Rohan D. S. Wells

Successive macrophyte invasions within the submerged flora of Lake Tarawera, Central North Island, New Zealand

Abstract Lake Tarawera is the lake with the clearest water in New Zealand to be invaded by four particularly successful invasive adventive weeds: Ceratophyllum demersum, Egeria densa, Elodea canadensis, and Lagarosiphon major. E. canadensis was the first to invade the lake followed by L. major which became the dominant tall‐growing species to 6 m depth. Comparisons of lake survey data for macrophytes in Lake Tarawera from 1988 and 1993/94 show that marked changes occurred in the composition and patterns of submerged vegetation as a result of a recent invasion by C. demersum. This species was first recorded in 1988 as infrequent, within a restricted area of the lake. Within 5 years it occurred in 52% of the lake profiles, had a depth range of 0.5–15.5 m, and a median average cover estimated to be between 51 and 75% throughout this depth range. The spread of C. demersum has been at the expense of native vegetation (particularly Characean meadows) and E. canadensis. E. densa was first recorded at the same ti...

Spread and status of seven submerged pest plants in New Zealand lakes

Abstract The distribution of seven submerged aquatic pest plants is reported. Lake vegetation surveys recorded pest plants in 27.9% of 344 lakes, with two species co‐occurring in 5.8%, and three species in 2.6% of lakes. Egeria densa was most frequent (15.4% of lakes), followed by Ceratophyllum demersum (9.0%), Lagarosiphon major (7.3%), and Utricularia gibba (5.5%). Spread since 2000 has continued for five pest plants, with 34 lakes invaded by U. gibba over 2004–08 alone. Early regional sites in proximity to human population centres were likely plant liberations and numerous potential founder colonies remain in garden ponds. Human activities were important for inter‐lake dispersal, with the exception of bird‐dispersed U. gibba. Significant lake associations between pest plants, and with presence of six exotic fish species, suggest common dispersal pathways and similar introduction risks. Therefore, predictions of future spread should be possible based on sources, dispersal pathways, and identifying key risk factors for lakes.

Effects of suspended solids on the establishment and growth of Egeria densa

Abstract To identify levels of suspended solids (SS) in Lake Waahi (Huntly, New Zealand) favourable for the re-establishment of Egeria densa Planchon, shoots were grown in 2 m deep tanks filled with water transported from the lake and maintained at selected SS loadings between 5 and 40 g m −3 (predominantly compromised kaolinitic clays). Plant growth was measured over periods of 44–47 days in spring, summer and autumn. Highest relative growth rates (RGR) were recorded in summer (maximum 40 mg g −1 day tt-1 ). Egeria propagules growing from a depth of 1.85 m showed positive growth responses at SS levels up to approximately 25 g m −3 ( K d ≈ 1.75) in spring and autumn, and approximately 35 g m −3 ( K d ≈ 1.95) in summer. At SS levels up to 15 g m −3 ( K d = 1.5 m −1 ), growth was little affected during summer, while during spring and autumn biomass accumulation was reduced to about 60% and RGR values to about 70% of the maximum seasonal values recorded. Egeria shoots showed maximum elongation at about 10–20 g m −3 SS ( K d = 1.3−1.65). Plant architecture became less branched with increasing SS and the number of roots produced decreased. Implications for the establishment of Egeria in turbid waters are discussed.

Submerged vegetation and spread of Egeria densa Planchon in Lake Rotorua, central North Island, New Zealand

Abstract SCUBA records on the distribution and abundance of macrophytes in Lake Rotorua show marked changes between 1980 and 1988. Extensive weed beds of Lagorosiphon major (Ridley) Wager, present since the mid‐1950s, disappeared from the lake by 1982. The loss of the L. major weed beds coincided with marked reductions in water clarity in the early 1980s and with a storm in April 1982. Egeria densa Planchon spread rapidly from a small infestation in July 1983 to become the most abundant macrophyte in the lake by December 1988 (occupying about 475 ha), equivalent to the historic L. major weed beds. We predict that E. densa will be dispersed to become a major problem weed in other Rotorua lakes in the future.

