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Home , Hydrocharitaceae, Ottelia, Vallisneria, Vallisneria spiralis

Tane 36: 181-195 (1997)

A REVIEW OF THE AQUATIC MACROPHYTE FAMILY (ANGIOSPERMAE) EN NEW ZEALAND

Clinton D. McCullough Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton

SUMMARY

The aquatic macrophyte family the Hydrocharitaceae has no examples in New Zealand's depauperate native aquatic flora, but is now represented by seven naturalised species; all of which carry a pest rating with the exception of () and (). All but one (ottelia) also reproduce solely vegetatively from either fragments, turions, tubers, or ; either due to only a single sex of the plant being in the country or environmental conditions being unsuitable for viable production. Representatives of the Hydrocharitaceae in New Zealand are the most troublesome introduced aquatic weed species of this country's waterways (especially the "oxygen weed" species), and although future control methods are promising, all naturalised members of the family, clearly look here to stay.

Keywords: Hydrocharitaceae; aquatic weeds; ; elodea; ; ; ottelia; oxygen weed; .

INTRODUCTION

"The that pose a major problem are those which grow to a considerable length and also occur in dense, pure communities" V.J. Chapman (1970a).

Above all other aquatic weed species in New Zealand, the family Hydrocharitaceae demonstrates the above quote well with its frequently tall growing "forest monocultures". The Hydrocharitaceae or "Frogbit" family consists of herbaceous aquatic perennials (largely freshwater, partly or completely submersed), and is not represented in the New Zealand native flora. It is taxonomically classified as follows: Division: Anthrophyta (angiosperms) Class: Subclass: Alismatidae Order: Hydrocharitalles Family: Hydrocharitaceae

181 Member species of this widespread family occur predominantly in warmer regions and are mostly submerged. Numerous genera within this family are monotypic and largely occur in the old world (Sculthorpe 1967). are actinomorphic, and commonly dioecious. The family contains approximately 16 genera, almost all of which are native to warmer regions of the world (Healy & Edgar 1980) and around 200 species (Lockley 1988) although Muhlberg (1982) suggests a very conservative (that is, relative to the majority of the literature) of only around 100 species. Of these, all the members found adventive in New Zealand are freshwater fully-submerged perennials. Within the family another clear division can be made with the so-called "oxygen-weeds" which, being so similar in morphologically and in habit, that they are frequently confused with each other (Mason 1960). These are all of the tribe Anachriteae and are probably the best known, most well-dispersed, and nuisance causing (greatest biomasses attained) of the family in New Zealand and much of the world where the Hydrocharitaceae family is adventive. Following in alphabetic order of genera, are individual descriptions of the biology of the species of Hydro• charitaceae known to be naturalised in New Zealand.

Egeria densa (Fig. 1) Also known as/synonyms: egeria, oxygen weed, Elodea densa, giant elodea, leafy elodea, Anacharis densa, brazilian elodea, Argentinian water weed, lake weed, dense waterweed. Description: A submerged, bottom-rooted dioecious freshwater perennial. Stems simple or dichotomously branched, brittle, to 5m long and 3mm diameter. Internodes elongated lower down the stem, becoming increasingly compact toward the shoot apex as seen in Fig. 1. are numerous, sessile, whorled with 4-5 leaves each to 5cm long by 2-5mm wide with minute Fig. 1. Apex of a growing stem of Egeria serrations. densa alongside a lower section displaying a Flowers are unisexual and fragile more open arrangement of whorls.

