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Water weeds in Southern Africa
Aquatic Botany, 6 (1979) 377--391
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
WATER WEEDS IN SOUTHERN AFRICA
A. JACOT GUILLARMOD
Institute for Fresh Water Studies, Rhodes University, Grahamstown (S. Africa) (Accepted 17 May 1979)
ABSTRACT Jacot Guillarmod, A., 1979. Water weeds in Southern Africa. Aquat. Bot., 6: 377--391. In Southern Africa, introduced water weeds have become serious pest plants in fresh and even brackish water systems only within the past twenty years, although in one case (water hyacinth) a warning of what was likely to happen was printed as early as 1913. The four main species involved are: Eichhornia crassipes (Mart.) Solms-Laubach, Myriophyllum aquatieum (Vell.) Verdcourt, Salvinia molesta Mitchell and Azolla filiculoides Lam. Between their introduction and the present, these plants, with the exception of Azolla, have spread widely over the whole region in suitable habitats and now pose threats to all the main river systems, hydro-electric and other water impoundments. No indigenous plants in these four genera are so far considered dangerous weed species, though some may have an occasional nuisance value.
INTRODUCTION
The four dominant exotic weed species affecting the fresh water supplies of Africa south of the Kunene and Zambezi rivers are: Eichhornia crassipes (Mart.) Solms-Laub. ( water hyacinth); Myriophy llum aq uaticum (Veil.) Verdcourt (parrot's feather, latterly more often written parrotfeather -- see Fernald (1950)); Salvinia molesta Mitchell (Kariba weed); Azolla filiculoides Lam. (water fern). All have, apparently, been imported deliberately by man during this century as ornamental plants for fish ponds or aquaria: Until recently, they have been more or less ignored as economically unimportant. In the past t w e n t y years, however, these plants have spread widely in the main river systems and impoundments, these latter providing relatively still bays and inlets suited to the explosive growth of the weeds. Even brackish water may be infested with water hyacinth (C.C.T.A./C.S.A., 1957) and it is often found in very slightly saline coastal lagoons in Natal (Hailer et al., 1974). There is still lack of awareness of the danger these aquatic weeds can pose for the fresh water supplies of Southern Africa and also, ultimate responsibility for control or eradication of such weeds is difficult to determine. Stutterheim (1978) has stated that South Africa will be using its entire available water
378 supplies b y the year 1995 and there is a possibility of a 30% deficit by 2000 A.D., while Botswana is already in urgent need of more water supplies now for agricultural expansion. The distribution maps accompanying this article represent data compiled from herbarium records, printed reports and articles on the plants concerned, information from reliable observers and personal collecting b y the author. P. Ashton, B.R. Davies (B.R.D.), C. Howard-Williams (C.H.-W.), D.J. Steyn and P.A. Thomas have provided much useful data. HISTORY OF INTRODUCTION AND PRESENT DISTRIBUTION In three cases (Eichhornia, Myriophyllum, Azolla) a few plants of each were imported into Southern Africa for use in fish tanks or lily ponds (Stent, 1913; Oosthuizen and Walters, 1961; Jacot Guillarmod, 1976, 1977a, b,c, 1978b; Kluge, 1978). No explanation, except for a statement in an unpublished report (Mitchell, 1972b) has been found for the presence of Salvinia molesta in the Zambezi River system, where it existed at least a decade before the completion of the Kariba Dam (Edwards et al., 1973). However, as natural spread from this plant's original home in South America (Mitchell, 1972a) seems highly unlikely, man is almost certainly responsible for the introduction of this pest into Southern Africa. Water hyacinth was brought to Cape Town as an ornamental shortly after 1900 (Phillips, 1938; note that the date 1884 given by Kluge, 1978, is incorrect -- it refers to introduction into North America). By 1912 (Stent, 1913), the plant was already a problem for some municipalities and the solution was the dumping of excess material into the nearest stream. Its rapid growth near such towns as Worcester and Wellington (Western Cape Province) caused Stent to plead for eradication, if still possible. Later, Stephens (1929) noted this plant as having been a permanent inhabitant of Langevlei (Cape Peninsula) for a number of years. Although Edwards and Musil (1975) do not discuss its presence in the Western Cape, and also s h o w on their map only one locality for this area, the author has during the past five years seen this weed growing in abundance and flowering well, near and within Cape Town itself (see Fig. 1). C. Howard-Williams (personal communication, 1978) reports its presence in an i m p o u n d m e n t near the Wilderness Lakes, South West Cape. According to verbal reports, water hyacinth was introduced into the Eastern Cape Province more than sixty years ago, reputedly from the Transvaal. It became a problem plant in this area only after large impoundments were created and bridges with massive supporting pillars were constructed. Such impoundments provide still, sheltered backwaters for rapid growth of the plant, while the bridge pillars hold back floating mats of Eichhornia and tangled debris so that water becomes dammed up behind this. An example of such a bridge is that on the 8wartkops River near the towns of Uitenhage and Despatch. There, during excessive rains, severe flooding of low-lying areas is c o m m o n and such flooding is due, in part, to blocking of the river channels
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Fig. 1. Distribution of Eichhornia crassipes in Southern Africa, 1978. (Compiled from herbarium data and observation by A.J.G., C.H.-W. and B.R.D.) Key to numbering of countries: 1, Angola; 2, Zambia; 3, Malawi; 4, South West Africa; 5, Botswana; 6, Rhodesia; 7, Moqambique; 8, Swaziland; 9, South Africa; 10, Lesotho; 11, Transkei.
by water hyacinth. In Natal, this plant is common on all types of water body, especially in the subtropical areas along the coast (Musil, 1973). It also occurs inland (see Fig. 1) where frost is frequent in winter, as, for example, along the Tugela River near Colenso (Edwards and Musil, 1975). The plant has also been sight-recorded in the Transkei, close to the border with Natal. In the Transvaal, though apparently not yet a problem weed when Phillips (1938) published his work on the weeds of South Africa, water hyacinth has now become a serious pest on streams and in many large and small impoundments, especially near the bigger centres such as Johannesburg and Pretoria. The Vaal River was first reported as seriously affected in 1959 (Edwards and Musil, 1975). Now many other river systems are infested. These include the Crocodile (Appleton, 1974) and Letaba, Eastern Transvaal and the Crocodile and Limpopo, North Western Transvaal. The Zambezi and its tributaries on both sides are heavily affected from below Lake Kariba to the Indian Ocean. Already, clearing operations against this and other aquatic weeds have had to be employed on the recently filled Cabora Bassa Lake (Bond and Roberts, 1978). First reported from Rhodesia in 1937 (Edwards and Musil, 1975), Lake McIlwaine, much of the Hunyani River system and the lower reaches of the Sabi are infested (Bond and Roberts, 1978). In southern Moqambique the Incomati, Limpopo and Save (Sabi) rivers are at times densely lined by water hyacinth (B.R. Davies, personal communication, 1978), while those rivers leading into Delagoa Bay all support flourishing populations of Eichhornia. The author has also seen (1968) living plants, washed down these rivers by flood waters, floating far out to sea between Maputo and Inhaca Island, some still flowering.
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No confirmed records of the presence of this plant in South West Africa, Swaziland or Lesotho are known, but only this last country is likely to remain unaffected because of its rigorous winter climate. Swaziland is already threatened by infestations close to its northern border, on the Crocodile River system (E. Transvaal) and other sources in the rivers of southern Moqambique. In Botswana, at Maun, some unknown person had imported a few plants for a pond but these were destroyed several years ago (P.A. Thomas, personal communication, 1979}. Myriophyllum aquaticum (=M. brasiliense Cambess. and M. Proserpinacoides Gillies) (Verdcourt, 1973) was apparently introduced into Southern Africa in about 1918--1919 or perhaps slightly earlier (Jacot Guillarmod, 1977a, 1978b) and the first recorded specimen is from Paarl, Western Cape Province (see Fig. 2). A few years after its introduction, portions were apparently taken to a trout hatchery and in consequence its distribution was at first closely connected with artificially fish-stocked rivers and ponds. It now occurs in many river systems of the Western Cape Province (e.g. Berg and Bree rivers) and in farm ponds, especially those near the rivers. It also grows, luxuriantly at times, in the Amalinda fish hatchery (Anon., 1960) near East London, E. Cape Province and in the Kariega River system, in the same area, this system having been stocked with fish fry from the hatchery about 15 years ago. In Natal, the earliest known record is 1950; the plant infests the water bodies around Durban and in the upper Tugela River system. In the Transvaal it is most prolific in water bodies around the two most heavily populated centres, Johannesburg and Pretoria, but there is an outlier locality in the Crocodile River, E. Transvaal, and the Hartbeespoort reservoir on the Crocodile River, N.W. Transvaal, is also affected. This river leads into the Limpopo eventually. This last locality (Hartbeespoort reservoir) became
Fig. 2. Distribution of Myriophyllum aquaticum in Southern Africa, 1978. (Herbarium data and observations by A.J.G.) C o u n t r i e s n u m b e r e d as i n Fig. 1.
