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Draft Guidelines for the Selection of Methods to Determine Ecological Flows and Water Levels

Acknowledgements

Executive Summary – Recommendations

1 Introduction

2 Rivers

3 Lakes and Wetlands

4 Groundwaters

5. References of Main Text and Appendices

Appendix 1: Relationship between Total Allocation and Ecological Flow Requirements in Rivers

Appendix 2: Effects of Flow Regime on Stream Ecology

Appendix 3: Methods Used in Ecological Flow Assessments of Rivers

3 Lakes and Wetlands

3.1 Ecological values, factors and principles

3.1.1 Values

Lakes

In lakes, the hydrological regimes that influence inflows and outflows result in changes to lake levels, and levels rather than flows per se have the greatest influence on lake ecosystem values, especially in larger lakes. Water level fluctuations control aquatic biota in lakes primarily through their effects on the species composition, distribution and productivity of the shallow-water littoral zone communities. In addition, the inflow-outflow regime influences the lake water residence time, a key parameter in setting lake water quality over long time frames. Changes in residence time are most likely to affect smaller lakes. A reduction in through-flow and increase in residence time increases the likelihood of algal blooms. At the other extreme, large decreases in residence time can reduce the productivity of lake communities.

Rules for hydrological management for lakes should be based on permitted ranges in water level and rates of water level fluctuation that protect lake values. There have already been cases where local communities have identified lake values to be protected by management rules. In the case of Lake Taupo (Taupo-nui-a-Tia 2004), 12 values were clearly identified and recognised, especially clear water, diverse plants and animals, good trout fishing, high-quality inflows and a weed-free lake. The guidelines for the protection of Lake Manapouri relate particularly to water level fluctuations in the context of hydro-electric extraction, and these can be applied to other lakes where water abstractions may affect lake level and shoreline fluctuations. Table 3.1 lists ecological values associated with lakes that are related to water levels.

Table 3.1: List of lake values and factors related to water levels

Wetlands

Hydrology is the fundamental driver of all ecological processes in wetlands and directly or indirectly controls all aspects of nutrient cycling and availability, water level, primary and secondary productivity, and habitat availability (eg, relative areas of vegetated habitat versus open water). Water level fluctuations are particularly critical for ecological values and the controls on the distribution of organisms in most wetlands. Aspects such as the depth and duration of flooding periods, depth to water table, and duration of drawdown periods when the water level is below the soil surface, are those that control distributions of organisms in most wetlands. Flow velocity may directly affect some wetlands that are very closely linked to a river channel, but most riverine wetlands are relatively quiescent backwaters or adjacent depressions that are indirectly controlled by groundwater inputs and overland flooding.

Vegetation is sensitive to water regime because plant species differ in their tolerance to flooding. Species diversity is generally highest in wetlands with moderate water level fluctuations. It decreases if the water level remains constant or fluctuates widely. For individual wetlands, a variety of water depths within a site, with a mixture of open water and shallows, allows emergent and submerged plants to grow. The water regime is also very important for preventing terrestrial weed invasion and limiting mammalian predator access, giving the wetland system its robustness to withstand external pressures. Many examples of loss of natural character in New Zealand wetlands relate to water abstraction and drainage that lower water tables, allowing competitive terrestrial flora to invade and displace flood-tolerant wetland species. The effects of water levels on the structure (ie density, height, and physical complexity) of lake littoral zones and wetland vegetation are important for invertebrate and periphyton values, and affect the availability of habitat for fish and birds. Different bird species, in particular, have different vegetation preferences for nesting and feeding.

There are other ways in which the water regime can control biotic composition. The timing and duration of the connectivity of wetlands with their parent lake and rivers control migration patterns of fish and breeding cycles of birds. Wetting and drying cycles determine the hatching of invertebrates, the flowering of many plants and the availability of areas of shallow open water for wading birds.

Table 3.2 lists values associated with wetlands and related to water levels.

Table 3.2: List of wetland values and factors related to water levels

3.1.2 Principles for determining significance of the values

Lakes

The significance of lake values determines the level of protection lakes are given and the methods used to assess flow and level requirements for maintaining their values.

A classification of New Zealand lakes similar to the River Environment Classification (currently under development) may form a further basis for setting values. This work is ongoing, but a recent snapshot of New Zealand lake water quality’ (Sorrell et al 2006) produced for the Ministry for the Environment provides a first step in comparing the state of a lake’s quality in relation to regional and national averages.

When considering ecosystem values of lakes, there are two distinct zones that respond quite differently to changes in lake levels. These are the littoral zone and the pelagic – open water or pelagial – zone (Figure 3.1). The littoral zone is the shallow-water zone around the edges of lakes (in contrast to the open water pelagic zone) that is occupied by submerged plants attached to the bottom. The littoral zone is characterised by high biodiversity and multiple ecosystem functions and is the important interface between land and lake. It can easily be envisaged from Figure 3.1 that the littoral zone will be affected by lake level variation. The situation is exacerbated in turbid lakes where the depth of the littoral zone is restricted by low light penetration.

