An exception to the rule? Light, not species interactions, sets the lower limit to the high intertidal seaweed, Fucus distichus.

Background/Question/Methods

Rocky intertidal habitats on the West Coast of North America show strong vertical zonation. Climate change forecasts predict a rise in sea level along the US West Coast, highlighting the immediate need for understanding both the underlying causes of vertical distribution patterns in rocky intertidal habitats and the physiological consequences of distributional shifts. The paradigm of rocky intertidal zonation is that biological factors set the lower limits of a species’ vertical distribution and abiotic factors set the upper limits, though exceptions have been documented. For the common rocky intertidal seaweed, Fucus distichus, we tested the hypothesis that biological, not physical or physiological factors limit its lower distributional limit.

In 2011 we performed reciprocal transplants of F. distichusindividuals in the field, removing individuals from the high zone and replacing them there and in the low zone. The factors evaluated in this experiment included stress, herbivory, competition, and recruitment limitation. We manipulated herbivore abundance and canopy competition at both tidal heights, and surveyed recruit abundance along vertical transects between the high and low zones. In laboratory experiments we manipulated immersion time and light level to test potential mechanisms for observed differences in field experiments.  

Results/Conclusions

Field transplantation of F. distichus showed marked differences in growth and form of low zone and high zone individuals. Individuals transplanted to the low zone had significantly less growth than high zone individuals, irrespective of treatment (ANOVA, P= 0.001). These results indicate a role of low zone abiotic conditions in F. distichus vertical zonation. Manipulating immersion time and light level in laboratory experiments pointed to a role of light in contributing to the observed patterns in the field (ANOVA, P=0.02), with shaded treatments exhibiting lower growth on average (95% Confidence Interval: -0.87, 0.11). We also took physiological measurements, such as photosynthesis versus irradiance curves to investigate the consequences of such shifts. Taken together, these results suggest a role of physical factors in setting the lower distributional limit in this species, providing an exception to the common model of rocky intertidal zonation.