182 with only male flowers (therefore no ) being found in New Zealand; the male flowers having three white 4-12mm long, nine (golden) and typically flowering in late summer (pers. obs.). Female flowers are even rare in the native habitat (apparently only found in a single pond (Lockley 1988) although Cook & Urmi-Konig (1984) say otherwise), with fruits only being known from artificially pollinated plants and never in the wild (Cook 1987). Propagation is therefore wholly vegetative in New Zealand (and in almost all the world) and is largely by stem fragments that contain lateral buds giving rise to new shoots and . Dormant stem apices will also facilitate new clonal growth (Howard-Williams et al. 1987). These are dispersed either by water currents, boats (an especially common vector for fragment dispersal (Clayton et al. 1981; Johnstone et al. 1985), pond and aquaria escapes (this species is perhaps the most universally available aquarium plant) (Cook 1987). Previously one of the oxygen weeds introduced by the aquarium trade, it was first discovered naturalised in 1946 (Mason 1960) in the hydroelectric lakes in the Waikato River, its native origin being still waters in South America. It has the second most restricted distribution of this group (apart from the related Hydrilla verticillata), being known only from the north half of the North Island (including Great Barrier Island, from where it is expected to be eradicated within the next five years) (L. Vervoort pers. comm.), with the exception of some Foxton, Canterbury, Marlborough (recent record), and a few other isolated South Island occurrences. This limited distribution in the South Island may be a consequence of limited dispersal, or also possibly of inhibition by climatic factors (Chapman 1970b) as an unreferenced French study indicated that egeria had an optimal temperature for growth 2°C higher than elodea. In contrast elodea has a wider distribution in the South Island as shown by Healy & Edgar (1980) and Cook & Urmi-Konig (1984). Egeria is abundant in the Waikato district, where it contributes significantly, albeit less so than hornwort (Ceratophyllum demersum) to problems of blockage of the coolant water intakes with the hydroelectric dams on the Waikato River. Unlike lagarosiphon (Coffey & Clayton 1988a), egeria performs well in the low light conditions associated with eutrophic conditions (Tanner et al. 1993, Wells & Clayton 1991) which may have assisted its spread in many waterbodies often noted to be at the expense of other less low-light tolerant species including already present populations of elodea and Lagarosiphon (Johnstone 1981, Cook & Urmi-Konig 1984, Coffey & Clayton 1987, Tanner et al. 1990a). This also may be seen to predispose many highly anthropogenically modified waterbodies to its invasion. Egeria was probably introduced into New Zealand by the aquarium trade, and may have been deposited initially into one of the natural lakes around Huntly (Chapman 1970b). From here it has spread throughout the lower Waikato

183 (probably by water motion), and even up the river no doubt via boat activity as it was first noted in Lake Karapiro in 1965 at a boat launching ramp (Chapman 1970a). The North Island distribution of both egeria and elodea have been found to be highly correlated with boating, access and fishing activity within a lake (Howard-Williams et al. 1987). Where it is found it often may form a complete monoculture, contributing to the elimination of many of the native species previously present, e.g. McCullough (1995), and decreasing the conservation values of that body. The morphology of egeria appears to be very plastic in response to its environment in much the same way (e.g. leaf density, stem rigidity, etc.) as are many of the other Hydrocharitaceae oxygen weeds. Growth is most rapid in early summer and autumn, excessively warm surface water temperatures inhibiting growth in late summer, and a over-wintering period of quiescence (no true winter dormancy) being observed where very little growth takes place (Lockley 1988). Egeria is now classified by all regional councils with a regional plant pest strategy under the new Biosecurity Act (1993) as a plant pest, making its sale or distribution illegal. This act replaces the previous Noxious Plant Act (1982), and decentralised control, placing the onus of aquatic and terrestrial weed control onto regional council whom have been developing and introducing their plant pest management strategies since this date. At the time of writing, all regional councils except the Chatham Islands and West Coast regional councils have developed/are developing a plant pest management strategy under this new act.

Elodea canadensis (Fig. 2) Also known as/synonyms: elodea, Canadian pond weed, oxygen weed, Elodea canadense, Babington's curse, American water weed, water thyme, blackweed, riverweed, Anacharis canadensis, Anacharis alsinastrum. Fig. 2. Elodea canadensis demonstrating an Description: A tall-growing, unbranched adventitious and a young dioecious freshwater perennial with bud on the right-most stem.

