Cross-depth connectivity shows that deep kelps may act as refugia by reseeding climate-vulnerable shallow beds

Abstract

Refugia are habitats where species can survive or retreat to during environmental disturbances. One key assumption of habitats that constitute refugia is that they may assist in the persistence of impacted populations through the provision of reproductive propagules. This “reseeding” hypothesis assumes that demographic connectivity exists between refugia and impacted habitats. We tested this hypothesis for the kelp Ecklonia radiata, a temperate marine foundation species dominant in the temperate coast of Australia. Our study site was the coast of Western Australia, a system where deep habitats are considered to act as refugia for their shallow counterparts, yet, for which estimations of population connectivity have overlooked propagule dispersal across depth. Here, we simulated the dominant ocean circulation conditions in a three-dimensional oceanographic model and the dispersal of kelp propagules by incorporating physical properties of zoospores within the model. The trajectories of kelp propagules were tracked and analyzed to identify their probability of settlement within the domain of the study. Measurements of kelp fecundity across depth were obtained within the study region, and used to estimate zoospore settlement densities. Here, we show that deep populations of E. radiata have the capacity to supply zoospores to shallow reefs. Our results show that zoospores released at deeper beds (40 m of depth) are transported to shallow reefs (<20 m of depth) and that settlement densities are sufficient to drive kelp fertilization. Analysis of particle exchange among depths also indicates that mid-depth reefs (~25 m of depth) act as important “stepping stones” for cross-depth connectivity, enhancing dispersal between the shallowest and deepest locations, which is especially important when reefs of different depths are separated by tens of kilometers. We show that after an environmental disturbance, persistent deep kelps may act as refugia by sourcing propagules and reseeding impacted shallow reefs, provided that environmental conditions are adequate for kelp recruitment and growth. This study demonstrates that through propagule exchange, populations of habitat-building species living in areas undamaged by biophysical disturbances may reseed their impacted counterparts, aid their recovery if conditions are adequate for regrowth, and ensure their persistence.