General Impact
In New Zealand, the plant has blocked intakes of hydro-electric systems and has formed dense floating mats in deep-water reservoirs and other water bodies. L. major has the potential to become a troublesome weed of lakes and slow-moving streams throughout temperate and sub- tropical regions of Australia. Under favourable conditions, dense growth of the plant can block light penetration into waterways, eliminating growth of native water plants and affecting associated populations of aquatic invertebrates and vertebrates. Once widespread, control would be extremely difficult (as is the case for most submerged aquatics) (Csurhes and Edwards, 1998). James et al. (1999) state that, "L. major creates progressively stressful conditions of high pH and low CO2 content. L. major may be successful in out-competing Elodea spp. as a result of its ability to photosynthesize and consequently grow, particularly under very stressful conditions of high pH and low free CO2, perhaps through more efficient bicarbonate utilization than the other species. There is some indication that the competitive success of L. major may be a consequence of greater toleration to pH stress. McGregor and Gourlay (2002) state that, "L. major replaces native vegetation; dense infestations restrict the passage of boats and limit recreational activities like swimming and angling; storms can tear loose the weed and deposit large masses of rotting vegetation on beaches, spoiling their amenity value. Rattray (1994) states that, "L. major has successfully out-competed native species wherever it has colonized." James et al. (1999) report that, "L. major has been reported to be actively displacing E. nuttallii and appears to be competitively superior to Elodea spp. in at least some habitats."
Location Specific Impacts:New Zealand 
Human nuisance: Csurhes and Edwards (1998) state that, "In New Zealand, the plant has blocked intakes of hydro-electric systems and has formed dense floating mats in deep-water reservoirs and other water bodies."
Reduction in native biodiversity: "In New Zealand L. major not only produced roots faster, it also grew faster, both in length and biomass, than the native Myriophyllum triphyllum. This suggests that L. major has an early competitive advantage which may explain the dominance this species now has in the littoral zones of many New Zealand lakes" (Rattray 1994). Lake Dunstan (New Zealand)
Human nuisance: In Lake Dunstan, the adverse effects of lagarosiphon relate more to recreational and amenity values. These are nuisance rather than environmental effects, with plenty of scope for mitigation (Strikland et al 2000).
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