The Density and Survival of Fucus vesiculosus L. (Fucales, Phaeophyta) on Different Bedrock Types on a Baltic Sea Moraine Coast

Abstract

The glacial moraine along the Baltic Sea of the south-eastern coast of Sweden is composed mainly of porous and soft Lower Cambrian sandstone and hard granite together with small amounts of alkaline amphibolite and diabase. Our objective was to determine whether the chemical and structural properties of these bedrock types, in combination with wave exposure, affect germling survival and adult plant density of Fucus vesiculosus L. commonly found growing on stones and boulders in the area. The number of surviving F. vesiculosus germlings was found to be similar on the alkaline amphibolite and diabase (1.0x10⁴±0.8x10³ ind.m⁻²). It was significantly higher than on the acidic porphyry, sandstone, granite, and gneiss (0.6x10⁴±0.5x10³ ind.m⁻²) after two months of growth. These results indicate that small-scale differences in surface structures such as porosity, holes and fissures may affect the survival of early stages of F. vesiculosus germlings in the Baltic Sea. Also the natural density of F. vesiculosus individuals at the wave-exposed site was shown to differ depending on bedrock type, with the highest number of plants on sandstone (1893±125 ind. m⁻²) followed by granite (405±28 ind.m⁻²) and amphibolite (374±33 ind.m⁻²). Contrary to the wave-exposed site, the density of plants was similar on all three types at the sheltered site. Further, the attachment strength of adult F. vesiculosus specimens was similar, i.e. 57.6±4.1 N and 61.8±6.8 N on granite and sandstone, respectively, and significantly higher than on amphibolite (36.3±4.2 N). Thus we suggest that the distribution and proportion of different bedrock types on a moraine coast may affect both the survival of Fucus germlings on a very small scale and may affect the density of plants due to differences in attachment strength and impact of waves.