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

► Macroalgae reduce both the amount and spectral quality of light in seawater. ► Photosynthesis by algae increases pH of seawater, this affects seagrass productivity. ► We evaluated effects of Ulva on the seagrass Zostera marina when incubated together. ► Ulva-generated pH increases decreased the photosynthetic capacity of Zostera. ► Ulva induced light changes (green light) did not decrease Zostera productivity. 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 520–570nm). 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 effects on photosynthesis of the seagrass, reducing the electron transport rates with up to 75%.