When Zostera marina is intermixed with Ulva, its photosynthesis is reduced by increased pH and lower light, but not by changes in light quality

Abstract

To evaluate the effects of Ulva on the photosynthetic capacities of the temperate seagrass Zostera marina, we compared it in the laboratory under normal light and light filtered through layers of Ulva intestinalis, and repeated the experiments with the addition of pH-induced changes in carbon speciation and availability. One thallus of Ulva reduced photosynthetically available irradiance to underlying seagrass by about 50% and shifted the quality of remaining light towards the green part of the spectrum (approximately 520–570 nm). There was no significant difference in photosynthetic performance between Z. marina under normal light and under Ulva-filtered light when adjusted to the same irradiance as for the control plants, indicating that the green spectrum transmitted through Ulva layers was being absorbed by the seagrass and was as efficient in driving photosynthesis as was the normal light. On the other hand, algae-generated pH shifts which could extend up to pH 10 had significant negative