184 slender, brittle, branched stems to 6m long (Healy & Edgar 1980). Leaves are opposite lower down the stem where inter-nodal length is greatest, and in whorls of three above (very occasionally four (Coffey & Clayton 1988b) although a curious all four-leafed whorled specimen is currently being grown by the author). Flowers have been recorded in New Zealand (summer and early autumn) being unisexual with three sepals 3-4mm long, and three petals 9-12mm long. Of the oxygen weed group, elodea has been established the longest in New Zealand with first records dating back to 1872 (Healy & Edgar 1980). Our elodea is no doubt a clone of original Tasmanian stock (Chapman 1970b). Elodea is the most widely distributed, adventive tall-growing submerged macrophyte in New Zealand, occurring throughout the country, and over a wide habitat range (Coffey & Clayton 1988b), and is cold tolerant; still thriving in areas where winter surface water temperatures are close to freezing (Sainty & Jacobs 1981) with growth occurring in New Zealand in the winter (Hughes 1976). It is, however, probably also the least nuisance of the introduced aquatic oxygen weeds, usually giving way when it comes into competition with either egeria or lagarosiphon (Mason 1960) although dense, tall-growing monospecific communities can still occur in its most competitive habitat on fertile, silty sediments in clear water (Chapman 1970b, Sainty & Jacobs 1981) although there is less dependence on the substrate if nutrient levels in the water are high (Hughes 1976). Both sexes are now present in New Zealand, with what appears to be many of the original male establishments being replaced by the more dominant female sex in an intersexual mode of competition. Although both sexes are present in New Zealand, flowering is rare and there is no evidence of any seed being set (P. Champion pers. comm., 1996). Vascular tissue in elodea is not well-developed and the xylem soon collapses giving rise to a lacuna. The floating shoots when damaged, or if prostrate, give rise to slender unbranched adventitious roots, typical also of the other oxygen weeds in New Zealand, which give rise to new centres of infection (Chapman 1970a). Nutrient studies have shown that good growth can be obtained with very low concentrations of nitrogen and phosphorus, although the presence of dissolved iron (Fe3+/Fe2+) in the water appears to affects the growth rates of elodea (and of the oxygen weeds in general), and this has been implicated as the prevailing major potentially limiting nutrient in most invaded waterbodies (Chapman 1970b, Chapman et al. 1974). The now widespread distribution of this North American plant within New Zealand, and its lesser aggression and (although still great) nuisance factor, compared with other oxygen weeds in New Zealand, have enabled it to avoid being classified both as a noxious plant in 1982 (Noxious Plants Act 1982: preventing sale and distribution through commercial outlets). It also appears to

185 have been avoided being classified as a Plant Pest by all regional councils under the new Biosecurity Act (1993) with elodea being the only oxygen weed continuing to be readily distributed through the aquarium trade. This does not imply it is not still a realised or potential menace to much of the country's waterways still remaining (Johnson 1982), merely that it has such a broad distribution now, that attempted control would not be feasible.

Hydrilla verticillata (Fig. 3) Also known as: hydrilla, oxygen weed, water thyme. Description: A monospecific , in New Zealand this plant is dioecious although elsewhere, especially in climatically tropical habitats, it is usually monoecious (Cook & Luond 1981). It is a fully submerged aquatic, freshwater perennial with stems to 2m long usually closing to a canopy just below water's surface. Internodes are 5cm apart in deep water, but much closer near surface. Leaves are arranged in whorls of 3-8 (typically 4-5), up to 4cm long and 5mm wide, linear to lanceolate with margins serrulate to naked eye (cf. other oxygen weeds where this is either not present, or not visible if present) (Cook & Luond 1981). Flowers are unisexual, floating, solitary, axillary, with three petals 1-2.5mm long and three sepals l-3mm long; three stamens. No are produced in New Zealand as only one sex (male) is present. It is a native to the North-Eastern parts of Australia, eastern and southern Asia, Europe and parts of Africa (Swarbrick et al. 1981). Hydrilla roots in the mud and spreads with rhizomes and stolons; water quality seldom appears to be limiting growing well under a wide range of nutrient conditions. It may, however, often do better under nutrient-enriched conditions, e.g. with surrounding agricultural land-use, Fig. 3. The typically standard "oxygen weed" although there are records of shape of Hydrilla verticillata with its whorls extinction following eutrophication in of up to eight leafs. a temperate habitat (Cook & Luond

