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The use of pulse-amplitude modulated (PAM) fluorometry in situ and in vivo as an indi- cator for nutrient and light status of microphytobenthos communities was investigated on the Great Barrier Reef. The 10 reefs investigated were located along a previously described water quality gra- dient, confirmed by sediment chemistry parameters (Corg, N) and incident light measurements. Esti- mates of the efficiency of Photosystem II (PSII) were biased when not corrected for background fluo- rescence that increased with increasing inorganic C content of the sediments. Uncorrected values mimicked a decline in quantum efficiency towards areas with presumed lower nutrient levels. Upon correction, maximum and effective quantum efficiency (ca. 0.59) did not vary significantly along the gradient. The maximum quantum efficiency (Fv/Fm,corr) measured after dark-adaptation did not differ statistically from effective quantum efficiency in the field (φPSII,corr). F0 values measured in the labora- tory were much lower than in situ F readings, indicating a higher concentration of biomass on the sur- face of the sediment in the field. Rapid light curve parameters (initial slope α, relative electron trans- port rate at light saturation rETRmax, irradiance at onset of light saturation Ek) were lower during in vitro readings than in situ. rETRmax and α values exhibited no statistical differences along the gradi- ent. In contrast, Ek increased towards midshelf reefs and correlated with incident PAR, indicating that microphytobenthos communities were highly adapted to incident light conditions. Because coral reef microphytobenthos was previously shown to be nitrogen limited, it was unexpected that efficiency measurements would not vary. It is hypothesised that, due to potential for differential vertical migra- tion, the actual species involved in photosynthesis at a given time are nutrient sufficient. Higher nutrient supply may change the community composition on the surface to dominance by species with higher productivity.

Photosynthetic efficiency and rapid light curves of sediment-biofilms along a water quality gradient in the Great Barrier Reef, Australia