Feature

Clear Water and Cryptic Ecosystems

Lakes in New Zealand's South Island have some very interesting special features.

Dr Anne-Maree Schwarz
and the Lakes Research group
NIWA Christchurch

The deep glacial lakes of the South Island form the centrepiece for much of our tourist trade whether viewed from the walking tracks of Fiordland or the restaurants of Queenstown.

These lakes, primarily due to their locations amongst mountain ranges and, in the case of the Fiordland lakes, in a National Park, tend to have catchments that have been little modified by humans. Even where farming has been established it has often been at a lower intensity than in many other parts of the country. Less intensive farming practices and relatively low population density has restricted nutrient input to the lakes, resulting in the majority of them maintaining high water quality and clarity. Some lakes in the area such as Tekapo and Pukaki, have reduced water clarity as a result of inputs of glacial flour via their tributary streams.

The state of New Zealand's environment report emphasises the relatively pristine nature of the South Island lakes in a summary statement:

New Zealand's 30 or so large deep lakes appear to be of high quality. However more than 700 (of New Zealand's) lakes are shallow and between 10% and 40% of these are nutrient enriched (eutrophic). Most of the eutrophic lakes are in the North Island and in pasture dominated catchments.

With some exceptions, most of the group of large lakes in terms of both area (100km²) and depth (300m) are glacial lakes in the South Island. They include the deepest lake, Lake Hauroko in Fiordland, with a maximum depth of 462 m, and, in many respects, these are the "jewels" amongst New Zealand's lakes.

The significance of deep glacial lake ecosystems is enhanced by the fact that they harbour increasingly rare native aquatic plant community types. While the ecology below the surface is not obvious to most visitors, by venturing below the water surface New Zealand scientists have recorded features of the biology and plant species diversity that rate these lakes highly on both national and international scales.

Biodiversity can be described at a range of levels and includes diversity within and between species, as well as at the broader scales of communities and ecosystems. New Zealand's biodiversity strategy has a number of specific goals but generally proposes actions that are aimed at halting the decline in indigenous biodiversity. Pressures on indigenous biodiversity within terrestrial ecosystems are widely publicised, yet the extent to which the native flora and fauna of New Zealand's lakes are being protected has received less attention. This is despite the significant cultural, economic and recreational value of these lakes to the country.

The shallow zone of lakes around the shoreline where submerged aquatic plants (macrophytes) grow attached to the substrate is termed the littoral zone. The littoral zone acts as an important interface between inputs from activities in the catchment (i.e. via rivers) and the open water of lakes. Submerged macrophytes in the littoral zone have long been recognised as an important three dimensional surface to which smaller algae (epiphytes) can attach and grow, and the micro- and macroscopic plants in this zone may contribute significantly to whole lake productivity.

Stylised diagram of native species of submerged aquatic macrophytes in a lake littoral zone. In shallower water, the zone is colonised by turf species and other flowering plants and at greater depths characean macroalgae dominate.

Macrophytes and their associated epiphytes are particularly important primary producers in low nutrient (oligotrophic) lakes, where extensive growths of algae in the open water are uncommon. In such lakes, the concentration of food amongst the littoral zone plant community can be an oasis of production in an otherwise sparse environment.

Recent studies by NIWA (the National Institute of Water and Atmospheric Research) using isotopic ratios, a technique which traces food sources through various trophic levels, have established that epiphytes are the dominant food source for macroinvertebrates. These are in turn the food source for fish, including native species and introduced trout, which feed in the littoral zone for part or all of their life cycles. In the areas where submerged macrophytes are absent, this littoral food web is vastly different, with low diversity and productivity. Therefore we can surmise that the presence of a diverse, productive submerged macrophyte community, within lake littoral zones, is a fundamental indicator of a healthy lake, potentially capable of supporting invertebrate and fish populations.

Changing Times,
Changing Species

As a result of the dedicated efforts of a small group of scuba-diving scientists in New Zealand since the 1970s, there are valuable records of aquatic macrophyte species composition and depth limits in many New Zealand lakes. This essential background information has enabled recent research to focus more on the processes determining how such plants grow and survive, as well as how they respond to changing growth conditions.