186 1981). Growing from water's margin to 6.5m, stems are frequently stiffer close to shore than in deeper water where more structural support is required to meet wave-action (B.T. Coffey pers. comm.). Tubers and turions (overwintering vegetative buds) are produced in profusion, the turions developing in the leaf axils, and after a few weeks of development (usually in the autumn) breaking off and sinking to the bed where they will then grow. Tubers (an annual, swollen, food storage portion of the underground stem and root) (Coffey & Clayton 1988) are formed on the rhizomes in autumn and overwinter up to 20cm deep in the sediment; tuber densities of up to 5000 per square metre have been recorded (Sainty & Jacobs 1981). Hydrilla also readily propagates by fragmentation, with a small piece of stem that includes one node growing into a mature plant. Genetic isozyme analyses indicate that all of the New Zealand material is of a single clonal type (implying a single introduction event to the country), typically being relatively slender and delicate (Verkleij et al. 1983) and of a different genetic type than that of populations investigated from elsewhere in the world (P. Champion pers. comm.). First recorded in New Zealand in Lake Tutira, hydrilla is of a limited distribution in New Zealand, being found only in the Hawkes Bay region of the North Island, (Lake Tutira, Lake Opouahi - still spreading, Lake Eland) (Coffey & Clayton 1988b) where it grows to a depth of up to 6.5m and easily dominates the vegetation; piles of dislodged material being up to a metre or more thick on the shoreline following storms (pers. obs.). These waterbodies are relatively isolated, with little anthropogenic activity that frequently characterises the waterbodies of New Zealand experiencing aquatic weed problems, e.g. the Waikato River's hydroelectric lakes, and this isolated distribution may explain why, as of yet, no other sites of establishment outside of these waterbodies have become known. A serious weed in other countries, it has also been recognised as a serious threat in New Zealand as well; being classified as a plant pest by all regional councils, but the West Coast, in order to try and prevent its spread from Hawkes Bay.

Lagarosiphon major (Fig. 4) Also known as/synonyms: lagarosiphon, oxygen weed, Elodea crispa. First naturalised record in New Zealand by Mason (1960). Description: Stems to 2m, very fragile. Leaves tend to be spaced below and crowded above; alternate or spirally arranged (a distinguishing feature from the other oxygen weeds), and are recurved, stiff, opaque, acute to acuminate, c.16 x 2mm. All New Zealand collections so far have been female. Female flowers are 0.25mm diameter to 6mm long, transparent and usually produced in from

187 Fig. 4. showing compact Fig. 5. A typical open-patterned Otellia leaf arrangement at the shoot apex, and a ovalifolia showing juvenile strap-like leaves, more diffuse leaf arrangement toward the adult leaves, a single , and flower lower stem. Also note the single female buds. flower in the top left corner. spring to late summer (pers. obs.). Spirals of leafs are more compact towards the apex of the growing shoot in a similar manner to egeria (Fig. 1.). As all New Zealand plants are of the same sex, propagation is, as for the other oxygen weed species in New Zealand, purely vegetative, by means of bud-bearing stem fragments serving as new centres of infection. These bud containing fragments may stay alive floating in the water for many months with a piece only a few centimetres long providing sufficient reserve food materials to permit growth, and sending out shoots over this time (Mason 1960). The plants appear to grow equally vigorously whether in mud, or more solid substrate (Chapman 1970a) and also in oligotrophic waters, e.g. Lake Taupo (Hughes 1976), although silty sand appears to be the preferred sediment type (Chapman et al. 1971). In addition to the expected limiting effects of light (commonly a significant limiting factor for submerged aquatic macrophytes) to its maximum attainable depth of 6.5m in New Zealand (Howard-Williams et al. 1989), water pressure also appears to be a

