Lake Mokoan Project: Strategy for Macrophyte restoration

"August 1991".

Project Number: Lake Mokoan Project - M/04/189.

MDFRC item.

Water quality of Lake Mokoan has deteriorated in recent years as a consequence of sustained high turbidity and algal blooms. Habitat conditions for other biota have declined at the same time, as exemplified by the death of tortoises and the virtual disappearance of submerged aquatic macrophytes. It is not known whether the latter is a cause or an effect of the high turbidity. We were engaged as consultants to consider whether this turbidity could be reversed by re-establishing vegetation on the lake bottom and shoreline, and if so, what procedures and plants should be employed. We visited the lake, met appropriate officials and members of the local community and consulted available reports. We concluded that revegetation of the shoreline was feasible and estimated the probability of success was at least 50%. Establishing and maintaining this vegetation would require careful management of water levels and of exposed shorelines. We estimated the probability of reducing turbidity by re-planting aquatic plants was no more than 25%. This is a guess-timate, there being no hard information on ecology of recommended species and their responses on lake floor sediments. The procedures we suggest for revegetating the lake and for restoring habitat conditions for lake biota in general are outlined in the following recommendations. RECOMMENDATIONS l. The whole lake should be fenced to contain the lake and a buffer zone with a minimum width of 20 m above maximum water level. 2. Lake levels should be gradually drawn down by 1-2 m per year to a target minimum, estimated to be about 4.5% of the total volume of the lake when full. 3. Lake sediments exposed by draw-down should be revegetated by pasture grasses, according to local practice for establishing pasture. 4. The species or combination of species that grows most under the prevailing shoreline conditions should be investigated possibly by experimental use of the Duckponds. Possible species are perennial rye grass (Lolium rigidum), couch grass (Cynodon dacrylon) for the upper shoreline, and water couch grass (Paspalum disrichum), for the lower part of the shoreline. 5. The established pasture should be managed to obtain maximum plant density and root/rhizome growth and should not be grazed unless no adverse effects are likely. 6. When the target draw-down level is reached, the remaining water should be clarified. Advice should be obtained on the most appropriate chemical to use, and on timing in relation to planting schedules. 7. Propagules of submerged macrophytes should be planted by hand in shallow water. Suitable species are ribbonweed (Vallisneria), pond weed (Potamogeton) and milfoil (Myriophyllum). Appropriate propagules include portions of stem and rhizome. 8. Populations of emergent aquatic plants should be established, also by hand planting portions of stem or rhizome, in proximity to the water line, once this is reasonably well stabilised. Suitable species for sheltered areas include rush (Juncus sp "0") and for exposed areas common reed (Phragmites australis). 9. The potential for natural revegetation from the seed bank should be examined and assessed, and procedures adapted if necessary to take advantage of this. 10. Carp may undermine young, submerged and emergent aquatic plants, and so promote resuspension of particulate matter into the lake water. It will be necessary to determine whether carp are likely to prejudice a planting program at Lake Mokoan, and if so what management strategies can be implemented to ensure safe establishment and maintenance of vegetation. 11. Volunteers from organisations such as the Australian Trust for Conservation Volunteers and from members of the local community may be willing to assist in carrying out these recommendations where appropriate.