Seaweed aquaculture requires reliable hatchery protocols to ensure a consistent supply of seedstock for cultivation. Durvillaea spp. (order Fucales), are large canopy-forming brown seaweeds native to the cool waters of the southern hemisphere and have been identified as a promising candidate for cultivation. However, components of the hatchery methods remain poorly characterised. This study investigated key factors influencing fertilisation success in Durvillaea potatorum determining: 1) whether storing sperm on ice enhances sperm motility and fertilisation success, 2) the effect of sperm concentration and source (different males) on fertilisation success and, 3) how the timing and volume of sperm addition affects fertilisation success. First, sperm kept on ice were motile for longer and had higher fertilisation rates over 180 minutes compared to sperm stored at room temperature. Second, sperm concentration strongly affected fertilisation success and the highest concentrations of 10⁶–10⁷ sperm mL⁻1 resulted in much higher fertilisation rates compared to lower concentrations. There were also differences in fertilisation success among males suggesting that male-specific factors are important. Third, both the timing and volume of sperm addition affected fertilisation success. Generally, higher amounts of sperm added early (at 0 min) resulted in higher fertilisation compared to when higher amounts of sperm were later (at 10 minutes) reinforcing that fertilisation in D. potatorum is likely be constrained by sperm availability. Overall, our findings highlight four important factors that enhance fertilisation outcomes in D. potatorum, providing guidance for improving fertilisation protocols in the hatchery to support the development of a sustainable aquaculture industry.

Seagrasses are key habitat-forming species that support essential ecosystem services, yet their condition is influenced by multiple stressors operating across varying spatial scales. Poor water quality is a major anthropogenic stressor affecting seagrass health, but its effects on seagrass condition and associated microbial communities remain poorly understood. We examined how the condition of the seagrasses Zostera muelleri and Heterozostera nigricaulis and their below-ground bacterial community structure (bulk sediment and rhizosphere) were influenced by large-scale abiotic factors (e.g. salinity) and localised changes in water quality (e.g., proximity to stormwater outfalls) along the Derwent estuary, Tasmania. Seagrass total biomass and leaf length were reduced closer to stormwater outfalls. Several putative “core” bacterial taxa (e.g. Desulforhopalus and Thiodiazotropha) were abundant in the seagrass rhizosphere, whereas other taxa (e.g. Saccharicrinis, Lacinutrix, Vallitalea) were associated with degraded seagrass meadows, characterised by lower biomass and higher epiphyte cover. Among large-scale abiotic factors, only salinity was negatively related with leaf length and below-ground bacterial community structure. Our findings highlight the stronger influence of localised changes in water quality on seagrass condition and below-ground microbial communities compared to larger-scale abiotic factors, and provide evidence for the role of both potentially beneficial and detrimental bacteria. This study establishes baseline knowledge of multiple stressors affecting Tasmanian seagrasses and their microbiomes in a highly urbanised temperate estuary, with implications for stress thresholds that influence seagrass health, distribution and restoration potential.

The large fucoid alga Durvillaea potatorum (southern bull kelp) has been identified as a suitable species for offshore seaweed aquaculture due to its large size, preference for wave-exposed sites, and high levels of bioproducts. However, there is a limited understanding of its reproduction and early life-cycle biology and a lack of scalable nursery techniques. We conducted experiments assessing: 1) gamete release, fertilisation, survivorship and early growth of D. potatorum juveniles in the nursery across three months during the austral winter, 2) any differences in the density and size of juveniles seeded onto four distinct substrata: synthetic polymer twine, jute, granite, and glass microscope slides, and 3) any differences in the density and size of seeded juveniles between three similar substrata: synthetic polymer twine, synthetic polymer ribbon and polyvinyl alcohol (PVA) twine. Gamete release and fertilisation increased throughout winter (June—August) while survivorship and maximum juvenile length at 35 days post-fertilisation did not differ among months. Synthetic polymer twine was the best substratum amongst the four distinct substrata, having significantly higher densities of juveniles compared to granite and jute after 28 days post-fertilisation and these juveniles also grew to a larger size. Compared to synthetic polymer twine, synthetic polymer ribbon and PVA twine maintained higher densities of juveniles 56 days post-fertilisation. This study identified techniques to release and fertilise gametes and determined suitable twine substrates for nursery-seeding juvenile D. potatorum.

