The most common eelgrass along the Swedish coast is Zostera marina. It is an ecosystem engineer which provides both nature and humans with important ecosystem services and various ecosystem functions. Eutrophication is one of the main reasons why the Swedish coast has lost a lot of Z. marina meadows. When there is more nitrogen in the ocean, it is an advantage for epiphytic algae so that they can grow faster. The epiphytic algae grow on Z. marina and reduce Z. marina from receiving light. And therefore, the aim of my thesis is to investigate the effect of eutrophication on the growth of epiphytic algae on Z. marina. I took water samples at 8 different locations (spanning 7.21 km) along the coast of Halmstad, from the mouth of river Nissan, which I hypothesize to be a main source of nitrogen, to the open sea. From the water samples nitrogen, absorbance and TOC was measured to see if there was any difference in the water composition from Nissan's outlet and seawards. Z. marina leaves were taken from two different sites on one occasion to investigate the degree of epiphytic algae on the leaves. I found that the nitrogen concentration was decreasing from the outlet of Nissan River along coast towards the open sea. Also, the growth of epiphytic algae on Z. marina was decreasing from the outlet of Nissan River along the coast towards the open sea. The growth of epiphytic algae on Z. marina was not solely dependent of the concentration of nitrogen. In Scandinavia, Z. marina meadows are one of the most diverse coastal ecosystems. On otherwise sandy and species-poor substrates, Z. marina is an important habitat for a rich epiflora and fauna so a more integrated understanding in both management and science of Z. marina's stress responses is needed.