Abstract:
The physical effects of causeways on both tidal circulation and velocities are well documented, but less is known about the ecological effects of causeways, with reported impacts varying between the sites and species In northern New Zealand estuaries causeways have been identified as a potential contributor to the expansion of the native mangrove species A. marina within the enclosed area behind the causeway. This thesis aimed to investigate this link between causeways and the associated habitat change, particularly in the area above causeways. Causeways are more prevalent in northern New Zealand estuaries than has been previously reported, a total of 186 causeways were identified in 160 estuaries. However, they generally affect only small percentages of the total area of estuaries as a majority were found to enclose small areas around the estuaries shorelines. Some estuary categories were found to be over-represented in terms of containing causeways due to their unique morphological features. The effects of causeways on sediment characteristics and benthic fauna were highly variable, with only one site found to have statistically different percentages of fine sediments above the causeway. However, some evidence existed to suggest a general pattern of greater fine sediment percentages near to causeways. At some sites evidence for differences in macrobenthic fauna assemblages was also found. Some evidence for greater retention of propagules above the Omaha causeway in the Whangateau Harbour was found. Retention between sites was found to be statistically different but this was mainly due to the differences between the above Omaha causeway and control sites. Propagule retention appeared to occur in greatest proportions within close proximity to the causeway in areas that are above the mid-tide. Propagule retention below the causeway in these same areas was greater than expected. No increase in rates of seedling establishment and survival was found above Omaha causeway. The presented data highlights some of the potential effects of causeways in estuaries and suggests directions for future research.