Effects of opportunistic algae on the early life history of a habitat-forming fucoid: influence of temperature, nutrient enrichment and grazing pressure

Fragmentation and loss of extensive populations of habitat-forming macroalgae have occurred worldwide as a result of human-driven coastal habitat degradation. Contributing to such declines may be the influence of opportunistic algae, which quickly respond to stresses such as nutrient loading and warmer sea temperatures. The early life stages of fucoids are particularly likely to be affected by the combined impacts of altered physical conditions and increased competition with stress-tolerant species. In this study, we examined the interactions between the early life stages of the intertidal fucoid Hormosira banksii and fast-growing, opportunistic green algae Ulva spp. Through a series of laboratory experiments, we tested (1) the impact of Ulva spp. on the settlement of H. banksil zygotes; (2) the impact of Ulva spp. on the rates of post-settlement survival and growth of H. banksii germlings under ambient and increased levels of temperature and nutrient concentration; (3) the interactions between the 2 taxa under grazing pressure of the abundant intertidal trochid Lunella smaragdus. Ulva spp. impaired the settlement of H. banksil zygotes, through both physical and chemical mechanisms, and the growth of the germlings; but Ulva spp. was also preferentially consumed by the grazers, thereby indirectly facilitating H. banksii.. In addition, Ulva spp. responded positively to increased temperature and nutrients, while we found an indication that post-settlement survival of H. banksii may decline with warmer waters. These findings augment extensive field-based research addressing the loss of algal beds and shed light on biotic and physical mechanisms affecting the most critical phases of the life cycle of a key habitat-forming species.