Knowledge of macrophyte requirements and tolerances holds the key to successful shallow lake restoration – a New Zealand perspective

Many shallow lakes are degraded to the point where they are permanently turbid and macrophytes no longer grow. Without the macrophytes, wave action resuspends lakebed sediments, and a feedback loop is set up, trapping the lake in a degraded state. Multiple restoration actions are required to reverse this process, including catchment management and in-lake actions to remove barriers to establishing native vegetation. In particular, when macrophytes have been lost for a long time, the native seedbank may be depauperate and limit re-establishment opportunities. Re-establishment of submerged vegetation is critical to the restoration of lakes, but to date there have been few efforts to actively do this. There are significant barriers preventing this goal, relating to poor water quality, low-density sediments and the browsing impacts of fish and waterfowl, that will be further exacerbated by the warmer temperatures and more extreme weather events forecast under climate change. A combination of experimental and field studies determining optimal environmental ranges of different macrophyte species, methods to culture en masse and techniques to promote sufficient water clarity and sediment density to sustain macrophytes are necessary. Once minimum vegetation cover thresholds are exceeded, sustainable restoration to a macrophyte-dominated clear-water state should eventuate.