The southern bull kelp Durvillaea potatorum is a key habitat-forming macroalga in southeastern Australia and has been identified as a species of interest for sustainable seaweed aquaculture. However, the species is threatened by rising ocean temperatures and other anthropogenic factors. Assessing the thermal limits across different life stages of D. potatorum is therefore crucial for understanding its response to warming and optimizing future aquaculture practices. Using a full two-factorial design, we tested the effects of a wide range of temperatures (~3.5-30.0°C) and two light regimes (zero/low light: 0/~40, and high light: ~120 μmol photons · m-2 · s-1) on the reproductive and early life stages of D. potatorum from Eaglehawk Neck, Tasmania. Gamete release, fertilization, and early growth after 15 min, 24 h, and 7 days, respectively, were assessed. Thermal performance curves revealed that fertilization was the most thermally sensitive stage, exhibiting the lowest thermal optimum (Topt = ~12.85°C), and narrowest thermal breadth (Tbr = ~2.5°C) independent of light conditions. Temperature and light had little to no effects on egg and sperm release, whereas early germling growth exhibited thermal optima of 16.33 and 14.50°C under low and high light treatments, respectively. These results indicate that fertilization conditions need to be closely monitored during the hatchery phase of aquaculture. In addition, natural D. potatorum populations may become increasingly susceptible to ocean warming due to impaired fertilization, potentially leading to demographic shifts and range contractions toward cooler waters.

Durvillaea spp. (Fucales) occur on wave-exposed coasts in parts of the southern hemisphere. In Tasmania, Australia, a small but economically valuable industry (> US1.5 million GVP) harvests beach-cast Durvillaea potatorum, primarily for alginates and liquid plant biostimulants. Currently, demand for D. potatorum biomass exceeds supply, prompting interest in aquaculture to sustainably increase production. However, fundamental hatchery methods required for its cultivation are not well understood. We conducted a series of experiments to determine optimal conditions for gamete release, fertilisation, and juvenile growth in a hatchery setting. Releasing gametes for 10 min at 14 °C produced consistently high egg densities (mean ± SE: 12,720 ± 1020 cm−2 tissue) and fertilisation rates peaked (89 ± 4

Lessonia corrugata is found in moderate to high wave-exposed environments in Tasmania, Australia, and has recently become of commercial interest for aquaculture. However, knowledge about its fundamental biology is lacking. Here, we studied the reproduction of L. corrugata at three sites with different levels of wave exposure for five consecutive seasons. Temporal reproduction was assessed by determining the percentage of fertile blade tissue which was classified into four maturation stages: vegetative tissue, pre-mature sorus, mature sorus, empty sorus. Zoospore viability was evaluated by determining the number of spores released, their size and the length of time that they were able to swim. We found that L. corrugata was reproductive in all seasons except Austral spring. During winter, thalli exhibited a larger area of mature sori and released more zoospores than autumn and summer seasons, a trend that aligns with the seawater's elevated nutrient concentrations. After 8 h, more than 50% of zoospores stopped swimming, although swimming zoospores were observed up to 52 h after release, and those from the most exposed site swam longer than those from the sheltered sites. In conclusion, L. corrugata was found to be a 'seasonal anticipator' for reproduction, and this appears mediated by wave exposure. These findings provide critical insight that will enhance the collection of high-performing brood stock and facilitate the cultivation of this species.

We assessed the palatability of four Tasmanian Caulerpa sea grape species: C. geminata , C. hodgkinsoniae , C. sedoides , and C. simpliciuscula , and quantitatively evaluated the palatability profiles of these species with commercially available C. lentillifera . Initial palatability was assessed through a hedonic human sensory evaluation, followed by quantitative instrumental sensory evaluation of tastes and volatile organic compounds using an electronic tongue (e-tongue) and headspace solid-phase microextraction gas chromatography–mass spectrometry (GCMS). The hedonic human sensory evaluation found three species to have a positive overall liking of approximately seven out of nine, while the fourth species, C. hodgkinsoniae , was significantly less liked overall. This initial evaluation found that the attributes of taste and texture were most influential in driving the separation in overall liking between C. hodgkinsoniae and the other, more palatable Tasmanian species. However, the e-tongue found few differences among the tastes of the five species, with most of the significant differences being small in magnitude, and as such the non-volatile component was unlikely to be the reason why C. hodgkinsoniae was disliked. Analysis with GCMS, however, found significant differences among the five species, indicating that the aroma profiles of these seaweeds play an important role in their palatability and hence potential culinary acceptance. Overall, our results highlight the potential for the Tasmanian sea grape species C. geminata , C. sedoides , and C. simpliciuscula as comparable to the benchmark for edible species of Caulerpa , C. lentillifera , given their positive hedonic human sensory evaluation in combination with comparable flavour profiles determined instrumentally by e-tongue and GCMS.

