Differential effects of sediments on survival and growth of Fucus serratus embryos (Fucales, Phaeophyceae)

Recruitment of seaweeds through small reproductive stages is limited on sediment inundated rocky shores and largely unsuccessful in soft sediment environments. Burial in sediment has several potentially negative effects for seaweed propagules, and these effects were differentiated in a laboratory experiment. We investigated how light deprivation, sediment type (grain size, organic content, and origin), and sediment chemistry (oxygen presence and toxicity through hydrogen sulfide) affected survivorship and growth of Fucus serratus L. embryos. Presence of hydrogen sulfide had overriding negative impacts on both survivorship and growth of Fucus embryos, independently of sediment type and light availability. In contrast, simple anaerobiosis generally did not impair survival or growth of the embryos. Fine sediments, 3 mm thick, significantly reduced embryo survivorship, presumably through accumulation of metabolic waste products in the immediate vicinity of the embryos as a consequence of constrained diffusion. This effect was equally pronounced in the presence of a 1-mm layer of organically rich biodeposits. Irradiance levels did not affect survival of embryos but influenced growth. Decreasing thickness and increasing coarseness of sediments together represented a gradient of enhanced light penetration and diffusion. Growth of embryos increased along this gradient. In nature, soft sediment environments with organically enriched muds (e. g. tidal flats and salt marshes) represent habitats least favorable for colonization through small reproductive stages of seaweeds.