Lagoons

Okarito Lagoon, Westland (Janet Wilmshurst)

In the broadest sense, a lagoon is a shallow lake that is intermittently connected with a river, a deeper lake, or the sea. In New Zealand the term is applied mostly to coastal lagoons impounded behind barrier beach ridges, usually in association with a river or stream mouth, and is used in that sense here. Their salinity ranges from almost fresh water, through brackish, to almost pure sea water and it can change over time. The depth and salinity of many lagoons are influenced by tidal movement. The barrier bars are often not permanent but are formed and re-formed by wave action, flooding rivers, or a combination and changes are often associated with storm events. Lagoons range in extent from less than a hectare to hundreds of hectares. Lagoons also form behind coral reefs but these are not found in New Zealand.

Synonyms

Coastal lagoons

Notable flora and fauna

Threatened plants include the nationally critical sebaea (Sebaea ovata) and stalked adder’s tongue fern (Ophioglossum petiolatum); nationally endangered pygmy clubrush (Isolepis basilaris) and swamp hibiscus (Hibiscus diversifolius subsp. diversifolius); nationally vulnerable Lepidium naufragorum and Gratiola concinna. Declining species include sand spike sedge (Eleocharis neozelandica) and tufted hair-grass (Deschampsia cespitosa). Native musk (Mimulus repens) is naturally uncommon.

Lagoons are extremely important habitat for bird conservation and are important refuges for birds during the shooting season. They support many threatened bird species including the nationally critical South Island brown teal (Anas chlorotis “South Island”), southern New Zealand dotterel (Charadrius obscurus obscurus), and white heron (Ardea modesta), nationally endangered Australasian bittern (Botaurus poiciloptilus), nationally vulnerable banded dotterel (Charadrius bicinctus bicinctus), and declining South Island fernbird (Bowdleria punctata punctata).

Some lagoons are important breeding grounds for native fish, particularly Galaxias spp. (juveniles are whitebait), including the threatened short-jawed kokopu (Galaxias postvectis).

Threat status

Endangered (Holdaway et al. 2012)

Threats

Weed invasion is high, several salt-tolerant weeds, e.g. grasses such as Puccinellia spp., sickle grass (Parapholis incurva), tall fescue (Schedonorus phoenix), creeping bent (Agrostis capillaris). In the north, kikuyu grass (Pennisetum clandestinum), buffalo grass (Stenotaphrum secundatum) and saltwater paspalum (Paspalum vaginatum) are common. Many exotic herbs are present on lagoon margins, e.g. Aster subulatus. Lagoon margins are a fragile system and the greatest threat to unfenced lagoon margins is grazing and trampling by domestic and feral animals. There are potential problems of nutrient enrichment from fertiliser and stock and from urban runoff into lagoons close to human settlements. Logging in the headwaters threatens some lagoons although this is now largely historical. Lagoon systems under greatest development threat are near centres of population. There is little threat from on-site erosion because this occurs naturally anyway, but it is influenced by upper catchment erosion, and river flooding, which can be related to human activity. Smaller lagoons are under greatest threat from drainage of their headwater supply streams. Also unnatural opening of bars across lagoon mouths when lagoon levels are high, to alleviate flooding of adjacent agricultural land, can be damaging. Damage by off-road vehicles can be severe and demand for access high because of the recreational value of lagoons.