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Dissolved organic carbon (DOC) released by macroalgae is an intrinsic component of the coastal ocean carbon cycle, yet knowledge of how future ocean warming may influence this is limited. Temperature is one of the primary abiotic regulators of macroalgal physiology, but there is minimal understanding of how it influences the magnitude and mechanisms of DOC release. To investigate this, we examined the effect of a range of temperatures on DOC release rates and physiological traits of Ecklonia radiata, the most abundant and widespread kelp in Australia that represents a potentially significant contribution to coastal ocean carbon cycling. Juvenile sporophytes were incubated at eight temperatures (4–28°C) for 14 days, after which time, DOC concentrations and physiological traits (growth, photosynthesis, respiration, Fv/Fm, photosynthetic pigment content, and carbon, and nitrogen content) were analyzed using thermal performance curves (TPCs) or regression analyses. Thermal optima were 15.63°C for growth and 25.84°C for photosynthesis, highlighting vulnerability to future ocean warming. Dissolved organic carbon concentrations increased when the temperature was above ~22°C, being greatest at the highest temperature tested (28°C), which was likely driven by photosynthetic overflow and thermal stress. Mean Fv/Fm, total chlorophyll, and total fucoxanthin content were lowest at 28°C. The C:N ratio of blades increased linearly with temperature from 23.9 ± 1.30 at 4°C to 33.0 ± 1.22 at 28°C. We demonstrate increased DOC release by E. radiata under elevated seawater temperatures and discuss potential implications for coastal carbon cycling under future ocean warming given the complex and uncertain fate of macroalgal DOC in the marine environment. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) released by macroalgae supports coastal ocean carbon cycling and contributes to the total oceanic DOC pool. Salinity fluctuates substantially in coastal marine environments due to natural and anthropogenic factors, yet there is limited research on how salinity affects DOC release by ecologically important macroalgae. Here we determined the effect of short‐term salinity changes on rates of DOC release by the habitat‐forming fucalean seaweed Sargassum fallax (Ochrophyta). Lateral branches (~4 g) cut at the axes of mature individuals were incubated across a salinity gradient (4–46) for 24 h under a 12:12 light:dark cycle, and seawater was sampled for DOC at 0, 12, and 24 h. Physiological assays (tissue water content, net photosynthesis, respiration, tissue carbon, and nitrogen content) were undertaken at the end of the 24‐h experiment. Dissolved organic carbon release increased with decreasing salinity while net photosynthesis decreased. Dissolved organic carbon release rates at the lowest salinity tested (4) were ~3.3 times greater in the light than in the dark, indicating two potential DOC release mechanisms: light‐mediated active exudation and passive release linked to osmotic stress. Tissue water content decreased with increasing salinity. These results demonstrate that hyposalinity stress alters the osmotic status of S. fallax, reducing photosynthesis and increasing DOC release. This has important implications for understanding how salinity conditions encountered by macroalgae may affect their contribution to the coastal ocean carbon cycle. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]