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Fig. 3. Distribution of Salvinia molesta (o) and Azolla filiculoides (%) in Southern Africa, 1978. (Herbarium data and observations by A.J.G.) Countries numbered as in Fig. 1.
apparent when, in 1977--1978, water hyacinth was destroyed by chemical treatment over large areas of this water b o d y (W.R. Alexander, letter, 1978). The Vaal River near Vereeniging and many small tributaries leading into it, as well as farm ponds in that area, are affected (Jacot Guillarmod, 1976). In Rhodesia it was recorded from the Hunyani River in 1960 (specimen in Rhodes University Herbarium, Grahamstown) and has been reported from the Gwai River near Victoria Falls and also from the Lundi River above the junction with the Sabi. No localities have so far been noted for other countries in Southern Africa, although the same species is present in East Africa (Verdcourt, 1973). In both regions, this plant is present in the female form only. The lack of other records is n o t surprising, as few k n o w what this plant looks like. It may be more widely distributed than is at present realised. Although it has been in several Western Cape rivers for almost 50 years, attention was focussed on it in South Africa only in 1973 (Piaget and Schliemann, 1973}. Of the four plant species discussed, it is the only one rooted in soil, either on the banks or in shallow water; the other three are floating aquatics. Salvinia m o l e s t a (formerly identified as S. auriculata Aubl. in many reports Schelpe, 1961; Mitchell, 1969) is widespread in Southern Africa (see Fig. 3). As a pond or fish tank ornamental it is c o m m o n l y grown and was, until recently, often sold in Southern Africa. This plant, a sterile hybrid (Mitchell, 1972a), was probably brought into the Zambezi area before 1948, the date of the earliest collection of it in the Victoria Falls portion of the river (Schelpe, 1961). It has thus spread from a northern point of introduction, unlike the other three species which apparently were first brought into the subcontinent through a southerly entry point. Natural spread of Kariba weed from its presumed point of origin in South America (Mitchell, 1972a) seems -
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382 out of the question and in an unpublished report, Mitchell (1972b) states that it was introduced by a mission worker in the Zambezi area. In 1960 this weed underwent explosive growth on the newly impounded waters of Lake Kariba and its spread was noted with alarm. An aerial survey was made (Schelpe, 1961) and by 1962 the plant was estimated to cover 21.5% (1003 km 2) of the total lake surface (Mitchell and Tur, 1975). Since then, there has been a fluctuating b u t downward trend in coverage and latterly only about 6% of the lake is reported by these authors as affected, the main areas being sheltered bays and river mouths. In 1973, one observer reported less than 3% coverage (Davies et al., 1975). The heaviest infestation of this weed occurs along the course of the Zambezi and some of its tributaries. The countries most affected are Botswana (mainly preventive measures), South West Africa (Caprivi Strip), Zambia, Rhodesia and Mozambique. Besides the downstream extension which was to be expected (it was collected in the Mana Pools area in 1960 -- specimen in Rhodes University Herbarium) and which Bond and Roberts (1978) state now encompasses the whole Zambezi system from Kariba to the mouth, there is a steady upstream spread (Edwards et al., 1973). This upstream extension is due to wind and water currents at certain seasons, as well as man's activities (movements of boats, fishing gear, etc.). The weed n o w threatens the Okavango swamp areas, of such importance to Botswana, as well as the whole Caprivi Strip, and there is a possibility of spread into Angola. A rigid cordon sanitaire is being maintained to keep the weed o u t of the Okavango swamps because, once in this area, it can never be eradicated and will completely upset this very delicately balanced environment (P.A. Thomas, personal communication, 1979). The Sabi River system, Rhodesia, was also at one time moderately infested but has latterly shown few affected areas. Kariba weed is also recorded from three (Transvaal, Natal, Cape) of the four provinces of South Africa and there is no indication of h o w it was introduced into these areas. It is in the Kruger National Park (Gertenbach, 1975), a protected area, so that introduction b y fishermen or boats is most unlikely, and so far removed from other known infested areas that an explanation for its presence is very difficult to find. A heavy infestation on an isolated farm dam, 11 ha in extent, has been reported from near Doveton, Natal; this lies over 50 km in a direct line from the nearest previously recorded site. The Baakens River (Eastern Cape) and the Knysna/George area of the south-eastern Cape are also affected and many parts of the Western Cape Province (Jacot Guillarmod, 1977d, 1978c). In the Knysna/George area, all feeder streams leading into the saline lake, Swartvlei, carry a heavy growth of this plant except after floods (A. Jacot Guillarmod, 1972 to date; C. Howard-Williams, 1974 to data --personal observations). Spread into Swaziland is possible soon, since this country has a suitable climate and many water bodies, but no records have yet been established. The weed is about 50 km distant from the border with the Republic of South Africa, on the Crocodile River near Nelspruit (E. Transvaal) (D.S. Steyn, letter,
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1978). There are no records of the presence of Salvinia molesta for Lesotho or Transkei. Of the four weeds, Azolla filiculoides is at present the most restricted in distribution (see Fig. 3} although Mitchell (1978, p. 1118) states that this plant, with water hyacinth and Kariba weed, has probably caused "the major difficulties to the utilization of southern African waters". The plant has, with Salvinia molesta, the shortest known history in the subcontinent, being recorded first in the late 1940's {Oosthuizen and Waiters, 1961) and identified then as A. pinnata R. Br., an indigenous species. Noted as plentiful in the Oorlogspoort stream by Dr. R.A. Jubb (1971) and later that same year by the author, it was shortly after that, through work undertaken on the autecology of the plant, that it was proved to be A. filiculoides (Ashton, 1974). It is present only in the immediate vicinity of Colesberg (Northern Cape) and the Hendrik Verwoerd Dam on the Orange River, with an outlier locality some 450 km downstream at Upington (P.J. Ashton, personal communication, 1978). There is also an unconfirmed report of a new locality in the Orange Free State, on a farm beyond Springfontein, about 40 km from the nearest known site and another, also as yet unconfirmed, from the Great Fish River (E. Cape), below the Grassridge reservoir. The plant occurs both in water and on land occasionally inundated, and is common in irrigation furrows, pools and farm reservoirs and small tributaries of the Orange River both in the Northern Cape and the Orange Free State (Jacot Guillarmod, 1978a). No records of this species from other parts of the subcontinent are known to the author, though the plant is sometimes seen in fish tanks and small garden pools. BIOLOGICAL NOTES
Eichhornia crassipes, under Southern African conditions, produces viable seed (Jacot Guillarmod and Allanson, 1978). This, given the right conditions, results in new plants capable of infesting water bodies. Such seed reproduction, known to be the case in the Sudan (Mohamed and Bebawi, 1973a,b), was suspected for Southern Africa but not fully documented (Edwards and Musil, 1975). Tests for length of viability have not yet been made in this region, but elsewhere Edwards and Musil (1975) report this as being at least 15 years. An example of possible seed regeneration occurred on the Swartkops River (E. Cape). In 1972 heavy floods had, as far as could be ascertained by the author and other observers, washed all living water hyacinth plants out of the river system. One year later, the banks of the river were lined in a previously infested area, with small, vigorous plants, some flowering. Even if a few adult plants had escaped destruction by the floods, regeneration could scarcely have been so extensive; seed propagation seems likely for at least part of the growth. The more usual method of reproduction by vegetative offsets is rapid in most of Southern Africa. Bond and Roberts (1978) report a doubling rate for numbers of plants ranging from 5.67 days in the earlier part of the growing
384 season to 27.4 days in the later part, for the subtropical Cabora Bassa area. Near Grahamstown (E. Cape), a much cooler locality, a farmer placed " o n e or t w o " plants in a reservoir (of approximately 200 m 2) and within fifteen months the surface was completely covered with a dense mat of water hyacinth. The plants numbered several thousands and rendered the reservoir almost useless as a watering point for cattle. Most of the Southern African material appears to belong to the mesostylic form (Penfound and Earle, 1948; Sculthorpe, 1967; Jacot Guillarmod, in press). Bond and Roberts fi1978) report b o t h mesostylic and a small number of dolichostylic forms in material examined at Cabora Bassa. Flowering is c o m m o n and prolific during the warmer months in the southern extent of its range and may occur almost throughout the year in the subtropical parts. Flowering plants have been observed in July in Moqambique. The author has seen both honey bees (Apis mellifera L.) and a species of carpenter bee (Xylocopa sp.) visiting the flowers in the Eastern Cape Province and there is also the possibility of self-pollination. During fading, the perianth segments twist and force stigma and anthers closely together. Seed-set appears to be high, at least in the Eastern Cape Province, as large numbers o f firm, fully formed and apparently viable seeds can be found in capsules on fruiting stems. Weed mats or sudd islands (Sculthorpe, 1967; Trivedy et al., 1978) have been reported b y Bond and Robertson (1978) on Cabora Bassa Lake and similar mats have been seen b y the author on Hartbeespoort reservoir, Transvaal, and on the Swartkops River, Eastern Cape. The dioecious Myriophyllum aquaticum, fortunately for Southern Africa, has been introduced only in the female form. Spread is thus limited to vegetative propagules only, as is the case with Salvinia molesta. A few years ago, it was stated verbally by some botanists and farmers that hybrids might have formed between M. aquaticum and an indigenous species, in parts of the Western Cape Province where parrot's feather is particularly prevalent. No hybrid plants have been seen to date by the author and it is thought that the impression that hybrids might have developed was due to the underwater growth form of M. aquaticum. Usually, this plant has obvious aerial axes floating on the water surface with the distal portions curving up into the air. In some cases in the laboratory, after a period of cultivation, such aerial axes die and the plant assumes an underwater form (a habit of growth c o m m o n in the genus) for a time -- up to fifteen months in one case. The normal growth form is then resumed, with production of aerial shoots. In the field, with careful search, the same t y p e of underwater leaf-bearing shoots can be found at times. For at least t w o other reasons also, it is thought unlikely that hybrids occur (Jacot Guillarmod, 1976). The indigenous species, of which there may be two, tend to flower from late summer into autumn, after a period of restricted growth during winter. M. aquaticum mostly flowers, under Southern African conditions, during late winter into early summer. Also, the indigenous species generally occur in areas where M. aquaticum has
