The endemic kelp Lessonia corrugata is being pushed above its thermal limits in an ocean warming hotspot.

Kelps are in global decline due to climate change, which includes ocean warming. To identify vulnerable species, we need to identify their tolerances to increasing temperatures and determine whether tolerances are altered by co-occurring drivers such as inorganic nutrient levels. This is particularly important for those species with restricted distributions, which may already be experiencing thermal stress. To identify thermal tolerance of the range-restricted kelp Lessonia corrugata, we conducted a laboratory experiment on juvenile sporophytes to measure performance (growth, photosynthesis) across its thermal range (4-22°C). We determined the upper thermal limit for growth and photosynthesis to be ~22-23°C, with a thermal optimum of ~16°C. To determine if elevated inorganic nitrogen availability could enhance thermal tolerance, we compared the performance of juveniles under low (4.5 µmol · d-1) and high (90 µmol · d-1) nitrate conditions at and above the thermal optimum (16-23.5°C). Nitrate enrichment did not enhance thermal performance at temperatures above the optimum but did lead to elevated growth rates at the thermal optimum. Our results indicate L. corrugata is likely to be extremely susceptible to moderate ocean warming and marine heatwaves. Peak sea surface temperatures during summer in eastern and northeastern Tasmania can reach up to 20-21°C, and climate projections suggest that L. corrugata's thermal limit will be regularly exceeded by 2050 as southeastern Australia is a global ocean-warming hotspot. By identifying the upper thermal limit of L. corrugata, we have taken a critical step in predicting the future of the species in a warming climate.

Cool-edge populations of the kelp Ecklonia radiata under global ocean change scenarios: strong sensitivity to ocean warming but little effect of ocean acidification.

Kelp forests are threatened by ocean warming, yet effects of co-occurring drivers such as CO2 are rarely considered when predicting their performance in the future. In Australia, the kelp Ecklonia radiata forms extensive forests across seawater temperatures of approximately 7-26°C. Cool-edge populations are typically considered more thermally tolerant than their warm-edge counterparts but this ignores the possibility of local adaptation. Moreover, it is unknown whether elevated CO2 can mitigate negative effects of warming. To identify whether elevated CO2 could improve thermal performance of a cool-edge population of E. radiata, we constructed thermal performance curves for growth and photosynthesis, under both current and elevated CO2 (approx. 400 and 1000 µatm). We then modelled annual performance under warming scenarios to highlight thermal susceptibility. Elevated CO2 had minimal effect on growth but increased photosynthesis around the thermal optimum. Thermal optima were approximately 16°C for growth and approximately 18°C for photosynthesis, and modelled performance indicated cool-edge populations may be vulnerable in the future. Our findings demonstrate that elevated CO2 is unlikely to offset negative effects of ocean warming on the kelp E. radiata and highlight the potential susceptibility of cool-edge populations to ocean warming.

Safeguarding marine life: conservation of biodiversity and ecosystems.

Marine ecosystems and their associated biodiversity sustain life on Earth and hold intrinsic value. Critical marine ecosystem services include maintenance of global oxygen and carbon cycles, production of food and energy, and sustenance of human wellbeing. However marine ecosystems are swiftly being degraded due to the unsustainable use of marine environments and a rapidly changing climate. The fundamental challenge for the future is therefore to safeguard marine ecosystem biodiversity, function, and adaptive capacity whilst continuing to provide vital resources for the global population. Here, we use foresighting/hindcasting to consider two plausible futures towards 2030: a business-as-usual trajectory (i.e. continuation of current trends), and a more sustainable but technically achievable future in line with the UN Sustainable Development Goals. We identify key drivers that differentiate these alternative futures and use these to develop an action pathway towards the desirable, more sustainable future. Key to achieving the more sustainable future will be establishing integrative (i.e. across jurisdictions and sectors), adaptive management that supports equitable and sustainable stewardship of marine environments. Conserving marine ecosystems will require recalibrating our social, financial, and industrial relationships with the marine environment. While a sustainable future requires long-term planning and commitment beyond 2030, immediate action is needed to avoid tipping points and avert trajectories of ecosystem decline. By acting now to optimise management and protection of marine ecosystems, building upon existing technologies, and conserving the remaining biodiversity, we can create the best opportunity for a sustainable future in 2030 and beyond.

Ocean resource use: building the coastal blue economy.

Humans have relied on coastal resources for centuries. However, current growth in population and increased accessibility of coastal resources through technology have resulted in overcrowded and often conflicted spaces. The recent global move towards development of national blue economy strategies further highlights the increased focus on coastal resources to address a broad range of blue growth industries. The need to manage sustainable development and future exploitation of both over-utilised and emergent coastal resources is both a political and environmental complexity. To address this complexity, we draw on the perspectives of a multi-disciplinary team, utilising two in depth exemplary case studies in New Zealand and within the Myanmar Delta Landscape, to showcase barriers, pathways and actions that facilitate a move from Business as Usual (BAU) to a future aligned with the Sustainable Development Goals (SDGs) and the UN International Decade of Ocean Science for Sustainable Development 2021-2030. We provide key recommendations to guide interest groups, and nations globally, towards sustainable utilisation, conservation and preservation of their marine environments in a fair and equitable way, and in collaboration with those who directly rely upon coastal ecosystems. We envision a sustainable future driven by conflict mitigation and resolution,  where:(i)Change is motivated and facilitated(ii)Coastal ecosystems are co-managed by multiple reliant groups(iii)Networks that maintain and enhance biodiversity are implemented(iv)Decision-making is equitable and based on ecosystem services(v)Knowledge of the marine realm is strengthened-'mapping the ocean of life'(vi)The interests of diverse user groups are balanced with a fair distribution of benefits.

