Abstract:
This thesis examines the ecological effects of urbanisation on the estuarine bivalve Austrovenus stutchburyi (New Zealand cockle). The effects of changes in habitat quality on A. stutchburyi health were investigated using physiological (condition indices and glycogen content), individual (reproductive capacity) and population (abundance and size) indicators. Measuring responses across several levels of biological organisation was important in distinguishing patterns of biological responses to subtle changes in habitat quality. Utilising related indicators greatly aided the interpretation of individual biological responses. Knowledge of the reproductive cycle was particularly instructive.
Physical environmental variables were examined in four estuaries along the east coast of Auckland. New Zealand, and a gradient of anthropogenic impact established. Sediments in more highly urbanised estuaries contained higher percentages of fine silts (<63µm) and higher levels of contaminants. Less urbanised estuaries had higher percentages of fine sand (l25µm). Densities of A. stutchburyi populations were lower and average size of individual A. stutchburyi was either smaller or larger in the more impacted estuaries. Recruitment of A. stutchburyi in impacted estuaries was also lower. Gametogenesis in A. stutchburyi began around June (winter) and peak breeding activity occurred in summer (November-March). Austrovenus stutchburyi from the most impacted estuary did not spawn completely, with ripe individuals found throughout the year. Glycogen content and condition of A. stutchburyi were generally lowest in this estuary, except during the resting period when higher condition and glycogen coincided with this retention of gametes.
The effects of changes in habitat quality were experimentally investigated by transplanting A. stutchburyi along a pollution gradient. Survival of A. stutchburyi was greatly reduced at the most polluted site. Over one year, survival of A. stutchburyi indicated a temporal limit in their tolerance to unsuitable habitats or stressful conditions. Patterns in condition, glycogen content and reproduction reflected those patterns found in four Auckland estuaries.
Changes in population structure, physiology, reproduction and survival measured along a pollution gradient provide strong evidence that existing changes in habitat quality, associated with urban development, negatively affect the ecology of A. stutchburyi. Such changes have important implications for benthic community structure and sustainability.