385 not y e t penetrated. They may have died out, if they were ever present, where parrot's feather nQw grows. There is one exception known to the author where M. aquaticum overlaps with an indigenous species, in the Vaal River near Vereeniging (Transvaal). In spite of careful search, nothing resembling a hybrid h a s b e e n found. No examples of mature, fully formed fruits have been found on any plants of parrot's feather examined, although the small axillary flowers are borne profusely and continuously during the flowering period. Salvinia molesta depends entirely on vegetative means for propagation, since it is a sterile hybrid (Mitchell, 1972a). Many attempts have been made in the laboratory (Rhodes University, Grahamstown) to germinate spores of this plant b u t these have all been unsuccessful. However, this plant is able to colonise large areas of suitable water bodies vegetatively, and boats, fishing equipment and other factors play a great part in its spread. It is n o t as successful on Cabora Bassa as it was during the first few years of existence of Lake Kariba (Bond and Roberts, 1978) as, in the former impoundment, water hyacinth impedes its growth. Sudd island formation has taken place on Lake Kariba (Schelpe, 1961; Mitchell, 1969, 1978). These islands float freely on the water surface and sometimes support other, rooted, plant species such as Ludwigia spp. In the Wilderness Lakes area (south-western Cape), the feeder streams of Swartvlei, a saline lake, are at times covered bank to bank for considerable distances b y this plant (A. Jacot Guillarmod and C. Howard-Williams, personal observations). The entangled growth then supports other plant growth such as Scirpus species. During floods, portions of the mat are torn off and float out into the lake. These remain for some time, b u t the fern eventually dies when the salinity of the surface water reaches approximately 11%0 ; this agrees with the findings of Hailer et al. (1974) for S. rotundifolia Willd. The second floating fern, Azolla filiculoides, is fertile under Southern African conditions. I t reproduces by means of spores, repeatedly shown in laboratory tests (Rhodes University, Grahamstown) to be capable of germination. Although this method of propagation is possible in the field, vegetative increase is probably the c o m m o n e r means on water bodies during the growing season. However, on farms in the Colesberg area, near the Orange River, it is often found on land only occasionally inundated, and persists in moist hollows. The mature plants die when the soil dries out, so that spore reproduction is more likely in such areas. Transfer of small portions of plants from nearby infested water bodies by domestic or wild animals cannot be ruled out, however. The climate where this fern is found in Southern Africa is harsh and the plant is often frozen into a layer of ice on streams or reservoirs during winter. In summer it is subjected to high temperatures. In the cold season, the plants assume a deep wine-red colour (Jacot Guillarmod, 1978a), making it conspicuous on streams or pools. Ashton (1974) reports its growth as inhibited on the open waters of the shallow Verwoerd reservoir, probably due to wind and wave action. This effect parallels that found for Salvinia on Lake Kariba
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(Mitchell, 1976). As spore reproduction is c o m m o n in Azolla and as the mature plant itself is small, farther spread can be expected through the recently opened system of tunnels leading Orange River water into t h e Sundays and Great Fish river systems of the Eastern Cape Province, especially where there are impoundments providing sheltered bays. ECONOMICS
Water quality and climate in Southern Africa are well-suited to the growth of the four weeds discussed. The scarcity of river systems and their use as enriched-effluent disposal bodies, as well as extra enrichment from run-off from agricultural land, has resulted in varying degrees of eutrophication in at least the lower reaches of the main rivers and in most of the impoundments. Three of the four species (Eichhornia, Myriophyllum, Salvinia) are n o w widespread and only Azolla is so far confined to a short portion of one river system, according to present knowledge. The effect these weeds have on the e c o n o m y of Southern Africa is not easily quantified. However, the water loss in this region due to evapotranspiration by water hyacinth is reported by Edwards and Musil (1975) as being 3.2--3.7 times that from an open water surface. In the survey by Pieterse (1978), figures ranging from 1.3 to 6 times the water loss from an open surface are given, depending on the country concerned. The Southern African figure is thus average. It represents a considerable loss of stored water in an area already faced with a shortage within a few decades. Added to this loss is the direct cost of controlling the weeds where they interfere with agricultural, recreational or other uses. Stent (1913) reported the efforts already being made in the Western Cape at the beginning of the century to deal with the excessive growth of water hyacinth. In 1977/78 almost a quarter of a million rands were spent clearing only a portion of the Hartbeespoort reservoir, Transvaal (Anon., 1978). A small impoundment near Alicedale, Eastern Cape, has cost the South African Railways thousands of rands over several years to control Eichhornia b u t the plant has increased its hold and much more will have to be spent in the future. Figures for Cabora Bassa have not been given (Bond and Roberts, 1978) b u t weed control was begun early and will be continued. Where biological control has been attempted, such as the introduction of the weevil species, Neochetina eichhorniae Warner and N. bruchi Hustache, or the mite, Orthogalumna terebrantis Wallwork (Bond and Roberts, 1978), costs of such introductions must also be considered, yet so far results have not been promising. The insects seem to have spread b u t the weeds grow at a faster rate than the control organisms (Mitchell, 1976, p. 171). Flood damage is a factor difficult to estimate and fouling of water supplies in various ways after chemical treatment, with possible effects on other life in the water bodies, has n o t so far been assessed. Myriophyllum aquaticum has been tackled, where present, b y chemical and mechanical means, but not by biological control so far. From experience, the
387 Rand Water Board (Transvaal) has discontinued herbicide use as being expensive and ineffective, and n o w employs mechanical and manual means to control this weed. The a m o u n t spent on labourers' wages alone must total many thousands of rands annually. Elsewhere, although the plant is recognised as troublesome and the problem is increasing, only sporadic attempts are made to control it and no exact figures of expenditure are available. Tests made with herbicides in the Eastern Cape have been disappointing b u t neither here nor in the Transvaal and Western Cape, do farmers give costs of work done on controlling this plant. Spread of M. aquaticum is usually from large portions torn off during clearing operations. These may float away to r o o t elsewhere while the rooted portions left in the soil readily regenerate. Unlike the indigenous species, parrot's feather does not shatter readily when handled, b u t the force needed to remove any large amount of this weed is considerable and shearing-off of plant portions is inevitable. Further, it has been demonstrated (Schelpe, 1976) that a piece of axis with node, only 5 mm long, is capable, under suitable conditions, of forming a new plant. Also, the plant, if left in heaps on the banks after removal, may still show some portions able to grow, even after several months (information from Amalinda Warm Water Fish Hatchery, E. Cape, 1975). This plant is so dense on and in some Western Cape rivers as to constitute a danger to canoeists. This hazard has been written a b o u t in a novel under the title Duisendblaar, the Afrikaans c o m m o n name for this plant (Nortje, 1977). Costly control programmes were proposed for Salvinia molesta on Lake Kariba during the earlier stages of infestation b u t as the weed stabilised to cover eventually only a small proportion of the lake surface (Mitchell and Tur, 1975), and as it proved beneficial for fish (Bowmaker, 1975), these were not carried out. Small experiments were done, however, with various sprays (Begg, 1971; Wild and Mitchell, 1970). Biological control by use of phytophagous insects, e.g. the grasshopper, Paulinia acuminata De Geer, have added to expenditure on this weed but, although this insect and others have been recovered downstream at Cabora Bassa (Bond and Roberts, 1978), such control has had little impact on mature plants. However, Bond and Roberts (1978) and Bowmaker (1975) consider that such insect control may be helpful in the early stages of invasion. For some years, a joint Botswana/Republic of South Africa project has been in operation in the Okavango swamps area. This consists of a cordon sanitaire, the cost of which amounts to R100 000 annually, shared equally b y the participants (P.A. Thomas, personal communication, 1979). Herbicide sprays are used with rigorous monitoring. Control of Azolla filiculoides has been attempted, mostly by farmers, through use of diesoline (Oosthuizen and Walters, 1961). Rising costs of this product and the ineffectiveness of the control have caused this practice to be abandoned. No figures of costs are obtainable, though many farmers complain of losses of stock through drowning under mats of this plant.