Kelps have a long history of human use and exploitation. Knowledge of past harvesting practices offers insights into environmental baselines and the contemporary management and conservation of these critically important ecosystems. In Tasmania, Australia, giant kelp ( Macrocystis pyrifera ) was commercially harvested for alginate production from 1964 to 1973, but those forests have since undergone precipitous declines due primarily to climate change. We reviewed a collection of archival data and sources to describe the history, methods, and scale of this understudied and largely forgotten industry. We calculated that >65,000 tonnes (wet weight) of Macrocystis were harvested from eastern Tasmania over a decade (mean annual harvest = 6531 t), making it one of the largest wild harvest industries to ever exist in the region. However, the industry had challenges finding sufficient biomass to sustain operations, ultimately driving its closure in less than a decade. Feasibility surveys prior to harvesting suggested much greater kelp availability than was ultimately realized, perhaps motivating overexpansion. Against a backdrop of climate change in this ocean warming hotspot, harvest efforts grew wider and more intensive, and during summer months when stocks were lowest, almost all exploitable biomass was harvested. It remains unclear whether harvesting contributed to the decline of Tasmanian Macrocystis forests, but it may have reduced their resilience and exacerbated other stressors, particularly in heavily harvested areas. This historical review provides a rare opportunity to examine the past scale and use of now-endangered Macrocystis forests and also to help inform the contemporary management and conservation of seaweed resources.

Rapid ocean warming is affecting kelp forests globally. While the sporophyte life stage has been well studied for many species, the microscopic life stages of laminarian kelps have been understudied, particularly regarding spatial and temporal variations in thermal tolerance and their interaction. We investigated the thermal tolerance of growth, survival, development, and fertilization of Ecklonia radiata gametophytes, derived from zoospores sampled from two sites in Tasmania, Australia, throughout a year, over a temperature gradient (3-30°C). For growth we found a relatively stable thermal optimum at ~20.5°C and stable thermal maxima (25.3-27.7°C). The magnitude of growth was highly variable and depended on season and site, with no consistent spatial pattern for growth and gametophyte size. Survival also had a relatively stable thermal optimum of ~17°C, 3°C below the optimum for growth. Gametophytes grew to single cells between 5 and 25°C, but sporophytes were only observed between 10 and 20°C, indicating reproductive failure outside this range. The results reveal complex effects of source population and season of collection on gametophyte performance in E. radiata, with implications when comparing results from material collected at different localities and times. In Tasmania, gametophytes grow considerably below the estimated thermal maxima and thermal optima that are currently only reached during summer heatwaves, whereas optima for survival (~17°C) are frequently reached and surpassed during heatwaves, which may affect the persistence and recruitment of E. radiata in a warmer climate.

ABSTRACTThe polewards range expansion of tropical herbivorous fish into temperate latitudes is leading to overgrazing of marine habitats and community phase shifts in some regions. Here, we test the potential effects of increased herbivory on the temperate habitat‐forming seagrass Posidonia australis. We used a series of simulated herbivory experiments to predict the potential impacts of climate‐mediated increases in seagrass consumption along P. australis entire latitudinal range (~9° latitude) in eastern Australia (1700 km of coastline). We subjected treatment plots to two levels of simulated herbivory (10% or 80% of leaves clipped) and compared them to unclipped controls. We measured seagrass leaf growth rates and tissue chemical traits: carbohydrates in rhizomes, leaf phenolics, and nutrients (carbon, nitrogen, and C:N ratio) in leaves and rhizomes. At the warmest range‐edge population, we also tested how responses to increased herbivory may vary between summer and winter, or with repeated clipping events. Clipped shoots maintained growth rates similar to unclipped controls despite losing up to 80% of leaf biomass. This was consistent along the full latitudinal range and after repeated simulated herbivory at the northernmost location. One‐off clipping events impacted plant architecture, increasing the number of subdividing shoots. At the species range edge, leaves grew more in winter than in summer, and clipping tended to lower seagrass growth only in winter; however, higher levels of shoot subdivision were produced over summer than in winter. Plant chemical traits could not explain consistently the growth patterns observed despite some traits varying with latitude (e.g., leaf nitrogen content decreased with latitude and C:N ratio increased) and/or simulated herbivory. Synthesis: P. australis growth is not affected by increases in simulated herbivory and may be relatively resilient to future increases in seagrass consumption, suggesting that this species could be a relative ‘winner’ under future climate change conditions that lead to enhanced herbivory.

