotoiti over the last decade (Chris G McBride, Brendan J Hicks and Michael van den Heuvel) and that the area of the lake habitable for fish over the summer months has decreased.
While the exact cause for the decline in size is not clear, it is important that this work continues as it is valuable for the tourist industry and it provides another link in the relationship between the decline in the lakes and the responses of the biota.
Recommendation 36: That EBOP continues to work with Fish and Game New Zealand and the University of Waikato to support investigations into the impact of the decline of Lakes Rotorua and Rotoiti on the trout fishery.
Ecosystem targets and management
While it is not directly part of this report, the overall health of Lakes Rotorua and Rotoiti must be considered and this is more than just the frequency and severity of algal blooms. Important also is the overall ecological health of the lakes, including the aquatic plants, the native fish and the fringing vegetation. Discussions with both EBOP and the Department of Conservation indicate considerable concern at the deterioration of the ecosystems in both lakes.
This is exemplified by the decline in native aquatic plants which have been smothered by invasive exotic aquatic weeds. NIWA (John Clayton and Tracey Edwards) supported by EBOP has developed 'lake health indicators' based on both the condition of native aquatic plants and the degree of impact by invasive weed species. A combined index, the Lake SPI or lake 'Submerged Plant Indicators' is a new management tool that uses submerged plants from within lakes to indicate lake ecological condition.
The results of this work for Lakes Rotorua and Rotoiti as set out in a poster presented by NIWA in 2003 are:
Lake Rotorua
- lake condition very poor
- lake SPI Index increased slightly due to declining invasive species
- blue-green algae covering plants
- worst invasive weed (hornwort) present but having little effect
- lake condition moderated by high exposure that helps to minimise invasive weed impact and water quality features.
Lake Rotoiti
- overall condition - very poor
- lake SPI Index has decreased
- blue-green algae smothering plants
- invasive weed species close to maximum impact
- worst rating weed (hornwort) dominant
- native condition at only 18% of its potential.
Overall, this work supports the TLI measurements and Professor Hamilton's work that shows the worst lakes are Lake Rotoiti, Lake Okaro and Lake Rotorua. While the toxic blue-green algal blooms have taken the limelight because of their immediate public impact, it is the overall ecological health of the lakes that is most important.
EBOP is to be commended for supporting the work to develop the Lake SPI Index and should continue to support this type of work to develop and refine broader ecological and ecosystem targets to aid management of the lakes. The following recommendation is perhaps a fitting place to end this report as it is about overall ecosystem health, for this is what we all, iwi and pakeha alike strive for.
Recommendation 37: That EBOP works with other organizations, such as the Department of Conservation and the Ministry for the Environment, to develop readily measurable lake ecosystem health indicators as an aid to measure the success of short- and longer-term management measures.