388 INDIGENOUS SPECIES
There is one species of Eichhornia native to Southern Africa, E. natans (Beauv.) Solms-Laub. (Dyer, 1976}. This occurs in the northern parts of South West Africa and in Botswana. There are two other plants of the family Pontederiaceae, to which water hyacinth belongs, native to Southern Africa. These are Monochoria africana (Solms) N.E. Br., found in the lowveld area of the Transvaal and Heteranthera callifolia Reichb. ex Kunth, occurring from South West Africa to the Northern Transvaal, including Botswana and Rhodesia (Dyer, 1976). At least one and possibly t w o indigenous species of Myriophyllum exist in Southern Africa (Dyer, 1975; Jacot Guillarmod, 1977a). The indigenous specimens have until n o w all gone under the name M. spicatum L., a species widespread in Europe and North America and n o w proving a troublesome weed in this latter region. Under Southern African conditions, the native species is not a nuisance b u t is even beneficial (Bruton and Boltt, 1975). It protects fish fry from predators and provides a substrate for periphyton on which some fish species feed. Nowhere in the Southern African region does the local Myriophyllum species grow as abundantly as M. aquaticum. Salvinia hastata Desv. is the indigenous Salvinia species in Southern Africa (Schelpe, 1969). It is recorded from the warmer parts of the region and is nowhere a c o m m o n nor a troublesome plant. Schelpe (1969, 1977) also records two indigenous species of Azolla, A. pinnata and A. nilotica Decne. The former occurs from northern Natal (Pongolo River flood-plain) and Botswana northward, in the warmer parts of the region, while A. nilotica is in the subtropical portion, being found in South West Africa, Malawi and Moqambique (Bond and Roberts, 1978). Both are thus plants of much warmer areas than the Orange River valley where A. filiculoides is present. LEGISLATION
Although weed control acts exist, as for example in the Republic of South Africa (1964) and Botswana, enforcing the provisions of such ordinances is difficult. There are t o o few weed inspectors, the general public knows t o o little of the provisions of the law and even less of the dangers these water weeds can pose, while the great majority do n o t know what the plants look like. Some of the countries concerned do not even list any proclaimed water weeds, while in other cases, the provisions of the acts are t o o wide. For example, the Republic of South Africa Weeds Act (1964) includes all species of Eichhornia and all members of the family Pontederiaceae, all Myriophyllum species and all Salvinia species as proclaimed weeds. Ownership of water bodies, such as rivers flowing through private property, is often a matter of dispute and ultimate responsibility for removal and destruction of water weeds is hard to define. Again, where river systems are concerned, these often pass from one country to another in their courses.
389 U n i t e d a c t i o n is n e e d e d in s u c h cases b u t , as t h e e x a m p l e o f t h e E u p h r a t e s s h o w s ( B e a u m o n t , 1 9 7 8 ) , such a g r e e m e n t a n d a c t i o n is unlikely.
USES Possible uses f o r t h e s e a q u a t i c w e e d s h a v e b e e n suggested b y , n o t a b l y , an A d H o c Panel ( 1 9 7 6 ) , b u t such use, e x c e p t o n a small scale, is s c a r c e l y p r a c t i c a b l e in S o u t h e r n Africa. Large-scale use requires a c o n s t a n t s u p p l y , s o m e t h i n g t o b e avoided. Utilising t h e s e plants as f o d d e r o r f o d d e r supplem e n t s and, in t h e case o f t h e n i t r o g e n - r i c h A z o l l a filiculoides ( A s h t o n a n d Walmsley, 1 9 7 6 ) , as green m a n u r e , a l t h o u g h e c o n o m i c a l l y s o u n d , has dangers. T h e r e is t h e possibility o f s p r e a d o f at least s o m e o f t h e s e weeds, t h a t o n e is a t t e m p t i n g t o use as well as c o n t r o l , t o o t h e r w a t e r b o d i e s a n d an even f u r t h e r lessening o f fresh w a t e r supplies.
ACKNOWLEDGEMENTS M y thanks are due to Professor B.R. Allanson, Director, Institute for Fresh Water Studies, Rhodes University, for critical reading of the manuscript; to P.J. Ashton, National Institute for Water Research, and D.S. Steyn, Department of Water Affairs, both of Pretoria; to C. Howard-Williams, Institute for Fresh Water Studies, Rhodes University, Grahamstown, all for help with distribution records. The work was done during tenure of a Senior Bursary from the Council for Scientific and Industrial Research, Pretoria, and facilities were provided by Rhodes University, Grahamstown. REFERENCES
Ad Hoc Panel, 1976. Making aquatic weeds useful: some perspectivesfor developing countries. National Academy of Sciences, Washington, DC, 175 pp. Anon., 1960. Myriophyllum eradication.Cape ProvincialAdministration Department of Nature Conservation, Report 25, p. 104. Anon., 1978. The "dam" big cleanup. Conservationist, 1:1. Appleton, C.C., 1974. A check listof the floraand fauna of the Gladdespruit, Nelspruit district, Eastern Transvaal. Newsl. Limnol. Soc. South. Aft., 22: 40--58. Ashton, P.J., 1974. The effect of some environmental factors on the growth of AzoUa filiculoides Lain. In: E.M. van Zinderen Bakker, Sr. (Editor), The Orange River: Progress Report. Bloemfontein, pp. 123--138. Ashton, P.J. and Walmsley, R.D., 1976. The aquatic fern Azolla and its Anabaena symbiont. Endeavour, 35: 39--43. Beaumont, P., 1978. The Euphrates River -- an international problem of water resource development. Environ. Conserv., 5: 35--43. Begg, G., 1971. Preliminary tests using formalin as a possible means of chemically controlling Salvinia auriculata and other undesirable floating plants in freshwater impoundments. Newsl. Limnol. Soc. South. Aft:, 17: 22--26. Bond, W.J. and Roberts, M.G., 1978. The colonization of Cabora Bassa, Mozambique, a new man-made lake, by floating aquatic macrophytes. Hydrobiologia, 60: 243--259. Bowmaker, A., I975. Fisheries productivity of Lake Kariba. Tob. Forum, 2: 17--25.
390 Bruton, M.N. and Boltt, R.E., 1975. Aspects of the biology of Tilapia mossambica Peters (Pisces: Cichlidae) in a natural freshwater lake (Lake Sibaya, South Africa). J. Fish Biol., 7: 423--445. C.C.T.A./C.S.A., 1957. Report of the symposium on Eichhornia crassipes, Leopoldville, 1957. C.C.T.A./C.S.A. Publications, 2 7 : 3 1 pp. Davies, B.R., Hall, A. and Jackson, P.B.N., 1975. Some ecological aspects of the Cabora Bassa dam. Biol. Conserv., 8: 189--201. Dyer, R.A., I975. The genera of Southern African flowering plants: 1: Dicotyledons. Department of Agricultural Technical Services, Pretoria, 756 pp. Dyer, R.A., 1976. The genera of Southern African flowering plants: 2: Monocotyledons. Department of Agricultural Technical Services, Pretoria, 284 pp. Edwards, D. and Musil, C.J., 1975. Eichhornia crassipes in South Africa - - a general review. J. Limnol. Soc. South. Aft., 1: 23--27. Edwards, D., Musil, C.J., van Robbroeck, T.P.C., van der Merwe, W., Waugh, W.F., Bennison, R.H., Irving, N.S., Smith, P.S., Wilson, B.H. and Raffle, J.A., 1973. 1972 Botswana--South Africa survey of Salvinia molesta in Chobe--Linyanti--Kwando River system. Report, Botanical Research Institute, Pretoria, 14 pp. (unpublished). Fernald, M.L., 1950. Gray's Manual of Botany (8th edn). American Book Company, New York, 1632 pp. Gertenbach, W.P.D., 1975. Kariba weed now in Kruger Park. Custos, 4: 39--41. Hailer, W.T., Sutton, D.L. and Barlowe, W.C., 1974. Effects of salinity on growth of several aquatic macrophytes. Ecology, 55: 891--894. Jacot Guillarmod, A., 1976. Myriophyllum, an increasing water weed menace for South Africa. Proc. S. Aft. Assoc. Advan. Sci., Nelspruit, 1: 93--100. Jacot Guillarmod, A., 1977a. Myriophyllum, an increasing water weed menace for South Africa. S. Afr. J. Sci., 73: 89--90. Jacot Guillarmod, A., 1977b. Some water weeds of the Eastern Cape Province. East. Cape Nat., 60: 4--8. Jacot Guillarmod, A., 1977c. Some water weeds of the Eastern Cape Province -- II. Myriophyllum. East. Cape Nat., 61: 14--17. Jacot Guillarmod, A., 1977d. Some water weeds of the Eastern Cape Province - - III. Salvinia. East. Cape Nat., 62: 10--13. Jacot Guillarmod, A., 1978a. Some water weeds of the Eastern Cape Province - - IV. Azolla. East. Cape Nat., 63: 16--18. Jacot Guillarmod, A., 1978b. Parrot's feather. In: C.H. Stirton (Editor), Plant Invaders, Beautiful but Dangerous. Cape Provincial Administration, Department of Nature and Environmental Conservation, Cape Town, pp. 96--99. Jacot Guillarmod, A., 1978c. Kariba weed. In: C.H. Stirton (Editor), Plant Invaders, Beautiful but Dangerous. Cape Provincial Administration, Department of Nature and Environmental Conservation, Cape Town, pp. 132--135. Jacot Guillarmod, A. and Allanson, B.R., 1978. Eradication of water hyacinth. S. Aft. J. Sci., 74: 122. Kluge, R., 1978. Water hyacinth. In: C.H. Stirton (Editor), Plant Invaders, Beautiful but Dangerous. Cape Provincial Administration, Department of Nature and Environmental Conservation, Cape Town, pp. 68--71. Mitchell, D.S., 1969. The ecology of vascular hydrophytes on Lake Kariba. Hydrobiologia, 34: 448--464. Mitchell, D.S., 1972a. The Kariba weed: Salvinia molesta sp. nov. Br. Fern Gaz., 10: 251--252. Mitchell, D.S., 1972b. Environmental management and the control of Eichhornia and Salvinia. In: Workshop on Aquatic Weeds (Especially Eichhornia) and Problems which they CTeate. Unpublished. Mitchell, D.S., 1976. The growth and management of Eichhornia crassipes and Salvinia spp. in their native environment and in alien situations. In: C.K. Varshnay and J. Rz6ska (Editors), Aquatic Weeds in South East Asia. Dr. W. Junk, The Hague, pp. 167--176.