Global seaweed aquaculture production has more than tripled since 2002 and is dominated by Asian countries with farming operations that typically occur in relatively wave-protected, nearshore areas. To meet future demand, production must move to "non-traditional" regions and into less contested waters offshore. However, the technological complexities and uncertainties in the performance of seaweed cultivated in high-energy offshore environments are substantial and must be overcome. Here, we identify knowledge gaps and suggest a research roadmap to inform the advancement of a commercial offshore seaweed aquaculture industry using southern Australian species as case studies: (1) Durvillaea spp. (order Fucales); (2) four kelps (order Laminariales); and (3) the rhodophyte Asparagopsis. These groups lie along a spectrum of commercial viability and readiness for offshore aquaculture, and key knowledge gaps are cultivation technology and the suitability to offshore conditions. Cultivation of Durvillaea is restricted by a low level of biological and technical understanding, but there is high market potential and readiness. For laminarian kelps, commercial production in nearshore conditions is already occurring elsewhere, which make them the most likely candidate for offshore cultivation in the medium term. Asparagopsis is least suited to offshore conditions, with substantial gaps in general cultivation knowledge, and its cultivation is likely to be restricted to land-based systems or relatively sheltered nearshore waters. The knowledge gaps identified here will inform research and development programs to advance offshore seaweed aquaculture in southern Australia and globally.

We found that an innovative nursery approach, where Lessonia corrugata seeded spools were cultivated by spinning to increase the water motion relative to non-spinning spools, had higher growth in both the nursery and at-sea stages. Using this method, we compared the at-sea growth of sporophytes cultivated on spinning spools at different depths (1 m, 3 m, 5 m) and seasons (timing of out-planting). Finally, we compared the at-sea growth of sporophytes cultivated on spinning spools vs. non-spinning sporophytes at 3-m depth. In the nursery, sporophytes on spinning spools developed significantly faster than those on non-spinning spools: blade length was 4.6 and 2.5 cm, and holdfast area was 0.10 and 0.03 cm 2 for spinning and non-spinning spools, respectively. At-sea L. corrugata in spring had significantly greater biomass production at 3 m and 5 m (3.0 kg m −1 and 2.4 kg m −1 , respectively) and up to 96% survival. In summer, 100% of deployed kelps died at all depths. Growth was faster at 5 m (0.3 ± 0.06 kg m −1 ) in autumn and at 3 and 5 m (1.1 ± 0.1 kg m −1 and 0.8 ± 0.1 kg m −1 , respectively) in winter. At sea, sporophytes from the spinning spools grew significantly 60% larger, and survival was ~ 3 times greater over 3 months than sporophytes from non-spinning. Overall, this study shows that spinning seeded spools in the nursery improves the growth at sea, spring is the best season for out-planting L. corrugata , and 3 or 5 m depth is best for production.

Kelp forests provide vital ecosystem services such as carbon storage and cycling, and understanding primary production dynamics regarding seasonal and spatial variations is essential. We conducted surveys at three sites in southeast Tasmania, Australia, that had different levels of water motion, across four seasons to determine seasonal primary production and carbon storage as living biomass for kelp beds of Lessonia corrugata (Order Laminariales). We quantified blade growth, erosion rates, and the variation in population density and estimated both the net biomass accumulation (NBA) per square meter and the carbon standing stock. We observed a significant difference in blade growth and erosion rates between seasons and sites. Spring had the highest growth rate (0.02 g C · blade-1 · d-1 ) and NBA (1.62 g C · m-2 · d-1 ), while summer had the highest blade erosion (0.01 g C · blade-1 · d-1 ), with a negative NBA (-1.18 g C · m-2 · d-1 ). Sites exhibiting lower blade erosion rates demonstrated notably greater NBA than sites with elevated erosion rates. The sites with the highest water motion had the slowest erosion rates. Moreover, the most wave-exposed site had the densest populations, resulting in the highest NBA and a greater standing stock. Our results reveal a strong seasonal and water motion influence on carbon dynamics in L. corrugata populations. This knowledge is important for understanding the dynamics of the carbon cycle in coastal regions.

