Introduction

Few Biological studies of thermal waters have been made in New Zealand, but it has been known for many years that a thermal fauna is present. Over 60 years ago, Poynton (1903) described ephydrid (Diptera) larvae from the Black Terraces, Taupo and in 1923, Stoner made brief observations on insects present in hot pools and rivulets at Ohinemutu. More recently, Winterbourn and Brown (1967) have investigated the faunas of two warm streams near Taupo, and zoologists at the University of Auckland are at present making an ecological study of the warm lake, Rotowhero (34°C approximately) at the base of Rainbow Mountain, Waitotapu (J. G. Pendergrast, pers. comm.).

The present account is based primarily upon field work done by the author in the Rotorua-Taupo-Tongariro region of the North Island during the period 1965-67. Observations were made in the 10 thermal areas listed in Table 1.

Thermal waters and hot springs occur most frequently in areas of volcanic activity. Important ones are found in Europe, Iceland, Algeria, the United States and New Zealand, the largest and most varied group of hot springs in the world being in Yellowstone Park, Wyoming. The biological characteristics of thermal waters depend on several factors of which the most obvious is the high temperature of the water. Because of this, only a small amount of dissolved oxygen is found (Ruttner 1953). and frequently large quantities of dissolved salts are also present. With reference to temperature, thermal waters are normally considered as those having temperatures sufficiently high so that members of the general freshwater fauna do not usually live in them. Pennak (1953) considers that, in general, 25-30°C represents the dividing line between a rich fauna at low temperatures and a poor fauna at high temperatures, and that 35-39°C represents the normal upper temperature limit for most members of the general freshwater fauna. In this account the term hot is used to denote waters having temperatures greater than 39°C, whereas those of a lesser temperature, but still greater than ambient air temperatures, are referred to as warm.

Chemical and Geological Characteristics of Thermal Regions

According to the detailed account given by Grange (1937), thermal waters can conviently be classified according to their chemical characteristics as chloride, sulphate, mixed chloride and sulphate, or bicarbonate springs. In the Rotorua-Taupo district, sulphate and chloride springs predominate. The former are typically acid, but the latter are generally alkaline although a few are weakly or strongly acid. Creamy-grey sinter is deposited freely by alkaline springs and to a small extent or not at all by acid ones. Characteristically, chloride springs are located in valley bottoms or on wide flats with a good supply of ground water, and from them strong or moderate overflows are common. By comparison, sulphate springs are commonly found on steep slopes or wide flats with probably limited ground water supply. Sodium is the principal cation for chlorides, sulphates and carbonates, and with potassium is present in considerable amounts. Silica is highest in chloride springs whereas aluminium and iron are found mainly in sulphate waters. Magnesium and calcium are generally present in small quantities.

The thermal areas considered here can be classified as follows:—

The Macrofauna

Because the taxonomic status of many of the invertebrates found is not clear (in common with much of the general fresh water fauna) identification of a number of species has not been attempted below the generic or even familial level. Members of the most important groups found are illustrated in Figs. 1 and 2.

Members of 8 phyla have been found in New Zealand thermal waters. Of these, the Protozoa, Aschelminthes, and Nematoda, all containing hot water species, will not be discussed here (See Winterbourn and Brown, 1967, for an account of their distribution in relation to observed temperatures in two Taupo warm streams). The Chordata is represented by a single species of fish. Mollienesia sp. Members of the Platyhelminthes, Mollusca and Annelida, although found in warm waters, were never present at high temperatures. Composition and temperature relations of the macrofauna of all warm and thermal waters examined is shown in Table 1. All page 113 temperatures cited were taken in close proximity to the animals at the time of collecting, but they should be considered only as a general indicator of the environmental temperature under which the animals live, as even thermal waters do not maintain a constant temperature. For example, continuous recordings were made with a maximum-minimum thermometer at single stations in Waipuwerawera and Waipahihi Streams, Taupo, over a 4 day period in March 1967. Rain fell continuously for 24 hours of this period and temperatures ranged between 22-24°C and 34-41.5°C respectively. A comparison between maximum temperatures at which New Zealand species have been found and maxima recorded elsewhere can be made by referring to Table 2 in which data from a number of sources have been collated.

