Tropical marine shallow-water areas are highly productive systems that promote important ecological functions and biodiversity. Stressors on these systems are intensifying due to increasing anthropogenic disturbances on multiple scales. The need to increase understanding of ongoing patterns and processes within the near-shore seascape is therefore imperative.

In Tanzania in the Western Indian Ocean (WIO), introductions of South East Asian (SEA) strains of the red macroalgae Eucheuma denticulatum are made through seaweed farming, with unknown environmental consequences. Because this species occurs naturally in East Africa (EA), an ongoing introduction is difficult to assess. Also, there is limited knowledge about the extent of a possible spread and environmental factors that regulate this. Hence, ecological consequences are difficult to predict. The aims of this thesis are therefore to 1) identify and address important knowledge gaps concerning environmental effects of introduced E. denticulatum on the surrounding tropical seascape, and 2) study ecological processes and factors that influence spread, distribution and interactions with indigenous species. In doing so, the thesis also includes herbivorous fishes associated with macroalgal habitats and environmental variables influencing these fish assemblages.

Paper I reviews the current scientific knowledge on introductions of SEA E. denticulatum in the WIO, and identifies knowledge gaps such as potential competition with native benthic taxa and environmental factors impacting spread. Paper II investigates environmental factors that affect the distribution and presence of SEA E. denticulatum in two geographical locations; one where E. denticulatum has turned into a nuisance and one where this is still unknown. We found that hard substrate and distance to areas of introduction best predicted SEA algal presence. Paper III examines the potential effects of E. denticulatum on corals. A field experiment showed that E. denticulatum did not induce any stress responses in corals, nor could it attach to live corals. Fish herbivory was the strongest factor controlling biomass of algae. In Paper IV and V, we focus on how seascape configuration and environmental factors influence the distribution and herbivory of reef fishes. In Paper IV, we show that macroalgal consumption was density dependent (inversely related to macroalgal cover) and that different habitats held distinct herbivorous fish communities. Moreover, Paper IV and V showed that presence and traits of macroalgae had a positive influence on the abundance of juvenile parrotfish, suggesting the potential of macroalgae to provide important nursery functions.

In conclusion, this thesis indicates that negative effects by farming of  SEA E. denticulatum in the WIO are minor, and might be problematic only in areas already subjected to environmental disturbances. In marine systems with high cover of live coral and healthy populations of herbivorous fishes, competition with indigenous benthic taxa is limited. Furthermore, the thesis highlights that the abundance and ontogeny of herbivorous fishes can be affected by the presence of macroalgal habitats and plant traits, suggesting macroalgal beds are key habitats with important ecological functions to be included in marine spatial planning and conservation efforts.