188 possible factor in limiting its maximum depth with decreasing growth being demonstrated with increasing depth and therefore water pressure (Coffey & Cnu 1988). This species is a native of tropical Zimbabwe, Zambia, and South Africa. Overall the plant is larger than elodea, but smaller of egeria, but the habit is essentially the same - attachment by a mass of roots with a number of erect stems arising from the root mass forming. As with the other oxygen weeds (not so much with the more open Vallisneria) a veritable monocultural forest may often be found due to the, erect nature of the shoots of the weeds which contain abundant air spaces (lacuna) -the contained air keeping the shoots floating erect (Chapman 1970a). Lagarosiphon also appears to be the more shade-tolerant of oxygen weed species, with maximum photosynthesis on a summer day typically occurring at a greater depth of around 2-4m (depending on water turbidity) with above this photoinhibition taking place (Chapman 1970b). Lagarosiphon is also rated as a serious plant pest by all but the West Coast regional councils.

Ottelia ovalifolia (Fig. 5) Also known as: ottelia, swamp lily. The first naturalised records in this country were by T.F. Cheeseman (1899) from Lake Pupuke (Takapuna) collections. Description: A robust perennial, leaves are all basal; juvenile leaves submersed, linear, transparent, short-petiolate and older leaves ovate or oblong, obtuse, floating with seven strong nerves connected by fine cross-veinlets to 40cm long. Flowers are large, fragile, white and bisexual, petals (3-white) to 4cm long, sepals (3-green) 1.5-3cm long (green). Ottelia also produces cleistogamic flowers which remain submerged, closed, and are self-pollinated (Coffey & Clayton 1988b). Many seeds are produced, the capsule being narrowly ovoid, 2-5cm long, and enclosed within the spathe (Healy & Edgar 1980). The seeds begin growth while still attached to floating and decaying flower capsules, thus aiding dispersal before settling in marginal areas, largely as a consequence of prevailing wind-activity (pers. obs.). Plants flower in summer and autumn; the seeds germinating the following spring with seedbanks appearing to be maintained for a long time afterwards as new plants are often seen arising from previously infected areas that have undergone an extinction. Ottelia appears to be solely propagated by seed in New Zealand with plants producing numerous seeds (Coffey & Clayton 1988b). Waterfowl have also been implicated to explain much of the distribution of this species as ottelia has become established in lakes to which there is no access for boating or fishing. Waterfowl have been noted to disperse aquatic plants that can set seed and ingestion then passing through

189 undamaged of the seeds through the guts of waterfowl migrating between waterbodies would seem to be the only satisfactory explanation for these otherwise perplexing distributions. Passage through the gut of a waterfowl may even provide a required conditioning process for the germination of the seeds to occur, common to many seed dispersed aquatic plants, e.g. mechanical degradation of the seed testa (P. Champion pers. comm.). Closely resembling other naturalised aquatic species Aponogeton distachyus and Hydrocleys nymphoides, and no doubt brought here originally as a pond decoration also, ottelia has only established over a limited habitat and geographical distribution, and is therefore only a local nuisance sometimes, in slow moving waters such as ponds and dams. Relative to the other members of its family little ecological information has been noted on this species in either the literature, or in internal government reports (J.S. Clayton pers. comm), and it seems that it does not present as a great menace in relation to many of the other aquatic weeds already present in New Zealand. It is, however, still spreading in both the North and the South Island (e.g. a recent discovery by the author on the Awhitu Peninsula, North Island, with plants found in a slow flowing pasture stream along with Potamogeton cheesemanii) where most known localities are in farm dams in the North Island where stock access to drinking water may be impeded (Cook & Urmi-Konig 1984). Ottelia grows to a depth of 3m in slow-moving/ static waters in typically rich-organic or silty sediments (Coffey & Clayton 1988b) although it is seldom troublesome; generally not growing very densely as typically do the other members of the Hydrocharitaceae in New Zealand. Occasionally it can become very abundant in fertile and slow flowing water (Coffey & Clayton 1988b), and is frequently one of the first colonisers of a newly created body of water. Ottelia is not regarded as a serious pest by any of New Zealand's regional councils.