391 Mitchell, D.S., 1978. Freshwater plants. In: M.J.A. Werger and A.C. van Bruggen (Editors), Biogeography and Ecology of Southern Africa. Dr. W. Junk, The Hague, pp. 1113--1118. Mitchell, D.S. and Tur, N.M,, 1975. The rate of growth of Salvinia molesta (S. auriculata Auct.) in laboratory and natural conditions. J. Appl. Ecol., 12: 213--225. Mohamed, B.F. and Bebawi, F.F., 1973a. Burning as a supportive treatment in controlling water-hyacinth in the Sudan -- I. Routine burning. Hyacinth Control J., 11: 31--34. Mohamed, B.F. and Bebawi, F.F., 1973b. Burning as a supportive treatment in controlling water-hyacinth in the Sudan --If. Backburning. Hyacinth Control J,, 11: 34--37. Musil, C.F., 1973. Water plants of Natal. Wildl. Prot. Conserv. Soc. South. Afr., 62 pp. Nortje, P., 1977. Duisendblaar. Tafelberg Pers, Cape Town, 199 pp. Oosthuizen, G.J. and Waiters, M.M,, 1961. Control of water fern with diesoline. Farming S. Afr., 37: 35--37. Penfound, W.T. and Earle, T.T., 1948. The biology of the water hyacinth. Ecol. Monogr., 18: 448--472. Phillips, E.P., 1938. The weeds of South Africa. Pamphlet 195, Ser. 41, Botanical Section, Department of Agriculture and Forestry, Union of South Africa, Pretoria, 229 pp. Piaget, J. and Schliemann, G., 1973. Weeds choking Western Cape rivers -- appearance of Myriophyllum spp. in the Boland. Deciduous Fruit Grower, 23: 176--179. Pieterse, A.H., 1978. The water hyacinth (Eichhornia crassipes) - - a review. Abstr. Trop. Agric., R. Trop. Inst., Amsterdam, 4: 9--42. Republic of South Africa, 1964. Weeds Amendment Act 1964. Government Printer, Pretoria, pp. 15. Schelpe, E.A.C.L.E., 1961. The ecology of Salvinia auriculata and associated vegetation on Kariba Lake. J. S. Afr. Bot., 27: 181--187. Schelpe, E.A.C.L.E., 1969. A revised check list of the Pteridophyta of Southern Africa. J. S. Afr. Bot., 35: 127--140. Schelpe, E.A.C.L.E., 1976. Report on autecology of MyriophyUum aquaticum. Unpublished report to Council for Scientific and Industrial Research, Pretoria, 2 pp. Schelpe, E.A.C.L.E., 1977. Pteridophyta. In: R.B. Fernandes, E. Launert and E.J. Mendes (Editors), Conspectus Florae Angolensis. Junta de Investiga~oes Cientificas do Ultramar, Lisbon, pp. 1--197. Sculthorpe, C.D., 1967. The biology of aquatic vascular plants. Edward Arnold, London, 610 pp. Stent, S.M., 1913. Water hyacinth (Eichhornia crassipes). Union S. Afr. Dept. Agric., Bull., 68: 1--4. Stephens, E.L., 1929. Freshwater aquatic vegetation of the South Western districts. In: Botanical Features of the South Western Cape Province. Speciality Press, Wynberg, pp. 81--95. Stutterheim, N., 1978. In: L. Dellatola and C. Groenewaid (Editors), Social Laboratory. S. Aft. Panorama, 23: 34--35. Trivedy, R.K., Sharma, K.P., Goel, P.K. and Gopal, B., 1978. Some ecological observations on floating islands. Hydrobiologia, 60: 187--190. Verdcourt, B., 1973. A new combination in Myriophyllum (Haloragaceae). Kew Bull., 28: 36. Wild, H. and Mitchell, D.S., 1970. L'influence du ®Bayluscide sur un ptSridophyte aquatique, Salvinia auriculata et sur d'autres plantes aquatiques. Pflanzenschutz-Nachr. Bayer 23/1970, 2: 110--115.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
WATER WEEDS IN SOUTHERN AFRICA
A. JACOT GUILLARMOD
Institute for Fresh Water Studies, Rhodes University, Grahamstown (S. Africa) (Accepted 17 May 1979)
ABSTRACT Jacot Guillarmod, A., 1979. Water weeds in Southern Africa. Aquat. Bot., 6: 377--391. In Southern Africa, introduced water weeds have become serious pest plants in fresh and even brackish water systems only within the past twenty years, although in one case (water hyacinth) a warning of what was likely to happen was printed as early as 1913. The four main species involved are: Eichhornia crassipes (Mart.) Solms-Laubach, Myriophyllum aquatieum (Vell.) Verdcourt, Salvinia molesta Mitchell and Azolla filiculoides Lam. Between their introduction and the present, these plants, with the exception of Azolla, have spread widely over the whole region in suitable habitats and now pose threats to all the main river systems, hydro-electric and other water impoundments. No indigenous plants in these four genera are so far considered dangerous weed species, though some may have an occasional nuisance value.
INTRODUCTION
The four dominant exotic weed species affecting the fresh water supplies of Africa south of the Kunene and Zambezi rivers are: Eichhornia crassipes (Mart.) Solms-Laub. ( water hyacinth); Myriophy llum aq uaticum (Veil.) Verdcourt (parrot's feather, latterly more often written parrotfeather -- see Fernald (1950)); Salvinia molesta Mitchell (Kariba weed); Azolla filiculoides Lam. (water fern). All have, apparently, been imported deliberately by man during this century as ornamental plants for fish ponds or aquaria: Until recently, they have been more or less ignored as economically unimportant. In the past t w e n t y years, however, these plants have spread widely in the main river systems and impoundments, these latter providing relatively still bays and inlets suited to the explosive growth of the weeds. Even brackish water may be infested with water hyacinth (C.C.T.A./C.S.A., 1957) and it is often found in very slightly saline coastal lagoons in Natal (Hailer et al., 1974). There is still lack of awareness of the danger these aquatic weeds can pose for the fresh water supplies of Southern Africa and also, ultimate responsibility for control or eradication of such weeds is difficult to determine. Stutterheim (1978) has stated that South Africa will be using its entire available water
378 supplies b y the year 1995 and there is a possibility of a 30% deficit by 2000 A.D., while Botswana is already in urgent need of more water supplies now for agricultural expansion. The distribution maps accompanying this article represent data compiled from herbarium records, printed reports and articles on the plants concerned, information from reliable observers and personal collecting b y the author. P. Ashton, B.R. Davies (B.R.D.), C. Howard-Williams (C.H.-W.), D.J. Steyn and P.A. Thomas have provided much useful data. HISTORY OF INTRODUCTION AND PRESENT DISTRIBUTION In three cases (Eichhornia, Myriophyllum, Azolla) a few plants of each were imported into Southern Africa for use in fish tanks or lily ponds (Stent, 1913; Oosthuizen and Walters, 1961; Jacot Guillarmod, 1976, 1977a, b,c, 1978b; Kluge, 1978). No explanation, except for a statement in an unpublished report (Mitchell, 1972b) has been found for the presence of Salvinia molesta in the Zambezi River system, where it existed at least a decade before the completion of the Kariba Dam (Edwards et al., 1973). However, as natural spread from this plant's original home in South America (Mitchell, 1972a) seems highly unlikely, man is almost certainly responsible for the introduction of this pest into Southern Africa. Water hyacinth was brought to Cape Town as an ornamental shortly after 1900 (Phillips, 1938; note that the date 1884 given by Kluge, 1978, is incorrect -- it refers to introduction into North America). By 1912 (Stent, 1913), the plant was already a problem for some municipalities and the solution was the dumping of excess material into the nearest stream. Its rapid growth near such towns as Worcester and Wellington (Western Cape Province) caused Stent to plead for eradication, if still possible. Later, Stephens (1929) noted this plant as having been a permanent inhabitant of Langevlei (Cape Peninsula) for a number of years. Although Edwards and Musil (1975) do not discuss its presence in the Western Cape, and also s h o w on their map only one locality for this area, the author has during the past five years seen this weed growing in abundance and flowering well, near and within Cape Town itself (see Fig. 1). C. Howard-Williams (personal communication, 1978) reports its presence in an i m p o u n d m e n t near the Wilderness Lakes, South West Cape. According to verbal reports, water hyacinth was introduced into the Eastern Cape Province more than sixty years ago, reputedly from the Transvaal. It became a problem plant in this area only after large impoundments were created and bridges with massive supporting pillars were constructed. Such impoundments provide still, sheltered backwaters for rapid growth of the plant, while the bridge pillars hold back floating mats of Eichhornia and tangled debris so that water becomes dammed up behind this. An example of such a bridge is that on the 8wartkops River near the towns of Uitenhage and Despatch. There, during excessive rains, severe flooding of low-lying areas is c o m m o n and such flooding is due, in part, to blocking of the river channels
379
Fig. 1. Distribution of Eichhornia crassipes in Southern Africa, 1978. (Compiled from herbarium data and observation by A.J.G., C.H.-W. and B.R.D.) Key to numbering of countries: 1, Angola; 2, Zambia; 3, Malawi; 4, South West Africa; 5, Botswana; 6, Rhodesia; 7, Moqambique; 8, Swaziland; 9, South Africa; 10, Lesotho; 11, Transkei.
