Summary The visual landscape of marine and terrestrial systems is changing as a result of anthropogenic factors. Often these shifts involve introduced species that are morphologically dissimilar to native species, creating a unique biogenic structure and habitat for associated species within the landscape. While community‐level changes as a result of introduced species have been documented in both terrestrial and marine systems, it is still unclear how long‐term shifts in species composition will affect habitat complexity or its potential to influence the biodiversity of species that occur at the base of the food web. We analysed quadrat photos collected at several subtidal sites in the Gulf of Maine over a 30+ year period, and collected individual seaweed species to determine their complexity and the biodiversity of meso‐invertebrates associated with each species. By coupling the relationship of 30+ years of shifts in seaweed assemblages, morphological structure of the seaweed assemblage, and their meso‐invertebrates, we determined introduced seaweeds have increased by up to 90%, corresponding to a rise in two‐dimensional (2D) structure, and a decline in canopy height of subtidal rocky habitats. The highly complex two‐dimensional habitat provided by introduced filamentous red seaweeds supports two to three times more meso‐invertebrate individuals and species that form the base of the food web than simpler forms of morphological habitat. Synthesis. The present study demonstrates a long‐term shift in foundation species towards a dominance of invasive seaweeds that directly reduce canopy height and increase the 2D biogenic structure of the habitat. These introduced seaweeds harbour greater biodiversity of species found at the base of the food web than seaweeds with simpler forms such as the native kelp species. Such shifts in habitat structure will propagate to food webs by influencing the structure of lower trophic‐level meso‐invertebrates and indirectly upper trophic‐level species that feed on these invertebrates and use the seaweed structure as refuge.

Supplement 1: Details of Seaweed Model Construction This supplement provides details of the methods used for seaweed model construction. The code for C.f. spp. fragile and D. japonica was written in C++ and OpenGL. The model for S. latissima additionally used the Bullet Physics Software Developer's Kit.

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2019. Temperature, phenology, and turf macroalgae drive seascape change: Connections to mid-trophic level species.Abstract. Landscape patterns created by the structure and form of foundational species shape ecological processes of community assembly and trophic interactions. In recent years, major shifts in foundation species have occurred in multiple ecosystems. In temperate marine systems, many kelp beds have shifted to turf macroalgae habitats with unknown consequences on seascape patterns or changes in the ecological processes that maintain communities. We investigated the effect of turf macroalgae on seascape patterns in three habitats dominated by kelp and turf macroalgae and those that have mixed species composition. We also examined decadal elevations in temperature with known growth and reproductive phenology of kelp and turf macroalgae to provide a mechanistic understanding of the factors that will continue to shape these seascapes. Our results indicate that turf macroalgae produce a more heterogeneous habitat with greater primary free space than those that are mixed or dominated by kelp. Further, we examined the relationship between seascape patterns and richness and abundance of fishes in each habitat. Results showed that patch size was positively related to the abundance of fish in habitat types, suggesting that turf-induced heterogeneity may lead to fewer observed fishes, specifically the mid-trophic level species, cunner, in these habitats. Overall, our results suggest that persistence of this habitat is facilitated by increasing temperature that shorten the phenology of kelps and favor growth and reproduction of turf macroalgae that make them poised to take advantage of free space, regardless of season.

Aim Environmental variables are strongly tied to species occurrence and population growth, but approaches to predicting the location of deep‐sea species or their ability to withstand a changing environment stem primarily from presence data. We coupled environmental data with observed densities of deep‐sea habitat‐forming corals and sponges to determine the environmental variables and geomorphology that contributed best to their occurrence. Location Northwest Atlantic. Time period 2013 and 2014. Major taxa studied Deep‐sea coral and sponge communities. Methods Multivariate and univariate analyses were used to determine significant environmental contributors to densities of genera and families of corals and sponges. We then assessed the relationship of densities of genera and families of corals and sponges with environmental variables found to be significant contributors to their occurrence and to geomorphology. Results Sponge and coral genera and families were influenced by different environment variables. Temperature, salinity and dissolved oxygen contributed to the occurrence of sponges, whereas seafloor properties of slope and substrate contributed to the occurrence of corals. Although individuals of corals and sponges were observed across a range of a contributing environmental variable, high densities were observed only in very narrow ranges. Main conclusions Geomorphic setting is an effective approach for discerning the associations of coral with seabed features. High densities of coral and sponge genera and families restricted to narrow environmental ranges might be at greater risk of local extinction. Differences in the occurrence of coral and sponge genera and families with environmental conditions suggest that they will differentially respond to predicted environmental changes. As conditions in the deep sea change with ongoing changes in climate, population expansion might be limited owing to suboptimal conditions, and established populations might persist but might have fewer individuals or species, which might lead to a loss in biodiversity.