Threat to natives increasing

BLUNT pondweed. It's not an exciting name and nor is it very familiar, even to many people who know native plants. Yet the aquatic blunt pondweed, Potamogeton ochreatus, is one of our region's special - and declining - native plants. Perhaps the easiest place to spot it locally is the pond at Bushy Park.

Blunt pondweed was once widespread through lowland New Zealand, including many of this region's lakes. David Kelly's 1978 survey of seven dune lakes from Marahau in the west to Alice in the east, revealed blunt pondweed in six of them.

From 1990 onwards, the Museum Botanical Group surveyed these lakes, as did staff of the National Institute of Water & Atmospheric Research (NIWA), and they found blunt pondweed in just two lakes. A similar pattern of decline has been found for other native aquatic plants.

A native water milfoil (Myriophyllum triphyllum) was in five of the lakes in 1978 and has not been found in any since then. Horse's mane weed (Ruppia) has not been re-found in the two lakes where it was in 1978. Fennel-leaved pondweed (Stuckenia pectinata) remains in just one of three lakes where it occurred in 1978; it is listed now as being 'at risk' nationally.

So what has caused these native plants to decline in range? An obvious difference in the lakes between 1978 and 1990 is an invasion of exotic aquatic plants, i.e. waterweeds. Kelly never found Canadian pondweed (Elodea), Egeria, or hornwort (Ceratophyllum) in any of the seven lakes. A hindrance to recreational users, all three are now rampant in lakes Pauri and Wiritoa but not in four of the others.

Canadian pondweed was recently (2007) found in Kaitoke Lake. This lake is also the only local one with parrot's feather (Myriophyllum aquaticum), a common waterweed in the Manawatu. It could have got to Kaitoke Lake in eel nets. Lake Kaitoke lacks the other weeds; it still has blunt pondweed and two other declining native species. Lake Westmere has two of these declining native species too - and no serious aquatic weeds.

Looking at a spreadsheet of data, we can see a striking coincidence of powerboat usage with more serious waterweeds. Conversely, lakes and ponds with rare native aquatic plants tend not to have power boats, witness lakes Kaitoke and Westmere. A small, private farm pond west of Waverley has blunt pondweed and another pond nearby has both Ruppia and Stuckenia. Small ponds are not usually used by powerboats. Bushy Park's pond, with blunt pondweed and no exotic water weeds, is another example.

The new boardwalk across the wetland at Bushy Park was officially opened last weekend. It gives visitors a duck's-eye view of blunt pondweed. Rotary North members have worked hard to raise funds and construct this route, that ends in a pond-side amphitheatre for education groups. Here the pond is ideally placed to become a case study for lessons on the benefits and consequences of public use of lakes for our indigenous aquatic life and for monitoring changes in these communities.

-Colin Ogle is a retired ecologist and member of the Bushy Park Trust's Farm & Forestry advisory committee.