by water hyacinth. In Natal, this plant is common on all types of water body, especially in the subtropical areas along the coast (Musil, 1973). It also occurs inland (see Fig. 1) where frost is frequent in winter, as, for example, along the Tugela River near Colenso (Edwards and Musil, 1975). The plant has also been sight-recorded in the Transkei, close to the border with Natal. In the Transvaal, though apparently not yet a problem weed when Phillips (1938) published his work on the weeds of South Africa, water hyacinth has now become a serious pest on streams and in many large and small impoundments, especially near the bigger centres such as Johannesburg and Pretoria. The Vaal River was first reported as seriously affected in 1959 (Edwards and Musil, 1975). Now many other river systems are infested. These include the Crocodile (Appleton, 1974) and Letaba, Eastern Transvaal and the Crocodile and Limpopo, North Western Transvaal. The Zambezi and its tributaries on both sides are heavily affected from below Lake Kariba to the Indian Ocean. Already, clearing operations against this and other aquatic weeds have had to be employed on the recently filled Cabora Bassa Lake (Bond and Roberts, 1978). First reported from Rhodesia in 1937 (Edwards and Musil, 1975), Lake McIlwaine, much of the Hunyani River system and the lower reaches of the Sabi are infested (Bond and Roberts, 1978). In southern Moqambique the Incomati, Limpopo and Save (Sabi) rivers are at times densely lined by water hyacinth (B.R. Davies, personal communication, 1978), while those rivers leading into Delagoa Bay all support flourishing populations of Eichhornia. The author has also seen (1968) living plants, washed down these rivers by flood waters, floating far out to sea between Maputo and Inhaca Island, some still flowering.
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No confirmed records of the presence of this plant in South West Africa, Swaziland or Lesotho are known, but only this last country is likely to remain unaffected because of its rigorous winter climate. Swaziland is already threatened by infestations close to its northern border, on the Crocodile River system (E. Transvaal) and other sources in the rivers of southern Moqambique. In Botswana, at Maun, some unknown person had imported a few plants for a pond but these were destroyed several years ago (P.A. Thomas, personal communication, 1979}. Myriophyllum aquaticum (=M. brasiliense Cambess. and M. Proserpinacoides Gillies) (Verdcourt, 1973) was apparently introduced into Southern Africa in about 1918--1919 or perhaps slightly earlier (Jacot Guillarmod, 1977a, 1978b) and the first recorded specimen is from Paarl, Western Cape Province (see Fig. 2). A few years after its introduction, portions were apparently taken to a trout hatchery and in consequence its distribution was at first closely connected with artificially fish-stocked rivers and ponds. It now occurs in many river systems of the Western Cape Province (e.g. Berg and Bree rivers) and in farm ponds, especially those near the rivers. It also grows, luxuriantly at times, in the Amalinda fish hatchery (Anon., 1960) near East London, E. Cape Province and in the Kariega River system, in the same area, this system having been stocked with fish fry from the hatchery about 15 years ago. In Natal, the earliest known record is 1950; the plant infests the water bodies around Durban and in the upper Tugela River system. In the Transvaal it is most prolific in water bodies around the two most heavily populated centres, Johannesburg and Pretoria, but there is an outlier locality in the Crocodile River, E. Transvaal, and the Hartbeespoort reservoir on the Crocodile River, N.W. Transvaal, is also affected. This river leads into the Limpopo eventually. This last locality (Hartbeespoort reservoir) became
Fig. 2. Distribution of Myriophyllum aquaticum in Southern Africa, 1978. (Herbarium data and observations by A.J.G.) C o u n t r i e s n u m b e r e d as i n Fig. 1.
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Fig. 3. Distribution of Salvinia molesta (o) and Azolla filiculoides (%) in Southern Africa, 1978. (Herbarium data and observations by A.J.G.) Countries numbered as in Fig. 1.
apparent when, in 1977--1978, water hyacinth was destroyed by chemical treatment over large areas of this water b o d y (W.R. Alexander, letter, 1978). The Vaal River near Vereeniging and many small tributaries leading into it, as well as farm ponds in that area, are affected (Jacot Guillarmod, 1976). In Rhodesia it was recorded from the Hunyani River in 1960 (specimen in Rhodes University Herbarium, Grahamstown) and has been reported from the Gwai River near Victoria Falls and also from the Lundi River above the junction with the Sabi. No localities have so far been noted for other countries in Southern Africa, although the same species is present in East Africa (Verdcourt, 1973). In both regions, this plant is present in the female form only. The lack of other records is n o t surprising, as few k n o w what this plant looks like. It may be more widely distributed than is at present realised. Although it has been in several Western Cape rivers for almost 50 years, attention was focussed on it in South Africa only in 1973 (Piaget and Schliemann, 1973}. Of the four plant species discussed, it is the only one rooted in soil, either on the banks or in shallow water; the other three are floating aquatics. Salvinia m o l e s t a (formerly identified as S. auriculata Aubl. in many reports Schelpe, 1961; Mitchell, 1969) is widespread in Southern Africa (see Fig. 3). As a pond or fish tank ornamental it is c o m m o n l y grown and was, until recently, often sold in Southern Africa. This plant, a sterile hybrid (Mitchell, 1972a), was probably brought into the Zambezi area before 1948, the date of the earliest collection of it in the Victoria Falls portion of the river (Schelpe, 1961). It has thus spread from a northern point of introduction, unlike the other three species which apparently were first brought into the subcontinent through a southerly entry point. Natural spread of Kariba weed from its presumed point of origin in South America (Mitchell, 1972a) seems -
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382 out of the question and in an unpublished report, Mitchell (1972b) states that it was introduced by a mission worker in the Zambezi area. In 1960 this weed underwent explosive growth on the newly impounded waters of Lake Kariba and its spread was noted with alarm. An aerial survey was made (Schelpe, 1961) and by 1962 the plant was estimated to cover 21.5% (1003 km 2) of the total lake surface (Mitchell and Tur, 1975). Since then, there has been a fluctuating b u t downward trend in coverage and latterly only about 6% of the lake is reported by these authors as affected, the main areas being sheltered bays and river mouths. In 1973, one observer reported less than 3% coverage (Davies et al., 1975). The heaviest infestation of this weed occurs along the course of the Zambezi and some of its tributaries. The countries most affected are Botswana (mainly preventive measures), South West Africa (Caprivi Strip), Zambia, Rhodesia and Mozambique. Besides the downstream extension which was to be expected (it was collected in the Mana Pools area in 1960 -- specimen in Rhodes University Herbarium) and which Bond and Roberts (1978) state now encompasses the whole Zambezi system from Kariba to the mouth, there is a steady upstream spread (Edwards et al., 1973). This upstream extension is due to wind and water currents at certain seasons, as well as man's activities (movements of boats, fishing gear, etc.). The weed n o w threatens the Okavango swamp areas, of such importance to Botswana, as well as the whole Caprivi Strip, and there is a possibility of spread into Angola. A rigid cordon sanitaire is being maintained to keep the weed o u t of the Okavango swamps because, once in this area, it can never be eradicated and will completely upset this very delicately balanced environment (P.A. Thomas, personal communication, 1979). The Sabi River system, Rhodesia, was also at one time moderately infested but has latterly shown few affected areas. Kariba weed is also recorded from three (Transvaal, Natal, Cape) of the four provinces of South Africa and there is no indication of h o w it was introduced into these areas. It is in the Kruger National Park (Gertenbach, 1975), a protected area, so that introduction b y fishermen or boats is most unlikely, and so far removed from other known infested areas that an explanation for its presence is very difficult to find. A heavy infestation on an isolated farm dam, 11 ha in extent, has been reported from near Doveton, Natal; this lies over 50 km in a direct line from the nearest previously recorded site. The Baakens River (Eastern Cape) and the Knysna/George area of the south-eastern Cape are also affected and many parts of the Western Cape Province (Jacot Guillarmod, 1977d, 1978c). In the Knysna/George area, all feeder streams leading into the saline lake, Swartvlei, carry a heavy growth of this plant except after floods (A. Jacot Guillarmod, 1972 to date; C. Howard-Williams, 1974 to data --personal observations). Spread into Swaziland is possible soon, since this country has a suitable climate and many water bodies, but no records have yet been established. The weed is about 50 km distant from the border with the Republic of South Africa, on the Crocodile River near Nelspruit (E. Transvaal) (D.S. Steyn, letter,
383
