Non-trophic responses of algal communities to nutrient enrichment: interactions among coralline turfs, ephemeral algae and perennial fucoids

摘要

Coastal eutrophication is known to promote the spread of fast-growing, ephemeral algae, with severe consequences for ecosystem health. Blooms of ephemeral algae within hard-bottom communities are facilitated by shifts from canopy to turf domination in degraded environments. In contrast to this pattern, however, turfs of articulated coralline algae have been shown to limit the development of ephemeral species, but the role of benthic habitat modification by coralline turfs in preventing nutrient-driven assemblage alterations has been scarcely investigated. Here, we tested whether coralline turfs can limit the growth of ephemeral algae in response to increased nutrient availability, and whether this can facilitate the recruitment of perennial canopy-forming fucoids. We manipulated coralline turf abundance and nutrient availability on an intertidal reef for 1 yr. The growth of ephemeral species was stimulated by nutrients, but only transiently during spring, and did not affect fucoid recruitment and final assemblage composition. The presence of the turfs was a major barrier to fucoid recruitment, and, although coralline turfs limited the growth of some ephemerals (Ulva spp.), they also facilitated the spread of others (Colpomenia spp.). These results highlight the resistance of fucoid assemblages to short-term nutrient stress and the threat posed by turf domination to the persistence of important habitat-forming species. Habitat modification by coralline turfs may both mitigate and exacerbate the biotic consequences of organic enrichment. Comparisons with the findings of previous studies indicate that local assemblage composition and the duration of disturbance can contribute to producing complex idiosyncratic responses to nutrient enrichment.