Demography of early life stages of habitat-forming intertidal fucoid algae

The intertidal zone is finely partitioned in species distributions and abundances. The demographies of key species over varying spatial and temporal scales are fundamental to understanding the population structure and overall dynamics of habitats and assemblages. In this thesis, settlement, dispersal and early life stage survival and growth were examined in several habitat-forming intertidal fucoid algae in New Zealand and Oregon, U.S.A. Natural settlement patterns of Hormosira banksii, Cystophora torulosa and Cystophora scalaris were quantified for over three years at a semi-protected shore in southern New Zealand. Settlement was monitored in four tidal zones, within bare rock and algal habitats. Settlement was synchronous between tidal zones but the density of settlement varied spatially and temporally. There were significant differences between tidal zones, habitats and times of the year. For H. banksii, small pulses of settlement occurred year-round with greatest densities during spring and early summer. Greatest settlement occurred at low tidal zones and under adult canopies. Both Cystophora species also reproduced year-round, but had much lower settlement densities than H. banksii. Most settlement occurred during spring and summer, while only small pulses occurred in autumn and winter months. Most settlement was in the lowest tidal zone (0.4 m above chart datum), with only a few zygotes settling at higher shore zones. Canopy cover had no significant effect on settlement densities. Dispersal was examined in Durvillaea spp., H. banksii, C. torulosa and Fucus gardneri. For all species, settlement densities declined with distance from the source populations, but densities were variable between species. Durvillaea spp. dispersal was more extensive than expected, with significant settlement occurring 32 m from the source population, the maximum sample range of the study. However, settlement densities were much higher within 8 m from the source. The extensive dispersal of Durvillaea spp. is a result of the combination of small, slowly sinking eggs and the presence of buoyant mucilage. The other species studied showed far more restrictive dispersal, and much lower settlement densities. Settlement occurred 2 m from the source, but most settlement occurred under or near the canopy. The eggs of these species are much larger and sink faster than the eggs of Durvillaea spp. The consequences of settling at different shore heights and seasons were examined in H. banksii and D. antarctica in New Zealand, and F. gardneri and Pelvetiopsis limitata in Oregon. Transplant experiments tested the effects of grazing and heat/desiccation stress on survival and growth of germlings at different shore heights, during different seasons. High germling mortality was a feature of all species, but rate of mortality depended on conditions and species. There is a trade-off for settling at different times of the year; overall, growth was faster in warmer seasons, but survival was better in cooler seasons. During cooler seasons, germlings are exposed to less heat/desiccation stress, but their slow growth exposes them to grazing and competitive interactions for longer periods. For New Zealand species, shore height had large effects, with better survival and growth in the low shore. Grazers were very effective in the low shore, and heat/desiccation stress had strong effects in the mid and high shores. For Oregon species, effects of grazing and heat/desiccation stress were generally weaker than for New Zealand species. Shore height had weak effects, but ultimately low shore germlings had poor survival, primarily because of overgrowth by ephemeral algae. This is in contrast to the generalisation that survival and growth in the low shore should be better due to a more benign environment. In this study, species had specific demographies that related to their life history characteristics and responses to the local environment. Differences in settlement, dispersal abilities, survival and growth over small spatial and temporal scales clearly underpinned large scale differences in recruitment and adult distribution and abundances.