Morphology and photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient

Increasing turbidity in coastal waters reduces benthic irradiance levels and consequently poses a fundamental threat to benthic primary producers and the food webs they support. It is therefore necessary to better understand how important primary producers, such as kelp, can adapt both physiologically and morphologically to these declining light conditions. This study examines how variation in turbidity influences the morphology of the kelp Ecklonia radiata, and how this relates to photosynthetic performance, by measuring multiple morphological characteristics, photosynthesis-irradiance response, and photosynthetic pigments in adult kelp across a large-scale turbidity gradient. Kelp morphology was strongly related to variation in benthic irradiance among sites, but this was not consistent with morphological acclimation. Kelp at the most turbid sites had short stipes (< 0.4 m) and lower surface area to volume ratios (SA:V) than kelp at moderate to high-light sites, which had long stipes (similar to 1 m) and high SA:V. Photosynthetic parameters and pigment content also varied considerably in relation to benthic irradiance, with increased photosynthetic efficiency and pigment content at low-light sites. However, maximum rates of photosynthesis were highest at high-light sites and strongly positively related to higher SA:V of kelp at these sites. These results demonstrate that while physiological adaptation may increase photosynthetic efficiency at low-light sites, maximum photosynthetic output is strongly related to thallus morphology rather than cellular acclimation. This suggests that the large-scale variation in kelp morphology observed across the turbidity gradient likely has important consequences for overall primary production as well as other important ecosystem functions.