The shape of the lake basin, the lake shoreline length and the water clarity dictate the extent of the littoral zone versus the pelagic zone. In deep steep-sided lakes, the littoral zone may be relatively less important in overall lake functioning than in shallow lakes with gently sloping shorelines. Even in the former, there may be distinct and specific high values associated with individual species in littoral zones (eg, spawning sites for some fish species, cover and nesting sites for bird species).

Wave-cut platforms are common features in large lakes where wave action is significant and the bottom profile of the littoral zone is modified by waves. These are commonly ideal areas for the development of a diverse and productive littoral community and their characteristics (and existence) depends on the interactions of waves and lake levels. Any alteration to lake levels in such situations may have profound effects on lake edge ecosystems. It should be recognised that in many cases when lake levels change over long time periods (ie, a long-term change in mean lake level) littoral communities may adapt over time to the new levels by moving up or down the lake bed profile. Alternatively, if the magnitude in the fluctuation of lake-levels changes at shorter (seasonal) time scales, this can have adverse effects on littoral zone communities (James and Graynoth 2002).

Natural lakes with high water quality are often those with relatively high oxygen concentrations in the lake water column throughout the year. Maintenance of oxygen throughout the water column ensures habitats for a wide variety of organisms. Oxygen is maintained through wave action and wind mixing, and through inflows that sink to the lake bed while carrying dissolved oxygen down with them.

Wetlands

Different types of wetlands have characteristically different hydrology. Rainwater-dominated wetlands such as peat bogs rarely have standing water, and the water level is usually close to the surface, responding primarily to rainfall events. Groundwater-fed wetlands will respond to adjacent rivers, but often with pronounced time lags between changes in river flow and changes in wetland water level. Wetlands with surface connections to rivers may mimic the river fluctuations closely, or there may be sudden changes in wetland water level, eg, if the river needs to overtop natural or artificial banks and levees before the wetland floods, as applies to many floodplain wetlands.

Wetland ecological significance can be judged on factors such as national or regional significance, rarity, and representativeness. Wetland significance is judged against a background of widespread loss of wetland environments in New Zealand: over 90% of the pre-European wetland area has been lost to landscape development. Much of the remaining wetland area is alpine, and a disproportionate number of remaining sites are peaty bogs and fens not associated with rivers. Riverine wetlands are highly likely to have considerable significance in regions where large areas of wetlands have been lost. Wetlands are also important for values including hydrology, nutrient retention, recreation and culture. The historic loss and reduced current extent of wetlands in New Zealand justifies a very strict standard for any flow alteration that would lead to loss of wetland area or condition.

A number of resources are available for assessing significance of wetlands. An important first step in determining significance of a wetland is classification of the type of wetland (described by the MfE-sponsored classification scheme of Johnson and Gerbeaux (2004)). The condition of a wetland, which relates to its significance, can be assessed from the condition assessment methods of Clarkson et al (2003). Important wetlands of New Zealand were compiled in the WERI (Wetlands of ecological and representative importance) database held by the Department of Conservation, recently updated as part of the Waters Of National Importance (WONI) project, which has also documented the historical extent of wetlands. The NIWA Freshwater Fish Database includes records from many wetland habitats. Botanical values of wetlands have been documented in many regional flora and there are many reports on specific wetlands produced for a range of local authorities. Landcare Research and NIWA are currently developing an environmental database that will allow species diversity of all major taxonomic groups to be related to wetland types, nutrient regimes and hydrological regimes. For assessment principles of representativeness, rarity etc, the methods of Whaley et al (1995) were developed specifically for wetlands and have proven to be robust in a number of regulatory and environment court hearings.

As so much of the biodiversity value hinges on vegetation type and structure, maintaining vegetation properties is the most critical factor when assessing flow (ie, flooding and water level) requirements for wetlands. Wetlands can be very difficult to restore once soil drying has changed vegetation character, especially if large ‘transformer’ weeds such as willows have invaded. Connectivity requirements for fish access and habitat requirements for birds are the other main critical flow-related factors.

3.1.3 Principles for determining potential change to flow regime/levels

Lakes and wetlands are characterised by natural water level fluctuations. No natural lake or wetland has a precisely constant water level. Seasonal drawdown and recovery of water levels are ubiquitous, and there can also be wide inter-annual variability in water regimes. In the case of wetlands, at one extreme, there are ephemeral wetlands that remain dry for months or years at a time, with no apparent aquatic character, and flood irregularly. Even in permanent wetlands, water levels can disappear below the surface for prolonged periods, and standing water may only occur during brief floods. Many wetlands are a mosaic of depths, with permanent water in some areas, intermittently wet areas, and semi-wet margins that often support highly diverse mixtures of wetland and terrestrial species.

