Effects of filamentous algae and sediment on recruitment and survival of Fucus serratus (Phaeophyceae) juveniles in the eutrophic Baltic Sea

Eutrophication of the Baltic Sea has increased both the growth of annual filamentous algae and the rate of sedimentation. Together these factors may have a detrimental effect on the survival of perennial macroalgal populations. The aim of this research was to study how these factors affect the colonization success of Fucus serratus at the local level. We investigated the settlement and survival of F. serratus germlings through a combination of field studies and laboratory experiments. The study area lay off the east coast of Oland, along the central Baltic Sea, where consistently-submerged, extensive fucoid stands occur at depths of 2- 10 in, together with turf-forming filamentous algae, mainly Polysiphonia fucoides. Very few F. serratus juveniles were observed outside the sweeping radius of adult F. serratus individuals at a 7-m depth, while abundant recruits were found inside F. serratus stands. Amounts of filamentous algae and sediment were significantly larger outside the F. serratus stands than inside, and the amount of fine sediment (< 0.25 mm) was positively correlated with the amount of filamentous algae, suggesting that fine sediment accumulates in the turf. Experimental studies showed that both filamentous algae and sediment negatively affect the settlement ability of F. serratus eggs and zygotes and the subsequent survival of the germlings, with sediment having the strongest effect. Thus, the prior existence of an adult F. serratus population may allow for continuing recruitment of juveniles, while colonization of new areas unaffected by the sweeping effect of larger individuals seems very difficult under current conditions.