Spatial and temporal variation in beach-cast Durvillaea antarctica rafts

Durvillaea antarctica is a robust, buoyant macroalga which is found growing throughout the high latitudes of the Southern Hemisphere. This kelp grows on exposed, rocky coastlines, and their holdfasts act as an important habitat for a wide variety of intertidal invertebrates. The internal structure of the blades traps air and gasses, allowing the entire structure to remain highly buoyant for extended periods of time. Thus, when kelp becomes detached following storm surges or intense wave action, floating rafts of D. antarctica can traverse the Southern Ocean, acting as an important transport vector for the associated invertebrates which inhabit the holdfasts. Previous work investigating the phylogeographic structure of growing, attached D. antarctica populations has revealed a high degree of genetic variation throughout the species’ range. The mitochondrial marker, cytochrome c oxidase subunit 1 (COI), is highly variable in attached populations, displaying a number of unique haplotypes, some exclusive to narrow geographic areas. With this molecular knowledge of attached populations, it is possible to assign a probable source location to rafts of D. antarctica which have washed ashore on the New Zealand coastline. This study collected beach-cast rafts of D. antarctica from a number of sites along New Zealand’s east coast and assigned each sample to a known haplotype (and thus, source population) by using established information regarding the species’ phylogeographic structure. By examining the assemblage of haplotypes sequenced from a particular beach, general patterns in rafted assemblages were evaluated. In particular, this study attempted to investigate how haplotype frequency found at a particular location changed across both space and time in relation prevailing weather patterns and ocean currents. On a spatial scale, significant changes were detected in haplotype assemblages across New Zealand’s east coast. Two distinct genetic breaks were found; one at Cape Campbell, and one in Pegasus Bay. A similar genetic break has previously been documented at Cape Campbell for a range of intertidal invertebrates with dispersing larval life-stages (e.g. limpets, sea stars, mussels, chitons). Data from this study confirm this genetic break; beach-cast D. antarctica assemblages showed dramatic genetic turnover between sites on a very fine scale, suggesting that the currents in this area act as a significant dispersal barrier to both D. antarctica rafts (and associated invertebrates) and intertidal invertebrates with larval life-stages. A second genetic break in the haplotype assemblages of beach-cast D. antarctica rafts was found in Pegasus Bay. Again, this break in haplotypes found mirrors a change in ocean currents of the region. This study demonstrates how the prevailing ocean currents of a particular region are intrinsically linked to the assemblage of beach-cast D. antarctica found on any given stretch of coastline. When changes in haplotype assemblages were examined across time, temporal patterns were only detectable after spatial variation was accounted for. The Cook Strait region showed the largest changes in haplotype assemblages found over time, which was expected, due to the variable nature of the ocean currents in this region. Changes in prevailing wind patterns can have profound effects on the flow of water in the Cook Strait region; prolonged shifts in prevailing wind direction can actually reverse the net flow of surface water. Therefore, the temporal shifts in quantity and source location of D. antarctica assemblages observed in this study are most likely a result of fluctuating wind patterns. This study found that the haplotype assemblages of beach-cast D. antarctica rafts show significant patterns across both space and time. D. antarctica rafts travel with major current systems, and thus, different assemblages of haplotypes are found in different regions of the country, reflecting the input of local currents. However, prevailing wind speed and direction are also critical components that influence which assemblages of D. antarctica are found on a particular beach at a particular time. The genetic breaks and temporal shifts identified in this study have profound implications for a wide variety of marine taxa, including the ‘hitch-hiking’ invertebrates present on D. antarctica rafts