Lake Whangapē

In 1988 Lake Whangapē was eutrophic with reasonably clear water.

Submerged aquatic plants (macrophytes) covered more than 95 per cent of the lake bed.

Oxygen weed (Egeria densa) was the dominant plant, although there was also:

• hornwort (Ceratophyllum demersum)
• blunt pondweed (Potamogeton orchreatus)
• Canadian pondweed (Elodea canadensis).

However, in 1987 populations of Egeria densa started to die off and Lake Whangapē began to lose most of its submerged aquatic plants.

Our monitoring results during 2002 to 2004 show the lake is now hypertrophic with very high levels of Chlorophyll a and nutrients.

In principle, controlling stock access to streams in the lake’s catchment and better management of sediment and nutrient losses from pastoral farming should reduce the sediment and nutrient load entering Lake Whangapē. Such reductions would be expected to reduce the amount of phytoplankton and other particulate material present in the lakewater, thereby improving underwater lighting and improving conditions for the re-establishment of macrophytes.

However, at this stage it is not clear just how large a reduction in the sediment and nutrient loads from the catchment would be necessary to improve underwater lighting sufficiently for vigorous macrophyte regrowth. Furthermore, it is possible that even if a sufficiently large improvement in underwater light did occur, then macrophyte re-establishment could be restricted by other factors (for example, disturbance and grazing by introduced fish).

Therefore at this stage all that can be said is that improved management of the catchment may help to maintain the existing ecological values and may promote a recovery of the lake from its current algal-dominated state towards its previous macrophyte-dominated state. However, it is expected that such a recovery will be difficult to achieve because the current state of Lake Whangapē appears to be rather stable.