Fig. 1: “Warm’ water species.
(a) Antiporus larva; (b) Simlimnaea fomentosa; (c) Ischnura aurora larva; (d) Ischnura larval gill; (e) Physa fontinalis; (f) Chironomus zealandicus; (g) Odontomyia larva; (h) Sigara; (i) Anisops wakefieldi; (j) Chironomus cylindricus larva. Length of scale represønts 1 mm.

The Insecta are the single most important group represented and include members of eleven families belonging to four orders. Two of the most important are the Ephydridae (Diptera) and Hydrophilidae (Coleoptera I. the only families containing species living at temperatures in excess of 39°C. Three hemipterans, Microvelia and Hydrometra, members of the supraneuston, and the terrestrial Saldidae. live in close association with thermal waters.

Fig. 2: “Hot’ water species and supra-aquatic associates
(a) Ephydrella larva; (b) Ephydrella pupa; (c) Erioptera larva; (d) Erioptera larval spiracular disc; (e) Enochrus adult; (f) Enochrus larva; (g) Saldula late nymphal instar; (h) Microvelia nymph (i) Scatella laiva; (j) Scatella, pupal respiratory siphons; (k) Scatella pupa. Length cf scale represents 1 mm.

The thermal fauna of New Zealand can conveniently be subdivided into three broad categories with respect to environmental temperature. Group 1 consisting of species of Ephydridae and Hydrophilidae are often present in large numbers at high temperatures and are not found in adjacent fresh waters at normal temperatures. Other closely related species of both families are found in non-thermal waters. Species constituting group 2 inhabit warm waters and in many cases are probably living close to their upper temperature limits. These species are also found regularly in normal fresh water habitats. This group can conviently be divided into two subgroups, the first containing the Chironomidae, Stratiomyidae. Anisops, Physa and Simlimnaea which may often be present in large numbers, whereas members of the second subgroup are rarely abundant and include Culicidae, Sigara, Cyprinotus, Antiporus and Ischnura. Group 3 contains species inhabiting spring waters which are no warmer than normal freshwaters in summer. It includes Potamopyrgus, Dugesia, Glossiphonia and the Oligochaeta.

Invertebrates commonly encountered in the littoral region of lakes in the Rotorua-Taupo district but not represented in warm waters include larvae of the damselflies Austrolestes and Xanthocnemis, larval Trichoptera. and the crayfish Paranephrops. Significantly, typical inhabitants of cold running waters (e.g. many larval Ephemeroptera, Plecoptera and Trichoptera, most, if not all of which live in water with a high dissolved oxygen content) are absent from warm running waters.

Habitats of the Fauna

Characteristics of the Fauna

Two of the main problems encountered by inhabitants of mineral and thermal waters are osmoregulation in an environment often of high salinity, and respiration in water which is generally low in oxygen. With regard to the first of these points, Brues (1927) has shown that there is a striking similarity between the faunas (of freshwater origin) of thermal waters, brackish ponds and the littoral zone of the sea. It is evident that freshwater groups which have developed species adapted to these environments have done so through their ability to adjust their metabolism to the increased osmotic pressure of the medium. Wise (1965) has listed the New Zealand insects which have been found inhabiting the marine littoral zone and saline ponds and these include flies of the families page 118 Tipulidae, Culicidae and Ephydridae as well as dytiscid beetles, all of which have representatives in mineral warm spring waters. Similarities can also be seen between the macrofaunas of thermal waters and the saline lakes of South Eastern Australia studied by Bayly and Williams (1966). They recorded the hydrophilid Enochrus, Culicidae. Chironomidae. Tipulidae, Ephydrella, and, of special interest, larvae of the zygopteran Ischnura aurora described by O'Farrell (1965) as an “opportunist’ species often found in temporary pools and from time to time in saline waters.

