Macroalgal rafts frequently occur floating in coastal waters of temperate regions of the world's oceans. These rafts are considered important dispersal vehicles for associated organisms with direct development. However, environmental factors may limit the floating potential of kelp and thereby the dispersal of associated organisms. To examine the effect of water temperature and grazing on growth, reproductive output, and survival of floating Macrocystis spp., experiments were conducted in outdoor tanks during austral summer 2006/2007 at three sites along the Chilean Pacific coast (20° S, 30° S, 40° S). At each site, Macrocystis spp. was maintained individually at three different water temperatures (ambient, ambient - 4°C, ambient + 4°C) and in the presence or absence of the amphipod grazer Peramphithoe femorata for 14 d. High water temperatures (>20°C) provoked rapid degradation of Macrocystis spp. rafts. At moderate temperatures (15°C-20°C), algal survival depended on the presence of associated grazers. In the absence of grazers, algal rafts gained in biomass while grazing caused considerable losses of algal biomass. Algal survival was the highest under cooler conditions (<15°C), where raft degradation was slow and grazer-induced biomass losses were compensated by continuing algal growth. Our results indicate that floating kelp rafts can survive for long time periods at the sea surface, but survival depends on the interaction between temperature and grazing. We suggest that these processes limiting the survival of kelp rafts in warmer temperatures may act as a dispersal barrier for kelp and its associated passengers.

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Floating macroalgae play a fundamental role for the connectivity of littoral marine habitats. They can be found throughout the world's oceans but information on their abundance and dispersal abilities is scarce. This information, however, is essential to evaluate their ecological role and the potential connectivity between populations of both the algae and associated biota. Here, we survey the abundance and spatiotemporal dispersal patterns of the floating brown alga Fucus vesiculosus in the Northern Baltic Sea. By incorporating a biophysical modeling approach, based on ocean circulation and surface winds, we demonstrate potential dispersal ranges, such as floating distances and directional trajectories of F. vesiculosus at different times of their reproductive season. Abundance of floating algae varied both spatially and temporally, with the highest densities being found during reproductive seasons. Evidenced by the fact that many algae were held back and accumulated in archipelagoes, the model suggests that a combination of local oceanographic features and seascapes strongly influences the dispersal of floating algae. However, a relatively high fraction of rafts spilled to more open sea areas, causing them to travel in multiple directions (e.g., Baltic Proper, Bothnian Bay, and Gulf of Finland) that are hundreds of kilometers away from their original sources, implying long-distance dispersal. We propose that the Northern Baltic Sea rocky littoral habitats are potentially well connected through abundant and long-reaching floating algae, with a chance to contribute to gene flow among F. vesiculosus populations.

Chemical defense is assumed to be costly and therefore algae should allocate defense investments in a way to reduce costs and optimize their overall Wtness. Thus, lifetime expectation of particular tissues and their contribution to the Wtness of the alga may aVect defense allocation. Two brown algae common to the SE PaciWc coasts, Lessonia nigrescens Bory and Macrocystis integrifolia Bory, feature important ontogenetic diVerences in the development of reproductive structures; in L. nigrescens blade tissues pass from a vegetative stage to a reproductive stage, while in M. integrifolia reproductive and vegetative functions are spatially separated on diVerent blades. We hypothesized that vegetative blades of L. nigrescens with important future functions are more (or equally) defended than reproductive blades, whereas in M. integrifolia defense should be mainly allocated to reproductive blades (sporophylls), which are considered to make a higher contribution to Wtness. Herein, within-plant variation in susceptibility of reproductive and vegetative tissues to herbivory and in allocation of phlorotannins (phenolics) and N-compounds was compared. The results show that phlorotannin and N-concentrations were higher in reproductive blade tissues for both investigated algae. However, preferences by amphipod grazers (Parhyalella penai) for either tissue type diVered between the two algal species. Fresh reproductive tissue of L. nigrescens was more consumed than vegetative tissue, while the reverse was found in M. integrifolia, thus conWrming the original hypothesis. This suggests that future Wtness function might indeed be a useful predictor of anti-herbivore defense in large, perennial kelps. Results from feeding assays with artiWcial pellets that were made with air-dried material and extract-treated Ulva powder indicated that defenses in live algae are probably not based on chemicals that can be extracted or remain intact after airdrying and grinding up algal tissues. Instead, anti-herbivore defense against amphipod mesograzers seems to depend on structural traits of living algae.

Nutrient enrichment in coastal marine systems can have profound impacts on trophic networks. In the Baltic Sea, the population of Great Cormorant (Phalacrocorax carbo sinensis) has increased nearly exponentially since the mid-1990s, and colonies of these seabirds can be important sources of nitrogen enrichment for nearby benthic communities due to guano runoff. In this study we used stable isotope analyses and diet mixing models to determine the extent of nitrogen enrichment from cormorant colonies, as well as to examine any possible changes in herbivore diet preferences due to enrichment. We found significantly higher levels of δ15N in samples from colony islands than control islands for producers (the dominant macroalga Fucus vesiculosus, filamentous algae, and periphyton) and herbivores, as well as a positive correlation between enrichment and nest density in colony sites. We also found that enrichment increased over the breeding season of the cormorants, with higher enrichment in late summer than early summer. While the amount of total nitrogen did not differ between colony and control sites, the amount of guano-based nitrogen in algae was >50% in most sites, indicating high nitrogen enrichment from colonies. Herbivores (the isopod Idotea balthica and the gastropod Theodoxus fluviatilis) preferred feeding upon the dominant macroalga Fucus vesiculosus rather than on filamentous algae or periphyton in both control and colony, and there was a significant increase in periphyton consumption near colony sites. Overall, guano from cormorant colonies seems to have effects on both producers and herbivores, as well as the potential to modify algae-herbivore interactions.