Physiological and Reproductive Responses of an Intertidal Rockweed to Conditions Associated with Sea Level Rise: Assessing the Potential for Local Adaptation to a Changing World.

Organisms in the intertidal zone experience terrestrial conditions during low tide emersion and are submersed in seawater during high tide. These organisms are threatened by sea level rise (SLR) which could reduce emersion time by over 50% over the next 100 years. Using a manipulative field experiment, we are assessing the impacts of SLR conditions on the canopy-forming seaweed Pelvetiopsis californica. We hypothesized that individuals would perform more poorly under SLR conditions than normal conditions. At two southern California sites, we established three treatments: marked in place individuals (transplant controls: MP) were not moved; middle-zone transplant individuals (MZ) were relocated at the same tidal elevation; and below-zone transplant individuals (BZ) were moved to a lower tidal elevation, exposing them to shorter emersion times (= SLR conditions). We assessed health through monthly measurements of length, branch number, cover of epiphytic seaweeds (which grow on the host and block it from gaining light and nutrients) and Maximum Quantum Yield, a measure of photosynthetic potential. After six months, we saw reduced performance in the SLR condition treatment relative to the non-SLR treatments. The proportion of individuals infected with epiphytic seaweeds in the BZ treatment was over three times the proportion in the MZ or MP treatments, and individuals in the BZ treatment were reduced, on average, to 75% of their original length: 8.75% greater loss than in the MZ treatment. These data indicate substantial risk of SLR to seaweed populations and may help managers develop mitigation efforts for this important intertidal producer.