Variability in water level is what promotes the high species diversity in wetlands, and the littoral zones of lakes. This is demonstrated in Figure 3.2.

The key principle in predicting change is understanding the current water level variability of a site, how it relates to the current distribution of plants and animals, and how much the water level variability can change before species are lost or weeds and pests invade. The seasonal pattern of water levels (ie, the depth, timing and duration of standing water and drawdown below the surface) is termed the hydroperiod. The maintenance of a period of levels high enough to ensure connectivity between wetlands and adjacent water bodies is often fundamental to their existence as discrete ecosystems, and essential for migrations and other movements of fish that use both habitats.

Some knowledge of the existing hydroperiod is therefore essential in predicting the likely effects of flow abstractions or, in some instances, additions. Where available, historical records can be used very robustly to do this. Regular water level monitoring is a feature only of those lakes that are used for commercial or drinking water purposes. Water level monitoring is rare in New Zealand wetlands. Often it may be necessary to interpret the water regime from topographic information, soil types, and mapping of connectivity to rivers. This approach can provide a simple indication of water-biota interactions, but in any situation where the site is of high value or where large changes are likely, direct monitoring of hydrological characteristics before the proposed changes occur are essential. Methods for monitoring wetland water levels include capacitance probes and piezometers, weirs, flow sensors, dipwells and tipping gauges. Campbell and Jackson (2004) provide a useful overview of these techniques and their data interpretation. Multiple recorders are essential in order to characterise the water regime of different communities in larger wetlands with many vegetation types.

How much change to the hydroperiod can most wetland and littoral communities tolerate? Models developed for lakeshore turf communities and littoral macrophytes are likely to be applicable to wetland environments with submerged plants and algae. For the taller vascular plants, wetland communities are complex and how individual species change differs, depending on the existing water regime. In general for wetlands with predominantly subsurface hydrology, permanent changes of < 20 cm in water table depth are unlikely to have much effect on species composition, whereas lowering the water table > 30 cm from its existing level typically leads to changes in species composition. This is because the live roots of most wetland species occur mostly in the top 30 cm, and drying this zone allows terrestrial species to invade. In some sites the maximum depth over the season is the important feature determining species composition, in others it may be the median depth, or some other feature. Continuous water level records provided by equipment such as capacitance probes can be invaluable for making these assessments.

Many shallow-water plants that inhabit the littoral zone of New Zealand lakes require a dry period, when they are exposed to the air at some stage to allow for flower and seed production. In some locations this dry period may be only every few years, but in general, the shallow littoral zone of New Zealand lakes is adapted to some degree of water level variability. However, where the range in lake levels exceeds 2 metres and/or when the water levels vary too frequently, the littoral vegetation may disappear. For example, Lake Hawea has high clarity and an extensive littoral zone should be supported; yet it has a very limited littoral zone due to artificial water level fluctuations of greater than 8 m.

3.2 Determining the degree of hydrological alteration

3.2.1 Lakes

The distribution and occurrence of healthy lake littoral habitats and communities varies with lake size, depth and water clarity. The risk that changing lake levels decrease available habitat or adversely affect communities depends on the lake bed profile (bathymetry), substrate type, water clarity, wave action as well as size and depth. The risks of deleterious effects are greater in shallower systems than in deep water bodies. Within a lake level range, impacts arise from changing seasonality in levels and the proportion of time spent at different levels (level duration).

Three parameters relating to lake level change are considered in Table 3.3. These are: median lake level (m), mean annual lake level fluctuation (difference (m) between maximum and minimum lake level), and seasonality of levels (seasonal pattern of relative summer vs winter lake levels). Two types of lakes are distinguished: deep lakes (> 10 m maximum depth) and shallow lakes ≤ 10 m maximum depth) as the sensitivity to level alteration depends on the lake depth.

Table 3.3: Descriptors of hydrological change for lakes

3.2.2 Wetlands

The distribution and occurrence of healthy wetlands varies with size and depth and connectivity to other hydrological systems. The risk of changing lake levels decreasing available habitat or adversely affecting communities depends on the depth and the bathymetry and the dominant species present. The risks of deleterious effects are greater in shallower than in deep-water wetlands.

Table 3.4: Descriptors of hydrological change for wetlands

The effect of changing inflows and/or outflows and therefore changing levels depends not only on the magnitude of change, but also on the periodicity (hydroperiod) and duration of the levels.

