By Philip Grove
Canterbury has a wide variety of wetland types in a range of landscapes from the mountains and high country through to the foothills, plains and the coast. The biological productivity of coastal wetlands and their ecological importance in the life cycles of many native fish and birds is well recognised. A national database of inland freshwater wetlands has been developed recently, but it does not cover saltmarsh or brackish wetland habitats, and is of limited accuracy in areas where freshwater wetlands adjoin or grade into brackish coastal lagoons and estuaries.
To complement the freshwater wetland database for Canterbury, Regional Council staff surveyed and mapped the vegetation of coastal wetlands over the period 2004-2011. The survey area included wetlands associated with estuaries, dunes, coastal lagoons and river mouths – that is wetland forms that are related directly or indirectly to coastal processes. Results of the survey were entered into a database recording type and extent of vegetated coastal wetland habitats following a standard wetland classification system.
The database contains information on 50 vegetated coastal wetlands in Canterbury Region, from the Tirohanga River mouth in the north to the Waitaki River mouth. The largest contiguous area of wetland vegetation in the database, more than 4000 ha, is located around the margins of Te Waihora/Lake Ellesmere.
Estuarine habitats supporting saltmarsh or brackish wetland vegetation comprised the majority of the mapped area – 4602 ha. However, only 341 ha of saltmarsh vegetation was recorded from within true estuaries subject to sea-water intrusion in daily tidal cycles. The greater part of Canterbury’s vegetated saltmarsh habitats are instead associated with brackish coastal lagoons that are not permanently open to the sea but are still affected to some degree by salt water.
Freshwater wetland vegetation and habitats formed a smaller but significant proportion of the coastal wetlands surveyed; some 1140 ha or about 20% of the total wetland area. Freshwater wetlands were commonly present on or adjoining the inland margins of estuaries and brackish coastal lagoons, as well as along the edges of freshwater coastal lagoons and river mouth lagoons (hāpua).
Native plant species remain the dominant element in the vegetation cover for the majority of the region’s saltmarsh habitats. The most abundant native vegetation types are saltmarsh herbfield, marsh ribbonwood (Plagianthus divaricatus) shrubland and sea rush (Juncus krausii ssp. australiensis) rushland. However, most of the freshwater wetland habitats within the coastal survey area were dominated by introduced plants, particularly willows and grasses, although native rushes and raupō are also common.
Prior to European settlement, large freshwater wetlands occupied the low plains connecting the region’s coastal lagoons, estuaries and hāpua. For the most part these links no longer exist. Drainage of this low-lying land for agricultural and urban development has reduced the formerly extensive complex of freshwater and estuarine wetlands to the isolated fragments that remain around river mouths, estuaries and coastal lagoons. Human-induced loss of saltmarsh and other coastal wetland vegetation in the region is on-going. There have however also been examples of successful restoration of some coastal wetland habitats over the last decade, such as at Charlesworth Reserve on the Avon-Heathcote Estuary and Otipua Wetland near Timaru. The importance of Te Waihora/Lake Ellesmere margins, which support more than 80% of the region’s remaining saltmarsh habitats, has been recognised with the inclusion of vegetation values in the National Water Conservation Amendment Order of 2011.
Following the Canterbury earthquakes of September 2010 and February 2011, significant changes have been recorded in bed level and hydrology in and around the Avon-Heathcote Estuary and Brooklands Lagoon. Repeating the vegetation survey and mapping of these estuaries in a few years when plant distributions have adjusted to the new conditions will help provide a measurable record of changes to extent and type of wetland habitats.
Philip Grove is a terrestrial and wetland ecologist at Canterbury Regional Council.