Plant-life in Streams

Aquatic plant life in rivers and streams in Tasman are made up of algae, mosses, and submerged vascular plants with leaves.

Before people settled most waterways were very shady because of the native forest surrounding them. They would have supported a range of native aquatic plant species. Today many of the smaller lowland streams are surrounded by agricultural or horticultural land and are dominated by introduced plants such as the oxygen weed (Elodea canadensis), swamp willow weed (Polygonum decipiens) and watercress (Nasturtium microphyllum).

These introduced plants can provide in-stream cover for other plants and animals. However when stream flows are low during summer they can reduce the dissolved oxygen in the streams.

In some cases native plants, like the fragile water plant Nitella have been displaced by aggressive exotic plants, such as oxygen weed. Many of the weedy exotics are causing reductions in stream oxygen levels in summer. This can cause problems for fish and invertebrates within the stream.