The neuston can be defined as ‘those plants and animals inhabiting the surface film of the sea’. The living conditions in the upper layer of oceans and seas are considerably different from those in deeper layers. Consequently, the neustonic zone forms a restricted ecological niche inhabited by a wide range of permanent or temporaryinhabitants, which are adapted to a stressful way of life in a region of a high organic matter supply. The distribution of neustonic organisms is mainly influenced by vertical migration and temporal and spatial variation. Additionally, winds, currents and the presence of floating objects can give rise to local accumulations of neustonic organisms. Of special interest is the neustonic community associated with the permanently floating seaweed Sargassum, which is abundantly found in the Sargasso Sea. Floating mats of these seaweed species significantly contribute to the primary production in the neustonic zone of the North Atlantic and consequently support a diverse community of marine organisms that use the seaweeds as food source, shelter, foraging grounds or surface for attachment.Next to the permanently floating Sargassum, the neuston is also strongly influenced by the occurrence of smaller, and usually ephemeral floating patches composed of detached coastal seaweed fragments. Ephemeral floating seaweeds harbour a diverse fauna originating from attached seaweeds, the strandline of beaches, the surrounding and underlying water column, the seafloor or the air. These organisms colonise the seaweeds for various reasons, usually including the provision of shelter, food or attachment substrate. The association behaviour of these organisms and their use of the resources offered by floating seaweeds potentially have important ecological consequences, like the possibility of passive dispersal of associated fauna to new, distant locations by means of rafting.The overall aim of this PhD study was to assess the ecological impact of floating seaweeds as ephemeral habitats and potential rafts in the North Sea. Because the information about the neuston and floating seaweed clumps in the North Sea was, up till now, very scarce, different aspects of raft-associated ecology were addressed.The results of this PhD thesis demonstrate that the habitat formed by floating seaweeds is very complex. Although the presence of floating seaweeds in the neuston can, to a certain degree, be seasonally predicted (storms, seasonal release of fertile structures), the habitat that they form is still very patchy and unstable. Consequently, most species found in association with ephemeral floating seaweed patches are opportunistic of nature. However, some species display a higher level of adaptation to this habitat than other species, which is manifested in the reproduction by continuous brooding in Idotea baltica, the diet consisting of seaweed-associated macrofauna in Cyclopterus lumpus and the behavioural shift towards surface pecking and dipping in Sterna hirundo. The association behaviour of the encountered species and their (optimal) use of the transient resources offered by floating seaweeds potentially have important ecological consequences, like for example the passive dispersal of associated fauna to new, distant locations by means of rafting. The process of rafting strongly depends on the longevity of the seaweed raft, which is in turn significantly influenced by temperature and grazing pressure. In favourable conditions, seaweed rafts can potentially cover great distances, carrying with them rafting fauna that are able to survive a long journey in the neuston.