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 intesti-nalis, and repeated the experiments with the addition of pH-induced changes in carbon speciation andavailability. One thallus of Ulva reduced photosynthetically available irradiance to underlying seagrassby about 50% and shifted the quality of remaining light towards the green part of the spectrum (approx-imately 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 thecontrol plants, indicating that the green spectrum transmitted through Ulva layers was being absorbedby the seagrass and was as efficient in driving photosynthesis as was the normal light. On the otherhand, algae-generated pH shifts which could extend up to pH 10 had significant negative effects onphotosynthesis of the seagrass, reducing the electron transport rates with up to 75%