Grazing in Macroalgae Communities of the Baltic Sea

Sammanfattning: Fucus vesiculosus and Fucus serratus are the only large, structuring perennial brown macroalgae in the low salinity waters of the Swedish coast of the Baltic Proper. Since the 1970s there are reports of declining Fucus vesiculosus stands from several locations around the Baltic Proper, but also indications of recoveries e.g. in Finland, in the Askö area of the northern Baltic proper and in the south east coast of Sweden. There are probably several causes for these changes. This thesis focuses on how, among biological and other factors, grazing may be the proximate factor to structure the rocky phytal zone of the Baltic Sea.Destructive effects on F. vesiculosus may be connected to extreme densities of grazers. In a field survey large densities of the potential mesograzer Idotea baltica correlated positively with grazing injuries on and reduced depth penetration of Fucus vesiculosus. In a grazing experiment biomass of F. vesiculosus was halved within two weeks at a grazer density found in the field (80 I. baltica per 100 g of Fucus wet weight). In a second survey large numbers of the gastropods Theodoxus fluviatilis and Lymnaea peregra coincided with unsuccessful recruitment of F. vesiculosus. In an experiment these gastropods grazed germlings of F. vesiculosus up to sizes of 0.8 and 1.0 mm respectively. Abundances of gastropods were highest in autumn, indicating that autumn reproductive F. vesiculosus might be more influenced than spring reproductive F. vesiculosus as the latter would have surpassed the critical size in autumn.Grazing may be modified by environmental factors. Comparatively small densities of Theodoxus fluviatilis correlated positively with abundances of Fucus vesiculosus and negatively with abundances of filamentous algae. Grazing effects advantageous to F. vesiculosus were tested in a field experiment with manipulation of grazers, nutrients and propagules of filamentous algae. At low nutrient loads grazers tended to clear the substrate from filamentous algae and strengthen the competitive ability of F. vesiculosus. At high nutrient loads the grazers could not prevent dominance of filamentous algae with detrimental effects to F. vesiculosus. Physical factors like wave-action may also modify grazing effects. In a wave-exposed coastline with mixed stands of perennial brown algae and frequent occurrences of grazing isopods, F. vesiculosus declined faster than F. serratus. In an experiment F. vesiculosus was more heavily grazed than F. serratus, but only at intense water motion. Thus F. serratus seem to have a competitive advantage to F. vesiculosus in wave exposed coasts where severe grazing occurs.The fact that the grazer I. baltica often has been connected with F. vesiculosus, might indicate that this animal would be dependent on F. vesiculosus as habitat and/or food. In a wave-exposed habitat on the eastern coast of Öland, probably earlier dominated by F. vesiculosus but now holding discrete patches of either red algae {Polysiphonia fucoides) or saw-wrack (F. serratus) I. baltica used both species as habitat and food. In addition to the two macroalgae faecal pellets contained 30-40 % microalgae, indicating that mixed diets may be important for this species.In monitoring programs of rocky bottom communities fixed sites may be revisited on a yearly bases to estimate the development of the community. There is an underlying assumption that the observations at a fixed site are representative to surrounding areas. This assumption was tested along a 350 km coastline in SE Sweden. 18 years of observations were evaluated and compared to complementary sites. Within areas of similar wave exposure there was good agreement between fixed sites and surrounding areas.When all fixed sites within wave-protected areas were analysed together for long-term trends, there was an agreement in development between sites över the whole geographic area. The Fucus community increased its distribution during the 1980s, decreased during the early ]990s (probably from grazing as described above), to increase again in 2000-2001, that is an oscillating pattern is suggested. In wave-exposed sites, along a 100 km coastline, the F. vesiculosus stands were destroyed, seemingly by grazing as described above, around 1992-1994, with no signs of recovery as late as in 2002.The results are discussed in the context of grazing, regulation of grazing by predation, eutrophication and physical factors. The importance of the lack of fish predators is suggested for further research.