The phytobenthic zone

Abstract

1. Phytobenthic communities consist of macrophytes (macroalgae, vascular plants and mosses) with their accompanying fauna and microorganisms. 2. The phytobenthic communities occur in the photic zone, which in the Baltic Sea extends from the water surface down to a 20 m water depth, but in turbid coastal waters only down to 5 m. 3. The type of vegetation is determined by the available substrate, which is a result of geography and geology in combination with currents. Most macroalgae grow attached to hard substrates whereas vascular plants and charophytes grow on sandy or soft (silt and mud) substrates. 4. Generally, the coastal areas of the Baltic Sea consist of mixed substrates with an intermingled vegetation of vascular plants and algae. In the northern Baltic Sea hard substrates dominate in the outer archipelagos, and in the southeastern Baltic Sea sandy and muddy substrates dominate. 5. Luxuriant stands of macrophytes provide food, shelter and spawning habitats for the associated sessile and mobile micro-, meio- and macrofauna, including fish. 6. On an ecosystem-wide scale, the phytobenthic communities vary along the large-scale Baltic Sea gradient. Biomass decreases with lower salinity and colder climate, while the proportion of freshwater species increases. 7. On a local scale, the phytobenthic communities are mainly, directly or indirectly, shaped by water movement (e.g. By the occurrence of sandy beaches and rocky shores) and winter ice cover. Light and substrate availability give rise to typical depth zonation patterns, ending with soft-substrate communities deepest down. 8. On a small scale (patches), phytobenthic community structure and composition is influenced by microhabitat structure and biotic interactions. 9. The phytobenthic communities in the brackish Baltic Sea are more sensitive to disturbance than their marine counterparts due to low diversity, physiological stress and the loss of sexual reproduction when species approach their salinity limit.