THE PQ?! EXPLORING THE IMPACT OF REDUCED IRRADIANCE ON THE PHOTOSYNTHETIC QUOTIENTS OF WELLINGTON HARBOUR MACROALGAE

<p><strong>Anthropogenic greenhouse gas emissions are driving global warming and ocean acidification, threatening ecosystems, and prompting urgent attention to carbon sequestration strategies. Blue Carbon, the sequestration of carbon by marine vegetated ecosystems, is an important mitigation approach. As one of the most productive organisms in the world, macroalgae could have significant potential for carbon sequestration through the uptake of dissolved inorganic carbon (DIC) during photosynthesis. Photosynthesis involves either the active, energy consuming uptake of the abundant bicarbonate (HCO3-) or the passive, less energy consuming uptake of the less abundant carbon dioxide (CO2). The product of photosynthesis is that oxygen (O2) is evolved, with the ratio between DIC and O2 evolution described as the photosynthetic quotient (PQ). Typically, a 1:1 ratio is assumed based on early photosynthetic stoichiometry, yet this assumption can introduce errors if environmental and species-specific factors alter this ratio. Reduced irradiance could influence macroalgal PQ by favouring passive CO2 uptake, therefore changing energetic requirements and thus PQ values. However, the effect of reduced irradiance on PQ is not well studied, highlighting a key research gap addressed in this thesis. Locally, within Wellington Harbour, there is possible impacts of sedimentation, but no published baselines of seaweed cover. I aimed to 1) Assess macroalgal cover in Wellington Harbour across a gradient of sites impacted by sedimentation, 2) Investigate the impact of reduced irradiance (simulating sedimentation) on PQ across five macroalgal species, and 3) Examine interspecies variation in PQ and discuss implications for carbon sequestration. I surveyed six Wellington Harbour sites showing significant variability in macroalgal coverage, diversity, and richness related to environmental stressors like sedimentation. Historical data comparison revealed substantial ecological changes in the status of macroalgae. Key species were chosen based on these surveys and were used in closed chamber incubation experiments under controlled light treatments (“Low” = ~120 µmol photons m-2 s-1, “Normal” = ~220 µmol photons m-2 s-1) representative of local sedimentation. I found that reduced irradiance had no significant impact on PQ, but PQ values differed from the assumed ratio 1.0, ranging from 0.90 (Pyropia plicata) to 1.36 (Macrocystis pyrifera). These interspecies differences are likely due to varying physiological and structural carbon assimilation strategies. Applying species-specific PQ values to global carbon sequestration models revealed potential inaccuracies – overestimations of up to 36% and underestimations of up to 10%, emphasizing the need for species-specific PQ data into carbon sequestration estimates.</strong></p>