One such change in conditions has been the invasion, by introduced species, of native submerged macrophyte communities. Macrophytes that grow beneath the water surface of lakes often tend to come to the attention of the public only when they are causing problems for activities associated with use of the water. Many native species tend to have a relatively short growth form or, if taller-growing species, seldom reach nuisance proportions for recreational activities. However, where macrophytes interfere with recreation or hydro-power generation, submerged macrophytes are viewed as weeds, for example the conspicuous, surface-reaching exotic oxygen weeds in Lake Dunstan on the Clutha River.

New Zealand's native submerged vegetation is susceptible to invasion, and competitive exclusion, by introduced species which grow extremely well in our temperate climate, as is the case for many examples in the terrestrial flora. In some North Island lakes, introduced species have completely displaced native assemblages. The introduction and establishment of exotic species in South Island lakes has lagged behind that of the North Island, perhaps because of the relative isolation of sites, and perhaps because of the different environmental conditions for growth. However the prolific growth of the oxygen weed Lagarosiphon major in Lake Dunstan, and its occurrence in other sites around the South Island, serves to illustrate that these species have the ability to dominate the submerged aquatic vegetation of many South Island lakes.

The extensive modification of native submerged macrophyte communities elsewhere in the country increases the significance of those that still dominate in the South Island glacial lakes. The susceptibility of native communities to invasion by introduced species, and therefore their increasing rarity, means that they assume great significance on a national scale, in terms of the biodiversity of New Zealand's aquatic flora.

What determines where submerged macrophytes grow? There is another reason why the glacial South Island lakes can be considered as special. Not only has the flora of these lakes largely escaped dominance by introduced weed species to date, but they also boast some of the deepest records to which aquatic macrophytes grow in New Zealand, and rate as some of the deepest freshwater records in the world.

There is a distinct zonation of different groups of submerged macrophytes which is related to increasing depth in lakes. The depth zonation can depend on a myriad of environmental factors including substrate, temperature, nutrient availability and grazing animals. In shallow water, the development of the littoral zone macrophyte community is strongly influenced by wave action and changes in water level. The effects of wave action include physical breakage and uprooting of plants, and are exacerbated at low lake levels when macrophytes which are usually submerged are more exposed to the elements. At extremely low water levels, shallow-growing plants may be exposed to the air. While some of the species in the shallow zone can tolerate periods of exposure, particularly if there is groundwater seepage in the area, many can not and will eventually die.

At depths which are greater than those affected by wave action, the dominant factor determining zonation is considered to be the availability of light for photosynthesis. Therefore, in general, depending on limitations imposed by plant growth form, the clearer the water, the deeper plants can grow.

The macrophytes that grow in shallowest water (approximately 0-5m water depth) comprise a low-growing assemblage which forms turfs (this includes species of Glossostigma, Isoetes, Lilaeopsis and many others). The depth range for tall-growing, flowering macrophytes overlaps with the turf community and, in the native state, is comprised of species of Potamogeton and Myriophyllum from depths of about 1 to 10 m (where light levels permit). It is this zone which is most heavily affected by introduced aquatic macrophyte species.

There are two groups of macrophytes which extend below depths of 10 m. The first is an association of macroscopic algae called characeans (Nitella spp. and Chara spp.) which grow attached to the sediment. They are often referred to as "characean meadows" owing to the extensive cover of plants extending below the maximum depth of the taller- rowing, flowering plants. In New Zealand lakes, characean meadows can extend to depths as great as 40m if light, substrate, slope and other factors are favourable.

The second, and deepest of these two groups, is the bryophytes: assemblages of mosses and liverworts considered internationally significant in terms of their depth range, development and diversity. These "deep water bryophytes" are known to occur in only a handful of lakes around the world and persist at depths greater than 50 m where light may be reduced to 0.01% of surface levels. Such low light is two orders of magnitude below the light requirements of the shade-adapted deepwater characeans and approaching the lowest recorded light levels for plant growth.

