Depth, nutrients and urchins explain variability in Ecklonia radiata (laminariales) distribution and cover across ten degrees of latitude

Foundation species support immense biodiversity and underpin stability and resilience of entire ecosystems. Understanding the physical and environmental conditions that mediate variation in the presence and abundance of foundation species is key to managing and understanding these ecosystems in the face of anthropogenic change. We examine potential drivers of the presence and percent cover of the kelp, Ecklonia radiata along ten degrees of latitude off mainland Eastern Australia. We used a model selection process to determine what environmental drivers explained the most variability in the spatial distribution and cover of Ecklonia. Interestingly, the distribution and cover of Ecklonia was not related to sea surface temperature or latitude. Instead, depth and the presence of urchins best explained the presence of Ecklonia with the addition of nutrients (in neighbouring catchments) and substrate in explaining percent cover of Ecklonia. The highest covers of Ecklonia occurred on reefs close to catchments with high and very high nutrient and sediment loads. The heterogenous distribution and the high covers of Ecklonia at warmer, low latitudes demonstrate the thermal tolerance of this kelp. Kelp forest presence and abundance can be complex and driven by spatially heterogeneous drivers rather than gradients in environmental conditions such as temperature. Understanding the interplay of drivers in structuring kelp forest distribution and cover is critical for predicting how changing climates and anthropogenic stress impacts foundation species and the ecosystems they support.