Water quality deterioration is expected to worsen the light conditions in shallow coastal waters with increasing human activities. Temperate seagrasses are known to tolerate a highly fluctuating light environment. However, depending on their ability to adjust to some decline in light conditions, decreases in daily light quantity and quality could affect seagrass physiology, productivity, and, eventually, survival if the Minimum Quantum Requirements (MQR) are not reached. To better understand if, how, and to what extent photosynthetic adjustments contribute to light acclimation, eelgrass (Zostera marina L.) shoots from the cold temperate St. Lawrence marine estuary (Rimouski, QC, Canada) were exposed to seven light intensity treatments (6, 36, 74, 133, 355, 503, and 860 μmol photons m–2 s–1, 14:10 light:dark photoperiod). Photosynthetic capacity and efficiency were quantified after five and 25 days of light exposure by Pulse Amplitude Modulated (PAM) fluorometry to assess the rapid response of the photosynthetic apparatus and its acclimation potential. Photoacclimation was also studied through physiological responses of leaves and shoots (gross and net primary production, pigment content, and light absorption). Shoots showed proof of photosynthetic adjustments at irradiances below 200 μmol photons m–2 s–1, which was identified as the threshold between limiting and saturating irradiances. Rapid Light Curves (RLC) and net primary production (NPP) rates revealed sustained maximal photosynthetic rates from the highest light treatments down to 74 μmol photons m–2 s–1, while a compensation point (NPP = 0) of 13.7 μmol photons m–2 s–1 was identified. In addition, an important package effect was observed, since an almost three-fold increase in chlorophyll content in the lowest compared to the highest light treatment did not change the leaves’ light absorption. These results shed new light on photosynthetic and physiological processes, triggering light acclimation in cold temperate eelgrass. Our study documents an MQR value for eelgrass in the St. Lawrence estuary, which is highly pertinent in the context of conservation and restoration of eelgrass meadows.

Les forêts de laminaires sont des écosystèmes riches et productifs, longeant les côtes des zones tempérées à polaires. Dans les eaux du golfe du Saint-Laurent, les platiers rocheux de l’île d’Anticosti abritent des forêts de laminaires qui sont encore très peu caractérisées. En 2021 et en 2022, 2 échantillonnages en plongée sous-marine ont permis de caractériser les communautés de laminaires présentes sur 14 sites au sud-ouest de l’île d’Anticosti. Cinq espèces de laminaires (Saccharina latissima, Alaria esculenta, Hedophyllum nigripes/Laminaria digitata, Agarum clathratum, Saccorhiza dermatodea) ont été recensées, avec une grande variabilité d’assemblage entre les sites. Les densités (de 10 ± 5 à 99 ± 20 individus·m−2) et les biomasses (de 0,3 ± 0,1 à 6,4 ± 1,0 kg·m−2) étaient semblables à celles dans d’autres écosystèmes à laminaires de l’est du Canada. Des relations allométriques sur S. latissima ont permis de mettre en évidence des différences entre les sites, probablement dues aux conditions environnementales locales. Cette caractérisation des forêts de laminaires du sud-ouest de l’île d’Anticosti ouvre des perspectives sur le potentiel écologique et économique de cet écosystème.