Kelp forests collapse reduces understorey seaweed ß-diversity.

BACKGROUND AND AIMS: Kelps are the primary foundation species in temperate subtidal rocky shores worldwide. However, global change is causing their decline with consequences for the organisms that rely on them. An accurate assessment of these consequences may depend on which attributes of the associated community are considered. This study shows that conventional α-diversity approaches may overlook some of these consequences compared to spatially explicit approaches such as with ß-diversity. METHODS: A 1-year seasonal study was conducted to compare the macroalgal understorey between healthy reefs with a Laminaria ochroleuca canopy and degraded reefs where the canopy collapsed years ago due to excessive fish herbivory. At each reef, the understorey seaweed assemblage was recorded in five replicate quadrats to estimate α-diversity (total richness, species density, Shannon index) and ß-diversity (intra- and inter-reef scale). KEY RESULTS: The understorey assemblage exhibited a distinct seasonal dynamic in both healthy and degraded reefs. α-Diversity attributes increased in spring and summer; turf-forming algae were particularly dominant in degraded reefs during summer. ß-Diversity also showed seasonal variability, but mostly due to the changes in degraded reefs. None of the α-diversity estimates differed significantly between healthy and degraded reefs. In contrast, spatial ß-diversity was significantly lower in degraded reefs. CONCLUSIONS: Although the loss of the kelp canopy affected the composition of the macroalgal understorey, none of the conventional indicators of α-diversity detected significant differences between healthy and degraded reefs. In contrast, small-scale spatial ß-diversity decreased significantly as a result of deforestation, suggesting that the loss of kelp canopy may not significantly affect the number of species but still have an effect on their spatial arrangement. Our results suggest that small-scale ß-diversity may be a good proxy for a more comprehensive assessment of the consequences of kelp forest decline.

Physiological response to warming in intertidal macroalgae with different thermal affinity.

Changes in the abundance and distribution of seaweeds have been reported worldwide. In Northwest Iberia, cold and warm affinity seaweeds have respectively decreased and increased their abundance. To improve our understanding of their vulnerability to future warming scenarios, the effects of warming on the photosynthetic and respiratory performance of seaweeds with cold-water (Fucus serratus and Vertebrata lanosa) and warm-water (Padina pavonica and Gigartina pistillata) affinities were compared in a highly resolved temperature gradient (7-31 °C) under controlled laboratory conditions. While neither the optimum temperature nor the photosynthetic rate at the optimum temperature showed consistent differences between water affinity groups, the temperature dependence of net photosynthesis was significantly higher in the warm-affinity group. Photosynthesis was less responsive than respiration to warming in cold-affinity seaweeds while both rates showed similar responsiveness in warm-affinity ones, suggesting that the relative responsiveness of respiration and photosynthesis to temperature may be indicative of warming susceptibility.