1978). There are no records of the presence of Salvinia molesta for Lesotho or Transkei. Of the four weeds, Azolla filiculoides is at present the most restricted in distribution (see Fig. 3} although Mitchell (1978, p. 1118) states that this plant, with water hyacinth and Kariba weed, has probably caused "the major difficulties to the utilization of southern African waters". The plant has, with Salvinia molesta, the shortest known history in the subcontinent, being recorded first in the late 1940's {Oosthuizen and Waiters, 1961) and identified then as A. pinnata R. Br., an indigenous species. Noted as plentiful in the Oorlogspoort stream by Dr. R.A. Jubb (1971) and later that same year by the author, it was shortly after that, through work undertaken on the autecology of the plant, that it was proved to be A. filiculoides (Ashton, 1974). It is present only in the immediate vicinity of Colesberg (Northern Cape) and the Hendrik Verwoerd Dam on the Orange River, with an outlier locality some 450 km downstream at Upington (P.J. Ashton, personal communication, 1978). There is also an unconfirmed report of a new locality in the Orange Free State, on a farm beyond Springfontein, about 40 km from the nearest known site and another, also as yet unconfirmed, from the Great Fish River (E. Cape), below the Grassridge reservoir. The plant occurs both in water and on land occasionally inundated, and is common in irrigation furrows, pools and farm reservoirs and small tributaries of the Orange River both in the Northern Cape and the Orange Free State (Jacot Guillarmod, 1978a). No records of this species from other parts of the subcontinent are known to the author, though the plant is sometimes seen in fish tanks and small garden pools. BIOLOGICAL NOTES
Eichhornia crassipes, under Southern African conditions, produces viable seed (Jacot Guillarmod and Allanson, 1978). This, given the right conditions, results in new plants capable of infesting water bodies. Such seed reproduction, known to be the case in the Sudan (Mohamed and Bebawi, 1973a,b), was suspected for Southern Africa but not fully documented (Edwards and Musil, 1975). Tests for length of viability have not yet been made in this region, but elsewhere Edwards and Musil (1975) report this as being at least 15 years. An example of possible seed regeneration occurred on the Swartkops River (E. Cape). In 1972 heavy floods had, as far as could be ascertained by the author and other observers, washed all living water hyacinth plants out of the river system. One year later, the banks of the river were lined in a previously infested area, with small, vigorous plants, some flowering. Even if a few adult plants had escaped destruction by the floods, regeneration could scarcely have been so extensive; seed propagation seems likely for at least part of the growth. The more usual method of reproduction by vegetative offsets is rapid in most of Southern Africa. Bond and Roberts (1978) report a doubling rate for numbers of plants ranging from 5.67 days in the earlier part of the growing
384 season to 27.4 days in the later part, for the subtropical Cabora Bassa area. Near Grahamstown (E. Cape), a much cooler locality, a farmer placed " o n e or t w o " plants in a reservoir (of approximately 200 m 2) and within fifteen months the surface was completely covered with a dense mat of water hyacinth. The plants numbered several thousands and rendered the reservoir almost useless as a watering point for cattle. Most of the Southern African material appears to belong to the mesostylic form (Penfound and Earle, 1948; Sculthorpe, 1967; Jacot Guillarmod, in press). Bond and Roberts fi1978) report b o t h mesostylic and a small number of dolichostylic forms in material examined at Cabora Bassa. Flowering is c o m m o n and prolific during the warmer months in the southern extent of its range and may occur almost throughout the year in the subtropical parts. Flowering plants have been observed in July in Moqambique. The author has seen both honey bees (Apis mellifera L.) and a species of carpenter bee (Xylocopa sp.) visiting the flowers in the Eastern Cape Province and there is also the possibility of self-pollination. During fading, the perianth segments twist and force stigma and anthers closely together. Seed-set appears to be high, at least in the Eastern Cape Province, as large numbers o f firm, fully formed and apparently viable seeds can be found in capsules on fruiting stems. Weed mats or sudd islands (Sculthorpe, 1967; Trivedy et al., 1978) have been reported b y Bond and Robertson (1978) on Cabora Bassa Lake and similar mats have been seen b y the author on Hartbeespoort reservoir, Transvaal, and on the Swartkops River, Eastern Cape. The dioecious Myriophyllum aquaticum, fortunately for Southern Africa, has been introduced only in the female form. Spread is thus limited to vegetative propagules only, as is the case with Salvinia molesta. A few years ago, it was stated verbally by some botanists and farmers that hybrids might have formed between M. aquaticum and an indigenous species, in parts of the Western Cape Province where parrot's feather is particularly prevalent. No hybrid plants have been seen to date by the author and it is thought that the impression that hybrids might have developed was due to the underwater growth form of M. aquaticum. Usually, this plant has obvious aerial axes floating on the water surface with the distal portions curving up into the air. In some cases in the laboratory, after a period of cultivation, such aerial axes die and the plant assumes an underwater form (a habit of growth c o m m o n in the genus) for a time -- up to fifteen months in one case. The normal growth form is then resumed, with production of aerial shoots. In the field, with careful search, the same t y p e of underwater leaf-bearing shoots can be found at times. For at least t w o other reasons also, it is thought unlikely that hybrids occur (Jacot Guillarmod, 1976). The indigenous species, of which there may be two, tend to flower from late summer into autumn, after a period of restricted growth during winter. M. aquaticum mostly flowers, under Southern African conditions, during late winter into early summer. Also, the indigenous species generally occur in areas where M. aquaticum has
385 not y e t penetrated. They may have died out, if they were ever present, where parrot's feather nQw grows. There is one exception known to the author where M. aquaticum overlaps with an indigenous species, in the Vaal River near Vereeniging (Transvaal). In spite of careful search, nothing resembling a hybrid h a s b e e n found. No examples of mature, fully formed fruits have been found on any plants of parrot's feather examined, although the small axillary flowers are borne profusely and continuously during the flowering period. Salvinia molesta depends entirely on vegetative means for propagation, since it is a sterile hybrid (Mitchell, 1972a). Many attempts have been made in the laboratory (Rhodes University, Grahamstown) to germinate spores of this plant b u t these have all been unsuccessful. However, this plant is able to colonise large areas of suitable water bodies vegetatively, and boats, fishing equipment and other factors play a great part in its spread. It is n o t as successful on Cabora Bassa as it was during the first few years of existence of Lake Kariba (Bond and Roberts, 1978) as, in the former impoundment, water hyacinth impedes its growth. Sudd island formation has taken place on Lake Kariba (Schelpe, 1961; Mitchell, 1969, 1978). These islands float freely on the water surface and sometimes support other, rooted, plant species such as Ludwigia spp. In the Wilderness Lakes area (south-western Cape), the feeder streams of Swartvlei, a saline lake, are at times covered bank to bank for considerable distances b y this plant (A. Jacot Guillarmod and C. Howard-Williams, personal observations). The entangled growth then supports other plant growth such as Scirpus species. During floods, portions of the mat are torn off and float out into the lake. These remain for some time, b u t the fern eventually dies when the salinity of the surface water reaches approximately 11%0 ; this agrees with the findings of Hailer et al. (1974) for S. rotundifolia Willd. The second floating fern, Azolla filiculoides, is fertile under Southern African conditions. I t reproduces by means of spores, repeatedly shown in laboratory tests (Rhodes University, Grahamstown) to be capable of germination. Although this method of propagation is possible in the field, vegetative increase is probably the c o m m o n e r means on water bodies during the growing season. However, on farms in the Colesberg area, near the Orange River, it is often found on land only occasionally inundated, and persists in moist hollows. The mature plants die when the soil dries out, so that spore reproduction is more likely in such areas. Transfer of small portions of plants from nearby infested water bodies by domestic or wild animals cannot be ruled out, however. The climate where this fern is found in Southern Africa is harsh and the plant is often frozen into a layer of ice on streams or reservoirs during winter. In summer it is subjected to high temperatures. In the cold season, the plants assume a deep wine-red colour (Jacot Guillarmod, 1978a), making it conspicuous on streams or pools. Ashton (1974) reports its growth as inhibited on the open waters of the shallow Verwoerd reservoir, probably due to wind and wave action. This effect parallels that found for Salvinia on Lake Kariba
386
(Mitchell, 1976). As spore reproduction is c o m m o n in Azolla and as the mature plant itself is small, farther spread can be expected through the recently opened system of tunnels leading Orange River water into t h e Sundays and Great Fish river systems of the Eastern Cape Province, especially where there are impoundments providing sheltered bays. ECONOMICS
