The effects of trace metals on juvenile cockles (Austrovenus stutchburyi)
Thesis DisciplineEnvironmental Sciences
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
This thesis investigated the population structure and spatial distribution of the cockle Austrovenus stutchburyi in estuaries and bays around the Canterbury coastline. Surveys investigated population attributes (average density and density of small cockles (< 10mm)) of Austrovenus stutchburyi in relation to physical environmental characteristics such as trace metals (copper, cadmium and zinc), sediment particle size, nitrogen and phosphorus levels, pore water and percentage organic matter of the sediment. Surface cover of flora and number of fauna present was also correlated with the density of cockles and small cockles < 10mm. MDS and PCA ordination showed that the biota was similar at 14 sites but differed significantly at the Pleasant Point Yacht Club (PPY) site. There was a positive correlation between fine sand (125 μm) and the average density of cockles and small cockles < 10 mm. High population densities of Austrovenus stutchburyi were also positively correlated with phosphorus levels, and percentage cover of Sea Grass (Ulva sp). However, Austrovenus stutchburyi density was negatively correlated with cadmium and zinc concentration, and percentage of mud present. The density of small cockles < 10 mm was negatively correlated with copper and cadmium concentration in the sediment and positively correlated with Topshell (Diloma subrostrata) numbers, Sea grass (Zostera muelleri) percentage cover, Sea lettuce (Ulva sp), percentage cover, and sediment particles sizes of < 63 μm (mud), 63 μm (very fine sand), and 125 μm (fine sand). Survival and behavioural changes of juvenile Austrovenus stutchburyi were investigated in relation to increased levels of copper, cadmium, and zinc in aqueous solution and sediment in the laboratory, and artificially increased levels in the field. In laboratory experiments in contaminated seawater it was found that, over time, copper and zinc had a detrimental effect on the percentage of juvenile cockles with their siphons extended as did copper concentration. Cockles 10 - 12 mm shell length exposed to different concentrations of copper had the lowest survival rate (25%) whilst cockles that were 5 - 7 mm in length had the greatest survival rate (69%). Cadmium did not affect survival or siphon extension in aqueous experiments. In the contaminated sediment experiments in the laboratory, the concentration of zinc (0, 20, 40, 80, 160 mg Zn/kg (dry weight)) and cadmium (0, 1.8, 5.6, 18, 36 mg Cd/kg (dry weight)) both decreased survival and burial of juvenile cockles in higher concentrations. Copper concentration (0, 5, 10, 25, 50 mg Cu /kg (dry weight)) decreased burial rates of juvenile cockles but did not affect survival. Transfer of juvenile Austrovenus stutchburyi within three sites in the Avon Heathcote Estuary during May 2007, found that site and exposure to copper, cadmium and zinc decreased the survival of the juvenile cockles. However, transfer of cockles between estuaries (Takamatua, Saltwater Creek and Avon – Heathcote Estuary) in May 2007 found that exposure to copper, cadmium and zinc had the main effect on survival of juvenile cockles. In July 2007 transfers of cockles between estuaries, site and exposure to copper, cadmium and zinc had an effect of survival on juvenile cockles. Cockle populations in the present research have shown a strong correlation with environmental variables, which can be used for management and conservation. The research in this thesis is a start to understanding the effects and implications of contaminants on survival, behaviour and recruitment of juvenile cockles. This research will benefit management strategies for increasing population numbers of Austrovenus stutchburyi.