Kāpiti Whaitua. Review of estuary ecological condition and habitat vulnerability
- Author
- Stevens LM, Forrest BM
- Year
- 2019
- Journal / Source
- Salt Ecology Report 028 prepared for Greater Wellington Regional Council.
- Pages
- 60 pp
- Summary
- In response to the National Policy Statement for Freshwater Management (NPSFM), Greater Wellington Regional Council (GWRC) has recently engaged in a community-led approach to provide advice and direction on how best to manage land and water to ensure that the future state of the regions streams, estuaries, beaches, rocky shores and waters meet the aspirations of those most closely connected with them. To support the Kāpiti Coast Whaitua process, Salt Ecology was commissioned by GWRC to re-assess the broad scale condition of nine Kāpiti Coast estuaries that had been previously assessed in 2007 and to evaluate any conspicuous changes in the pressures which may impact on coastal ecological values. The assessment of pressures draws extensively from existing information, supplemented with synoptic field assessments of the current ecological state of each estuary which were undertaken in January 2019. These field surveys were used to update broad scale maps of substrate, macroalgae, seagrass and salt marsh, and to collect point-in-time water quality data (e.g. chl-a measures) to support the calculation of NZ Estuary Trophic Index (ETI). The synoptic assessments are the main component described in the current report. All nine estuaries are Shallow, Short Residence time Tidal River (and adjoining lagoon) Estuaries (SSRTREs). Seven are low flow SSRTREs (Waitohu, Mangaone, Pekapeka, Waimeha, Tikotu, Wharemauku, Whareroa). They are primarily small, shallow, narrow and occur where stream outlets to the coast are restricted or blocked completely by a sand or gravel barrier just short of the ocean. They are characterised by having little or no intertidal habitat or salt marsh, and water and sediments experience regular cycles of degradation and rejuvenation. Most of these estuaries have been heavily modified by past drainage and channelisation, with mouths opened artificially to reduce flood risks. In the two moderate and high flow SSRTREs (Waikānae, Ōtaki), freshwater flows dominate over tidal flows and flushing is relatively extensive because the mouth is nearly always open and river flow relatively large. In addition, tidal flats are large relative to the smaller SSRTREs, but not particularly broad or expansive. The presence of larger areas of salt marsh adds to their ecological value, particularly for birdlife, and the estuaries often support healthy fisheries, including whitebait. The presence of intertidal flats and intermittent stratification of these moderate and high flow SSRTREs may result in fine sediment deposition and the growth of nuisance macro- and micro-algae, requiring targeted management of catchment nutrient and sediment inputs.