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

Vegetation pattern, soil attributes, and salinity regimes along with severa l other environmental factors were studied in a small (11 ha) but unmodifie d estuary in south Westland, New Zealand. Part of an unmodified 40 km(2) ca tchment within the South-west New Zealand World Heritage Area, the Hapuka E stuary, formed behind a 5-km long barrier beach, provides considerable ecol ogical value as well as baseline, conservation advocacy and educative poten tials both in a national and international context. Seventy-four quadrats w ere sampled randomly along five transects at right angles to the main Hapuk a River. Eight plant communities comprising 141 native and thirteen exotic vascular species, were differentiated using multivariate analyses, from bar e mud (mean elevation 1.23 +/- 0.24 m above Mean Low Water Spring (MLWS)), saltmarsh and shrubland associations through to the adjacent podocarp-broad leaved 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 ele vation association to a minimum of 0.15 g cm(-3) in the rain forest. pH sho wed 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% ODW t) in the rain forest substrate. The water which irrigates the saltmarsh at high tide reaches 15-19 parts per thousand salinity on calm days but may b e much less saline when moderate to strong southerly winds counter the tida l influence. Northerly winds, or a southern outlet through the barrier beac h, intermittently evident in the past, are likely to enhance salinity of th e tidal waters across the saltmarsh. Ordination of the vegetation samples indicated a very strong gradient assoc iated 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 conductiv ity, and negative correlations with elevation, soil water, organic matter a nd potassium contents.