Water quality and climate in Southern Africa are well-suited to the growth of the four weeds discussed. The scarcity of river systems and their use as enriched-effluent disposal bodies, as well as extra enrichment from run-off from agricultural land, has resulted in varying degrees of eutrophication in at least the lower reaches of the main rivers and in most of the impoundments. Three of the four species (Eichhornia, Myriophyllum, Salvinia) are n o w widespread and only Azolla is so far confined to a short portion of one river system, according to present knowledge. The effect these weeds have on the e c o n o m y of Southern Africa is not easily quantified. However, the water loss in this region due to evapotranspiration by water hyacinth is reported by Edwards and Musil (1975) as being 3.2--3.7 times that from an open water surface. In the survey by Pieterse (1978), figures ranging from 1.3 to 6 times the water loss from an open surface are given, depending on the country concerned. The Southern African figure is thus average. It represents a considerable loss of stored water in an area already faced with a shortage within a few decades. Added to this loss is the direct cost of controlling the weeds where they interfere with agricultural, recreational or other uses. Stent (1913) reported the efforts already being made in the Western Cape at the beginning of the century to deal with the excessive growth of water hyacinth. In 1977/78 almost a quarter of a million rands were spent clearing only a portion of the Hartbeespoort reservoir, Transvaal (Anon., 1978). A small impoundment near Alicedale, Eastern Cape, has cost the South African Railways thousands of rands over several years to control Eichhornia b u t the plant has increased its hold and much more will have to be spent in the future. Figures for Cabora Bassa have not been given (Bond and Roberts, 1978) b u t weed control was begun early and will be continued. Where biological control has been attempted, such as the introduction of the weevil species, Neochetina eichhorniae Warner and N. bruchi Hustache, or the mite, Orthogalumna terebrantis Wallwork (Bond and Roberts, 1978), costs of such introductions must also be considered, yet so far results have not been promising. The insects seem to have spread b u t the weeds grow at a faster rate than the control organisms (Mitchell, 1976, p. 171). Flood damage is a factor difficult to estimate and fouling of water supplies in various ways after chemical treatment, with possible effects on other life in the water bodies, has n o t so far been assessed. Myriophyllum aquaticum has been tackled, where present, b y chemical and mechanical means, but not by biological control so far. From experience, the
387 Rand Water Board (Transvaal) has discontinued herbicide use as being expensive and ineffective, and n o w employs mechanical and manual means to control this weed. The a m o u n t spent on labourers' wages alone must total many thousands of rands annually. Elsewhere, although the plant is recognised as troublesome and the problem is increasing, only sporadic attempts are made to control it and no exact figures of expenditure are available. Tests made with herbicides in the Eastern Cape have been disappointing b u t neither here nor in the Transvaal and Western Cape, do farmers give costs of work done on controlling this plant. Spread of M. aquaticum is usually from large portions torn off during clearing operations. These may float away to r o o t elsewhere while the rooted portions left in the soil readily regenerate. Unlike the indigenous species, parrot's feather does not shatter readily when handled, b u t the force needed to remove any large amount of this weed is considerable and shearing-off of plant portions is inevitable. Further, it has been demonstrated (Schelpe, 1976) that a piece of axis with node, only 5 mm long, is capable, under suitable conditions, of forming a new plant. Also, the plant, if left in heaps on the banks after removal, may still show some portions able to grow, even after several months (information from Amalinda Warm Water Fish Hatchery, E. Cape, 1975). This plant is so dense on and in some Western Cape rivers as to constitute a danger to canoeists. This hazard has been written a b o u t in a novel under the title Duisendblaar, the Afrikaans c o m m o n name for this plant (Nortje, 1977). Costly control programmes were proposed for Salvinia molesta on Lake Kariba during the earlier stages of infestation b u t as the weed stabilised to cover eventually only a small proportion of the lake surface (Mitchell and Tur, 1975), and as it proved beneficial for fish (Bowmaker, 1975), these were not carried out. Small experiments were done, however, with various sprays (Begg, 1971; Wild and Mitchell, 1970). Biological control by use of phytophagous insects, e.g. the grasshopper, Paulinia acuminata De Geer, have added to expenditure on this weed but, although this insect and others have been recovered downstream at Cabora Bassa (Bond and Roberts, 1978), such control has had little impact on mature plants. However, Bond and Roberts (1978) and Bowmaker (1975) consider that such insect control may be helpful in the early stages of invasion. For some years, a joint Botswana/Republic of South Africa project has been in operation in the Okavango swamps area. This consists of a cordon sanitaire, the cost of which amounts to R100 000 annually, shared equally b y the participants (P.A. Thomas, personal communication, 1979). Herbicide sprays are used with rigorous monitoring. Control of Azolla filiculoides has been attempted, mostly by farmers, through use of diesoline (Oosthuizen and Walters, 1961). Rising costs of this product and the ineffectiveness of the control have caused this practice to be abandoned. No figures of costs are obtainable, though many farmers complain of losses of stock through drowning under mats of this plant.
388 INDIGENOUS SPECIES
There is one species of Eichhornia native to Southern Africa, E. natans (Beauv.) Solms-Laub. (Dyer, 1976}. This occurs in the northern parts of South West Africa and in Botswana. There are two other plants of the family Pontederiaceae, to which water hyacinth belongs, native to Southern Africa. These are Monochoria africana (Solms) N.E. Br., found in the lowveld area of the Transvaal and Heteranthera callifolia Reichb. ex Kunth, occurring from South West Africa to the Northern Transvaal, including Botswana and Rhodesia (Dyer, 1976). At least one and possibly t w o indigenous species of Myriophyllum exist in Southern Africa (Dyer, 1975; Jacot Guillarmod, 1977a). The indigenous specimens have until n o w all gone under the name M. spicatum L., a species widespread in Europe and North America and n o w proving a troublesome weed in this latter region. Under Southern African conditions, the native species is not a nuisance b u t is even beneficial (Bruton and Boltt, 1975). It protects fish fry from predators and provides a substrate for periphyton on which some fish species feed. Nowhere in the Southern African region does the local Myriophyllum species grow as abundantly as M. aquaticum. Salvinia hastata Desv. is the indigenous Salvinia species in Southern Africa (Schelpe, 1969). It is recorded from the warmer parts of the region and is nowhere a c o m m o n nor a troublesome plant. Schelpe (1969, 1977) also records two indigenous species of Azolla, A. pinnata and A. nilotica Decne. The former occurs from northern Natal (Pongolo River flood-plain) and Botswana northward, in the warmer parts of the region, while A. nilotica is in the subtropical portion, being found in South West Africa, Malawi and Moqambique (Bond and Roberts, 1978). Both are thus plants of much warmer areas than the Orange River valley where A. filiculoides is present. LEGISLATION
Although weed control acts exist, as for example in the Republic of South Africa (1964) and Botswana, enforcing the provisions of such ordinances is difficult. There are t o o few weed inspectors, the general public knows t o o little of the provisions of the law and even less of the dangers these water weeds can pose, while the great majority do n o t know what the plants look like. Some of the countries concerned do not even list any proclaimed water weeds, while in other cases, the provisions of the acts are t o o wide. For example, the Republic of South Africa Weeds Act (1964) includes all species of Eichhornia and all members of the family Pontederiaceae, all Myriophyllum species and all Salvinia species as proclaimed weeds. Ownership of water bodies, such as rivers flowing through private property, is often a matter of dispute and ultimate responsibility for removal and destruction of water weeds is hard to define. Again, where river systems are concerned, these often pass from one country to another in their courses.
389 U n i t e d a c t i o n is n e e d e d in s u c h cases b u t , as t h e e x a m p l e o f t h e E u p h r a t e s s h o w s ( B e a u m o n t , 1 9 7 8 ) , such a g r e e m e n t a n d a c t i o n is unlikely.
USES Possible uses f o r t h e s e a q u a t i c w e e d s h a v e b e e n suggested b y , n o t a b l y , an A d H o c Panel ( 1 9 7 6 ) , b u t such use, e x c e p t o n a small scale, is s c a r c e l y p r a c t i c a b l e in S o u t h e r n Africa. Large-scale use requires a c o n s t a n t s u p p l y , s o m e t h i n g t o b e avoided. Utilising t h e s e plants as f o d d e r o r f o d d e r supplem e n t s and, in t h e case o f t h e n i t r o g e n - r i c h A z o l l a filiculoides ( A s h t o n a n d Walmsley, 1 9 7 6 ) , as green m a n u r e , a l t h o u g h e c o n o m i c a l l y s o u n d , has dangers. T h e r e is t h e possibility o f s p r e a d o f at least s o m e o f t h e s e weeds, t h a t o n e is a t t e m p t i n g t o use as well as c o n t r o l , t o o t h e r w a t e r b o d i e s a n d an even f u r t h e r lessening o f fresh w a t e r supplies.
ACKNOWLEDGEMENTS M y thanks are due to Professor B.R. Allanson, Director, Institute for Fresh Water Studies, Rhodes University, for critical reading of the manuscript; to P.J. Ashton, National Institute for Water Research, and D.S. Steyn, Department of Water Affairs, both of Pretoria; to C. Howard-Williams, Institute for Fresh Water Studies, Rhodes University, Grahamstown, all for help with distribution records. The work was done during tenure of a Senior Bursary from the Council for Scientific and Industrial Research, Pretoria, and facilities were provided by Rhodes University, Grahamstown. REFERENCES
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