Effects of reduced salinity and seston availability on growth of the New Zealand little-neck clam Austrovenus stutchburyi

Abstract

Estuarine bivalves are exposed to spatial and temporal variations in salinity and seston quality and quantity. This is the first study to investigate the effects of salinity reduction and phytoplankton availability on the survival, health and growth of a burrowing bivalve. The intertidal clam Austrovenus stutchburyi survived 6 wk of laboratory storage in dilute seawater (20 ) at 15C. In salinities below 14 , and at low phytoplankton concentrations (<10 g chl a l -1 ), clams lost weight and mortality occurred within 4 wk of exposure. With high phytoplankton availability (> 20 g chl a l -1 ), some clams survived 6 wk exposure to 7 . Salinity and quality of seston over 5 shellfish beds varied both amongst and within sites. Chl a concentration at high tide was variable between sites, but was generally below 8 g l -1 . Seston total particulate matter (TPM) ranged from 25 to 400 mg l -1 , and particulate organic matter (POM) from 3 to 31 mg l -1 . Clams transplanted to other sites in the estuary quickly adapted to new environmental conditions. Average growth rates of clams caged at 4 estuary sites for 6 mo (October 1997 to April 1998) were low, between 0.17 and 0.35 mm mo -1 . Total length increment correlated positively with low-tide temperature, salinity and average chl a concentration. After 2 mo, there was a direct linear relationship between transplanted clam standard condition index (CI shell ) and length increment. The integrated laboratory and field studies confirm that salinity dilution, together with low phytoplankton availability, reduces growth and condition of this shallowburrowing estuarine clam.