|This thesis aimed to describe the relationship between suspended sediment concentration stimulating the turbidity caused by natural events or human activities, and the feeding behaviour and energetics of scallops (Pecten novaezelandiae), mussels (Perna canaliculus) and pipis (Paphies australis). A wide range of suspended sediment concentration with different organic content was used. In experiment 1, sediment with high organic content added to natural seston (12 - 733 mg 1−1) was used, whereas in experiment 2, sediment with very low organic content added to natural seston (20 - 196 mg 1−1) was used. Scallops enhanced the organic content of ingested matter by producing pseudofaeces with low organic content when the diet organic content had been diluted to~ 14% in both experiments. For the diet with high organic content, the clearance rate (the volume of water cleared of suspended particles) decreased with increasing sediment concentration (12 - 733 mg 1−1) ,but when the diet was composed of low organic content, clearance rate increased up to a concentration limit of 120-140 mg 1−1. Positive scope for growth was attained until suspended sediment concentration exceeded 190 mg 1−1. Similarly, mussels also enhanced the organic content of ingested matter by producing pseudofaeces with low organic content when the diet organic content had been diluted to ~ 16% in both experiments. The enhancement of ingested organic fraction was accomplished by two different feeding behaviours. When the diet had high organic content, mussels clear less water, reduce the volume of pseudofaeces produced, and reduce the amount of energy expired via low respiration rates. Consequently, the net energy gained was uniformly high as sediment concentrations increased to 322 mg 1−1. The low organic diet prompted high clearance rates and increase in pseudofaeces production as sediment concentration increased. Due to high feeding activities, energy expenditure was also high which was reflected in a low net energy gain. Pipis can only feed efficiently at very low sediment concentrations (<39 mg 1−1) in experiment I and 20.1 mg 1−1 in experiment 2). Beyond these values, pipis were unable to compensate for increased sediment concentration and therefore had very low scope for growth.
From these results it was concluded that preferential selection of organic particles before ingestion, adjusting clearance and respiration rates, within limits, are the key elements that enable scallops and mussels to cope with high turbidity levels. Pipis have a low tolerance level to turbidity.
The second aim of the study was to investigate the effects of resuspended sediments caused by dredging on seston quality (organic content and pigment concentration) and quantity. This was accomplished through water sampling at various places while dredging was in operation. The results indicated that suspended sediments caused by dredging had no significant effect on the seston quality and quantity, in relation to the seston characteristics observed naturally.