Abstract:
This thesis describes the relationship of small (0.5 - 8.0 mm) mobile invertebrate (SMI) communities to environmental factors in a diverse range of coastal habitats (high shore to 20 m depth) around temperate north-eastern New Zealand. Environmental variables included habitat complexity, wave exposure and experimentally manipulated water turbidity as well as the effects of fishing exclusion zones. SMI density, biomass and productivity varied by 3 orders of magnitude across 34 diverse habitats, which included hard, soft and macroalgal substrates. Density, biomass and productivity tended to be highest in the structurally complex and food-rich coralline algal turf and beach seaweed wrack habitats, and lowest in intertidal sands, but showed only a weak negative relationship with habitat 'grain size,. Temporal consistency in results was demonstrated for 4 habitats sampled seasonally for 2 years. Total SMI density, biomass and productivity were higher in hard- than soft-bottom habitats, which tended to be numerically dominated by gastropods and gammarid amphipods respectively. Size-structure of communities from this survey was positively correlated with habitat grain size (i.e. the proportion of large animals was higher in coarsely-structured habitats) within biogenic and subtidal soft sediment habitats (Chapter 2). Proportions of large animals were lowest in the subtidal coralline turf, and, at a broader scale, rocky reefs, which was consistent with strong size-selective predation pressure in these habitats. A subset of habitats ranked consistently by size-structure as well as density, biomass and productivity over two years. Density, biomass and productivity within epifaunal communities associated with 3 variously structured macroalgal habitats (finely-structured to 'strap-like') were positively correlated with wave exposure in Ecklonia radiata and coralline turf, and this relationship was stronger in fished than marine reserve (MR) sites. In a mesocosm experiment, all epifaunal SMI community parameters showed evidence of a negative correlation with turbidity, although trends were not statistically significant at the P = 0.05 level. As turbidity increased, community dominance shifted from grazers towards filter-feeders and detritivores.