Physiological responses and ecosystem functions of key subtidal primary producer under local and global changes

Abstract

Ocean warming and acidification have altered the physical and chemical properties of marine environments by increasing ocean heat budget and lowering pH. Increased temperature threatens the survival of organisms by challenging their physiological limits, resulting in mortality or range shift. Reduced pH changes seawater carbon chemistry and can restructure marine communities, influencing ecosystem processes. As a result of these rapid changes, the distribution and phenology of marine producers are experiencing significant impacts - establishing baseline and mechanistic understanding behind these shifts allow for future assessment of systemic changes in ecosystems. Throughout this thesis, I examined effects of environmental conditions on the photophysiology and ecology of habitat-forming macroalgae in the genus Sargassum to provide insights on the shifts in ecosystem functions of a macroalgal-dominated ecosystem relevant to the global change setting.

Transition zones between tropical and temperate regions, such as the South China Sea, are characterized by large seasonal variability which affects algal phenology. Seasonal field surveys revealed that temperature negatively correlate with the proliferation of subtidal macroalgae in Hong Kong, especially Sargassum hemiphyllum, emulating the phenology of Sargassum forests in similar ecoregions. Biomass of this dominant species reached up to 33.8 kg m-2 in winter, contrasting the barren rocky substrates defined by encrusting algae and rock oysters during summer. Nevertheless, subtidal surveys are labour intensive and inefficient. Aerial drones equipped with high resolution cameras combined with deep learning to differentiate macroalgae from its environment are a potential replacement for field-based macroalgal surveys but are yet unproven. Therefore, I used drone surveys and deep learning (AI) combined with ground-truth sampling of biomass, to increase the sampling extent and accuracy with less labour than SCUBA surveys. Importantly, such monitoring systems may allow us to link local population productivity to regional patterns.

I then experimentally manipulated temperature and irradiance to test their effects on the productivity and growth of S. hemiphyllum. Macroalgae in the high biomass, rapid growth stage were limited by warm temperatures, however, their photosynthetic performance peaked in the same temperature range when growing in drastically different form (short and prostrate). This suggests a dimorphic adaptation to counter thermal stress may come at the expense of shortened growth season and overall lower productivity. Algal biomass will eventually enter detrital pathways mediated by microbes, which are sensitive to pH shifts. Therefore, I tested how ocean acidification changes microbial community composition and diversity and, subsequently, their degradation of algal detritus. My experiment revealed that increased microbial diversity near submarine CO2 vents and enhanced degradation rate, suggesting that restructuring of acidified microbial communities may reflect quicker carbon recycling in future oceans, ultimately limiting the sequestration capacity of macroalgal forests through biomass exportation.

As climate change alters the marine environment, physiological and ecological processes of macroalgal-dominated communities will change. However, since interactions between biotic and abiotic factors are complex, the resultant consequences may not always be straightforward. Here, I provided valuable insights on the mechanistic changes in a subtropical macroalgal-dominated biota, thus improving our understand on their functional shifts in the face of climate change.