Interpreting ecological patterns in an intact estuary, South-west New Zealand World Heritage Area

Vegetation pattern, soil attributes, and salinity regimes along with several other environmental factors were studied in a small (11 ha) but unmodified estuary in south Westland, New Zealand. Part of an unmodified 40 km² catchment within the South-west New Zealand World Heritage Area, the Hapuka Estuary, formed behind a 5-km long barrier beach, provides considerable ecological value as well as baseline, conservation advocacy and educative potentials both in a national and international context. Seventy-four quadrats were sampled randomly along five transects at right angles to the main Hapuka River. Eight plant communities comprising 141 native and thirteen exotic vascular species, were differentiated using multivariate analyses, from bare mud (mean elevation 1.23±0.24 m above Mean Low Water Spring (MLWS)), saltmarsh and shrubland associations through to the adjacent podocarp-broadleaved rain forest (4.53±0.74 m above MLWS). Bulk density of the upper 10 cm of substrate varied from a maximum of 1.15 g cm^?3 in the lowest elevation association to a minimum of 0.15 g cm^?3 in the rain forest. pH showed a similar trend with values of 6.97 and 4.31, respectively. The reverse pattern was evident with organic matter, with the highest content (53% ODWt) in the rain forest substrate. The water which irrigates the saltmarsh at high tide reaches 15–19» salinity on calm days but may be much less saline when moderate to strong southerly winds counter the tidal influence. Northerly winds, or a southern outlet through the barrier beach, intermittently evident in the past, are likely to enhance salinity of the tidal waters across the saltmarsh. Ordination of the vegetation samples indicated a very strong gradient associated with Axis 1 (eigenvalue=0.872) and Axis 2 (eigenvalue=0.461). Vector fitting of nine measured environmental factors indicated a strong positive correlation with Axis 1 of the ordination, of soil pH, sodium and conductivity, and negative correlations with elevation, soil water, organic matter and potassium contents.