Ecology of the Avon-Heathcote Estuary: Comparative Salt Marsh Survey 2006-2007. Estuarine Research Report 34

Abstract

Executive Summary: 1. As cities cope with increasing populations, salt marshes around the world are on the decline due to negative effects of high levels of development, draining and reclamation. Due to high levels of disturbance in estuaries, monitoring is essential to ensure that salt marshes are preserved for future generations through sustainable management. 2. During the summer of 2006/07, salt marsh vegetation surveys were undertaken in the Avon-Heathcote Estuary, Christchurch, New Zealand using the same study areas as in McCombs and Partridge (1992). Global Positioning Systems (GPS) were used to capture each of 14 study sites to enable a high degree of accuracy. GPS were also used to create a detailed map of the margins of the estuary and to determine the locations of salt marsh vegetation. 3. The main aims of the study were to evaluate the current area of salt marsh vegetation in the Avon-Heathcote Estuary, to describe the community types and assess if these have changed since 1991/92. 4. A total of 12 vegetation types were found in 2006/07 compared with 15 in 1991/92. Oioi Rushland (Type 1) with 340 sites and Sea Rush Rushland (Type 2) with 205 sites were the dominant community types in the Avon-Heathcote Estuary. 5. Salt marsh vegetation in the Avon-Heathcote Estuary has changed since 1991/92. Salt tolerant plants, such as New Zealand Musk (Mimulus repens) and Suaeda (Suaeda novae-zelandiae) have been replaced by less tolerant plants, such as sea rush (Juncus krausii) and Oioi (Apodasmia similis). This is most likely due to increased sedimentation of finer sediments from developments around areas such as the Heathcote River. Of the original 495 sites, 27 had no vegetation in 2006/07. 6. The salt marsh near the Avon River had the most stable vegetation, dominated by Oioi Rushland (Type 1) and Sea Rush Rushland (Type 2). They shared an index of stability of 0.83, that is 83% of the survey sites remained the same from 1991/92 to 2006/07. In contrast, Sandy Point and the study area above Ferrymead were the most unstable areas and had changed the most. 7. The GPS map confirmed a high proportion of built structures compared to natural substrates around the margins of Avon-Heathcote Estuary. The total area of salt marsh in the Avon-Heathcote Estuary, determined from GPS measurements, was 372163 m2 (0.37 km2). The largest area contiguous of salt marsh was on the Avon River (Study area 1), with the smallest area at Sandy Point (Study area 8). 8. To encourage protection of the salt marsh vegetation in the Avon-Heathcote Estuary the survey should be completed again within 8 to 10 years. The present study forms a baseline for future comparative research which focuses on changes to the estuary and its salt marshes, this may be particularly important after 2008 with removal of the treated wastewater from the estuary.