Control of macrophytes by grass carp (Ctenopharyngodon idella) in a Waikato drain, New Zealand

Abstract Hornwort (Ceratophyllum demersum L.) and other aquatic macrophytes have historically been mechanically removed from the Rangiriri drain and Churchill East drain to maintain drain efficiency. As an alternative control method for the high plant biomass that accumulates at the end of summer, the effect of stocking diploid grass carp (Ctenopharyngodon idella L.) on the aquatic vegetation was evaluated in these Waikato drainage systems. At the start of the trial, both drains had a low diversity of aquatic macrophytes, and of the nine species (including the emergents), seven were exotic. Two months after grass carp were released to Churchill East drain (the treated drain) the four submerged and floating macrophyte species became scarce in the main drain. Over the same period, these species increased in biomass in Rangiriri drain (the untreated drain), where hornwort became dense and surface‐reaching and remained so for the duration of the trial. However, grass carp did not control submerged vegetation in smaller side drains or the shallow, upper parts of the main drain, or the marginal sprawling species and emergent species. The cost of leasing the grass carp was similar to the cost of clearing the drains mechanically, but grass carp provided continuous weed control. However, subsequent to this trial, 62 dead grass carp were found in Churchill East drain in February 2001, and weed cover subsequently increased. This illustrates that grass carp management in New Zealand agricultural drains can be problematic due to periodic fish kills.

SUBMERGED VEGETATION OF LAKES TE ANAU, MANAPOURI, MONOWAI, HAUROKO, AND POTERITERI, FIORDLAND, NEW ZEALAND

Abstract The submerged vegetation in Lakes Te Anau, Manapouri, Monowai, Hauroko, and Poteriteri from the South Island of New Zealand, was surveyed in 1993 using SCUBA methods to record a range of botanical parameters to the deepest extent of the vegetation. This is the first detailed account of the submerged vegetation in these lakes and the first description for Lake Poteriteri. The vegetation of these lakes is comprised predominantly of native species. Isoetes kirkii dominates the shallow‐water assemblage in all lakes. Hydatella inconspicua, an endemic shallow‐water plant of rare status, is reported for the first time in the South Island, New Zealand. Formerly only reported from a limited number of North Island sand dune lakes, the discovery of these southern disjunct populations of H. inconspicua means that this species should no longer be considered endangered. The tall‐growing native vascular species, Potamogeton cheesemanii, Myriophyllum propinquum, and M. triphyllum were recorded in all five lakes....

Invasion of water net, Hydrodictyon reticulatum: The surprising success of an aquatic plant new to our flora

Invasion of water net, Hydrodictyon reticulatum: The surprising success of an aquatic plant new to our flora Ian Hawes a , Clive Howard‐Williams a , Rohan Wells b & John Clayton b a Taupo Research Laboratory, DSIR Marine & Freshwater , P.O. Box 415, Taupo b Aquatic Plant Section, Ruakura Agricultural Research Centre , Ministry of Agriculture & Fisheries , Private Bag, Hamilton Published online: 30 Mar 2010.

Submerged vegetation of Lakes Sumner, Marion, Katrine, Taylor, and Sheppard in Canterbury, New Zealand

Abstract The submerged vegetation of Lakes Sumner, Marion, Katrine, Taylor, and Sheppard was surveyed in May 1987. These high‐altitude lakes lie at c. 600 m a.s.l., within the largely unmodified upper Hurunui catchment. Submerged vegetation was diverse and included numerous short shallow‐water species, dense swards of Isoetes alpinus, low covers of taller native vascular plants, and charophyte meadows beyond the depth limit of vascular plants to a maximum of 15 m. A sparse deepwater bryophyte community was observed from 11 to 32 m depth in Lake Sumner. Displacement of native vegetation by dense growths of the adventive oxygen weed Elodea canadensis over mid‐depths of 3–6 m was noted in all lakes, except Lake Marion.