3.3 Which method? Decision-making framework

3.3.1 Principles for selecting methods

Lakes

The distribution and occurrence of healthy lake littoral habitats and communities varies with lake size, depth and water clarity. The risk that changing lake levels decrease available habitat or adversely affect communities depends on the lake bed profile (bathymetry), substrate type, water clarity, wave action as well as size and depth. The risks of deleterious effects are greater in shallower systems than in deep water bodies. Within a lake level range impacts arise from changing seasonality in levels and the proportion of time spent at different levels (level duration). Level duration profiles graphically and quantitatively demonstrate the lake level regime (Henderson and Clement 1995; ECNZ 1995; Genesis Power Limited 2000). However there is no easy way to use these in a generic rule-based format as they are generally calculated from absolute altitude. It may be possible to convert these to a relative level base on variance from a mean (or median lake level). This needs to be explored. We recommend that work be commissioned to provide scientific justification for this categorisation and provide an equivalent of mean annual low flow in rivers (and other flow statistics) based on level duration curves.

The decision as to which method to apply for assessing the ecological level requirements of a particular lake depends firstly on the significance of the value to be managed (Section 3.1.2a) and secondly on the potential for hydrological alteration (Section 3.2.1). This framework is presented in Table 3.5: one or more of the methods listed within each cell should be used to assess ecological flow and level requirements for the relevant degrees of hydrological alteration and significance of instream values. In situations with high lake values, two or more methods from each cell should be used, because the risks to ecology of making an incorrect ecological flow decision are greater.

The methods within each table cell are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. For example, if the stability of banks due to wave action, and subsequent effects of turbidity were the perceived major ecological problem of a proposed drawdown in level, then there would be little sense in using a hydrodynamic water quality model. In contrast, if the potential hydrological alteration had the potential to change hydrodynamic processes within the lake, thereby affecting phytoplankton production or oxygen depletion, then a hydrodynamic water quality model should be used where the potential degree of hydrological alteration justified it.

Table 3.5: Methods used in the assessment of ecological flow and water level requirements for degrees of hydrological alteration and significance of lake values

Wetlands

The distribution and occurrence of healthy wetlands varies with size and depth and connectivity to other hydrological systems. Changes in flow regime are likely to have their greatest effect on wetlands through effects on water level. Falling water tables dry and oxidise soil, leading to peat shrinkage, weed and predator invasion, and loss of specialised wetland flora and fauna. Raising water levels stress wetland plants and decrease plant species diversity, and increase sedimentation and eutrophication. The potential change of the flow regime is therefore defined in terms of its effects on the median annual water table of the site. There may be shorter-term effects on the hydroperiod (eg, changes in seasonality) that can also impact on wetland values.

The risk that changing wetland levels decrease available habitat or adversely affect communities depends on the depth and the bathymetry and the dominant species present. The risks of deleterious effects are greater in shallower than in deep-water wetlands.

The decision as to which method to apply to assess the ecological level requirements of a particular wetland depends firstly on the significance of the value to be managed (Section 3.1.2b) and secondly on the potential for hydrological alteration (Section 3.2.2). This framework is presented in Table 3.6 and one or more of the methods listed within each cell should be used to assess ecological flow and level requirements for the given combination of degrees of hydrological alteration and significance of wetland values. In situations with high wetland value, two or more methods from each cell should be used, because the risks to ecology of making an incorrect ecological flow decision are greater.

The methods within each table cell are not listed in hierarchical order and the choice of method(s) depends upon the perceived ecological problem affected by the flow regime. For example if connectivity with ground or surface waters was a critical factor, then this should be one of the assessment methods used for high-value wetlands; yet such assessment may not be necessary if the wetland is perched without hydrological connections.

Decision pathway to setting ecological water levels

The decision pathway that should be followed to set ecological water levels in lakes or wetlands is as follows:

1. identify the spatial boundaries of the lake/wetland, and the impacted area (depth range over which water regime will change)
2. identify the specific values that will be affected in the impacted area
3. quantify the range and seasonal timing of change in water level in the affected area
4. select appropriate methods from the values/change matrix in Tables 3.5 and 3.6
5. quantify the change in hydroperiod consistent with requirements for maintaining values.

Table 3.6: Methods used in the assessment of ecological flow and water level requirements for degrees of hydrological alteration and significance of wetland values

vi. if methods cannot give clear solutions, make ‘best information’ decision on change in hydroperiod that:

• is suitably conservative

• represents conditions of historical water levels in the system (eg, median historical water levels)

• represents time-varying conditions of water level fluctuations (ie, hydroperiod)

• considers long-term requirements of species for high and low levels (eg, for life cycle passage and recruitment events), including connectivity requirements for fish passage)

• maintains ecological flows in connected surface waters and groundwaters

• includes spatial and temporal limits on level variability.

3.4 Summary of methods

The following table summarises the methods shown in the decision-making framework, with their advantages and disadvantages.

Table 3.7: Summary list of methods for lakes and wetlands from the decision-making framework, with pros and cons for use

View summary list of methods for lakes and wetlands from the decision-making framework, with pros and cons for use (large table).

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