Vopel, KayAfening, Mary2026-01-122026-01-122025http://hdl.handle.net/10292/20475Ocean heatwaves are increasing in frequency and intensity, posing threats to coastal ecosystems. One potentially affected group is seaweeds, whose photosynthetic performance may be disrupted by elevated temperatures. Pulse-amplitude modulation (PAM) fluorometry is a non-invasive technique commonly used to assess species’ photosynthetic performance. However, its suitability for unattended and automated in situ monitoring of seaweed photosynthesis under heatwave conditions remains untested. To investigate this, three laboratory experiments were conducted using the invasive kelp species Undaria pinnatifida. In each experiment, one blade from each of three specimens (Experiment 1) or six specimens (Experiments 2 and 3) of U. pinnatifida, acclimated in the laboratory under constant temperature and a diel light cycle, was mounted in front of a PAM fluorometer. The blades were held in fixed positions while the fluorometers automatically conducted saturation pulse (SP) analyses at 30-minute intervals—initially during an acclimation period, and subsequently throughout a warming phase. The measuring lights of the PAM fluorometer remained off between analyses, although the blades remained in place for at least 10 days. SP analyses conducted during the first five hours after light onset each day were used to plot the relative electron transport rate (rETR) and photosynthetically active radiation(PAR) curves, with the slope (alpha) serving as a proxy for the efficiency of U. pinnatifida’s photosystem II (PSII). Inspection of the multi-day time series of alpha revealed a significant decline in PSII efficiency under constant temperature, suggesting that the experimental setup was unable to maintain stable PSII performance. Therefore, the second hypothesis remains inconclusive regarding the response of U. pinnatifida’s PSII efficiency to warming. Nevertheless, Experiment 1 indicated the possibility of a positive response to moderate warming from 14 to 19 °C, and a negative response to further warming to 23 °C. This was also evident in Experiments 2 and 3, despite an overall decline in PSII performance including blade discoloration. Our experiments highlight a critical measurement artefact that must be addressed before attempting to monitor PSII performance in the field. Seaweed blades should not remain fixed in position during unattended automated fluorometry. Instead, they should be mounted on the fluorometer only for the duration of an SP analysis. Although more labour-intensive, this approach enables adequate replication when the number of available PAM fluorometers is limited, and reduces confounding effects associated with prolonged blade immobilisation.enThesisOpenAccess