KELP SANDHOPPER INTERACTIONS ON A SAND BEACH IN NEW-ZEALAND .1. DRIFT COMPOSITION AND DISTRIBUTION

The composition of organic matter deposited on a New Zealand sand beach was measured monthly during neap tide periods over an annual cycle. The main components of the drift were wood, grass and two kelp species, Macrocystis pyrifera (L.) C.Ag. and Durvillaea antarctica (Chamisso) Hariot. Drift composition was measured from five transects, 5 m in width extending from the base of the dunes (EHWS) to the low tide level. The average wet weight of material collected from the 5-m beach strip was 11.23 kg (SE 3.78), with low values in summer (December and January) and highest biomass in July. Variability in organic matter was due to fluctuating levels of M. pyrifera, which dominated the summer periods, ranging from 0.07 to 2.2 kg . m-1 strip of beach. The biomass of D. antarctica decreased downshore, contrasting with M. pyrifera which was generally distributed evenly down to 20 m from the base of the dunes. The amphipod Talorchestia quoyana (Milne-Edwards) was closely associated with drift material, especially fresh M. pyrifera. Distribution patterns and density of T quoyana varied with season, shore level and substratum type. Exceptional densities of T. quoyana (xBAR = 121 750 m-2) were found associated with M. pyrifera on the drift line. There were two peaks of juvenile recruitment during which juveniles were also associated with sand away from drift material. Results from core samples suggest no direct relationship between sandhopper density and algal biomass on a seasonal basis. Annual deposition of kelp onto the sand beach was estimated as < 400 kg . m-1, characterised by regular input due to storm and seasonal events. The spatial distribution of T. quoyana was dependent on the availability of drift wrack and this is discussed in relation to its behavioural and physiological tolerances of the environmental factors operating on the sand beach.