Vallisneria gigantea and V. spiralis (Fig. 6) Also known as: vallisneria, eelgrass, wild celery. Previously all Vallisneria spp. in New Zealand were listed as being V. spiralis. Description: A submerged, dioecious perennial with floating flowers producing small rooted tufts at varying intervals on the stolons. Leaves all basal; from a few centimetres of shallow water, to 5.5m long and 8cm wide in deep water for V. gigantea, to 5cm wide for V. spiralis with this difference being maintained under culture (pers. obs.) with V. spiralis having the narrower lamina, all with 5-9 parallel veins. Leaf tips are obtuse to acute, with finely serrulate margins toward the apex. Plants may therefore be easily confused with young leaves of ottelia. Numerous adventitious roots, up to 40cm long, arise at

190 each leaf-bearing node on the rhizomes. Flowers are unisexual, axillary, with the male in several-flowered inflorescences, and the female solitary. Both flowers have three petals (minute) and three sepals (Lowden 1982). V. spiralis is often found flowering in mid-summer when under cultivation. Further taxonomic work has indicated that we probably have in fact two species: V. gigantea (native to Australia and Asia) (Sculthorpe 1967) found only in Lake Pupuke in the North island and probably the result of a single deliberate introduction (Cheeseman 1897), and V. spiralis (native to Europe and North Africa) found at more than one site, but only in the North Island. This divisional status of vallisneria as two separate species, or even as separate varieties, in New Zealand is not recognised unanimously, e.g. regional councils tend to adopt a personal view of this for their own use. Spread is chiefly vegetative under natural native conditions, and wholly in New Zealand by mode of extensions; although both male and female flowers are common during the summer in Lake Pupuke, no evidence of viable seed production appears to exist (Coffey & Clayton 1988b). Vegetative dispersal is by transfer of a rhizome fragment that includes a leaf node, this being less likely to occur by chance than for the transfer of stem fragments of the oxygen weeds, the spread of this species between waterbodies is usually by intentional plantings. The distribution of vallisneria in New Zealand is only in the North Island where it is found in Lake Pupuke (V. gigantea since 1897), Meola Creek (near Western Springs, Auckland), and Lake Whiritoa, Wanganui (V. spiralis). A previous record of in a farm pond at South Head, Kaipara (herbarium voucher AK 129804) (see Healy & Edgar 1980: 26) has been redetermined by P. Champion as Otellia ovalifolia; further demonstrating the ease of confusion between the juvenile foliage of otellia r. , „. . ,. ... J b Fig. 6. Vallisneria spiralis, with the narrower and that of adult vallisneria. It is an iamina of our two naturalised species in this important food source of the genus.

191 Australian (Cygnus atratus) in Lake Pupuke (Coffey & Clayton, 1988b) where intensive grazing along with that of coinhabiting ducks, maintains it roughly lm below the water's surface (pers. obs.). The two naturalised vallisneria species are regarded as high risk plant pests by all but West Coast Regional Council; whilst identification problems with other commercially distributed vallisneria species, not recognised as plant pests, causes difficulty with enforcement of their distribution (e.g. sometimes the case of V. spiralis in the Auckland Regional Council administered region) (L. Vervoort pers. comm.).

