New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington (Received for publication 20 March 1967)
The distributions of 19 species of polychaetes burrowing in the intertidal sand and mudbanks of the Heathcote Estuary, New Zealand, are related to salinity, sediment grade, and length of time of exposure at low tide. Only Nicon aestuariensis Knox and Scotecolepides benhami Ehlers appear to be truly estuarine species reaching their greatest population densities in low salinities. Incidental observations of feeding and reproduction are reported. INTRODUCTION
There have been very few studies of the biology of estuarine animals in New Zealand. The estuarine environment is characterised by steep and variable gradients in environmental factors, and the fauna tends to be specialised to tolerate these conditions (Emery and Stevenson 1957). In this survey of the polychaetes of the intertidal mudbanks of the Heathcote Estuary the distribution of each species was found to be related to the salinity of the water, the nature of the substratum, and the length of time of exposure to air at low tide. Some observations of feeding habits and reproduction were made. Systematic notes and a key to the species whose ecology is discussed in this paper have appeared elsewhere (Estcourt 1967), and also a description (Estcourt 1966) of the breeding biology of one of them. Comparatively little work has been done on the biology of the estuary of the Heathcote and Avon Rivers. Thompson (1929) made the first general study and there have since been studies by Bruce (1953), Williams (1960), and Rosenberg (1963) carried out for the Christchurch Drainage Board and principally concerned with the effect of pollution on the biota. THE HEATHCOTE ESTUARY
The estuary of the Heathcote and Avon Rivers (Fig. 1) near Christchurch lies immediately north of the volcanic mass of Banks Peninsula. Geologically it is a tiny remnant of the arm of the sea which once separated Banks Peninsula from the mountain backbone of N.Z' Jl mar. Freshwat. Res. 1: 371-394
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372 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [SEPT.
ESTUARY OF THE HEATH COTE AND AVON RIVERS
FIG. 1—The estuary of the Heathcote and Avon Rivers. A^Kerrs Reach; B—Avondale Road Bridge; C—Bower Bridge; D—Pages Road Bridge. the South Island. This bay was filled in when material eroded from the mountains formed the Canterbury Plains and spits built by currents cut off the lagoons of Lake Ellesmere and the Heathcote Estuary. The estuary now has an area of two and a half square miles and is approximately the shape of an equilateral triangle with the outlet to the sea at the south-east corner between the rock of Banks Peninsula and the end of the sandspit which separates it from Pegasus Bay. An isolated rock stack, Shag Rock, stands on the south side of the outlet channel. The Avon River enters the estuary at its northern corner and the Heathcote River at the south-west corner. Both rivers run through very flat country and are tidal for a considerable distance Downloaded by [184.108.40.206] at 18:50 29 February 2012
1967] ESTCOURT—HEATHCOTE POLYCHAETES 373
upstream. Their sources and upper reaches are described by Hogan and Wilkinson (1959). Both are largely spring fed, but receive stormwater drains from the city streets which cover a large part of their catchment areas. The effluent from the Christchurch Drainage Board's sewerage treatment plant is discharged into the western side of the estuary near the mouth of the Avon River. 0-5 mile
F;IC 2—The southern side of the estuary showing the position of the transects. Downloaded by [220.127.116.11] at 18:50 29 February 2012
374 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [SEPT.
The estuary is shallow and mostly intertidal. The...