Foundation kelps are key ecosystem engineers in coastal ecosystems across temperate to polar latitudes. Climate change, however, is driving marked alterations to their distribution, often leading to cascading effects on associated ecosystems. Predicting the future distributions of these foundation kelps has, therefore, become a key focus of scientific research. This thesis addressed key physiological questions that inform range predictions of two buoyant foundation kelps in the Southern Hemisphere, Durvillaea poha and Durvillaea antarctica. As buoyant, rafting species, both could theoretically access higher latitude climate refuges. Thermal constraints on reproduction, however, could compromise such an ability, and for D. poha, also its capacity to persist within its warming present range. The first data chapter (Chapter. 2) reports on the tolerance of D. poha gametes and post-settlement stages (germlings) both to elevated water temperatures, and to colder temperatures representative of its sub-Antarctic range limit and beyond. The second data chapter (Chapter. 3) evaluates the viability of D. antarctica gametes after exposure to sub-zero temperatures that rafts would encounter on poleward rafting journeys. Evidence suggests adult stages of the New Zealand endemic D. poha are sensitive to rising ocean temperatures, particularly marine heat waves (MHWs). Early life stages of seaweeds are often more vulnerable to environmental stressors than adults, yet the thermal tolerance of D. poha’s reproduction and post-settlement stages (germlings) remains poorly understood. Reproduction may be an overlooked bottleneck, potentially compounding adult summer mortality events and further threatening the persistence of the species on mainland New Zealand. A potential offset to the likely decline of mainland D. poha populations could be poleward range expansions to climate refuges via oceanic rafting. The likelihood of such expansion, however, is greatest with large populations that lead to high raft supply. With climate change-induced population decline, D. poha’s capacity for poleward expansion may, therefore, diminish. I conducted a series of short- (2-day) and long-term (3-week) manipulative laboratory experiments exposing D. poha gametes and post-settlement stages to elevated water temperatures (as projected under future warming scenarios during its winter reproductive period) and to colder temperatures representative of its sub-Antarctic range limit and beyond. I found the impacts of elevated temperatures differ among reproductive stages. An upper threshold of 14 °C was identified, beyond which fertilisation success and early post-settlement development declined significantly. In contrast, later post-settlement stages II were unaffected, showing resistance to the maximal winter warming scenario of 16 °C. Similar adverse impacts on fertilisation successes and the development of early post settlement stages were evident at temperatures below those of D. poha’s southern range limit (< 5 °C). Our findings suggest that D. poha’s reproduction may be robust in the face of some future warming across much of its present range. The species’ current poleward range, however, appears to be constrained by colder sub-Antarctic ocean temperatures. In turn, the need for research into processes outside of D. poha’s reproductive capacity that could affect the persistence of this endemic species with climate change is evident. Oceanic barriers were once thought to prevent passive dispersal to the Antarctic. However, recent evidence suggests that D. antarctica rafts reach Antarctica with relative frequency. As the climate warms, some researchers suspect D. antarctica could be among the first species to establish in Antarctica via non-anthropogenic means. Durvillaea antarctica’s establishment in novel environments, however, depends on the production of viable propagules, rendering the status of D. antarctica gametes following Antarctic rafting journeys a key research question. When rafting to Antarctica, individuals will encounter sub-zero temperatures, and modelled rafting trajectories suggest rafts are likely to encounter sea ice in which they could become encased. Sub-zero temperatures / freezing can induce severe disruptive stress in seaweeds, which for D. antarctica could render its gametes inviable. However, if D. antarctica’s freezing point is lower than seawater’s, rafts could be buffered from freezing impacts, increasing the likelihood of gametes remaining viable. I conducted laboratory experiments to test the viability of D. antarctica gametes following exposure of adult fronds to sub-zero temperatures in ‘rafting’ environments (floating in seawater) and also in sea ice environments (frozen). Impacts differed between the scenarios. Seawater appeared to buffer rafts from the freezing impacts of sub-zero air temperatures, with individuals exposed to temperatures as low as -20 °C, while floating on seawater, still viable and producing germlings. In contrast, in the absence of seawater, sub-zero temperatures led to negative impacts, with extensive reproductive failure at - 7 °C and complete failure at -18 °C. These results suggest that seawater may buffer rafts from freezing during Antarctic rafting journeys, supporting the possibility that the species could eventually establish in the region. Although laboratory experiments cannot entirely accurately represent ‘real world’ processes, these findings nonetheless give insights into how buoyant Southern Hemisphere kelp might respond to warming, and raise exciting new directions for future research.
Benjamin Graham (Thu,) studied this question.