Spatially explicit modelling of kelp-grazer interactions: a seascape approach for effective habitat enhancement using artificial reefs

Abstract

141. Artificial structures are sprawling along the coast affecting the aspect and the functioning of 15 shallow coastal seascapes. For years, the ecology of artificial structures has been investigated 16 mainly in contrast to natural coastal habitats. However, it is increasingly emerging that structuring 17 processes, such as trophic interactions, can depend on properties of the surrounding landscape. 18Heterogeneity of coastal seafloor and habitats could thus play a major role in determining the 19 variability of ecological outcomes on artificial structures. 20Artificial reefs are being used in coastal areas in attempts to restore and enhance marine habitats 21 and communities, including large brown seaweed ("kelp"), to offset habitat loss and mitigate 22 coastal development impacts. The outcome of enhancement projects using artificial reefs have not 23 always been either consistent or positive. Overlooking the effect of strong ecological interactions 24 adds a high level of uncertainty and can undermine the success of these efforts. 25In Eastern Canada, top-down control exerted by green sea urchins (Strongylocentrotus 26 droebachiensis) can seriously compromise the success of artificial reefs for kelp enhancement. 27Importantly, urchin interactions with macroalgae are likely to be influenced by the bottom 28 composition. A seascape approach could thus integrate behavior and habitat heterogeneity. 29 2. We investigated whether the local seascape could create zones of differential grazing risk for kelp 30 outplanting kelp (Alaria esculenta) on artificial blocks on an heterogenous bottom. Adopting a 31 spatially explicit framework, we determined how seascape affected the urchin use of the habitat 32 and used this information to map the grazing risk throughout the area.33 3. Kelp survival was a function of frequency of urchin presence throughout the study site. While 34 urchins avoided sandy patches, bottom composition and algal cover modulated the within-patch 35 urchin use of the habitat. This translated in the heterogeneity of grazing risk intensity.36 4. Synthesis and applications. The presence of discrete seascape features locally increased the 37 grazing risk for kelp by differentially affecting the urchin's usage of the habitat, even within the 38 same bottom patch. Incorporating this information when planning artificial reefs could minimize 39 the detrimental grazing risk thus increasing the rate of success and ensuring lasting results. 40 41 Keywords 42 Spatial; kelp; green sea urchin; artificial reef; survival; grazing risk 43 48 development and foster its sustainability (Bishop et al., 2017; Dyson & Yocom, 2015; Mayer-Pinto et 49 al., 2017; Perkol-Finkel, Hadary, Rella, Shirazi, & Sella, 2018). However, whether investigating 50 biodiversity, colonization and ecological succession, community ecology, or species genetic diversity, 51 so far the predominant approach to study artificial structures has been framed in the dualism typically 52 contrasting alternative categories, such as artificial-natural, vege...