Density-dependence and seasonal variation in reproductive output and sporophyte production in the kelp, Ecklonia radiata.

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.

Interactive effects of canopy-driven changes in light, scour and water flow on microscopic recruits in kelp.

Ecosystem engineering kelp forms habitat and influences associated communities by altering abiotic conditions. These conditions can also affect the engineer's own demographic rates but the mechanisms underpinning these feedbacks are not well known. Here, we tested the interactive effects of three abiotic factors engineered by the Australasian kelp Ecklonia radiata (light, water flow and scour) on the early survivorship and growth of its outplanted microscopic recruits. After six weeks, recruit survivorship was high in the absence of scour and low light (2-3 times higher than when scour was present) and under low water flow-ambient light conditions. Growth of sporophytes was strongly related to light, with recruits under ambient light approximately four times larger after six weeks. Overall, reduced scour (for survivorship) and ambient light (for growth) appear crucial for maximising E. radiata recruitment suggesting a healthy forest can provide microenvironments to enhance survivorship while gaps in the canopy enhance growth.

High kelp density attracts fishes except for recruiting cryptobenthic species.

As foundation species, kelp support productive and species rich communities; however, the effects of kelp structure on mobile species within these complex natural systems are often difficult to assess. We used artificial reefs with transplanted kelp to quantify the influence of kelp patch size and density on fish assemblages including the arrival of recruiting cryptobenthic species. Large patches with dense kelp supported the highest abundance, species richness, and diversity of fishes, with the addition of dense kelp tripling biomass and doubling richness. The abundance of recruits in artificial collectors declined with patch size and was halved on reefs with sparse kelp compared to reefs with dense kelp or no kelp. These results highlight the importance of dense kelp cover in facilitating biodiversity and indicate that kelp addition could support the recovery of degraded coastal ecosystems. Kelp also apparently drives complex interactions affecting the recruitment/behaviour of some cryptobenthic species.

Resilience and stability of kelp forests: The importance of patch dynamics and environment-engineer feedbacks.

Habitat forming 'ecosystem engineers' such as kelp species create complex habitats that support biodiverse and productive communities. Studies of the resilience and stability of ecosystem engineers have typically focussed on the role of external factors such as disturbance. However, their population dynamics are also likely to be influenced by internal processes, such that the environmental modifications caused by engineer species feedback to affect their own demography (e.g. recruitment, survivorship). In numerous regions globally, kelp forests are declining and experiencing reductions in patch size and kelp density. To explore how resilience and stability of kelp habitats is influenced by this habitat degradation, we created an array of patch reefs of various sizes and supporting adult Ecklonia radiata kelp transplanted at different densities. This enabled testing of how sub-canopy abiotic conditions change with reductions in patch size and adult kelp density, and how this influenced demographic processes of microscopic and macroscopic juvenile kelp. We found that ecosystem engineering by adult E. radiata modified the environment to reduce sub-canopy water flow, sedimentation, and irradiance. However, the capacity of adult kelp canopy to engineer abiotic change was dependent on patch size, and to a lesser extent, kelp density. Reductions in patch size and kelp density also impaired the recruitment, growth and survivorship of microscopic and macroscopic juvenile E. radiata, and even after the provisioning of established juveniles, demographic processes were impaired in the absence of sufficient adult kelp. These results are consistent with the hypothesis that ecosystem engineering by adult E. radiata facilitates development of juvenile conspecifics. Habitat degradation seems to impair the ability of E. radiata to engineer abiotic change, causing breakdown of positive intraspecific feedback and collapse of demographic functions, and overall, leading to reductions in ecosystem stability and resilience well before local extirpation.

Ectoparasites increase swimming costs in a coral reef fish.

Ectoparasites can reduce individual fitness by negatively affecting behavioural, morphological and physiological traits. In fishes, there are potential costs if ectoparasites decrease streamlining, thereby directly compromising swimming performance. Few studies have examined the effects of ectoparasites on fish swimming performance and none distinguish between energetic costs imposed by changes in streamlining and effects on host physiology. The bridled monocle bream (Scolopsis bilineatus) is parasitized by an isopod (Anilocra nemipteri), which attaches above the eye. We show that parasitized fish have higher standard metabolic rates (SMRs), poorer aerobic capacities and lower maximum swimming speeds than non-parasitized fish. Adding a model parasite did not affect SMR, but reduced maximum swimming speed and elevated oxygen consumption rates at high speeds to levels observed in naturally parasitized fish. This demonstrates that ectoparasites create drag effects that are important at high speeds. The higher SMR of naturally parasitized fish does, however, reveal an effect of parasitism on host physiology. This effect was easily reversed: fish whose parasite was removed 24 h earlier did not differ from unparasitized fish in any performance metrics. In sum, the main cost of this ectoparasite is probably its direct effect on streamlining, reducing swimming performance at high speeds.