All remaining animals have aquatic respiration only and must extract oxygen from the surrounding water. Two groups of insects are included in this category, chironomid larvae and larvae of the damselfly Ischnura. Both possess anal gills, and chironomids also contain the red respiratory pigment, haemoglobin, which appears to assist them to live in places deficient in oxygen (Walshe. 1950). Ostracods respire through the general body surface, and as a group are tolerant of wide fluctuations in temperature and water chemistry. The warm water species Cyprinotus incongruens is also commonly found in shallow puddles, an environment in which extreme high and low water temperatures are frequently encountered. Mason (1939). found that resistance to heat in Heterocypris balnearia was correlated with marked resistance to oxygen lack. The Oligochaeta. Hirudinea. Tricladida and prosobranch Mollusca are all confined to waters in which temperature rarely rises above that attained by general fresh waters. Temperature relations of the Mollusca have proved most interesting. Temperature acclimatisation has apparently been of considerable importance to P. fontinalis, as in England, Duncan (1959) observed them in the field at a maximum temperature of 18.8°C, and found it difficult to maintain them in cultures at 23-4°C which is 10°C below the maximum at which they have been found living in New Zealand. By comparison, the optimum temperature for Simlimnaea tomentosa in Australia was found by Boray (1964) to be about 26°C, although snails were tolerant of a wide range of temperatures, and he was able to keep snails alive for 6 weeks at a constant temperature of 36°C. A similar finding was obtained during the present study, and S. tomentosa from Waipahihi hot stream and local cool water habitats all survived in the laboratory for over a week at a constant temperature of 35°C. During this time, all snails were relatively inactive, however, and remained clustered around the air-water interface. Snails forced down into the water immediately crawled back out again. In a series of trials in which water temperature was increased 1°C every five minutes, snails began migrating to the surface when the temperature reached 31 °C. and all had arrived there at 35°C. It is interesting to consider these findings in relation to the observed distribution of S. tomentosa page 120 within Waipahihi Stream. In March 1967 snails were abundant in shallow water at the sides of the stream among grass and rushes where the temperature was 33.5°C, but no snails were taken in the adjacent main stream channel where a temperature of 37.5°C was recorded, although they had been found there a year previously, when temperatures recorded were several degrees lower.

The distribution of the prosobranch Potamopyrgus in relation to temperature is more restricted. Experimental work carried out by the writer (unpublished data) indicated that 28°C, the upper temperature at which snails were found in the field, coincides with the temperature at which a significant drop in the level of activity occurs prior to the onset of heat stupor. Further experimental evidence indicates that this decrease in activity is directly related to temperature increase and not to the associated fall in dissolved oxygen concentration of the water. Heat death occurs at 32°C. Not all prosobranchs are confined to cool waters however, and Paludestrina aponensis has been found living at 32-36°C in spring waters in northern Italy, and even surviving at 46°C (Hunter, 1964).

Heat death and resistance to high temperatures

According to Macan (1963) there are three main reasons why animals may be prevented from colonizing warmer water. (1) Because it is lethal, (2) because there is competition from more favourably adapted species which can move faster, eat faster, utilize food more efficiently and breed faster, and (3) because the temperature is never low enough to stimulate reproduction. Water temperature is obviously not lethal to occupants of warm springs or thermal waters although the animals may in many cases be living close to their thermal death points. Also, it seems highly likely that some of the species present in warm spring waters, e.g. Chironomus cylindricus, Simlimnaea tomentosa and Ischnura aurora, have been able to establish themselves because competition from more widely established cool water species is no longer found. As all the insects occupying warm springs and thermal waters have nonaquatic adults (except Coleoptera which are semi-aquatic) the environmental temperature under which larvae live should have no direct effect on reproduction. However, as the eggs of most species probably are laid in the larval habitat, they too must be able to develop normally at these higher than average temperatures.

Discussion

When compared with the thermal faunas of other countries it is clear that the New Zealand fauna is a fairly representative one. Brues (1927) found beetles and dipterous larvae, particularly Chironomidae, Stratiomyidae, Ephydridae and Tabanidae to be the most abundant aquatic macro-invertebrates in Yellowstone National Park, and Pennak (1953) has confirmed this for America as a whole, adding the Culicidae as another important insect group. Mani (1962) lists larval Ephydridae, Chironomidae, Stratiomyidae, Syrphidae and Ceratopogonidae but not Coleoptera from the North West Himalayas, while the most important groups found in Algerian hot springs by Mason (1939) were Ostracoda, Dytiscidae, amphibian tadpoles, Culicidae, Chironomidae and Notonectidae. Some of the overseas groups which do not appear to have warm water representatives in New Zealand are the Tabanidae, Syrphidae, and Ceratopogonidae (all Diptera), Gyrinidae (Coleoptera), Hydracarina, Copepoda, Trichoptera, Ephemeroptera, Amphibia and Crustacea-Malacostraca.