Hydrobiologia (2006) 570:183–188 Ó Springer 2006 J.M. Caffrey, A. Dutartre, J. Haury, K.J. Murphy & P.M. Wade (eds), Macrophytes in Aquatic Ecosystems: From Biology to Management DOI 10.1007/s10750-006-0179-z
Risk assessment method for submerged weeds in New Zealand hydroelectric lakes J. Clayton* & P. Champion National Institute of Water and Atmospheric Research, 11-115 Hamilton, New Zealand (*Author for correspondence: E-mail:
[email protected])
Key words: aquatic plants, weed potential, risk assessment, weed management
Abstract The latent potential for problematic weed growth in a hydro-lake is proportional to water level fluctuation, water clarity, lake shape, littoral gradient and exposure to wave action. These five factors were used to assess a score for measuring the latent potential for weed impact and each factor was allocated a score of 1–5, with a theoretical maximum of 25. Any hydro-lake scoring above 15 could be expected to present potential inconvenience to power generation, given the presence of suitable submerged weed species. Assessment of potential impact arising from weed invasion must firstly determine what species are already established, and whether it is possible for a weed species of greater potential impact to be introduced. The risk of this occurring is dependent upon a number of factors, including public accessibility, proximity to sources of those species and the desirability of the waterbody to potential weed vectors. Once these factors are quantified, appropriate surveillance strategies in high-risk hydro-lakes may then be developed.
Introduction New Zealand has an abundance of rivers, lakes and an oceanic climate providing year-round moderate rainfall. These have all contributed to the development of a network of hydroelectric lakes that supply around 80% of the nation’s electricity. The potential for any new hydro-lake to create suitable habitat for weed growth has often received only cursory regard, insufficient to warrant modification of station design prior to lake filling. Nearly all weed problems at power stations over the last 30 years or more can now be attributed to species that are not native to New Zealand. These weeds have resulted in world record biomass, significant loss of generation potential, and environmental and utilitarian impacts (Clayton, 1982, 1996; Howard-Williams et al., 1987). In order to assist power station managers to better appreciate the level of risk posed by weeds for their
hydroelectric lakes, a simple largely subjective scoring system was devised. Unlike many tropical or subtropical countries floating plant species such as salvinia (Salvinia molesta D. Mitch.) and water hyacinth (Eichhornia crassipes (Mart.) Solms-Laub.) do not thrive in New Zealand on account of the cool temperate climate. In addition, an effective national weed management strategy has virtually eliminated these two species from public waterbodies where they have previously occurred. Erect marginal species (e.g., phragmites (Phragmites australis (Cav.) Steudel) and Manchurian wild rice (Zizania latifolia (Griseb.) Stapf)), and sprawling species (e.g. alligator weed (Alternanthera philoxeroides (C. Mart.) Griseb.), reed sweet-grass (Glyceria maxima (Hartman) Holmb.), parrot’s feather (Myriophyllum aquaticum (Vell.) Verdc.), watercress (Nasturtium officinale R. Br.) and primrose willow (Ludwigia
184 peploides (Kunth) Raven)) potentially pose a weed management problem for small hydro-lakes. However, lake size and wave action result in limited suitable habitat apart from protected backwaters and sheltered embayments. Overall, these plant species tend not to be a problem to hydrolake managers and are unlikely to become problematic in the foreseeable future. Submerged aquatic plants have the most immediate and future relevance to New Zealand hydro-lake operations. Historically, we had no tall-growing, canopy-forming submerged native plant species and this has allowed a range of introduced species with alternative growth strategies to displace many native communities (Howard-Williams et al., 1987). Elodea canadensis Michaux and Ranunculus trichophyllus Chaix, two of the earliest aquatic plant arrivals, are widespread and now close to reaching their full potential impact. Fortunately they present minimal concern to managers of hydro-lakes, since they grow less dense and are less competitive than other less widely distributed weed species. Egeria densa Planchon, Lagarosiphon major (Ridley) Moss and Ceratophyllum demersum L. (hornwort) have proven to be the most problematic weed species on account of their substantial surface-reaching growth forms. C. demersum was the first weed species to temporarily close down a hydro-electric power station in 1965 (Widgery, 1967) and this same species continues to dominate in many North Island hydro-lakes. These species can tolerant low water clarity and can survive ice-cover during winter (Haramoto & Ikusima, 1988; Spencer & Wetzel, 1993). Hornwort does not produce roots so it is readily dislodged and can be found impaled on submerged obstacles, such as drowned or fallen trees or caught on hydro-lake booms and screen intakes, where it will keep growing until removed. Waikato hydro-lakes (a chain of eight lakes on the longest river in the North Island) have from 10 to 53% of their littoral margin supporting submerged weed growth. The larger the area of lake and the higher the proportion of vegetated area, the greater the likelihood of a weed problem. Nevertheless, the variation between stations on the same river system is considerable and the quantity of weed removed from any one station can differ significantly from year to year. For example, one hydro-lake (Whakamaru) in one year had 7286 m3
of weed removed from the station intakes during four Autumn months of peak efflux, yet during the following 2 years only 728 m3 and 1,259 m3 were removed for the same period. Other potential weed species in hydroelectric lakes include Hydrilla verticillata (L.f.) Royle, currently restricted to one geographically isolated region, and Myriophyllum spicatum L., which fortunately has not yet made an appearance in New Zealand.
Methodology Aquatic plants will only grow where they have access to light, nutrients and in most cases a suitable substrate for anchorage. The following factors collectively determine the vulnerability or latent potential of any waterbody to support problematic weed growths if invaded by a submerged weed species. Water level fluctuation Hydro-lakes with large routine (e.g. daily) or annual fluctuations in water level tend to support significantly less submerged plant growth compared to waterbodies with small fluctuations; however, disturbance tolerant species such as C. demersum and Najas marina L. can persist and show rapid recovery following changes in water level (Springuel & Murphy, 1991; Ali et al., 1995). In the 1970s, routine water level fluctuations of c. 5 m were used to avert nuisance growths of weeds on the Waikato River hydro-lakes. In these lakes problematic surface-reaching growths are normally restricted to water
