1. Phytobenthic communities consist of macrophytes (macroalgae, vascular plants andmosses) with their accompanying fauna and microorganisms.

2. The phytobenthic communities occur in the photic zone, which in the Baltic Sea extendsfrom the water surface down to a*20 m water depth, but in turbid coastal waters onlydown to*5m.

3. The type of vegetation is determined by the available substrate, which is a result ofgeography and geology in combination with currents. Most macroalgae grow attachedto hard substrates whereas vascular plants and charophytes grow on sandy or soft (siltand mud) substrates.

4. Generally, the coastal areas of the Baltic Sea consist of mixed substrates with anintermingled vegetation of vascular plants and algae. In the northern Baltic Sea hardsubstrates dominate in the outer archipelagos, and in the southeastern Baltic Sea sandyand muddy substrates dominate.

5. Luxuriant stands of macrophytes provide food, shelter and spawning habitats for theassociated sessile and mobile micro-, meio- and macrofauna, includingfish.

6. On an ecosystem-wide scale, the phytobenthic communities vary along the large-scaleBaltic Sea gradient. Biomass decreases with lower salinity and colder climate, while theproportion of freshwater species increases.

7. On a local scale, the phytobenthic communities are mainly, directly or indirectly, shapedby water movement (e.g.by the occurrence of sandy beaches and rocky shores) andwinter ice cover. Light and substrate availability give rise to typical depth zonationpatterns, ending with soft-substrate communities deepest down.

8. On a small scale (patches), phytobenthic community structure and composition isinfluenced by microhabitat structure and biotic interactions.

9. The phytobenthic communities in the brackish Baltic Sea are more sensitive to dis-turbance than their marine counterparts due to low diversity, physiological stress and theloss of sexual reproduction when species approach their salinity limit.