Abstract Tasmania is an island state in south-eastern Australia that has a long and rich history of seaweed use, research, and development. It is a cool-temperate system with 750 macroalgal species currently described. Tasmanian Aboriginal peoples have lived on this land for at least 40,000 years utilising seaweed as food, shelter, water carriers and medicine, as well as for ceremonial reasons. Modern taxonomic investigations began with French naturalist Jacques-Julien Houtou de La Billardière in 1791, and there are 184 type specimens of seaweeds originating from Tasmania. Ecological and physiological studies of seaweed in Tasmania have focussed on the dominant large brown seaweeds (Laminariales and Fucales) and have contributed significantly to the global understanding of these systems, particularly related to community resilience, seaweed-urchin interactions, their habitat-forming role for other species, responses to global change, and restoration of lost habitat. Ocean warming and changing oceanography have caused a 95% decline in surface canopy cover of Macrocystis pyrifera in eastern Tasmania since the 1950s and led to a focus on restoring these lost forests. Tasmanian seaweed communities have a uniquely high proportion (up to ∼90%) of seaweeds that rely solely on CO2 for photosynthesis, which has implications for responses to ocean acidification. Tasmania has industries that use brown seaweeds for fucoidan extraction and beach-cast harvest for alginates, fertilisers, and feeds for agriculture. New aquaculture initiatives include integrated multi-trophic aquaculture, offshore kelp mariculture and Asparagopsis cultivation for bioactive products to reduce methane emissions in ruminants, as and the development of unexploited species including Caulerpa spp. for food.

Lessonia (order Laminariales) is a kelp genus restricted to the temperate southern hemisphere, where species form dense forests from the low intertidal to 25 m depth at wave exposed sites. Lessonia spp. are among the most harvested kelps globally due to their importance in providing raw materials for food, cosmetics, bioactive and biomedical industries. Over-harvesting of natural beds can negatively affect Lessonia populations and the many species that depend on these habitats, including commercially important fish and molluscs, but good harvest management plans reduce these impacts on natural Lessonia stocks. However, the increasing demand for raw materials will likely only be met by aquaculture for which Lessonia shows high potential in pilot scale studies undertaken in Chile, New Zealand, and Australia. In this concise review, we highlight the current knowledge of Lessonia spp. taxonomy and distribution, life history, ecology and ecosystem services, wild harvest, aquaculture, and commercial applications. We discuss future research directions.

The kelp, Ecklonia radiata, is an abundant subtidal ecosystem engineer in southern Australia. Density‐dependent changes in the abiotic environment engineered by Ecklonia may feedback to affect reproduction and subsequent recruitment. Here, we examined: 1) how the reproductive capacity of Ecklonia individuals in the field (zoospores released · mm−2 reproductive tissue) varied with adult density and time, and 2) how the recruitment of microscopic gametophytes and sporophytes was influenced by zoospore density at two times. Zoospore production did not vary with adult density, with only one month out of ten sampled over a 2‐y period showing a significant effect of density. However, zoospore production varied hugely over time, being generally highest in mid‐autumn and lowest in mid‐late summer. There were strong effects of initial zoospore density on gametophyte and sporophyte recruitment with both a minimum and an optimum zoospore density for sporophyte recruitment, but these varied in time. Almost no sporophytes developed when initial zoospore density was <6.5 · mm−2 in spring or <0.5 · mm−2 in winter with optimum densities of 90‐355 · mm−2 in spring and 21‐261 · mm−2 in winter, which resulted in relatively high recruitment of 4‐7 sporophytes · mm−2. Sporophyte recruitment declined at initial zoospore densities >335 · mm−2 in spring and >261 · mm−2 in winter and was zero at very high zoospore densities. These findings suggest that although adult Ecklonia density does not affect per‐capita zoospore production, because there is a minimum zoospore density for sporophyte production, a decline in population‐level output could feedback to impact recruitment.