DISCUSSION

New Zealand is poor in submerged, non-marine, phanerogamic (gymnosperms and angiosperms) plants (especially tall-growing submerged macrophyte species) with only five indigenous species which always grow entirely submerged, most of these having fairly specialised habitat types (e.g. brackish or at least coastal waters) (Mason 1960, Mason 1975) and certainly being ill-adapted to much of the modified conditions (e.g. high light exposure) that now exist in the majority of New Zealand inland waters as a result of changing land practices (Howard-Williams et al. 1987, Lockley 1988). It therefore seems likely that there would be exotic species which can fill some fresh water habitats to which the few New Zealand species are poorly or not at all suited i.e. the concept of an "empty niche" being present in our aquatic macrophytes. One of these reasons may be that many of the adventive macrophytes in New Zealand arrived without their natural herbivores, which may explain why many of these species have been able to develop nuisance biomass levels here whilst the same species usually remain at non-nuisance levels within their native range (Howard-Williams et al. 1987). Herbivore diet switching to these often less-desirable introduced species, through selection for those species of grazers, e.g. Lymnaea snails that make greater use of these abundant supplies of resources may, however, now be helping to counteract this lack of predatory control with one of the introduced Hydrocharitaceae species (elodea) now showing either slight declines in previously recorded biomasses, or a slowing of distributions otherwise unaccountable for in both this country and others (Cheeseman 1897, Winterbourn & Mason 1983, Wardle 1985). Due to the many slight habitat preference differences between the species of the Hydrocharitaceae present in New Zealand (e.g. substrate type, light requirements, exposure tolerances, etc.), in addition to the obviously strong inhibitory effects prevailing on further colonisation once these species are established at a site at typically high biomasses (e.g. shading effects), it is very unusual to find mixed communities of these species. In general for the

192 Hydrocharitaceae, it would seem that there is a decreased rate of recovery from cropping with increasing depth of water, related to light quality and therefore photosynthetic rate (Chapman 1987b). This may allow species such as the charophytes inter alia to occupy the deeper regions of infested waterbodies (Clayton et al. 1981) although native littoral species will still be excluded. One of the major problems facing control of further spread of these species is that the initial invasion of a lake from a source of inoculum by submerged adventive species is often through many independent small plants growing inconspicuously beneath the water surface (Howard-Williams et al. 1987). These plants will usually have become established from stem fragments bearing buds or dormant stem apices that are frequently well established by the time growth has extended the plants to the water's surface where they are immediately visible to a casual observer and henceforth noted. The sudden visible appearance of a canopy at the water's edge is thus often preceded by a long period of usually undetected (by casual observation from the surface) sub-surface dispersal, growth, and (Howard-Williams et al. 1987). Control by use of mechanical extraction to be used as a food for live stock as occurs in some overseas areas where these same aquatic weeds are a problem (Cook & Luond 1981) is not viable in the Waikato River and many other areas in New Zealand (e.g. Hamilton Lake) where toxic heavy metals present in the water either through natural, e.g. geothermal, or anthropogenic activities, as our species of the oxygen weed tribe in New Zealand actively accumulate these chemicals (Chapman et al. 1974). In these areas use of chemical (e.g. diquat) or a biological control, e.g. grass carp (Ctenopharyngodon idella) may be a more practical and long-term solution (e.g. Hughes 1971, Mitchell 1980, Stevenson 1992, Tanner et al. 1990b, Anon 1992). However, even with these measures to control further dispersal of naturalised species, removal of some existing populations, and with more effective border control on incoming potentially weedy aquatic species, it is clear that the Hydrocharitaceae, complete with all its established and successful (some of them too successful) species, is in New Zealand to stay.

ACKNOWLEDGEMENTS

Thanks to Paul Champion and John Clayton of NIWA (Hamilton) for very helpful discussion, answers, and use of their personal libraries; Paul Champion further for reviewing and constructive comments, Lance Vervoort of Auckland Regional Council for the Plant Pest status of these plants within the various regional councils, and to my girlfriend Kylie Titchener for her patience. Acknowledgment and thanks is also given to Ewen Cameron and Peter de Lange for reviewing and constructive criticism of the manuscript.

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