Interactive Effects of Light, Temperature and Water Movement on Wellington Fucales and Laminariales Physiology and Community Structure

<p><strong>Habitat-forming macroalgae like Fucales and Laminarians are essential intertidal ecosystem engineers. However, the intertidal cover of brown algae in New Zealand could change due to anthropogenic effects. Environmental conditions vary significantly between the intertidal zones inside the harbour and on the south coast of Wellington, where the harbour has high sediment loads, causing coastal darkening and low water flow; the south coast has the opposite. Intertidal field surveys over six years showed a trend of increased bleaching of calcifying species and gradual invasion of non-native Undaria pinnatifida following several marine heatwaves in the summer of 2022–2023. Invasive Asian Kelp Undaria pinnatifida is increasing with the decrease in canopy-forming native algae, raising concerns that it will overrun ecosystems that have experienced mortality events from heat waves due to its fast proliferation. The bleaching of calcifying species and slow recovery from 2023 surveys indicate prolonged marine heatwaves significantly decrease species diversity and percentage cover where the gaps in the community are filled by Undaria pinnatifida. A multi-stressor marine heatwave, water velocity and light intensity experiment on Fucales species Carpophyllum maschalocarpum and Cystophora torulosa indicated temperature is the primary driver of physiological changes and mortality of these species. C. maschalocarpum experienced significant relative length and weight loss because of oxidation of the blades from overproduction of reactive oxygen species because of physiological stress to temperature. C. torulosa experienced significant oxidation, relative length and weight loss from exposure to temperature and lack of irradiance due to sedimentation. Physiological impacts extend far into the recovery period, suggesting the effects of a marine heatwave do not stop at the end of the marine heatwave, increasing the likelihood of mortality and long-term physiological damage. The species responses indicated that although both species are often described as occupying the same environment, the two species can withstand different ecological conditions, with C. maschalocarpum living in low light and low water velocity and C. torulosa living in high light and increased water velocity.</strong></p>