Effects of abiotic and biotic factors on gametophyte growth, survival, development, and sporophyte production of the laminarian kelps <i>Ecklonia </i><i>radiata </i>and <i>Lessonia corrugata</i>

<p dir="ltr">This thesis chiefly investigates the effects of abiotic and biotic factors on the microscopic life stages of the laminarian kelps (order Laminariales) <i>Ecklonia radiata</i> and to a lesser extent <i>Lessonia corrugata</i>. <i>E. radiata</i> is native to southern Australia, New Zealand and South Africa. The species forms large often monospecific stands and forest networks such as the Australian Great Southern Reef (GSR). <i>L. corrugata</i> has a more restricted range and is endemic to southern Australia, specifically the island Tasmania. Both species are of interest to aquaculture in their native ranges. Like all laminarian kelps they follow a biphasic haplodiplontic lifecycle with a microscopic dioecious gametophyte and a macroscopic sporophyte stage. While the adult sporophyte stage has been well studied the microscopic gametophyte has long been regarded as a black box and only has received increased attention recently. In a warming climate the thermal performance of adult sporophytes as well as gametophytes is an important knowledge gap that is of interest to both seaweed farmers as well as conservationist and kelp ecologists.<br><b>Chapter one </b>of this thesis serves as a general introduction to the topic. In <b>chapter two</b>, the impact of initial zoospore density on the growth and sporophyte production of <i>E. radiata</i> gametophytes is investigated. Gametophytes were grown in six-well plates at 6 zoospore densities and growth measurements were taken after 2 and 4 weeks followed by a sporophyte count after 6 weeks. Female gametophytes were largest when grown at 10 individuals/mm² and were smaller with increasing densities (up to 280 individuals/mm² ). However, these significant differences were only evident after four weeks of culture and less visible after two weeks. Sporophyte production was significantly reduced with increasing initial zoospore density, and the highest density culture (280 individuals/mm² ) did not develop any sporophytes. Regardless of the principal cause the results highlight the potential impact of culture density on gametophytes as well as the need for long term experiments to estimate effects of prolonged culture (i.e. in aquaculture nurseries).<br><b>Chapter three</b> of this thesis explores the complex effects and interactions of temperature and seasonality at two sites in Tasmania on zoospore germination, and thermal performance of <i>E. radiata</i> gametophytes (growth, survival, development, sporophyte production). Spores were released from fertile sori collected from both sites over the course of a year. The resulting gametophytes were cultured in a temperature gradient table (3–30 °C). Many gametophytes remained vegetative over the 6-week experimental period and gametophyte size, development and survival were measured after 4 weeks and sporophytes were counted after 6 weeks. To estimate parameters of thermal performance of gametophyte growth and survival at each site and for each season we used thermal performance curves.<br>The interactions were complex and gametophyte growth showed great variability between sites and among seasons without one site consistently outperforming another. Thermal optima for gametophyte growth were stable at ~20.5°C over the course of the year and at both sites. The same was true for gametophyte survival, however the optimum was at 17°C, 3°C below that of gametophyte growth. Sporophyte production occurred between 10 and 25°C, whereas unicellular gametophytes capable of reproducing were observed between 5 and 25°C. Overall the optimal temperatures for gametophyte growth are not commonly exceeded in Tasmanian waters. However, gametophytes experience temperatures at or above the optimum for survival in summer or during heatwaves. The results highlight the complex variability in gametophyte performance over time and space which can make comparisons and transfer of knowledge between studies difficult. Additionally, the results show the potential of selective breeding for both aquaculture and conservation if the observed variation has a heritable component.<br><b>Chapter four</b> is concerned with the interactive effects of light, nutrient supply and temperature on the thermal performance of <i>E. radiata</i> gametophytes. Using similar methodology as described for<i> chapter three</i> I assessed thermal performance of <i>E. radiata</i> gametophytes under two nutrient concentrations (3 and 882 μmol N L^-1 ) and two light levels (60 and 120 μmol photons m^-2 s^-1 ) in addition to the temperature gradient (3– 30°C). Performance metrics and measurement times were the same as in chapter three, and performance curves were fitted to the data in the same fashion. Thermal optima for gametophyte growth and density did not significantly vary between treatments. However, growth was significantly higher under the high nutrient concentrations at the optimum temperature relative to low nutrients and regardless of light levels. Additionally, high light seemed to suppress growth under nutrient enrichment but enhance growth under nutrient depletion. Sporophyte production followed a similar trend with high nutrient concentration being essential for the transition.<br><b>Chapter five</b> of this thesis illustrates the interactive effects of light, nutrient supply, and temperature as well as the effects of nitrate vs ammonium as primary nitrogen sources on the germination, growth, development and survival of <i>Lessonia</i> <i>corrugata</i> microscopic life stages. As in <b>chapter three and four</b> a temperature gradient table (3– 27°C) was used to assess thermal performance via thermal performance curves. Experiment one investigated the interactive effects and experiment two looked at the differences in growth, survival, development, and sporophyte production of gametophytes depending on the primary nitrogen source (nitrate vs ammonium). We observed a minor increased thermal tolerance of gametophytes under nutrient enriched conditions and a similar hierarchy of factors to that seen in <b>chapter three</b> (nutrients > light). Nitrogen source did not affect thermal optima or maxima of gametophyte growth or survival, but gametophytes grew significantly better with nitrate compared to ammonium.<br><b>Chapter six</b> summarises and discusses the general results in a broader seaweed context. Overall, I have shown the importance of culture density as well as site and season when planning experiments or collecting and culturing <i>E.</i> <i>radiata </i>gametophytes for aquaculture. The correct combination of light and nutrients will additionally facilitate better growth in a nursery setting, however, unlike for <i>L. corrugata</i> gametophytes I did not find shifting thermal optima over the different growth conditions in <i>E. radiata</i>. The results of the thermal performance curves in <b>chapters three to five</b> are furthermore applicable to ongoing climate change modelling and restoration efforts for both species in Tasmania and allow for more accurate predictions of the fate of microscopic kelp life stages in a warming ocean.</p>