The Awarua-Waituna Wetland complex encompasses approximately 20,000 hectares of wetlands, including a coastal lagoon (Waituna Lagoon), extensive peatlands, swamps, freshwater streams, Awarua Bay, Toetoes Harbour and New River Estuary.
It is recognised as a wetland of international importance under the Ramsar Convention because of its significant biodiversity values. Features of the vegetation are the ecological sequences across ecological gradients and between wetland types, from the wetland to the coast and to Awarua Bay and from wetland to forest.
The area provides habitat for a rich array of vegetation including
montane and subalpine species which are otherwise confined to mountainous areas,
with the nearest similar vegetation of this type found in the Blue Mountains in
West Otago. There is a rich flora that has been recorded which includes two hundred and thirty-six species of plants, moss and algae in Waituna Lagoon and surrounding wetland.
The wetland vegetation consists of a mosaic of vegetation which reflects the soils, fertility, drainage and other factors. The most widespread vegetation are peatland communities dominated by wirerush (Empodisma minus), with tangle fern, manuka and swamp inaka (Dracophyllum oliverii) and/or manuka shrubland. Along streams and natural drainage channels are areas of flax swamp, sedgeland and red tussockland. There are also many peat pools, ponds and lakes peatland. Forest stands were formerly much more extensive however have been lost to a combination of historic fires, logging and land development. The forest is generally dominated by rimu (Dacyrdium cupressinum), but locally by kahikatea (Dacrycarpus dacrydioides) or totara (Podocarpus laetus). Regenerating forest stands tend to be dominated by broadleaf (Griselinia littoralis) and kohuhu/black mapou (Pittosporum tenuifolium).
The reserve is important for its unique moor like vegetation (cushion bogs) characterised by herbs and shrubs adapted to cold peaty conditions. These include the cushion plants Donatia novae-zelandiae and Oreobolus pectinatus, along with Gentianella lineata and Actinotus novae zelandiae, sundews (Drosera spp.), grass lily (Oreostylidium subulatum) and comb sedge (Carpha alpina). The cushion bogs are a significant botanical feature of the area. Unfortunately they have become increasingly more and more restricted in area as a result of fires and invasion by woody species (notably manuka). Their best development is now restricted to a small area close to the coast west of the usual lagoon opening.
The coastal fringe of the wetland consists of a gravel beach. The gravel beach has a discontinuous vegetation of grasses, herbs and shrubs, including Muehlenbeckia axillaris, shore gentian, ramarama (Selliera radicans), stonecrop and silver tussock along with some locally uncommon species – mat daisy (Raoulia spp.), native daphne (Pimelea prostrata) and native iris (Libertia perigrinans). At the eastern end of Toetoes Bay, adjacent to the lower reach of the Mataura River and Fortrose Estuary is a dune system. The Fortrose dune system has one of the last populations of pingao (Ficinia spiralis) on the southern Southland coast. Associated with the dune are a rare and unusual, fragile mat daisy community. Altogether the coastal vegetation is a striking feature of the area, and forms a distinct assemblage not found north of Southland.
Vegetation in the peatland habitat is governed by the height of the water table and drainage. Waituna Lagoon has a fluctuating water level. The margin of Waituna lagoon has an ecological sequence from the shore which is frequently inundated to areas that are only occasionally inundated. The margin is dominated by oioi/jointed rush (Apodasmia similis). The oioi rushlands support the threatened plants tufted hair grass (Dechampsia cespitosa) and swamp nettle (Urtica perconfusa). The most extensive vegetation type is wire rush (Empodisma minus) with tangle fern (Gleichenia dicarpa), manuka (Leptospermum scoparium) and (Dracophyllum longifolium). Better drained areas are dominated by shrublands or red tussock (Chionochloa rubra), with some stunted coastal rimu (Dacrydium cupressinum) forest. Low lying sites are generally dominated by sedges, rush and bryophytes with ponds or pools.
Waituna Lagoon is considered unique because of its relatively intact macrophyte communities dominated by ruppia (both megacarpa and polycarpa spp.). The aquatic vegetation is dominated by predominantly freshwater species, although the brackish-water tolerant algae, Enteromorpha spp. and Bachelotia antillarum, can often be found in varying amounts within the lagoon.
Ruppia is an aquatic plant that grows in the water in the lagoon. It is a macrophyte, (the plant family that grows in or near water). Macrophytes provide cover for fish and a place for aquatic invertebrates to live. They also produce oxygen and act as food for some fish and wildlife. Ruppia has been identified as playing a key role in regulating Waituna Lagoon’s water quality as well as providing habitat for animals.
Slime algae which was has been discovered in the lagoon in the last decade, can smother ruppia. Phytoplankton, or tiny plants which naturally grow in the lagoon, can over-grow and cause a bloom, which also harms the ruppia. This is turn affects the animals that use the ruppia as a home.