Hornwort - The End of an Invasion

The noxious weed hornwort (Ceratophyllum demersum) has finally invaded the Waikato River from the most upstream headwater lake in the south, Lake Rotoaira, to the river mouth in the north. This invasion has taken just over 35 years. Hornwort (Ceratophyllum demersum) was first discovered in the Waikato catchment, in1963. It was found in Lake Ohakuri, one of eight hydrolakes lying between Lake Taupo and the sea.  Recently (February 1999), hornwort was found in the weed mass removed from the Tokaanu hydroelectric station’s intake screen at the northern end of Lake Rotoaira.  Quantities of hornwort were also seen floating beside the weed boom and growing amongst the oxygen weed (Lagarosiphon major) along the shore.  This lake had previously been free of this weed.

Hornwort invasion of the Waikato catchment has followed both expected and unexpected patterns. From Lake Ohakuri, hornwort rapidly spread downstream.  By 1966 it was established in the three hydroelectric lakes immediately downstream of Ohakuri  (Atiamuri, Whakamaru and Maraetai), and 5 years later it had reached the remaining downstream hydroelectric impoundments and, downstream of these, was found in the main river channel to the river’s mouth.  

Hornwort does not have roots and is readily transported by water currents.  It is not just found at the surface. A study using nets set at different depths in Lake Atiamuri found hornwort drifting with the current from the surface to at least 25 m depth (the depth of the deepest net), and underwater video has revealed hornwort caught on the screens of the penstock intakes at Maraetai hydroelectric station, which are 40 m below the lake’s surface.  The downstream colonization by this weed is, therefore, no surprise, because  even deep water intakes are no real barrier to its downstream dispersal.

Invasion upstream or to another catchment is, however, another matter.  Evidence strongly suggests that dispersal upstream normally happens by weed fragments either on boats that are transported between lakes, on fishing nets used in more than one lake, or in aquarium releases, rather than by natural means such as on waterfowl.  

A comprehensive aquatic weed survey of the Waikato River system in 1975 found that hornwort was not present in the lakes upstream of Lake Ohakuri (Aratiatia, Taupo and Rotoaira respectively).  A similar survey in 1980 found that although Aratiatia and Rotoaira were still free of hornwort, it was present in the Tokaanu hydroelectric power station tailrace and Waihi Bay at the very southern end of Lake Taupo.   

The presence of hornwort at this southernmost part of the lake was somewhat surprising, as the nearest source was about 75 km downstream.  Local residents provided a possible explanation. In 1976 or early 1977 an experimental NZED weed harvester was brought from Lake Ohakuri to the Tokaanu tailrace to test harvest the oxygen weed Lagarosiphon. Locals observed that when the harvester was launched, it was carrying quantities of hornwort.  Photographs of the exercise were taken too far from the harvester to determine whether this is how hornwort hitched a ride to the south end of Lake Taupo.  However, no other plausible explanations have been suggested for this significant upstream movement.  

Prevailing winds over Lake Taupo are from south to north, so it was only a matter of time until hornwort began to drift north, leave Lake Taupo and invade Lake Aratiatia via the Waikato River.  In June 1983 hornwort was found in the two major backwaters of Lake Aratiatia.  It is not surprising that this is where the weed first appeared. Studies had just been done to determine if much of the aquatic weed accumulating on the Aratiatia power station penstock intake screens could be coming from Lake Taupo.   The studies showed that this was not the case and that most weed from Taupo ended up in the backwaters of Aratiatia, exactly where hornwort first established itself.  In addition, neither of these areas is near a boat ramp or easily accessible by land, so the hornwort invasion of Lake Aratiatia appears to have been simply by natural means. 

In contrast, annual inspections of weed removed from the intake screens at lake Rotoaira, upstream of Lake Taupo, found no hornwort until 1999.  Thus, assuming it takes a year or two from introduction to becoming a noticeable component of the lake’s flora, it took over 20 years for hornwort to travel the approximately nine kilometers from the southern end of Lake Taupo to Lake Rotoaira. It is likely that the weed entered the lake through someone’s carelessness, not cleaning their boat or fishing nets of weed before using the lake.

In terms of “the ride”, Rotoaira is the end of the line for hornwort in the Waikato river system.

The only lake left is Rotopounamu, Although this land-locked lake, perched high on Mt Pihanga between Taupo and Rotoaira, does not have hornwort, it is unlikely to be successfully invaded.  Rotopounamu is extremely unusual in that it has virtually zero alkalinity. Thus, it is one of the few lakes in New Zealand that is not conducive to the growth of hornwort – even if that weed could climb mountains!

Contact:
Ian Johnstone, Chisholm Associates,
PO Box 12443, Chartwell, HAMILTON, 
Email: ipca@xtra.co.nz

Added August 1999