The South Iland glacial lakes boast some of the deepest records in the world for characeans and bryophytes. A recent study which compiled data from 153 lakes around the world noted that only Lake Tahoe in California had deeper records for characean algae than New Zealand examples. Although the deep bryophyte communities are probably not as well known world-wide, owing to the difficulty of access by scuba diving, similarly impressive statistics can be quoted for their depths.

Why Such Depths?

There are two main reasons that can be put forward to explain the deep records for macrophyte growth in clear South Island lakes. The first of these is high water clarity. The clarity of lake water, and therefore the depth to which sunlight can penetrate, depends on what is suspended or dissolved in the water column. Suspended sediment, phytoplankton, detritus and coloured compounds all affect water clarity and the relative contribution of each of these varies from catchment to catchment and lake to lake.

Previous work in the NIWA lakes research programme has focused on growth conditions for submerged macrophytes at the deepest limits, where light levels are so low as to become critical for growth should water clarity decrease for any reason. Extremely clear, blue water is a feature of lakes such as Wakatipu, Wanaka and Hawea. Because sediment inputs are relatively low for most of the year, and low nutrient concentrations mean a low phytoplankton biomass, there are very few constituents in the water to stop light from penetrating to great depths in these lakes. For example, enough light for characean photosynthesis to occur can penetrate to depths greater than 30-40 m in Lake Wakatipu.

Although they also have high water quality, the Fiordland lakes have more of a brown colouration owing to the tannins, from the predominantly beech forested catchment, that leach into the waters flowing into the lake. This restricts the depth to which light can penetrate, and the depth at which sufficient light penetrates for characean photosynthesis is closer to 15 m in Lakes Te Anau, Hauroko and Manapouri.

The Shallow zone of Lake Te Anau. There is a clear line of demarcation between the shallowest zone on the left which is most affected by changes in water level and the deeper water on the right where macrophytes have established. Periphyton-covered rocks can be seen to the left. The submerged macrophytes to the left are Isoetes alpinus and Myriophyllum triphyllum.

The second characteristic which helps explain deep records for macrophyte growth is the relative absence of grazing animals in South Island lakes compared to the North Island, and compared to other parts of the world. The New Zealand native fish fauna lacks an herbivorous component and the only significant native herbivore in New Zealand lakes is the koura (freshwater crayfish). In North Island lakes, koura have been observed grazing on characean algae and causing physical disruption by their foraging activities. However koura are rarely seen in South Island lakes and it has been suggested that their absence enables plants to extend to greater depths in clear South Island lakes compared to similar North Island examples.

To date we know less about the resilience to disturbance of native turf species and tall-growing flowering macrophytes than deep water species. The growth dynamics of aquatic macrophytes and periphyton in shallow water is the focus of part of NIWA's current lakes research programme funded by the Foundation for Research, Science and Technology. This programme specifically investigates the effects of water level fluctuations and resuspended sediment on the plants and animals of the littoral zone of lakes. We are investigating how different species in the shallow zone respond to physical stresses through wave action and how they photosynthesise, and therefore grow, under changing light conditions.

We are simultaneously investigating the ways in which macrophytes and algae are consumed by littoral zone invertebrates, and how this energy is passed on to predatory fish, both native and introduced. Through this programme, we are identifying assemblages of species which can be associated with specific habitats within lakes. Our aim is to be able to make predictions about the depth zones each species will occupy, their productivity and their interactions with other organisms, under a given light and water level regime.

The very features that make lakes such as Wanaka, Wakatipu, Manapouri, Te Anau and Hauroko attractive to view -- clear, nutrient-poor water and bush clad, mountainous surroundings -- are also the basis of their special ecological characteristics. By better understanding littoral zone productivity and the ecology of native species in the oligotrophic glacial lakes, we will be better placed to predict their susceptibility to human impacts and invasion by exotic species. The effective management of littoral zone plant communities is integral to the overall ecological health of our lakes and has implications for maintenance of biodiversity and aspects of the landscape valued highly by New Zealanders and tourists alike.