Wave-related mortality in zygotes of habitat-forming algae from different exposures in southern New Zealand: the importance of ‘stickability’

Using field and laboratory experiments, we investigated the relative ability of zygotes of three species of habitat-forming large brown algae from southern New Zealand to survive wave action and remain attached after settling times of 1, 6, and 12 h. Zygotes of Hormosira banksii (Turner) Decaisne and Cystophora torulosa (R. Brown) J. Agardh, intertidal algae common on sheltered and semi-protected shores in southern New Zealand, had between 24% and 35% survival 1 h after settlement and exposed to a single wave in the lab and field. In contrast, Durvillaea antarctica (Chamisso) Hariot, an exposed-shore species, had up to 75% survival under the same conditions. Survival increased with post-settlement time for all three species. When given 6 h to attach under the same conditions, the survival of Hormosira and Cystophora zygotes ranged between 50% and 60% and between 80% and 90% when given 12 h to attach. Durvillaea, in contrast, had between 90% and 100% survival at both 6- and 12-h setting times. In other experiments, H. banksii and D. antarctica zygotes were given either 1, 6, or 12 h to attach and then placed for 12 h into sites within three levels of wave exposure (sheltered, intermediate and exposed). Survival of Hormosira zygotes given 1 and 6 h to attach was poor, ranging from 1% to 8%. If given 12 h to set, however, 5–8% of Hormosira zygotes survived, even at the exposed sites. Under similar conditions, Durvillaea survival was significantly higher, ranging from 70% to 100% at all post-settlement times, in sites of all exposures. Wave action clearly affects the ability of sheltered shore species to settle in exposed sites. However, our study highlights the importance of factors other than wave action in determining the distribution and abundance of post-settlement stages of D. antarctica across exposures.