Processes influencing intertidal barnacle populations in the Wellington Region

Abstract

Marine intertidal communities are widely used as model systems for examining processes that influence population and community structure. This study documents natural patterns of abundance and distribution of barnacle species in the Wellington region and examines the role of settlement vs. post-settlement processes in structuring these populations. Surveys were conducted in three locations within the Wellington region (West Coast, South Coast, and Wellington Harbour) at high and mid intertidal zones. I found significant differences in the overall density and species composition of barnacles among the three locations: Chamaesipho columna had the highest abundance in the Harbour, was moderately high on the West Coast, and occurred in very low density on the South Coast. Although overall barnacle cover was greatest in the Harbour. Chamaesipho brunnea was more abundant on the South Coast than in the Harbour. To examine the role of settlement processes, I monitored monthly larval settlement rates at sites in the Harbour and on the South Coast. Additionally, I examined long-term colonisation onto natural rock substrate in cleared and un-cleared plots within existing barnacle patches. Peak settlement rates were more than three orders of magnitude greater in the Harbour than on the South Coast. After one year, barnacle colonisation was also greatest in Wellington Harbour. Colonisation was greater in cleared, mid zone plots in the Harbour, and in un-cleared high zone plots on the South Coast. I also examined the role of post-settlement juvenile and adult mortality in structuring populations at each location. Both juvenile and adult mortality were found to be non-significant between locations, but sources of mortality appear to be mainly from wave exposure on the South Coast and from predation in the Harbour. Therefore it is likely that a low settlement rate is the primary factor in limiting barnacle populations on the South Coast, and wave exposure is likely to be causing the species separation between the two locations.