We evaluated single and interactive effects of density reduction of Zostera marina L., a habitat-forming species, shading, and sediment nutrient enrichment on the response of Z. marina leaf and root-rhizomes carbohydrates (soluble sugar and starch content). Soluble sugars were extracted using 80%, centrifuged, treated with phenol (supernatant) and sulfuric acid then measured using spectrophotometry. Soluble sugars dry weight was calculated using glucose-standard curves. Enzymatic digestion of the remaining pellet was used to determine starch concentrations and measured using spectrophotometry. For full method, see related article (doi:10.3354/meps13546).