Effects of filamentous algae and deposited matter on the survival of Fucus vesiculosus L. germlings in the Baltic Sea

As a result of increased nutrient levels in the Baltic Sea during the past 50 years, mass developments of filamentous algae have become a common feature along the Swedish east coast and deposition of organic matter has also increased. To test whether these two factors have any effects on the early life stages of Fucus vesiculosus a number of laboratory and field studies were conducted. The amount of epilithic and epiphytic filamentous algae on F vesiculosus and the amount of deposited matter in the littoral zone were quantified during the two reproductive periods of E vesiculosus, early summer (May-June) and late autumn (September-October). Both filamentous algae (Cladophora glomerata) and deposited matter (introduced either before or after settlement of fertilized eggs) were shown to significantly decrease the number of surviving germlings. The survival of germlings seeded on stones with filamentous algae, or seeded on culture dishes concurrently with the lowest concentration of deposited matter (0.1 g dm(-2)), was 5% or less. In the field, the amount of filamentous algae was significantly higher during E vesiculosus summer reproduction, whereas the amount of deposited matter collected in traps was significantly higher during the period of autumn reproduction. The greatest biomass of filamentous algae was observed at sheltered sites. Based on the negative effects of filamentous algae and deposited matter on Fucus recruitment and the observation of local and seasonal differences in abundance of filamentous algae and deposition, we suggest that the prerequisites for the survival of either summer or autumn-reproducing populations of F vesiculosus in the Baltic Sea may differ locally.