Abstract Kelp aquaculture is an emerging industry outside of Asia. To be successful, this industry requires a reliable production of seedstock, the optimisation of which greatly benefits from a detailed physiological understanding of the microscopic life-cycle stages of the cultured species. This study investigated the impact of six zoospore densities (10–278 mm−2) on the subsequent development of Ecklonia radiata gametophytes and sporophytes. The results showed that germination rates and sex ratio were unaffected by initial zoospore density, but there were significant effects on gametophyte size and sporophyte production. After two weeks, female gametophytes were largest at an initial zoospore density of 40 mm−2 while male gametophytes grew largest at densities below 40 mm−2, but after four weeks gametophyte size showed a negative relationship with initial zoospore density. Significantly more sporophytes developed at initial zoospore densities below 40 individuals mm−2 and no sporophytes were observed at the highest density (271 zoospores mm−2). These results clearly show the importance of initial zoospore density in optimising the nursery stage of kelp aquaculture.

Ecosystem engineers often exert strong effects on the recruitment of other species through modification of the local abiotic and biotic environment. In 2015, artificial reefs in eastern Tasmania (− 42.64693, 148.01481) spanning seven different patch sizes (0.12–7.68 m 2 ) and supporting four densities of transplanted kelp ( Ecklonia radiata at 0, 4.1, 8.2 and 16.4 kelp m −2 ) were used to determine how the patch size and density of this ecosystem engineer influenced the recruitment of microphytobenthic (MPB) algae, and a secondary ecosystem engineer, the mussel Mytilus galloprovincialis . Increasing kelp density and patch size inhibited the establishment of subcanopy MPB algae on settlement slides and reduced the recruitment of mussels in standardised rope fibre habitats (RFHs). The productivity:biomass ratio (P:B) of mussel recruits tended to be lower on small reefs and reefs without kelp, relative to larger reefs with high densities of kelp. Canopy shading and reduced cover of turf algae appeared to negatively impact the recruitment of MPB algae and mussels, whilst reduced sediment accumulation on the reefs due to the kelp was also negatively associated with mussel recruitment. These findings highlight the role of ecosystem engineering by kelp in inhibiting the establishment of other species which may additionally impact community dynamics and primary and secondary productivity. The limited capacity of small kelp patches to inhibit the recruitment of other organisms supports the notion that fragmented patches of ecosystem engineers could be more suspectable to adverse outcomes from species interactions making them less resistant to shifts towards an alternative ecosystem state.

The patch dynamics of foundation species profoundly affects community assembly and thus has important implications for ecosystem restoration. However, it is unclear how restored kelp patches that vary in size and density will influence the establishment of mid-trophic level (MTL) macroinvertebrates, a key functional group in coastal ecosystems. Artificial reefs with transplants of the canopy-forming kelp, Ecklonia radiata, were used to quantify the effect of patch size and kelp density on the densities of MTL macroinvertebrates (primarily decapod crustaceans) and on the recruitment of an ecologically important and commercially valuable lobster species. Densities of MTL macroinvertebrates, which were dominated by hermit crabs, decreased with increasing patch size but responded inconsistently to kelp density. There was, however, an overall positive relationship between MTL macroinvertebrates and the density of small epifaunal grazers (a potential food source), along with a negative association with cover of understorey foliose algae. In contrast, the total abundance and density of lobster recruits was higher on larger reefs, and reefs with kelp had up to double the number of recruits relative to reefs with no kelp. After 12 months, most of the surviving lobster recruits occurred on reefs supporting low and medium densities of kelp. These results show that patchy reef substratum is effective in supporting high densities of some MTL macroinvertebrates, irrespective of kelp presence. Although conversely, larger reefs with restored kelp at natural - or even relatively low - densities appear critical to the recruitment of lobsters, which could motivate and provide positive feedback for kelp restoration projects in some locations. Patch dynamics may be used to support restoration efforts by helping to accelerate the recovery of key species and ecosystem services; however, trade-offs will exist through different taxa responding to patch characteristics in different ways, some positive and some negative.